ASNT NDT Handbook Volume 2_ Liquid Penetrant

Hydrogen Damage from 14 percent nickel, iron base material —
Sulfur Compounds in was evaluated in five such alloys with
Nonstainless Steels sulfur impurities: 80, 120, 240, 890 and
1700 µg·g–1. Nonstress corrosion rates
When an acid solution attacks a carbon or appeared to be directly related to sulfur
alloy steel surface, hydrogen is generated content. Corrosion was uniform rather
and deposited on the metal surface as an than localized. The extent of attack was
adsorbed film of atomic hydrogen. slight compared with that found for
Normally, the film builds up and bubbles commercial purity 14 percent chromium,
off the surface as molecular hydrogen gas. 14 percent nickel, 72 percent iron alloys
However, in the presence of even a few and of commercial, high temperature
parts per million of hydrogen sulfide in steels such as AISI 304.
the solution environment, much of the
atomic hydrogen enters high strength Stressed U-bend samples of 14 percent
steel. The presence of a sulfide in the steel chromium, 14 percent nickel, iron base
appears to catalyze the generation of alloy representing the five levels of sulfur
hydrogen. The formation of molecular were exposed to 42 percent boiling
hydrogen internally in voids produces magnesium chloride for 600 h. No
high pressure, creates blisters on the evidence of cracking was observed. Similar
surface and reduces the ductility. Delayed alloys of commercial purity cracked after
failures then can occur at stress levels well only 28 h exposure of the stressed
below the yield strength. The minimum samples to the strong chloride corrodent.
yield strength level for sulfide stress
cracking is about 600 MPa It was concluded that, to produce
(90 000 lbf·in.–2). The tendency for excellent corrosion resistance to aggressive
cracking is reduced as the pH rises from media, all impurities, rather than just
3 to 6 (solutions become less acid). sulfur, must be excluded from austenitic
Inorganic and aliphatic organic sulfur stainless steels. Although increasing
compounds are generally more reactive additions of sulfur increased the corrosion
than aromatic organics. Sulfide corrosion rate slightly in the high purity austenitic
of nonstainless steel is reduced by the matrix, the corrosion rate of the
presence of halides in the low 1700 µg·g–1 or 0.17 percent sulfur alloy
temperature aqueous phase. was much below that of commercial
purity materials. Residual sulfur
Corrosive Effects of Sulfur accumulations from liquid penetrant
on Various Alloys materials would not pose a special
corrosion or stress corrosion problem at
High Yield Strength Stainless temperatures below 100 °C (212 °F) for
Steels parts produced from austenitic stainless
steels.
The high yield strength AISI 635 stainless
steel (17 percent chromium, 7 percent Nickel and Nickel Base Alloys
nickel) is very susceptible to cracking in a
high sulfide environment (1000 µg·g–1 of Materials in which nickel predominates
hydrogen sulfide). Stainless steels with are subject to intergranular attack when
chromium contents less than 9 percent they are heated in the presence of sulfur
may corrode faster in a given sulfide and sulfur compounds. Grain boundary
concentration than carbon steel. stress corrosion cracking and nickel has
occurred in sulfide environments above
AISI 300 Austenitic Stainless Steels 643 °C (1190 °F), the melting point of the
nickel/nickel sulfide eutectic.
Adequate protection against sulfide
corrosion is provided by the lower Fuels used in the heat treatment of
strength standard AISI 300 series nickel base alloy materials must have a
austenitic stainless steels with a basic alloy low sulfur content. A marginal
content of 18 percent chromium and concentration of sulfur is stated to be
8 percent nickel. 0.73 g·m3 of gases manufactured from
coal and oil. Lubricants for deep drawing
Iron Base Alloy with 14 Percent and spinning nickel alloys generally
Chromium and 14 Percent Nickel contain sulfur or lead and therefore must
be removed before annealing.
The effect of sulfur as an impurity in a
completely stable austenitic structure —
a high purity 14 percent chromium,

Liquid Penetrant System Chemistry and Effluent Waste 293

Environmental Conditions
Affecting Intergranular
Attack

Intergranular attack by sulfur has been
reported only for nickel base alloys
subjected to elevated temperatures.
However, it seems probable that austenitic
stainless steels could also be adversely
affected by exposure to sulfur or its
compounds in its sensitizing temperature
range of 590 to 650 °C (1100 to 1200 °F).
At temperatures below 150 °C (300 °F),
steels with a maximum yield strength of
520 MPa (75 000 lbf·in.–2) should not be
subject to sulfide stress cracking. The
tendency for hydrogen absorption and
embrittlement is reduced by raising the
pH of the environment to 6 or greater.
High yield strength austenitic such as
17 percent chromium, 7 percent nickel
alloys should not be susceptible to
cracking if the sulfide content of the
environment is kept well below
1000 µg·g–1.

294 Liquid Penetrant Testing

PART 2. Mechanisms of Stress Corrosion
Cracking of Austenitic Stainless Steels

Restrictions on Halogens in Mechanisms of Stress
Expendable Test Materials Corrosion Cracking of
Used on Austenitic Austenitic Stainless Steels
Stainless Steels
Austenitic stainless steels can fail by stress
For years, it was common practice in the corrosion cracking, a brittle failure in
aerospace industry to use halogenated ductile material which occurs as a result
cleaners, acids and degreasers in the of the combined action of tensile stress
treatment of AISI 300 series stainless and corrosion. Tensile stress may be
steels. However, pressure vessel codes and externally applied or residual but it must
other standards in the United States be sustained or static. Where allowable,
prohibit expendable cleaners, liquid shot peening to alleviate surface tensile
penetrants and processing materials stresses will reduce the tendency for
containing more than one percent cracking. Although the minimum stress to
halogens. Investigations of cause and initiate stress corrosion cracking will vary
prevention of stress corrosion cracking of with the environment and with steel
stainless steel have demonstrated that less composition differences, stresses
than 50 µg·g–1 of certain hydrolyzable reportedly as low as 14 MPa
chlorides in combination with 1 to (2000 lbf·in.–2) have resulted in stress
200 µg·g–1 of oxygen can cause cracking corrosion cracking of AISI 347 austenitic
under low stress and low temperature stainless steel. It would seem that
conditions. The likelihood of stress significant stress concentrators would
corrosion will diminish with decreases in have to be operating to bring about such
concentration of halides and/or oxygen, low stress failure in AISI 347 surrounded
with decreases in stress and with decreases by vapors at 200 °C (400 °F) containing
in temperature. Laboratory specimens of 50 µg·g–1 of chloride.
stressed austenitic stainless steels failed
when immersed in a 5 µg·g–1 chloride Other Mechanisms of
solution at 93 °C (200 °F) and in a Cracking of Austenitic
10 µg·g–1 chloride solution at 74 °C Stainless Steels
(165 °F).
Not all cracking occurring in fabrication
Because chloride concentrations in or service is halide induced. Three other
normal environments and residual stresses modes of failure exist that can take place
from fabrication and heat treatment at stress levels below the yield strength of
generally exceed these levels, one might austenitic stainless steel in a noncorrosive
wonder at the usual long life and high environment.
performance characteristics of most
stainless parts. There may be a 1. Fatigue failure may be induced
relationship between hydrogen thermally, mechanically or by
embrittlement and stress corrosion vibration during operation.
cracking. A further restriction of hydrogen
generation in the right form and location 2. Long time, elevated temperature stress
could limit the chance for stress corrosion rupture may occur.
cracking to occur.
3. Differences in coefficient of expansion
The fact that many of the halogen and in thermal conductivity between
compounds are volatile does not negate cladding and base metal may lead to
the potential danger. Cyclic evaporation failure.
and subsequent condensation is actually a
means of concentrating the halogen in a In boiling water reactors, non–stress
restricted area such as a crack or crevice. corrosion cracks were found to be
Entrapment and contamination with longitudinal in cold worked tube and
surface adherent material can reduce the circumferential in solution annealed
tendency of halogen compounds to tubing. Although all potential causes
become volatile. should be considered in a failure
investigation, good design should
minimize the likelihood of cracking from
mechanisms other than stress corrosion.

Liquid Penetrant System Chemistry and Effluent Waste 295

Fracture Paths Resulting concentration (pH) of the solution. The
from Stress Corrosion concentration effect of intermittent
wetting and drying will shorten the time
Both intergranular fracture and required for crack initiation. Under a
transgranular fracture may occur as a constant stress level, cracking will occur
result of stress corrosion. Usually, cracking most easily at 85 °C (185 °F), the
produced in stainless steel exposed to maximum temperature for corrosion
chlorides is transgranular. However, where acceleration at a high enough level for
there is a continuous grain boundary crack initiation. The temperature
segregation, intergranular cracking will limitation of 85 °C provides a minimum
take place. There is a disposition toward a driving force for progressive corrosion
chemical concentration differential such that cracking must occur within 48 h
between grains and grain boundaries or the rate of passive film repair will
because impurities and alloy constituents exceed the rate of penetration.
tend to diffuse to the boundaries.
Test for Susceptibility to
Segregated solute such as silicon or Stress Corrosion
phosphorus can occur in the grain
boundaries of properly solution annealed The establishment of a standard corrosion
AISI 304. On the other hand, a medium could serve as a tool for
continuous grain boundary part of second measuring the susceptibility of material or
phase, alloy carbides is formed by a parts to stress corrosion cracking. Because
sensitizing treatment at 663 °C (1225 °F) 875 µg·g–1 of sodium chloride vapors at
for 2 h followed by slow cooking. The 200 °C (400 °F) will produce severe
likelihood of forming continuous carbide cracking in a short time, this might be
at the grain boundaries is a function of used as such a standard corrosion
grain size and carbon percent. medium.

The tendency for grain boundary Electrochemical Theory of Crack
attack by a corrodent is independent of Propagation
pH between 4.7 and 9.4. Surface pickling
with boiling nitrate chromate can cause The electrochemical theory of stress
cracking only on sensitized material. corrosion assumes crack advancement by
Intergranular attack on an oxide free anodic dissolution of stained metal at the
surface will only happen on stressed crack tip. It has been postulated that
material or in crevices. nitrogen may be responsible for the
corrosion attack on strained, crack
Causes of Crack Initiation sensitive areas. Based on work with low
nitrogen, 18 percent chromium, 8 percent
Plastic deformation may be a necessary nickel and 20 percent nickel alloys,
condition for establishing a propensity for investigators suggest that nitrogen, under
cracking. In austenitic stainless, even conditions of lattice strain, readily diffuses
elastic strain is associated with a small to lattice imperfection sites. Nitride
amount of creep (plastic strain). Cracks precipitation then forms a cathodic area
can originate from elongated pits or from that stimulates corrosion of the adjoining
eroded grain boundaries. Reducing the alloy, thereby favoring crack propagation.
temperature of the environment extends
the life of the part before crack initiation In nonsusceptible high nickel alloys,
but does not alter the minimum stress nickel apparently inhibits the formation
level requirements for cracking. of the cathodic precipitates. Electron
microscopy has revealed what appear to
Stress corrosion cracking always be precipitated nitrides on slip planes. Slip
produces corrosion products. Apparently, increases the effective area of the cathodic
reaction of the chloride on bare metal nitride precipitates and thereby increases
exposed at breaks in the protective oxide galvanic corrosion and aids crack
film on the stainless steel produces local propagation. Cracking may occur at
anodic areas. The latter set up galvanic anodic paths because of reaction of
action leading to pitting and thence to stainless steel with corrosion produced
crack initiation when under tensile stress. hydrogen. This hydrogen diffuses under
The minimum stress to initiate stress an applied load to form stress oriented
corrosion cracking depends on (1) the transgranular bands of hydrogen strained
nature of the corrodent medium, ferrite. Such transformed bands, highly
(2) temperature, (3) anion availability, anodic to the austenitic matrix, dissolve
(4) history of the material and in a corrosive halide such as magnesium
(5) availability of minimum amounts of chloride to cause cracking. In
chloride and oxygen or minimum nonsusceptible alloys, hydrogen
amounts of hydroxide. precipitates as a hydride.

The rate of corrosion is unaffected by
varying the initial hydrogen ion

296 Liquid Penetrant Testing

Some investigators suggest that quantities of transformed austenite at the
cracking originates from a condition of tip of the crack or as a network of strain
reduced surface energy brought about by bands within the austenite grains.
the adsorption of some ion species from
the corrosive medium. Electrochemical The trace of a crack on the surface does
processes might contribute to failure by not necessarily indicate that the same
removing existing films at localized areas propagation process fracture path has
that would otherwise prevent adsorption occurred in the bulk. The three
of the critical ion species or, at higher dimensional path of a crack is determined
stress levels, might prevent healing of mainly by the stress distribution near the
oxide films that become ruptured during advancing edge. This is not necessarily
loading. related to microstructural features (grain
size and orientation) or crystallographic
Electrochemical/Mechanical planes.
Theory of Crack Propagation
Effect of Hydrogen Embrittlement
The electrochemical/mechanical theory of on Stress Corrosion Cracking
stress corrosion presupposes a two stage
repeating cycle consisting of a short Hydrogen embrittlement must be
period of chemical attack, which advances assumed to have relation to stress
the crack very little but which triggers a corrosion cracking. It is known that
sudden mechanical fracture over a longer protons (hydrogen nuclei) can move
distance and which is stopped apparently along with dislocations during plastic
by a soft slip plane. Once a crack is deformation because of the mobility of
initiated, it is not permanently arrested by the proton. Hydrogen embrittlement can
contact with grain boundaries, twin only occur when the proton diffusion rate
planes, or inclusions, nor is its is equal to or greater than the rate of
propagation influenced by changes in movement of dislocations. Hydrogen
applied stress or by changes in the nature collects preferentially in the tetrahedral
of the medium. lattice positions in the (112) planes, in the
discontinuities associated with these
At a low stress level, cracking occurs planes and probably in the dislocations
only in the area of the highest stress. related to this crystallographic system.
Studies indicate that cracks extend in Movement of the most unstable
short bursts at average rates up to dislocations can be accomplished in the
4 mm·h–1 in the condensing vapors above presence of hydrogen by stresses well
an 875 µg·g–1 sodium chloride solution at below the elastic limit, resulting in
204 °C (400 °F) or up to 10 mm·h–1, in a delayed failures.
boiling magnesium chloride environment.
Some investigators have recorded acoustic Stress Corrosion Immunity,
emission, suggesting association with Susceptibility and
crack extension. Other investigators were Prevention
unable to detect acoustic emission during
crack propagation. Six general rules postulated for
susceptibility to stress corrosion are listed
These observations suggest that very below.
little acoustic emission activity
accompanies intergranular stress corrosion 1. A pure metal is immune to stress
cracking whereas the opposite is indicated corrosion cracking.
for transgranular stress corrosion cracking.
Perhaps high stacking fault energy, 2. Alloys made from pure metals may be
indicating tangled dislocations and easy susceptible.
cross slips in an alloy, inhibits cracking.
Rapid stress corrosion crack propagation 3. Corroding conditions (media or
occurs in alloys having highly oriented environment) that produce cracking
dislocations (low stacking fault energies) are specific to an alloy or alloy system.
and restricted slip.
4. Cathodic protection can prevent stress
Effect of Wedging Action of corrosion cracking or even stop crack
Corrosion Products propagation if applied while cracking
is in process.
Deposition of corrosion products within
progressing cracks may provide a wedging 5. One or more minor impurity elements
action that causes brittle fracture beyond in a metal or alloy can affect its degree
the tip of the crack. Pressures in excess of of immunity or susceptibility.
50 MPa (7000 lbf·in.–2) have been
measured for the wedging action of the 6. Changes in structure or homogeneity
corrosion product. It has been proposed of an alloy by heat treatment can
that the chemical aspects of cracking are influence its immunity or degree of
due to preferential attack on small susceptibility.

Liquid Penetrant System Chemistry and Effluent Waste 297

Effect of Cathodic TABLE 1. Effects of chlorides on stainless steels.
Protection on Stress
Corrosion Cracking Chlorides General Attack Stress Corrosion

A small amount of cathodic protection Ammonium chloride pitting slow (long exposure)
may inhibit stress corrosion cracking Calcium chloride nil rapid
whereas a large amount may induce it. Chromium (III) chloride heavy nil
Arrangement of anodes in reactors, piping Iron (III) chloride heavy nil
and heat exchangers for sufficient Magnesium chloride nil rapid
protection is frequently impossible and Mercury (II) chloride heavy nil
sacrificial anodes may be undesirable Potassium chloride pitting slow (long exposure)
because of the corrosion products Sodium chloride pitting slow (long exposure)
introduced into the coolant. Where the Zinc chloride nil rapid
highly stressed component is the cathode,
stress corrosion cracking may be stopped Corrosive Environments
or restarted at will by applying an
external potential. The rate of corrosive attack on stressed
stainless steel by various chlorides is
Conditions Conductive to essentially inversely proportional to their
Stress Corrosion Cracking rate of attack on unstressed stainless.
Information on the comparative effect of
Wherever combinations of high residual specific chlorides on stressed versus
stress, crevices for entrapment of halides unstressed AISI 300 series stainless steels is
or hydroxides, heat transfer, evaporation given in Table 1.
and concentration occur, stress corrosion
cracking is likely. Chlorides can also It would appear that stress corrosion
concentrate in aluminum corrosion requires selective localized attack, which
products by an ion exchange mechanism. can only be provided by corrodents that
Thus, fabricated stainless steel parts that do not affect the pure alloy.
have been tested with liquid penetrants or
cleaned by chlorinated solvents are prime Chloride Removal
targets for stress corrosion. Following Liquid Penetrant
Testing of Stainless Steels
Conditions That Inhibit
Stress Corrosion Cracking Stainless steel components should be as
free as possible of surface residual
Any approach to prevention or inhibition chlorides after cleaning or liquid
of stress corrosion cracking requires an penetrant testing, particularly before heat
understanding of the mechanism of treatment, welding or long term storage.
cracking. A reduction of stress level will A quantitative chloride determination will
reduce the likelihood of rupture and will be required to establish the accuracy of a
increase the life of the part. As previously given flushing procedure. Acid treatment
stated, reducing the temperature will should be avoided to ensure that no
extend part life but will not alter the intergranular corrosion produces a site for
minimum stress level for cracking. Virtual liquid sodium attack in a breeder reactor
elimination of either halides or oxygen or for stress concentration. Final flushing
from the environment (including liquid after liquid penetrant testing of AISI 304
penetrant testing fluids) will prevent stress stainless steel can be done with alcohol,
corrosion as long as the hydroxide acetone or an aromatic hydrocarbon
concentration is under five percent. solvent such as benzene or toluene. The
Cathodic protection will be an effective latter should provide effective removal of
deterrent where it is practicable. Changes adsorbed and entrapped chlorides.
in design and operating conditions will be However, all are highly flammable.
helpful if, for example, opportunities for
fluid entrapment are reduced and Fluids for Cleaning of
operating temperatures and stress levels Stainless Steels
can be lowered. A substitution of less
susceptible material such as carbon steel, Several chlorinated solvents such as
nickel base alloys or corrosion resistant nonflammable, low toxicity
nickel-copper alloys for stainless steel is trichlorotrifluoroethane (refrigerant-113)
another preventive measure. Tensile stress can be used to clean austenitic stainless
relief by peening vibration or heat steels regardless of heat sensitization and
treatment can reduce the effect of a degree of stress. The accompanying low
corrosive medium.

298 Liquid Penetrant Testing

boiling point of 45.8 °C (114 °F) facilitates to 200 µg·g–1 or less of oxygen can result
complete removal by evaporation.2 in stress corrosion cracking.
However, these solvents are no longer
available for cleaning. They were Low concentrations of total halides
regulated because of their high potential (5 to 50 µg·g–1) cannot be accurately
for ozone depletion. determined by current standard analytical
procedures. Ion chromatography can
It has been reported that sulfamic acid, analyze halide concentrations in this
HSO3NH2, can be used for cleaning range (see ASTM E 1654 for procedures).
austenitic steel without causing stress
corrosion cracking. A five percent solution Final cleaning is recommended before
of this acid at 70 °C (160 °F) can be heat treatment, forming, fabrication,
applied for periods up to 24 h. No data storage or shipment to remove water,
have been reported to suggest that dissolve grease and oil, float away
nonhydrolyzable halides such as insoluble particulate — without
fluorocarbons were associated with stress promoting stress corrosion cracking.
corrosion cracking.

Superior resistance to stress corrosion
has been reported for a high alloy,
columbium stabilized, iron base,
nickel-chromium-molybdenum austenitic
stainless steel. This has a nominal analysis
of 30 percent chromium, 34 percent
nickel, 2.5 percent molybdenum and
3.5 percent copper. The high nickel
content is credited with supplying the
improved resistance to stress corrosion
cracking Laboratory tests in the most
severe corrodent, boiling magnesium
chloride, show more than a tenfold
increase in life for the columbium
stabilized nickel-chromium-molybdenum
austenitic stainless steel over AISI 304;
however, the cost of the high alloy
material is about four times that of
AISI 304 for tubular products.

Conclusions on Control of
Stress Corrosion Cracking

As long as the ASME Boiler and Pressure
Vessel Code3 limits total halogens in liquid
materials for nondestructive testing
methods to one percent, materials for use
on austenitic stainless steel components
should be selected for minimum halide
content consistent with effectiveness and
cost. The lower the halide residual
concentration in the environment, the
lower the possibility of stress corrosion.
Thorough cleaning for removal of halide
containing materials after their use will
effectively eliminate any potential for
stress corrosion.

Stainless steels containing a nominal
8 percent nickel — such as AISI 304,
AISI 316 and AISI 347 — have maximum
sensitivity to chloride stress corrosion
cracking. Attainment of minimum stress
levels in stainless steel components is
therefore desirable. Thermal, vibrational
or peening stress relief can reduce residual
stresses. Residual and/or applied tensile
stresses as low as 14 MPa (2000 lbf·in.–2)
in combination with 74 °C (165 °F) or
higher fluid environments containing 1 to
50 µg·g–1 or less of certain chlorides and 1

Liquid Penetrant System Chemistry and Effluent Waste 299

PART 3. Methods for Sulfur and Halogen
Analysis of Liquid Penetrant Materials

Effects of Sulfur and techniques either/or ion
Halogens on Liquid chromotography,4 when liquid penetrant
Penetrant Testing testing materials are applied to nickel base
Materials alloys, austenitic stainless steels and
titanium. To accommodate requirements
Liquid penetrant testing is used on most that are more restrictive than those
nonferrous metals, particularly aluminum, specified in the ASME Boiler Code,3
magnesium, titanium, stainless steel and consideration has been given to lower
high nickel alloys. Stainless steels and limits on impurities in expendable
titanium have been found to be subject to materials that will contact such metal
corrosion and embrittlement because of surfaces.
contact with chloride ions. High nickel
alloys have been found subject to Contaminants levels are the result of
corrosion and embrittlement by contact impurities in the bulk chemical
with sulfur in the form of sulfide ions at ingredients used to formulate liquid
elevated temperatures. penetrant materials. Therefore, some
variations may be observed between
Specifications Limiting materials from different suppliers,
Sulfur and Halogen in between different types of materials from
Liquid Penetrant Testing a single supplier and between samples of
Materials the same type of material purchased from
a single supplier at different times.
Because of these findings, limits have
been placed on the amount of sulfur and Specifications for Analysis
halogen that may be present in liquid for Sulfur or Halogens
penetrant testing materials used on high
nickel alloys, stainless steel and titanium, Analyses for sulfur or halogen impurities
particularly in components of nuclear usually are conducted by the following
power plants. The specifications have methods although other methods are
usually been for total sulfur content and known to be equivalent or superior in
total halogen content. Here it was accuracy, reproducibility and repeatability.
assumed that the worst possible case
would be where the liquid penetrant 1. Analysis for water leachable chlorides
testing materials are not cleaned off the is conducted according to
part after inspection and the test part is ASTM D 24417 (modified by water
heated to temperature that will reflux and potentiometric titration).
decompose the test materials, releasing
the elements present for reaction with the 2. Analysis for halogenated compounds
metal of the part. is conducted according to
ASTM D 808.6 Chloride compounds
Impurity Level are absorbed in sodium carbonate
Requirements for Liquid (Na2CO3) solution. The chloride
Penetrant Testing content is determined by
Materials potentiometric titration.

Impurity level requirements for liquid 3. Analysis for sulfur is conducted
penetrant testing materials are restricted according to ASTM D 1295 or
by the ASME Boiler Code, Section V, ASTM D 1552.8
Article 6.3 This code limits maximum
residual to 1 percent total sulfur and Some specifications stipulate evaporation
1 percent total halogens as determined by of sample (preceding sample
ASTM D 1295 and ASTM D 8086 analytical decomposition) whereas other
specifications call for an untreated
(whole) sample. For example, Section V
of the ASME Boiler Code3 specifies
ASTM D 1295 and either ASTM D 8086
or ion chromatography.4

300 Liquid Penetrant Testing

Detrimental Sulfur and Halogen Analysis only for ionizable sulfur and
Content of Liquid Penetrant halogen is probably adequate as long as
Testing Materials the tested parts will not be subjected to
conditions such as extreme heat that can
There is some difference of opinion on break down the residual liquid penetrant
how much sulfur or halogen content is testing material. The products of thermal
really detrimental to any material. This is or chemical breakdown are usually
because the sulfur or halogen may be ionizable compounds that may cause
present in a number of forms and degrees attack.
of permanence and much is still unknown
concerning the interactions of these Sulfur Analysis of Liquid
elements with sensitive metals. Some Penetrant Testing
specification writers feel that if the Materials
undesired elements are present in any
form where they may be physically The most common sulfur test specified is
captured for analysis and in any amount the ASTM D 1295 method for total sulfur
over the slightest trace, the test material is content. This sulfur analysis can be made
unacceptable. These users require analysis on all organic materials. In this test, the
of the as-received liquid penetrant testing sample is first decomposed by burning in
material from its manufacturer. a high pressure oxygen bomb. The sulfur
present is thus converted to sulfur dioxide
Other specification writers note that and sulfur trioxide. These are absorbed in
liquid penetrant testing materials are a sodium carbonate solution, forming
often sprayed on the parts and, in any sodium sulfate. Barium chloride is added
case, are present only in thin layers on to convert the sulfur from soluble sodium
the part, often for considerable amounts sulfate to insoluble barium sulfate, which
of time. Thus, during normal testing, is filtered out and weighed. The amount
considerable evaporation of volatiles may of sulfur present is calculated from its
take place. The detrimental sulfur or molecular fraction in barium sulfate.
halogen content then is considered to be
that of the residue after normal This method can reliably detect sulfur
evaporation has taken place. This contents as low as 0.1 percent
evaporation can either increase or (1000 µg·g–1). It becomes very unreliable
decrease the apparent sulfur or halogen below this level because of weighing
content, depending on the form in which inaccuracies and losses of materials that
the contaminant is present. If the pass through the filter used to collect the
contaminant is present as part of a final yield. Further problems can arise
volatile vehicle, its apparent amount is with many liquid penetrant testing
decreased by evaporation. Any materials containing elements such as
contaminant present in the nonvolatile iron, aluminum, calcium, silicon or lead
residue will be concentrated. or substances such as silica, asbestos or
mica. All these substances cause
Significance of Ionizable precipitates that may falsely be measured
Compounds of Sulfur or Halogens as the barium sulfate precipitate that
indicates sulfur by this test.
Still other specification writers feel that
there is a difference between ionizable Alternative Tests for Total
and nonionizable sulfur or halogen. Sulfur Content
Actually, no attack on or reaction with
metal can occur unless the element can Several sulfur tests can be used if preferred
ionize. Then it can leave the compound or where ASTM D 1295 is not applicable.
in which it was originally bound and This ASTM D 15528 test method also
produce new bonds with the metal. Sulfur measures total sulfur content. It operates
and the halogens are normally found in by burning the sample in a stream of pure
both inorganic and organic forms. The oxygen to form the sulfur dioxide and
inorganic forms of sulfur and the sulfur trioxide. A starch iodate solution
halogens usually ionize fairly easily and so normally blue in color is bleached clear by
are readily available for reaction with the the absorption of sulfur oxides. The
metal. The organic forms of sulfur and the titrant is often added and the color
halogens usually are very tightly bound. measured photoelectrically by an
As long as the organic compounds automatic or semiautomatic titrator. This
containing sulfur and halogen are stable, test can measure as little as 600 µg·g–1
these elements may be quite harmless to sulfur in samples on which it is
metal. For this reason, some specifications applicable.
require tests of aqueous extractions of the
test materials. Ionizable contaminants
that could attack parts are then detected
whereas nonionizable contaminants are
ignored.

Liquid Penetrant System Chemistry and Effluent Waste 301

Lamp Method for Total Sulfur anions of interest are separated on the
Content basis of their affinities for a strongly basic
anion exchanger. The separated anions are
A third method that has been proposed is measured by conductivity. They are
the ASTM D 12669 lamp method for total identified on the basis of retention time
sulfur content. Here the liquid sample is as compared to standards. Quantitation is
burned in a wick type lamp. An absorbent a measurement of peak area or peak
wick collects the combustion products height. For details of technique, see
including the sulfur oxidized to sulfuric Annex A4 of ASTM E 165.4
acid. Sulfur is then either determined
acidimetrically by titration against This method provides a single,
standard sodium hydroxide or instrumented technique for rapid,
gravimetrically by precipitation as barium sequential measurement of common
sulfate. anions such as bromide, chloride,
fluoride, phosphate etc. and must be
The lamp method is sensitive because considered an alternative technique for
large samples are used. It will easily find testing for those compounds as well as for
20 µg·g–1. However, it is usable only on sulfur. Ion chromatography is much easier
liquids that do not contain suspended and quicker to run than the standard
solids. Therefore, it would not work on ASTM D 1295 and is much more sensitive
liquid penetrant testing developers. with equivalent precision.
Further acid forming elements such as
phosphorus or halogens if present in Halogen Analysis of Liquid
substantial quantity will interfere with Penetrant Testing
acidometry. Phosphorus is often a Materials
constituent of penetrant materials.
Halogen is the family name for the group
Coulometric Measurement of elements including, from the lightest to
the heaviest, fluorine, chlorine, bromine,
Instrumentation is available that burns iodine and astatine. Astatine is not stable
the sample in a stream of oxygen and and is not normally found in measurable
inert gas (helium or argon). This converts or significant quantity in nature. For test
the sulfur to sulfur dioxide, which flows purposes, it can be disregarded. Of the
into a titration cell where it reacts with an other four elements, fluorine is the most
iodine solution. An electric current is run active and reacts differently from the
through the cell to regenerate the iodine remaining three. Therefore, it requires
and the current required is a measure of different methods of analysis than the
the sulfur content. others. In fact, fluorine is not measured as
part of the total halogen analysis that is
This test operates best with liquids but often reported. Although there is evidence
can be adapted to solids as well. The that fluorine, in the form of fluoride ions,
equipment for this test requires may attack some metals, only limited
substantial investment. work has been done in this area. Limits
on fluoride content are normally applied
Bomb Turbidemetric Sulfur Test to test materials by only a few
specifications, so fluoride analysis will be
A fifth test that has been found very briefly detailed here. The reporting of
useful for the measurement of sulfur in total halogens (not including fluorine) is
materials that are completely combustible required by most specifications (including
is bomb decomposition as in Section V of the ASME Boiler Code3 and
ASTM D 1295 followed by turbidemetric ANSI/ASME B 31.711) and is recommended
sulfur determination as in ASTM D 516.10 by ASTM E 165.4

For this test, samples are burned in an Difficulties in Analysis of Chlorine
oxygen bomb as in ASTM D 1295 except Content
that sodium hydroxide is used as the
absorber instead of sodium carbonate. The Chlorine is by far the most common and
bomb washings are diluted to a known possibly the most troublesome halogen.
volume; glycerin, sodium chloride and Most of the analytical procedures used for
barium chloride are added. Light the halogens are designated as chlorine
transmission measured photometrically tests although, in most cases, they are
through the mixture is proportional to sensitive to bromine and iodine as well.
the amount of barium sulfate precipitate. Thus, if the analysis must be for chlorine
only, it is necessary to use special
Ion Chromatography procedures to remove the unwanted
bromine and iodine before the chlorine
Still another method for sulfur testing is analysis is performed. Furthermore,
ion chromatography. After the sample is
prepared by bomb combustion, a filtered
aliquot is injected into a stream of
carbonate/bicarbonate eluant and passes
through a series of ion exchangers. The

302 Liquid Penetrant Testing

fluorine will probably not be reported The procedure detects all the inorganic
because most of the tests applied in 1981 chlorine and most of the organic chlorine.
for chlorine will not respond to fluorine.
Bomb Decomposition
The chloride ion, when present in Potentiometric Analysis for
inorganic form, is very soluble in water Chlorine
and so is readily available for a variety of
analytical methods. Refluxing the sample Another method for measuring chlorine is
is necessary to leach out all the chloride the bomb decomposition potentiometric
for analysis. When chlorine is present in titration method. This method
organic form, however, it binds very decomposes the sample in an oxygen
tightly to the rest of the molecule and bomb as in ASTM D 808.6 Analysis is done
cannot usually be obtained for analysis by potentiometric titration of the metallic
unless the molecule is destroyed. This sodium method described above.
destruction is usually accomplished by
heat, often in the presence of oxygen. The This method is sensitive, detecting
effluent gases are then absorbed in some chlorine in samples down to 3 µg·g–1. The
solution for analysis. method, in common with many others,
actually measures bromides and iodides
Bomb Decomposition Gravimetric along with the chlorides. This method is
Analysis of Chlorine described in detail in Annex A2 of
ASTM E 165.4
Probably the most common test for
chlorine is the bomb decomposition Characteristics of Various Methods
gravimetric analysis method of of Analysis for Chlorine Content
ASTM D 808.6 For this test, a weighed
sample is burned with oxygen. The All of the preceding methods can properly
gaseous combustion products are absorbed be used for the measurement of chloride
into a sodium carbonate solution, which ions, so long as each is used within its
converts the chlorides to sodium chloride. particular limitations. All actually measure
Silver nitrate is added to precipitate the total chloride, bromide and iodide. The
chloride as silver chloride, which is then bomb method of ASTM D 8086 is less
filtered out and weighed. sensitive to bromide and iodide so these
ions will not be fully detected. All may be
The inadequacy of ASTM D 8086 is subject to interference from other
generally recognized because this chlorine substances that may be present in liquid
test is not sensitive below 0.1 percent penetrant testing materials, so that test
(1000 µg·g–1) nor above 50 percent results may not always be accurate. The
content. This test does break down and best way to check for accuracy is to add a
measure all organic chlorides as well as known amount of chloride ion to the
the inorganic chlorides. It measures not sample and remeasure. The percent of this
only chlorine but also bromine and known amount actually recovered will
iodine, because the silver salts of these allow a good estimate of the accuracy of
elements are very similar to the silver the analysis to be made.
chloride. The test will not measure any
reportable quantity of fluorine, nor will it Ion Chromatography for Chlorine
pick up all the bromine and iodine that Analysis
may be present. Phosphorous causes
interference with this test, leading to Another method for halogen testing is ion
falsely high readings. chromatography. After the sample is
prepared by bomb combustion, a filtered
Metallic Sodium Method for aliquot is injected into a stream of
Chlorine Analysis carbonate/bicarbonate eluant and passes
through a series of ion exchangers. The
Another method that was developed to anions of interest are separated on the
measure chlorine content is the metallic basis of their affinities for a strongly basic
sodium method. Here the sample is anion exchanger. The separated anions are
refluxed with metallic sodium in a measured by conductivity. They are
mixture of butanol and toluene. The identified on the basis of retention time
chlorides and other halogens are stripped as compared to standards. Quantitation is
out and form sodium salts. Methanol and a measurement of peak area or peak
acetone are added to the mixture and the height.
chloride is titrated with silver nitrate. The
end point is potentiometrically
determined by using a glass electrode —
silver billet electrode system.

This sodium refluxing method of
chlorine analysis is quite sensitive,
detecting as little as 3 µg·g–1 of chlorine.

Liquid Penetrant System Chemistry and Effluent Waste 303

Fluorine Contamination of beryllium, iron, potassium, sodium,
Liquid Penetrant Materials ammonia, phosphate and sulfate.

Fluorine is the most active of the Two other titrimetric methods operate
halogens, so it can cause all the problems on the basis of adding titrant and
ascribed to the other halogens plus some detecting the first excess of titrant, thus
of its own. However, until 1981, most indicating that all fluoride has reacted.
specifications did not test for or limit One of these titrations uses calcium
fluorine content, because adequate tests nitrate titrant; the other is the lead
did not exist and sufficient work to nitrate–hydrochloric acid reaction. The
determine valid limits was not done. calcium method, of course, would not
work with developers that contain large
Fluorine can exist in either organic or amounts of calcium, and the lead nitrate
inorganic compounds. Only the ionizable method is subject to the same long list of
inorganics are likely to be reactive but the interferences as ferric alum, noted above.
organics can be converted to inorganics
by the application of heat, so the safest Probably the most promising of the
course is to test for total fluoride rather titrimetric procedures is the thorium
than just readily ionizable fluoride. For nitrate titration with sodium alizarin
this reason, it is normally necessary to sulfonate indicator. This method is only
decompose the test material, thus partially applicable to liquid penetrant
liberating the fluorides to inorganic, testing materials because of certain
ionizable form. This is best accomplished interferences. Even in small quantities,
by oxygen bomb decomposition. (See phosphate is a serious interference,
Annex 3 and Annex 4 of ASTM E 165.4) causing a red color which hides the end
point completely. Phosphorous is often a
Tests for Fluoride Content constituent of test materials. Another
interference is any acid insoluble solid,
A number of tests have been used with e.g., many developer ingredients; these
more or less success for the determination completely mask the end point.
of fluoride content. In all cases, a general
precaution is in order. Fluoride tests Photometric Method for Fluoride
cannot use glass apparati because the Analysis
fluoride ion reacts rapidly with silica,
forming a nonreactive complex resistant A third general procedure that has been
to all methods of analysis. This effect also used is the reaction of the dissolved
makes the analysis of silica containing fluorides with some color forming reagent
developers for fluoride very difficult. to produce a solution capable of being
measured photometrically. In these tests,
Possible Methods of Fluoride the depth of color is proportional to the
Analysis fluoride concentration. Probably the best
known of these analyses is the method of
The fluoride ion behaves quite differently ASTM D 1179,12 which uses a compound
from the other halogens, so special tests of sodium, 2-(parasulfophenylazo),
are necessary to obtain valid results. 1,8-dihydroxy and 3,6-naphthalene
Fluoride is not detected along with the disulfonate (SPADNS). This material,
other halogens by standard tests, such as dissolved in water and mixed with a
ASTM D 808.6 A number of other zirconyl chloride hydrochloric acid
analytical methods have been used with mixture, is bleached by fluoride. The
varying success, however. All of the amount of bleaching is then measured
described methods are assumed to be photometrically.
performed using the fully ionized product
of a bomb decomposition. The earliest The ASTM D 1179 method is subject to
fluoride analyses used gravimetric interference from aluminum, iron,
techniques, which are low in sensitivity, phosphate and sulfate, all of which may
tedious and subject to many interferences. be present in liquid penetrant materials.12
They were seldom used after 1981.
Fluoride Electrode Method for
Titrimetric Methods for Fluoride Fluoride Analysis
Analysis
A device available in the 1980s eliminates
Many titrimetric procedures have also most of the problems of the other
been developed. One of these involves methods and is now the preferred method
titration of the dissolved fluoride with a for some fluoride measurements. This
silver nitrate potassium thiocyanate device is the fluoride specific ion
reagent and a ferric alum indicator. electrode. When the electrode is
Unfortunately, the method is subject to immersed in a solution containing
many interferences, including aluminum, fluoride ions, an electrical potential is
developed between the fluoride electrode
and a reference electrode also placed in
the solution. The potential decreases with

304 Liquid Penetrant Testing

increasing fluoride content. Further, the
specific ion electrode responds only to
fluoride ion and hydroxide ion if
hydroxide is in greater concentration than
fluoride. The proportion can be controlled
easily by adjusting to a lower pH.

Fluoride ions can be lost through its
complexing with cations such as
aluminum (III) (Al3+); ferric ion or
iron (III) (Fe3+); or silicon (IV) (Si4+). This
interference can be eliminated by the
addition of an agent such as citrate,
(cyclohexylenedinitrilo)acetic acid (CDTA)
or ethylenediaminetetraacetate (EDTA).
Another limitation to fluoride
measurements with the specific ion
electrode is that the response is not
directly to concentration but to the ion
activity. The activity of an ion in solution
is modified by other ions in solution. If
the fluoride present is ionized and the pH
is properly controlled, there are essentially
no interferences with this method.

Ion Chromatography for Fluoride
Analysis

Another method for fluoride testing is ion
chromatography. After the sample is
prepared by bomb combustion, a filtered
aliquot is injected into a stream of
carbonate/bicarbonate eluant and passes
through a series of ion exchangers. The
anions of interest are separated on the
basis of their affinities for a strongly basic
anion exchanger. The separated anions are
measured by conductivity. They are
identified on the basis of retention time
as compared to standards. Quantitation is
a measurement of peak area or peak
height.

Liquid Penetrant System Chemistry and Effluent Waste 305

PART 4. Techniques for Control of Pollution
from Liquid Penetrant Waste

Liquid Penetrant 4. Apply emulsifier sparingly and allow
Constituents Contributing to dwell the required time. Spray
to Waste Pollution techniques use less than dip
techniques of emulsifier application.
Liquid penetrants may contain a
considerable variety of chemical 5. Rinse the part with water immediately
compounds. Among those fairly widely to stop emulsification.
used are petroleum products ranging from
relatively low boiling range solvents to 6. Rinse water washable or emulsified
high boiling oils. Another generic group postemulsifiable liquid penetrant
are nonionic surface active agents. residues in two stages: (a) the first
Anionic surface active agents are less rinse water may be recycled until
commonly used because their presence in contaminated beyond use and
liquid penetrants has disadvantages. These (b) second rinse water effluent can be
are generally considered to outweigh their added to the first rinse water as
excellent surface active properties and, makeup.
what is perhaps more important in the
present context, the comparative ease 7. Rinse waters can be treated to separate
with which they are biologically out the emulsifier and liquid
degraded. Other constituents present in penetrant by using special membrane
significant proportions may be alcohols, filters. Activated charcoal can be used
glycol ethers and esters, to name but a to remove the color, oil and most of
few. Finally, liquid penetrants contain the emulsifier (wetting agent).
dyes, both visible and fluorescent. These
dyes are the most troublesome 8. Filtration of rinse water will remove
contaminants because they are visibly developer residues, if any.
detectable at low concentration and can
be readily traced back to the source of Specific recommendations for waste
pollution. disposal treatment and the cost of such a
system depend on numerous factors, such
Techniques for Reducing as (1) the type of liquid penetrant or
Liquid Penetrant Process emulsifier used, (2) whether application is
Wastes by dip or spray, (3) the volume and type
of work being processed, (4) the facilities
The following chemical engineering and equipment in use and (5) the
and/or liquid penetrant processing steps particular restrictions of applicable city,
are suggested to reduce the amount of county, state or federal codes.
liquid penetrant product residues in the
waste effluent from liquid penetrant Treatment Processes
testing processing systems. Applicable to Liquid
Penetrant Effluents
1. Minimize the amount of liquid
penetrant applied. Electrostatic spray The actual effluent arising from a liquid
or spray application is preferable. penetrant process generally consists of a
dilute emulsion formed by direct
2. Drain the liquid penetrant as long as emulsification of the liquid penetrant
possible to reduce carry over. with water, in the case of water washable
liquid penetrants. Similar dilute emulsions
3. Rinse postemulsifiable liquid can be formed by the action of an
penetrant before applying emulsifier. auxiliary emulsifier or liquid penetrant
This liquid penetrant will separate remover in the case of postemulsifiable
from the rinse water and may be liquid penetrants. The auxiliary
skimmed off or otherwise separated. emulsifying agents may contain
This step can allow the rinse water to constituents broadly similar to those of
be recycled. the liquid penetrants themselves and the
surfactant system may be either lipophilic
or hydrophilic in nature. The effluent
emulsion generally contains less than one
percent by weight of nonaqueous matter
and is usually fairly stable. Conventional
treatment can be designed to break the

306 Liquid Penetrant Testing

emulsion and thus separate nonaqueous consume about 2.5 to 3.0 g of oxygen in
matter. It involves collecting and treating this manner. Thus, 1 L of rinsings
the effluent successively with strong containing 1 g of oily liquid penetrant
electrolytes and flocculating agents in a may show a chemical oxygen demand of
series of vats or tanks. 2500 to 3000 µg·L–1. More dilute rinsing
may show a lower chemical oxygen
The coagulated contaminants are demand. The chemical oxygen demand
separated by filtration and finally technique of monitoring pollution
incinerated, leaving water of acceptable concentration can be related empirically
purity for discharge or reuse. This is to the biological oxygen demand (BOD).
essentially a batch process, relying for its The biological oxygen demand test
efficiency on accurate pH control. It also measures the oxygen used during a
requires time and space: time for the specified incubation period for the
various additives to become fully effective biochemical degradation of organic
and space to accommodate the plant, the material and the oxygen used to oxidize
size of which is determined by the volume inorganic material such as sulfides and
of effluent requiring treatment in a given ferrous iron.
period of time. Where liquid penetrant
processes are used intensively and Avoidance of Environmental
continuously, correspondingly large Water Contamination by Liquid
volumes of water are required for rinsing. Penetrants
More recently, membrane filtration has
been used as a way of treating the effluent Where local regulations allow some oil to
on a continuous basis and feeding back be present in the plant effluent, the liquid
the purified water into the process and in penetrant rinsings, combined with other
effect creating a closed circuit system that nonoily waste streams from within the
is economical in the use of water. plant, may happen to yield a total
effluent within the acceptable range. In
Effects of Liquid Penetrant most cases, though, the situation is not
Waste Materials in Sewage this simple. Then something must be
and Streams done with the liquid penetrant removal
operation to lower the oil content of the
When an inspection liquid penetrant is effluent.
rinsed from the test surface, it enters the
rinse water as a fine, stable, oily emulsion. Worldwide, many attempts have been
Typically, this emulsion is not considered made to manage liquid penetrant removal
to be toxic but does make the effluent without contaminating the environment.
turbid. It can leave an oil slick on the Some of these techniques use liquid
water’s surface and even deplete some of penetrant compositions that meet
the oxygen supply in the water. All these prevailing pollution control regulations.
results lower the value of the water. Other control techniques alter the process
Sewage regulations in some localities so that the wastes never leave the test
allow plant effluent water to contain as area. Still other techniques treat the
much as 600 µg·g–1 of such oils whereas rinsings to concentrate the oily
other regulations allow none at all. The contaminants for easy disposal.
rinsings from liquid penetrant removal
operations normally contain from 200 to Compositions of Liquid
1000 µg·g–1 oil and are usually not an Penetrant Materials to
acceptable waste. Avoid Pollution

Measuring Concentration of Oily One approach to the pollution problem is
Contaminants in Waste Water to alter the liquid penetrant’s composition
so that the rinsings are less objectionable.
Oily contaminants can be detected either For instance, formulating a liquid
as hexane solubles or by the chemical penetrant to be completely water soluble
oxygen demand (COD). Extraction of would avoid the formation of turbidity
rinsings by hexane yields their total and oil slicks.13 Such rinsings might pass
nonvolatile oil content. The chemical all tests, even though the rinse water
oxygen demand test measures the would still be contaminated. However,
concentration of organic contaminants complete solubility makes effluent
(oil, surfactants etc.) by the amount of treatment more difficult. Liquid
oxygen used in oxidizing them penetrants that are completely water
completely by a dichromate reflux soluble have been available since 1953.
technique. The results are expressed in
milligram of oxygen per liter of test
solution or in parts per million. One gram
of a typical oily liquid penetrant may

Liquid Penetrant System Chemistry and Effluent Waste 307

Biodegradable Liquid theory may not be true. This very limited
Penetrants to Reduce study found that each of the studied
Pollution factors important to a publicly owned
treatment works (POTW) — treatment
Another way to alter a liquid penetrant’s time, inhibition, respiration rate, percent
composition is to make it biodegradable. biological oxygen demand (BOD) removal
Such a liquid penetrant need not be water — is more favorable with an oil based
soluble, yet it can have the same removal liquid penetrant than with a surfactant
properties as oily liquid penetrants. The based liquid penetrant. It further showed
rinsings consist of the same sort of fine that even oil free liquid penetrants test
emulsion but with an important positively for oil when tested in the
difference. Within a few days, the oily laboratory.
liquid penetrant is almost completely
decomposed by organisms in the water. Waste Water Disposal to Publicly
The liquid penetrant decomposition will Owned Treatment Works
use up a large amount of free oxygen
from the stream that contains the oily The greatest challenge to liquid penetrant
wastes. This behavior limits the locations users is knowing whether waste water
where such a liquid penetrant can be from liquid penetrant processes can be
legally used. sent out to publicly owned treatment
works through the sewer system.17 The
It would be objectionable if liquid first step in finding the answer is to ask
penetrant effluents were emptied directly the operators of publicly owned treatment
into natural bodies of water where the works if liquid penetrant waste water is
oxygen depletion could harm the acceptable for treatment at these facilities.
environment. Oily waste processed in a Waste water treatability depends on how
sewage treatment plant in the presence of microorganisms in waste water treatment
ample oxygen and a large population of plants respond to the waste stream.
hungry microorganisms will disappear Conducting tests on fluorescent dye
without harmful effects. This will increase liquid penetrants using a respirometer
the work load of the sewage treatment simulated the treatment of liquid
plant and perhaps might result in penetrant materials in the activated
increased sewer charges. Biodegradable sludge system. Activated sludge consists of
types of liquid penetrant testing materials microorganisms cultured in a controlled
are commercially available for use in environment where waste water enters
situations where suitable treatment plants and clean water (effluent) exits. The
are available. microorganisms are fed oxygen, mixed,
retained for a period of time, then settled.
Recent Developments in Most waste treatment plants use the
Liquid Penetrant Waste activated sludge treatment system to
Water Disposal reduce the soluble (dissolved) organic
strength waste before the waste water is
The increasing regulatory restrictions on discharged to a river, stream or lake.
water pollution brought greater scrutiny
of liquid penetrant materials getting into This reduction in organic strength is
waste water.14 Tests were conducted to accomplished not only because the
develop a better understanding of waste United States Environmental Protection
water characteristics resulting from liquid Agency requires it but also because the
penetrant materials introduced around waste water would quickly deplete the
1990.15 The focus was on two distinct oxygen in the receiving stream. The
formulation approaches — compositions publicly owned treatment works’ objective
based on petroleum distillates and those is to remove organic and solid and soluble
based on surface active agents (surfactants), organic waste. Anything that threatens
more commonly referred to as the achievement of this goal is a concern
biodegradable liquid penetrant. The study of the facility.
investigated the treatability of waste water
generated in the normal course of Drain Disposability
fluorescent liquid penetrant testing.
Drain disposability is a term loosely used
The popular theory at the time was to denote the environmental friendliness
that surfactant based liquid penetrants, of a particular waste when it is put in a
because they were oilfree, were more sewer system. In the case of activated
biodegradable than the traditional oil sludge systems, drain disposability would
based formulations.16 Hence making them mean that the waste water can be treated
more drain disposable. However, the study without hurting the microorganisms,
came to the conclusion that the popular within the capability of the air supply
system (aeration) and within the
treatment time (detention time). Drain
disposability largely depends on the

308 Liquid Penetrant Testing

strength of the waste. Strength of waste is ferrous chloride must be added to the
measured with the five day biological collected rinsings to decompose any
oxygen demand test, which measures the unreacted permanganate. The resulting
depletion of oxygen from primarily ferric oxide floc helps absorb more liquid
microbiological metabolism (respiration). penetrant.
The higher the biological oxygen demand,
the larger the tank capacity and air The liquid penetrant floc mixture can
requirements. Liquid penetrant materials be easily separated by filtration or
that result in low biological oxygen decantation but the technique is slow.
demand values are more likely to be Pound for pound, permanganate is more
treated within the detention time allotted expensive than liquid penetrant. Finally,
by publicly owned treatment works to the chemical additions must be
treat waste water. In some tests, all oil painstakingly balanced so that something
based materials tested resulted in oxygen more obnoxious than the original liquid
uptake rates below normal requirements penetrant does not go down the sewer
whereas most surfactant based samples unneutralized.
tested exceeded the maximum allowable
range. In summary, all liquid penetrant Reverse Osmosis Technique for
products exerted certain amount of Removal of Waste Liquid
organic strength measured as biological Penetrant
oxygen demand and thus may require
pretreatment before disposal to publicly In reverse osmosis, the impure water is
owned treatment works. forced through an osmotic membrane
under pressures up to 2.80 MPa
Techniques of (400 lbf·in.–2). In a single stage, from half
Posttreatment of Liquid to three fourths of the water passes
Penetrant Rinsings for through the membrane. The
Pollution Control contaminants that cannot pass through
the membrane are concentrated in the
Several liquid penetrant waste purification remaining waste water. Although such
processes result in water pure enough to equipment is primarily used to separate
reuse in the rinsing step. Most of these pure water from mineral laden water, it
processes rely on posttreatment of the also works well with water washable
rinsings. Posttreatment processes for liquid penetrants that are surfactant
liquid penetrant rinsings can be placed based.
into four groups: (1) chemical destruction
of the emulsion; (2) molecular filtration, Figure 7 shows schematically how a
such as reverse osmosis; (3) absorption of membrane separator works and includes a
the emulsion onto a suitable substrate; simple flow diagram. This is a soluble oil
and (4) a remover that forms an unstable waste treatment system in which the
emulsion that separates easily. Each of separation is nearly quantitative.
these techniques of pollution control is Beginning with municipal water laden
described in detail below. with 2000 µg·g–1 of emulsified liquid
penetrant (chemical oxygen demand
Technique for Chemical about 4000 µg·g–1), it is possible to salvage
Destruction of Water Liquid 65 percent of the water. This recovered
Penetrant water contains less than 10 µg·g–1
impurities, largely a water soluble
Surface active agents that allow liquid emulsifying agent, and had a chemical
penetrant removal by water also stabilize oxygen demand of 60 µg·g–1. The waste
the resulting emulsion. These surfactants water is about 35 percent of the total. This
can be destroyed by oxidizing agents. One waste water contains over 90 percent of
of the most effective oxidizing agents is the mineral content of the original sample
potassium permanganate. With the and all but a trace of the original liquid
surface active agent destroyed, the fine penetrant. Its chemical oxygen demand is
emulsion can separate. The oily around 18 000 µg·g–1. The reverse osmosis
constituent is absorbed onto the process concentrates the oily contaminant
manganese dioxide floc that forms and into 35 percent of the original waste
settles out when the permanganate reacts. water, which is still too bulky for disposal.
This oxidation is a slow process. If However, this concentrated waste can be
permanganate is added to the collected forced through another osmotic
rinsings in an amount equal to that of the membrane to salvage more pure water
liquid penetrant and the mixture is not and shrink the volume of the waste water.
heated, it takes about 2 h to break the
emulsion. At this point, a small excess of Performance of Osmotic Liquid
Penetrant Removal System

In one series of tests with the permeator
of Fig. 7, liquid penetrant rinsings were
subjected to three stages of purification.

Liquid Penetrant System Chemistry and Effluent Waste 309

Overall, about 94 percent of the rinsings depending on impurity level, pressure and
was salvaged as pure water. The efficiency separation ratio. Unfortunately, it is
of the separation, as well as the rate of damaged by traces of chlorine and oil in
output, dropped as more concentrated water. In addition, it still requires disposal
water was fed to the permeator. The of a significant volume of waste materials.
purified water from the first stage
separation had a chemical oxygen Filtration
demand of 60 µg·g–1. By the third stage,
enough impurities were passing to raise There are a variety of membrane materials
the chemical oxygen demand to and configurations. Membranes are made
110 µg·g–1. Table 2 shows the chemical from polymers, stainless steel and
oxygen demand values for the initial ceramics. Membrane selection is based on
rinsings along with the values for reject the size of the particles to be separated
and product water for each of the three and the chemistry of the effluent stream
successive stages. as it relates to the chemistry of the
membrane.
TABLE 2. Effect of successive rinses on
waste water purity. The standard membrane delivery
system is a dead end filtration, so called
Chemical Oxygen Demand because the feed fluid flow is directed at a
right angle to the membrane without any
__________(_µ_g_·_g_–_1)___________ attempt to control the thickness of the
concentrate boundary layer at the filter’s
Stage Waste Product surface. Rapid buildup of retained solids
to the membrane surface and continuous
Initial 4000 50 buildup of materials severely limit the
1 18 000 75 flow and separation of the effluent. The
2 32 000 110 most widely used dead end filters are the
3 66 000 cartridge filters. Generally the cartridge
filters are limited to feed streams of low
This does not represent much viscosity and solid content. Once the filter
contamination but there is another limit flow rate has dropped off to an
on the degree of waste concentration that unaccepted level, the cartridge filter must
can be obtained. The mineral content of be discarded.
the waste water also increases at each
stage. Eventually it exceeds the solubility To achieve relief from the
and precipitates out to form scale on the accumulation of rejected effluent
membrane, which stops further action. materials and the increasing pressure
The scale can be removed but only by drop, filtration systems have been
shutting down the permeator and developed to flow the effluent stream
flushing it out with scale dissolving parallel to the membrane’s surface in cross
chemicals. flow filtration, or tangential flow filtration.
In tangential flow filtration, the effluent
The permeator can treat from 6 to 10 L feed is pumped at a high velocity in an
(1.5 to 2.5 gal) of water per minute, attempt to shear away the concentration
polarization layer and minimize its effect
on the separation. Turbulent flow designs

FIGURE 7. Schematic cross section drawing of permeator showing membrane used to pass oil
free water radially outward and retain oily contaminants in tubular enclosure.

Dilute Membrane Oil concentrate
oil feed
Oil
free
water
Ultrafiltration process

310 Liquid Penetrant Testing

operate at high shear rates and achieve can be separated by any of the following
higher filtration efficiencies. techniques: (1) stratification, (2) filtration
or (3) centrifugation.
Conventional tangential flow filtration
configurations include plate and frame, Stratification for Recovery of
spiral wound (stacks of membrane sheets Waste Water
with turbulent promoting screens
between layers), hollow fibers (bundles of Flocculated solid particles settle loosely at
filament membranes) and tubular designs the bottom of the tank, allowing the
(membrane tubes). The spiral wound purified water to be decanted off. The
configuration is easily serviced and is the bottom layer (about 20 percent of the
most energy efficient tangential flow filter original volume) will then have to
design. It is used in the food, chemical undergo further processing such as
and environmental industries for evaporation or filtration to reduce it to a
ultrafiltration and reverse osmosis more easily disposable, compact, wet solid
applications. waste.

Reuse of Separated Pure Filtration for Recovery of Waste
Water Water

The separated pure water can be drained The filtration of the treated effluent batch
into the sewer or it can be reused in the can be accomplished by using a precoated
liquid penetrant removal process. The horizontal plate filter. The end products of
small amount of impurities that passed this technique are purified water (oil
through the first stage did not increase content is about 25 µg·g–1) and a wet solid
after six cycles of liquid penetrant reuse cake that has only slightly more bulk than
and contamination. Table 3 shows the the removed liquid penetrant itself.
chemical oxygen demand values of the
product water after six cycles of reuse (see Centrifugation for Recovery of
chart). Different liquid penetrants should Waste Water
be checked for compatibility and
processing characteristics. A successful extraction of solid particles of
clay from the treated batch of effluent was
TABLE 3. Purity of separated water after also achieved by means of a simple cream
six cycles of reuse. separator. However, a continuous
separation would require special
Cycle Chemical Oxygen Depletion equipment that, although industrially
Number (µg·g–1) available, is much more expensive than
an ordinary centrifuge.
1 60.8
2 57.6 Water Immiscible Solvent
3 57.2 Removers
4 64.0
5 48.0 Another approach to the effluent problem
6 64.0 is the removal of non–water washable
liquid penetrants by aqueous dispersions
Liquid Penetrant Removal of volatile, water immiscible solvents. This
by Adsorption results in an effluent whose typical
composition is as follows: (1) 99 percent
Studies have shown that the clarification water, (2) 0.98 percent solvent and
of effluent containing liquid penetrant (3) 0.02 percent liquid penetrant.
waste also can be accomplished by the
adsorption technique. It is essentially The mutual compatibility of the liquid
based on the affinity of certain absorbent penetrant and solvent remover and their
particles toward typical ingredients of combined immiscibility in water
liquid penetrants. In practice, the oil predetermines an easy separation of water
contaminant is extracted by stirring by centrifugation or gravity stratification
vigorously 7 kg (15 lb) of absorbent into in a holding tank. Apart from exhibiting a
10 000 L (2600 gal) of waste water slight bluish fluorescence, the recovered
containing about 0.06 percent liquid water is sufficiently uncontaminated (oil
penetrant. A solution of a flocculating content less than 100 µg·g–1) to be
agent is then added and the treated batch disposed of as a regular aqueous waste.
The density of the removing solvent must
be either lighter than water or heavier
than water in order for it to be separated
by centrifugation or gravity. The
considerably smaller volume of the

Liquid Penetrant System Chemistry and Effluent Waste 311

remaining remover liquid penetrant this and filtration. This is confirmed by
mixture can then be distilled to recover the fact that a combination of activated
the volatile solvent, leaving behind carbon and filter aid, such as for example,
proportionally minute quantities of pearlite or diatomaceous earth, is more
somewhat contaminated liquid penetrant. effective than either one of these on its
Or it can be skimmed off and collected for own. If only carbon is used, the oily
disposal as oily waste. The solvent fraction matter in the effluent quite soon forms an
will have mostly liquid penetrant oils and oily coating on the carbon particles that
traces of water and can be incinerated or effectively prevents their functioning as
used as a fuel blend. adsorbents for the dye. If only pearlite or
diatomaceous earth is used, only a
It follows that the solvent removal of filtration effect is attained, with little or
liquid penetrants represents a foundation no adsorption of dye taking place.
for a closed circuit system that eliminates
effluent entirely by reusing water and the Experimental work has shown that a
remover and by collecting the liquid sandwich arrangement, whereby the
penetrant waste in concentrated form. effluent flows first through the filter aid
and subsequently through the activated
Technique for Liquid carbon, practically doubles filter life, that
Penetrant Waste is, its capacity to function before reaching
Purification with Activated actual or apparent saturation.
Carbon
Recycling of Purified Water after
The technique of filtration through Carbon Filtration
activated carbon and other filter media for
decolorizing is a well known and Carbon filtration, with the correct grades
established technique. It is usually carried of carbon and filter aid, will produce
out by making a slurry of carbon in the clean water that may be discharged to
liquid from which the color is to be waste. The only contaminants that may
removed and by heating it if possible. still be present in the water are the
After a suitable reaction time, which may strongly hydrophilic elements of the
vary from a few minutes to several hours surfactant system of the liquid penetrant
depending on the nature of the coloring or emulsifier. If the water is to be
matter and its concentration, the carbon discharged to waste, it is important that
is separated by filtration. Where large only surfactants that can be readily and
amounts of color have to be removed, it is substantially broken down by biological
generally necessary to repeat the process organisms like biodegradable detergents
several times. It is clearly a batch process be used in the liquid penetrant or
suffering from limitations of time and emulsifier formulation.
space.
In the present state of knowledge, the
However, it has been found that it is choice of suitable nonionics that also
possible to achieve the same end by meet the other required criteria is limited.
allowing the effluent to flow through a However, a postemulsified liquid
bed of activated carbon and other filter penetrant process in which a hydrophilic
media on a continuous basis. Two major liquid penetrant remover is used presents
factors determine the success of this a further problem if the rinse water is
technique. First, the type and particle size recirculated. Under such conditions the
of carbon and filter medium is specific for concentration of liquid penetrant remover
a particular emulsion and apparently in the rinse water gradually builds up to a
depends on its hydrophilic lipophilic level where it causes excessive foaming.
balance (HLB). Secondly, the ratio of the This foam is deposited and dries on the
rate of flow of effluent to the volume of parts being processed, staining them and
carbon is highly critical in determining thus hindering inspection.
the rate at which color is removed. This in
turn varies with the amount of Adsorption of Surfactants
contamination present in the carbon and onto Carbon
filter medium, so that the rate of removal
of color reduces as the degree of Certain types of carbon are capable of
contamination of the filter media removing surfactants. However, it was
increases. discovered that types of carbon that
remove surfactants have no decolorizing
Mechanisms of Waste Purification properties with respect to the type of dye
by Carbon Filtration generally used in fluorescent liquid
penetrants. Therefore, one has to adopt a
Waste purification is not simply a matter sandwich arrangement of the two types of
of removing the organic dyes by a process carbon, separated by a perforated plate,
of adsorption but rather a combination of set up so that the effluent water passes

312 Liquid Penetrant Testing

first through the layer of decolorizing
carbon and thereafter through the layer of
carbon that removes the surfactants. It
was also found that if the order of the two
carbon types is reversed or if they are
intimately mixed or blended, the desired
effect is not produced. Reverting,
therefore, to what has been said
previously about filter aids, a three layered
sandwich consisting of one layer of filter
aid and two layers of carbon is required
for some applications. Disposal of the
saturated carbon is best carried out by
incineration, which destroys the adsorbed
organic matter. If the quantities justify it,
regeneration of the carbon may be
economically attractive.

Carbon Filters with Hydrophilic
Emulsifiers

In view of the relatively high price of
activated carbon, the cost of the carbon
filter process is not insignificant and ways
of improving the economics have
therefore been sought. One technique
that shows worthwhile savings uses a
postemulsifiable hydrophobic liquid
penetrant in conjunction with a
hydrophilic liquid penetrant remover. In
such a system, a preliminary water rinse is
carried out before immersion in the liquid
penetrant remover. This rinse removes the
bulk of the excess liquid penetrant by
mechanical action without forming an
emulsion. Then, this liquid penetrant may
be recovered from the water by a
centrifuge or coalescer. Both achieve
almost 100 percent separation, producing
clean water for reuse or discharge and
actually recovering usable liquid
penetrant. In one particular installation,
about 4 L (1 gal) of liquid penetrants
(representing about 65 percent by volume
of the total dragout) are recovered daily.
This process, which may be carried out
continuously, achieves a double effect. It
economizes in expensive liquid penetrant
and, by ensuring that far less liquid
penetrant is carried over to the second
rinse, reduces the amount of liquid
penetrant to be removed by the carbon
filter, thus increasing its service life and
reducing the operating costs of the
process.

Liquid Penetrant System Chemistry and Effluent Waste 313

PART 5. Recycling of Water Effluent and
Postemulsifiable Liquid Penetrant

Hydrophilic Concept for loop liquid penetrant processes where
Recovery of Liquid waste water and unexpended liquid
Penetrant penetrants are recycled through the
system rather than expelled into the
The prewash or prerinse concept, which environment.
uses a postemulsifiable liquid penetrant
and a hydrophilic emulsifier, is known as Development of Prewash Concept
the hydrophilic postemulsification liquid
penetrant process. This process permits In 1967, data were presented showing
recycling of the wash water. The that higher liquid penetrant system
technique calls for removing the bulk of performance and greater reliability were
the postemulsifiable liquid penetrant from possible with the prewash approach.18
the surface with a plain water wash before Test data support the hypothesis that
treatment with an emulsifier. Properly emulsifier contact time is not as critical in
formulated postemulsifiable liquid this system as in lipophilic
penetrant used in the prewash mode will postemulsification.
yield nonemulsified effluent that separates
by gravity when agitation ceases. The Potential Advantages of Prewash
liquid penetrant should float for easy Techniques
removal by skimming and for possible
reuse if carefully collected. The water that The prewash system answers two
remains on the bottom of the collection contemporary exigencies: conservation of
tank may also be reused in the prerinse petroleum derived products and water
step. Further processing may be required pollution control. Its use results in lower
if separation is incomplete as evidenced material costs. Also, lower pollution
by discoloration or organic abatement expenses will be experienced as
contamination. The prewash concept can the primary wash water is recycled
be used in closed loop liquid penetrant without treatment and the second wash
processes where waste water and water is recycled with only minimal
unexpended liquid penetrants are recycled treatment. This is accomplished with a
through the system rather than expelled liquid penetrant system that (according to
into the environment. some authorities) has higher levels of
performance and reliability than the
Prewash Concept for existing water washable and
Recovery of Liquid postemulsifiable techniques.
Penetrant
Potential Advantages of the
Another approach to control of liquid Hydrophilic Technique
penetrant waste pollution uses a
non–water washable liquid penetrant and The main advantage of the prewash
gravity separation. This system permits technique from an environmental
recycling of the liquid penetrant as well as standpoint is control of water pollution. If
the rinse water. The technique calls for recycling of the liquid penetrant is
removing the bulk of the non–water possible, the decrease in material costs is
washable liquid penetrant from the an added benefit. Pollution abatement
surface with a plain water wash before expenses will decrease also as the primary
treatment with an emulsifier. Therefore, wash water is recycled without treatment.
the principal effluent of this prewash
technique is a nonemulsified mixture of Example of Prewash Separation of
liquid penetrant oil and water; the Liquid Penetrant Oil and Water
mixture quickly and completely separates
by gravity. The liquid penetrant is Figure 8 compares the effluent generated
skimmed off. The water is drawn from the by the prewash process to the effluents of
holding tank and recirculated. The the conventional water washable
prewash concept can be useful in closed lipophilic and postemulsification
processes. The prewash effluent shown in
beaker 2 is in two layers: liquid penetrant
oil and water. The liquid penetrant floats
whereas the water is completely clean

314 Liquid Penetrant Testing

except for a few globules clinging to the FIGURE 8. Prewash liquid penetrant effluent floats (center
sides of the beaker. Effluents in the other beaker) whereas effluents from water washable (hydrophilic)
two beakers are stable, colored emulsions. liquid penetrant (left) and postemulsifiable (lipophilic) liquid
penetrant (right) form emulsions.
The first beaker contains the effluent of
a water washable fluorescent liquid
penetrant. The effluent in the third beaker
is a mixture of a postemulsifiable
fluorescent liquid penetrant and a
lipophilic emulsifier. For photographic
purposes, the effluents are 10 percent
solutions, much higher than would result
in practice. However, even though more
concentrated than found under actual
conditions, the results of stable, colored
emulsions from the nonprewash processes
and a floating liquid penetrant from the
prewash process are accurately portrayed
in the photograph.

Economic Feasibility of Comparison of Hydrophilic with
Liquid Penetrant Waste Lipophilic Liquid Penetrant
Water Clarification Techniques

In many localities, effluents from Using the hydrophilic technique is like
nonprewash liquid penetrant processes using the lipophilic postemulsification
cannot go directly to the sewer. They technique inasmuch as a postemulsifiable
require extensive treatment (1) to break liquid penetrant is used for both.
the emulsion, (2) to separate the organics Nevertheless, the two processes differ
and (3) to clarify the water. Although slightly. In the hydrophilic process, a
water may be reclaimed after expensive plain water wash precedes application of
processing, the separated liquid penetrant an emulsifying agent. Also, instead of a
and emulsifier oils will be chemically full strength emulsifier solution, the
altered and the economics of reclaiming hydrophilic system relies on a very dilute
these materials might be questioned. If solution of hydrophilic emulsifier. The
the effluent is not an emulsion, then hydrophilic and lipophilic systems are
treatment costs are reduced. A distinguished elsewhere in this volume.
nonemulsified effluent separates by Although an additional step is required to
gravity. Whether the pollutants float to perform the hydrophilic process, this
the top of the tank or sink to the bottom disadvantage may be more than offset by
depends on their specific gravity. In either savings in the costs of material and
case, separation takes place without reclaiming rinse water.
expensive filtration or chemical addition.
Therefore, a nonemulsified effluent can Principles of Operation of
minimize water clarification cost. Hydrophilic Liquid
Penetrant Systems
Equipment for Prewash
Separation of Liquid Penetrant Prewashing with plain water physically
and Water removes from the surface all but a trace of
the liquid penetrant. How completely the
The equipment required to capture and liquid penetrant is removed depends on
recycle liquid penetrant and rinse water factors such as (1) surface roughness,
from the prewash stage need not be (2) water pressure and scrubbing action,
elaborate. However, coalescers, (3) duration of the wash and (4) liquid
precipitators, aerators, flocculators or penetrant characteristics such as viscosity
similar devices are available to improve and adhesive properties. On a typical
and hasten separation, especially if the turbine blade, for example, a non–water
water is going to be reused. In addition, washable fluorescent liquid penetrant
there need be no chemical treatment of with balanced adhesive and cohesive
the effluent. The equipment needs only to properties and relatively low viscosity will
use the gravity separation principle. Such be removed from the surface with a water
equipment may involve holding tanks wash so that the remaining liquid
that allow the liquid penetrant to be penetrant is only a microscopic film, as
skimmed from the surface of the tank or
the water to be drained from the bottom.

Liquid Penetrant System Chemistry and Effluent Waste 315

judged from its appearance under either stages of the hydrophilic process on a set
white light or ultraviolet radiation. of relatively smooth turbine blades. Blade
1 is coated with fluorescent liquid
Removal of Microscopic Surface penetrant. Blade 2 has received a prewash.
Film of Liquid Penetrant after Blade 3 has gone through the complete
Plain Water Wash processing cycle, including treatment in a
5 percent solution of emulsifier. The
After removing all but a trace of the procedure for removing the residual liquid
surface liquid penetrant with water, the penetrant after the prewash is to immerse
next step is to convert this remaining the blade in the weak hydrophilic
non–water washable liquid penetrant to a emulsifier solution, remove it and allow it
water miscible product, so it can be to drain for 60 to 120 s. Then the blade is
washed completely from the surface. This flushed with water. Allowing the
is accomplished by applying a hydrophilic emulsifier solution to drain back to the
emulsifier. Because the remaining liquid tank conserves the solution and
penetrant film is microscopically thin, the minimizes waste water contamination by
coating of hydrophilic emulsifier can also reducing dragout. Also, because the water
be microscopically thin. This contrasts portion of the emulsifier solution
with the conventional postemulsifiable substantially evaporates during the drain,
process where a heavy layer of liquid the solution concentrates for maximum
penetrant requires a heavy layer of effectiveness.
emulsifier. With the hydrophilic
technique, only a thin coat of emulsifier Before using this procedure, allowing
is needed. the emulsifier to sit on the part, a test
should be run to determine that
The final wash effluent is produced by discontinuity detection is not degraded.
the thin film of liquid penetrant and
overlying film of hydrophilic emulsifier. Evaluation of Hydrophilic
Although it is an emulsion, it is highly Technique on Liquid
diluted. If it were determined that this Penetrant System Monitor
effluent required treatment, the cost of Panel
such treatment would be greatly reduced.
A charcoal filter may provide a practical Figure 10 is a composite ultraviolet
means of handling the final effluent. radiation photograph that again illustrates
the three stages of the process. The test
Example of Hydrophilic piece is a liquid penetrant system monitor
Treatment of Jet Engine panel. This stainless steel panel has a
Turbine Blades chrome strip with five induced crack
centers of varying magnitude whereas the
For demonstration purposes, the white balance of the panel has been sand
light photograph of Fig. 9 shows the three blasted. The panel facilitates simultaneous
sensitivity and washability evaluation. At

FIGURE 9. White light photograph shows three stages of FIGURE 10. Ultraviolet fluorescent photograph shows three
prewash system: blade 1, liquid penetrant coated; blade 2, stages of prewash system on liquid penetrant system
following plain water wash; and blade 3, after process monitor panel: liquid penetrant coated panel (left), following
completed. plain water wash (center) and after process completed
(right).

316 Liquid Penetrant Testing

the left of the composite photograph, the
panel is shown after being coated with
fluorescent liquid penetrant. In the
middle position, the panel has been
washed, substantially removing the liquid
penetrant from the smooth chrome plated
strip. The quantity of liquid penetrant
remaining on the blasted section may be
considered objectionable. The right
section shows the panel after full
processing; four or five crack patterns are
clearly delineated and the fluorescent
background on the blasted section is at an
acceptable level.

Liquid Penetrant System Chemistry and Effluent Waste 317

PART 6. Clarification and Distillation Recovery of
Waste Water

Clarification of Waste reached where they become unstable and
Water Containing Liquid break apart into oil-detergent fragments
Penetrant and Emulsifier that disperse in the water through
Contaminants dilution. Similar emulsification and
solution mechanisms occur in virtually
Modern inspection liquid penetrants are every soap and detergent composition.
commonly made in the form of the oil Hence, it is possible to group all soaps,
phase system. This oil phase structure emulsifiable oils and liquid penetrants
includes both postemulsifiable and water together, insofar as their solution
washable materials. The essential mechanism is concerned.
difference between the two types of
materials is that, in the postemulsifier Detergents Producing True
process, the emulsifier (detergent) is a Solutions of Oil in Water
separate process material whereas, in the
water washable liquid penetrant process, In some cases, where the detergent system
the emulsifier is included as a constituent in the emulsifier is designed for such
of the liquid penetrant. purposes, the oil constituent of the liquid
penetrant may be carried into true
The liquid penetrant process causes solution. According to the classic micelle
problems of water pollution generally theory, such materials remain as stable
produced by industrial and household oil-water micelle clusters even with very
detergents and by solubilized oils such as large additions of water. Resulting
industrial cutting oils and inspection mixtures often have the clarity of true
liquid penetrants. A suitable clarifier solutions and behave in all respects like
material acts to precipitate detergent true solutions.
materials out of solution and permits
separation from water by means of a When industrial process materials that
conventional settling tank clarifier contain detergents are dispersed in wash
(described below) or a continuous flow water, an unusual form of pollution
centrifugal separator. The clarifier material occurs. Many modern detergents of
completely precipitates and adsorbs nonionic nature do not respond to
dissolved detergents along with chemical treatment, as do some ionic
complexed oils, solvent couplers and solutes. Also, the water mixtures are
fluorescent dyes, leaving only a few parts essentially free from solid matter, except
per million of dissolved residue. for solid soil carried into suspension by
detergent action. Hence, detergent
Function of Detergents in containing water wastes possess a type of
Oil Phase Liquid pollution or contamination that is
Penetrants difficult to extract from the water.

The function of the detergent ingredient Pollution Resulting from
in oil phase liquid penetrants is to render Emulsifiers
the oil constituent of the material
emulsifiable in water. The detergent acts In the postemulsifier liquid penetrant
to combine with the oily liquid penetrant system, contamination of wash water
and when test parts coated with the liquid effluent by oil carryover on parts may be
penetrant/detergent mixture are washed minimized by removal of excess oil before
with water, the liquid penetrant oil is the step of emulsification by using a high
flushed from the test surfaces and is pressure spray of water. The water
emulsified and partially solubilized. insoluble liquid penetrant floats to the
surface of the water in a sump tank and
The mechanics of oil emulsification may be recovered by means of a simple
imply that water added to an emulsifiable weir arrangement. Although this helps in
oil mixture forms molecular clusters the overall problem of water pollution, it
known as micelles. These micelles become still leaves the water contamination that
enlarged as water is added, until a point is results from the emulsifier used in the
process. Lipophilic emulsifiers are
themselves mixtures of oil and detergent
materials.

318 Liquid Penetrant Testing

Effects of Detergent stirred into the waste water being treated
Contamination of Lakes and is allowed to settle, as in a settling
and Streams tank clarifier. Another technique of usage
is the filtration mode, in which the
Modern detergents, both industrial and clarifier powder is included as part or all
household, have presented tremendous of the filter bed. Conventional nylon
problems of water pollution. Several years fabric grids in a spin filter configuration
ago, the detergent manufacturers were may be used to retain the clarifier powder.
seeking commercially acceptable detergent After use, the clarifier powder along with
products that were readily biodegradable. precipitated detergents may be recovered
The idea was that all would be well if by conventional backwash and flushing
effluent waters containing such techniques.
compounds would readily decompose or
be eaten up by biological organisms. Chemical Behavior of Clarifier

Unfortunately, the idea of enhancing In many respects, the clarifier behaves in
biodegradability of detergents did not accordance with normal chemical
solve the pollution problem. Some lakes principles. The clarifier’s adsorption and
and streams have such high concentration precipitating action is a predictable
of detergent contaminants that thick function of mole ratios of the clarifier and
layers of foam or suds remain and act to the detergent material in the waste water.
choke off oxygen from marine life. In However, the reaction of the clarifier goes
certain cases of high biodegradability, beyond a simple precipitation reaction
algae proliferate to the point where the with detergents.
algae consume all the nutrients in the
water. Then the algae die and create It has been found that the effect of oil
further damage by exhausting the water solubilization in the presence of
of dissolved oxygen through the process detergents is such that molecular clusters
of decay and decomposition called (or micelles) of oil, detergent and water
eutrophication. It is becoming more and occur as more or less firmly bound
more evident that the only permanent complexes. The result is that when the
solution to the problem of detergent detergent substance becomes precipitated
pollution of water is to remove the by reaction with a clarifier, it carries down
detergent contaminants before the waste with it the various other constituents that
water is discharged into outfall sewers or may be present in the micelle structure.
into waterways. This phenomenon extends even to
fluorescent dyes and certain solvent
Some detergents contain nonionic couplers that may be present in an
surfactants that are easily biodegradable. emulsifier or water washable liquid
However, the issue of oxygen depletion by penetrant.
these surfactants has necessitated
regulatory controls for waste water A typical water washable liquid
disposal. penetrant consists of a mineral oil, an
oil-water emulsifying detergent, a solvent
Possible Techniques for coupler and one or more fluorescent dyes.
Clarifiers to Precipitate When clarifier is added to the water, a
Detergent Wastes precipitation of the detergent takes place.
In addition, many of the related
A possible useful approach to the constituents are carried down out of the
extraction of detergent contaminants solution along with the detergent, leaving
from water has been found in clarifier water with only a few parts per million of
materials that have a unique property of residual contamination.
causing the precipitation of the detergent
substances out of a water solution. Some pollutants in water take the form
Although there may be some detergent of colloidal dispersions or partial solutions
materials that do not respond to the of oil. Waste water of this kind may be
clarification action of a suitable clarifier, purified in a two-step process, in which a
no such material has yet been found. The detergent is first added to the waste water
clarifier serves to completely precipitate to initiate the formation of oil-detergent
all types of detergents, regardless of micelles, after which clarifier is added to
whether their chemistry has a linear or the water. When the detergent is
branched chain nature. precipitated, it carries down with it the
dispersed oil along with any other
In physical form, the clarifier is a fine, contaminants that can form micelle
white, inert powder that may be readily complexes with the detergent.
dispersed in water. If used in the
dispersion mode, the clarifier is simply Determination of Reaction Ratios
of Clarifier

If it is desired to provide an economically
attractive process of water purification, it

Liquid Penetrant System Chemistry and Effluent Waste 319

is necessary that the clarifier material be settled sludge in such a way that it passes
used at its highest possible efficiency. For out through an exit port in the bottom of
rapid and complete precipitation, a slight the tank. This type of equipment requires
excess of clarifier or some simple means a fairly large tank area and the movement
must be used to determine when the of the water must be slow enough to
clarifier is completely consumed. permit settling of the fine particles of
suspended matter.
The technique of mixing clarifier into
waste water and subsequent removal of Centrifugal separation may be
the precipitate by settling or centrifuging accomplished with equipment
provides a highly efficient process. The considerably smaller and somewhat more
clarifier particles normally remain efficient in operation than the settling
suspended in the water for a sufficient tank clarifier. Here the water is injected
time to permit the precipitation reaction into a vertical cylindrical tank rotating at
to go to completion. On settling or high speed. Centrifugal force drives the
centrifuging, the detergent and other heavy suspended particles toward the
complexed materials are separated from walls of the tank. A screw conveyor or
the water, leaving only a few parts per spiral scraper blade moves the separated
million of dissolved material. In this sludge toward the bottom of the tank and
mode of usage, it is desirable to determine out through a conical orifice. The purified
the percent of detergent contamination of water flows over a weir lip at the top of
the waste water in order to add the the tank and is recovered.
correct quantity of clarifier.
Recovery of Clarifier Material for
Technique of Filtering Waste Reuse
Water through Clarifier
The clarifier material is consumed while
In the filter mode of usage, waste water dissolved detergent is precipitated.
containing dissolved detergents and However, the reaction is reversible in the
complexed oils is pumped through a filter same way that water softening reactions
bed or clarifier. In this process, the waste are reversible. Thus, the exhausted
water passes through the filter bed clarifier may be reclaimed, reconstituted
rapidly, so that the precipitation reaction and reused.
may not go to completion in one pass
through the filter. Recirculation of the Onsite Clarification of Waste
water through the filter will serve to Water
extract the dissolved detergents fully. In
this mode of usage, it is desirable to know Waste water that contains detergents and
when the filter bed of clarifier becomes oil/detergent complexes and that is
exhausted. discharged into sewage lines may first be
treated by the clarifier process. Systems for
The detergency of the waste water may treating water by county sanitation
be determined by relatively simple departments or by municipalities must be
techniques. One such technique involves properly designed and engineered with
the evaluation of surface tension of the regard to appropriate projections of flow
water and comparison with samples rates. In addition, accessory systems must
having known detergency. Thus, it is not be included for recovery and recycling of
difficult to determine the detergent the clarifier material and the detergents
content of waste water, either before or and oils that are removed from the water.
after treatment with clarifier. Certain For such onsite clarification, small units
techniques of fluorescence analysis permit may be constructed from currently
the quantitative measurement of residual available components. Various bag or grid
contaminants by the degree of filters are available, as are various sizes of
fluorescence response. centrifugal separators. Recovery and
recycling of the clarifier material itself
Clarifier Equipment for Separating could be carried out in separate processing
Solid Precipitates from Water plants.

Dissolved detergents are precipitated by a Closed Loop Liquid
clarifier in the form of particulate solids. Penetrant Testing System
This solid material may be separated from
the water by means of a settling tank The closed loop concept evolved from the
clarifier or by means of a continuous flow desire to implement an apparently simple
centrifugal separator. The settling tank and obvious expedient of recovering the
clarifier consists of a large circular tank liquid penetrant process materials for
with a shallow sloping bottom. Water reuse. However, the implementation of
containing suspended particles is pumped this idea was not all that simple. First, it
into the tank where particles settle to the
bottom and clear water flows over a weir
at the top. A rotating scraper moves the

320 Liquid Penetrant Testing

should be noted that complete recovery of Recycling Liquid Penetrant after
the process materials will not be possible Solvent Distillation
if any of the materials undergo a
substantial physical chemical change The new slow solubility liquid penetrants
during use. may be recycled in a closed loop
operation through three stages. First,
One of the reasons for failure of water surface liquid penetrant is stripped from
purification and chemical recovery parts by means of a pressure spray of wash
systems in the case of emulsified oils is water. The liquid penetrant thus removed
the fact that the liquid penetrant oil does not dissolve rapidly in the water;
undergoes a substantial physical change instead, it tends to float on the surface of
by being emulsified in the wash water. the wash water and may be drawn off
The micelles or molecular clusters of oil over a drainage weir and recovered.
and detergent become tightly bound to
water molecules and then resist Recycling of Wash Water
separation.
The wash water is circulated from a
Slow Solubility Liquid Penetrants reservoir through wash nozzles and back
to the reservoir. After considerable use,
Some water washable fluorescent liquid the wash water becomes saturated with
penetrants, referred to as slow solubility dissolved liquid penetrant. It is necessary
liquid penetrants, disperse in water without to extract the dissolved liquid penetrant
undergoing emulsification. Thus, the continuously from the wash water to
dissolved or dispersed liquid penetrant is preserve its ability to dissolve liquid
not tightly bound to the water in the penetrant from test surfaces. This
conventional micelle emulsion structure. extraction is carried out as outlined above.

Another interesting feature of the slow If it were required to purify the used
solubility liquid penetrants is that they prewash water enough to drink, liquid
exhibit a high degree of discontinuity penetrant contamination would have to
entrapment efficiency. Their entrapments be reduced to a value of about 5 µg·g–1 or
in cracks are slow to dissolve and tend to less. For purposes of prewash treatment in
remain in the cracks throughout a the closed loop process, it is not necessary
relatively prolonged washing period. for wash water to be so pure.
Depletion time constants are controllable
within broad limits, so the liquid Process Diagrams for Closed Loop
penetrants can be designed to meet any System
desired condition of discontinuity
entrapment efficiency. (Depletion time For reasons of economic feasability, the
constants are described elsewhere.) closed loop, water washable liquid
penetrant testing technique has not been
Another, very important feature of the developed commercially, specified in
slow solubility liquid penetrants is that standards or implemented by industry as
they exhibit very little tendency to of 1999. The process would consist
become adsorbed onto fine porosity essentially of five interlocking loops
surfaces. Conventional emulsion forming (Fig. 11).
water washable liquid penetrants and
even postemulsifiable liquid penetrants, FIGURE 11. Interlocking loops of closed loop system for
are characterized by an effect of water washable liquid penetrant testing process.
adsorption at the liquid solid interface on
test parts. Where the solid surface has a Liquid
large area, as in anodized surfaces for penetrant
example, this feature of adsorption
produces an excessive amount of Water Solvent
background fluorescence, resulting in poor Distillation
signal-to-noise ratio. Adsorption and Process Liquid
unwanted background are minimized in Parts penetrant
the slow solubility liquid penetrants.
Inspect Skim Solvent
The first of the two new chemical extraction
categories is exemplified by liquid
penetrants characterized by a relatively
rapid rate of wash removal with a hot
water spray wash at temperatures in the
vicinity of 55 °C (130 °F). The second
category liquid penetrants are chemically
different and provide progressively larger
indication depletion time constants.

Liquid Penetrant System Chemistry and Effluent Waste 321

1. Test parts would be processed through
the steps of (a) liquid penetrant
application, (b) wash removal of
surface liquid penetrant and
(c) inspection for indications.
Development, of course, would be
included as part of the inspection step.

2. Part of the liquid penetrant would
float and would be skimmed and
retained for testing to determine its
suitability for continued use.

3. Used wash water would be cycled
through a solvent extraction stage and
the purified water would be returned
to the wash water reservoir.

4. The solvent that contains extracted
dissolved liquid penetrant would be
cycled through a distillation column
and recovered solvent would be
returned to the extraction column.

5. Liquid penetrant recovered by
distillation of extraction solvent would
be retained for testing to determine its
suitability for continued use.

322 Liquid Penetrant Testing

References

1. Spanner, J.C. [Sr.] “Methods and 13. Goff, R. and S. J. Robinson.
Reasons for Measuring the Chloride “Water-Base (WB) Penetrants —
Content in Liquid Penetrant Advantages and Disadvantages.” ASNT
Materials.” Materials Evaluation. Fall Conference and Quality Testing Show
Vol. 30, No. 6. Columbus, OH: Paper Summaries [Nashville, TN].
American Society for Nondestructive Columbus, OH: American Society for
Testing (June 1972): p 126-135. Nondestructive Testing
(October 1998): p 114-116.
2. ASTM A 380, Recommended Practice for
Cleaning, Descaling, and Passivation of 14. Robinson, S.J. “Issues Concerning the
Stainless Steel Parts, Equipment, and Disposal of Waste Penetrant Materials”
Systems. West Conshohocken, PA: (Back to Basics). Materials Evaluation.
American Society for Testing and Vol. 49, No. 8. Columbus, OH:
Materials (1996). American Society for Nondestructive
Testing (August 1991): p 962-967, 969.
3. ASME Boiler and Pressure Vessel Code.
New York, NY: American Society of 15. Holmgren, V. and M. Plamoottil.
Mechanical Engineers. “Testing of Detrimental Elements in
Penetrant Materials.” 1992 ASNT Fall
4. ASTM E 165, Standard Test Method for Conference and Quality Testing Show
Liquid Penetrant Examination. West [Chicago, IL]. Columbus, OH:
Conshohocken, PA: American Society American Society for Nondestructive
for Testing and Materials (1995). Testing (November 1992): p 179-180.

5. ASTM D 129, Standard Test Method for 16. Holmgren, V. “Penetrant Materials —
Sulfur in Petroleum Products (General Are They Biodegradable?” 1989 Fall
Bomb Method). West Conshohocken, Conference ASNT Program and Paper
PA: American Society for Testing and Summaries [Valley Forge, PA].
Materials (1995). Columbus, OH: American Society for
Nondestructive Testing
6. ASTM D 808, Standard Test Method for (October 1989): p 39-40.
Chlorine in New and Used Petroleum
Products (Bomb Method). West 17. Hessinger, P. and M.L. White.
Conshohocken, PA: American Society “Treatment Alternatives for Liquid
for Testing and Materials (1995). Penetrant Rinse Water.” Materials
Evaluation. Vol. 56, No. 8. Columbus,
7. ASTM D 2441, Standard Test Method for OH: American Society for
Hydrolyzable Chlorine Compounds in Nondestructive Testing (August 1998):
Chlorinated Aromatic Hydrocarbons p 969-970.
(Askarels) by Refluxing. West
Conshohocken, PA: American Society 18. Birley, R.E., N.H. Hyam and T.
for Testing and Materials (1995). Tebbenham. “Removal Techniques in
Liquid Penetrant Inspection Processes,
8. ASTM D 1552, Standard Test Method for Their Development and Effect on
Sulfur in Petroleum Products Sensitivity.” Proceedings of the Fifth
High-Temperature Method). West International Conference on
Conshohocken, PA: American Society Nondestructive Testing [Montreal,
for Testing and Materials (1995). Canada, May 1967]. Ottawa, Canada:
Queen’s Printer (1969): p 222-225.
9. ASTM D 1266, Standard Test Method for
Sulfur in Petroleum Products (Lamp
Method. West Conshohocken, PA:
American Society for Testing and
Materials (1991).

10. ASTM D 516-90, Standard Test Method
for Sulfate Ion in Water. West
Conshohocken, PA: American Society
for Testing and Materials (1995).

11. ANSI/ASME B 31, Code for Pressure
Piping. Washington, DC and New York,
NY: American National Standards
Institute.

12. ASTM D 1179, Standard Test Methods
for Fluoride Ion in Water. West
Conshohocken, PA: American Society
for Testing and Materials (1993).

Liquid Penetrant System Chemistry and Effluent Waste 323

11

CHAPTER

Filtered Particle Testing

Robert L. Crane, Air Force Research Laboratory,
Wright-Patterson Air Force Base, Ohio

PART 1. Principles of Filtered Particle Testing1

Nondestructive Testing of deposited on the part surface. The
Porous Materials particles may be colored or fluorescent.
Generally speaking in industry,
Visible and fluorescent liquid penetrant fluorescent filtered particle tracers are
test techniques can be used on most solid used to the exclusion of all others. These
materials. However, if the test materials fluid tracers may be of any base, although
are porous, the background haze or sharp boiling point hydrocarbons are
coloration of the entire surface by the preferred because of their economy and
penetrant reduces the contrast of reasonable safety hazards.
indications. The surfaces of such porous
materials trap so much test fluid that Forms of Filtered Particle
delineation of a discontinuity is no longer Testing Media
possible. For this sort of test condition,
the filtered particle test technique is Filtered particle test media commonly
effective. used in industry today are of two types:
(1) a vehicle containing solid tracer
The procedure for locating particles only and (2) a liquid vehicle with
discontinuities with filtered particles is to soluble dye tracer and tracer particles.
apply a liquid dispersion of properly sized
and shaped particles to a porous material The standard form of filtered particle
suspected of having discontinuities open testing medium consists of fluorescent
to the surface. If a crack exists, a highly particles and a base oil. When sprayed on
visible indication will appear almost at a porous material such as clay, carbon,
once. certain powdered metals and concrete, it
will produce a highly visible indication, as
This test is useful for testing of a wide shown in Fig. 1. The indication reveals
range of porous materials. Generally the location of the discontinuity and the
speaking, the filtered particle technique part may be rejected without further
operates on those materials whose particle work. This material is used where salvage
size is 150 µm (0.006 in.) (100 mesh) or is not possible or worthwhile.
smaller. Thus the area of effectiveness of
this technique is fairly well bracketed by The soluble dye form of filtered particle
the 150 µm (0.006 in.) limitation on one test medium is commonly used in certain
hand and the failure of liquid penetrants segments of the clayware industry where
to work on the other. The filtered particle salvage is desirable. In this case, soluble
test was developed by Taber DeForest and dyes are included in the test liquid in
Henry N. Staats in 1946.2-4 addition to the particles. The filtered solid
particles form the indication of the
Principles of Operation of discontinuity on the surface.
Filtered Particle Test
Technique FIGURE 1. Fluorescent filtered particle indication on unfired
clay insulator.
The filtered particle test technique
depends on selective motions of liquid
and solid matter. The test fluid consists of
a liquid vehicle (or tracer when it
contains soluble dyes) that carries solid
dyed particle tracers in suspension. This
fluid, generally oil based, is sprayed on a
porous material suspected of having a
discontinuity or anomaly. At the site of a
discontinuity, more liquid is absorbed
than anywhere else, because of the extra
liquid tracer absorption area in the
discontinuity. The liquid enters the
discontinuity opening while the
suspended particles are filtered out and

326 Liquid Penetrant Testing

Next the operator scrapes away or
chases the discontinuity and reveals the
soluble dye indication below the surface.
The particles indicate the discontinuity at
the surface, whereas the soluble dyes
penetrate the piece and concentrate in the
discontinuity. The discontinuity is chased
until no traces of the dye material remain
at the test site. This does not necessarily
mean that the discontinuity has been
completely removed.

Just as it is necessary to recheck in
other techniques, it is essential in this
case too. In continued salvage, water must
be added to the surface of the material.
This is known as prewetting and is always
essential in salvage work. After the water
has been absorbed, a fresh application of
test medium can be used and a new
surface particle indication developed.
Salvage work can then continue.

The test medium fires off during the
processing of clayware and leaves no
residue. Filtered particle test media
containing water have been developed
and can be used on portland cement
concrete.

Applications of Filtered
Particle Tests

The most significant use of filtered
particle testing is in the clayware
industry.5-8 This is particularly true of
sanitary ware and high tension insulator
manufacture.9 Here, the objects under test
usually are relatively expensive or involve
a great deal of workmanship. The filtered
particle technique is important in these
two industries, because a crack in unfired
clay always opens on firing. In the case of
sanitary ware, if the crack opens wide
enough to break open the glaze, the ware
will have unsightly marks on it and
cannot be sold. Opening of a crack during
the firing process will provide a path for
electrical breakdown on high tension
insulators so that they cannot be used in
service. Another application of filtered
particle testing is for testing carbide
inserts for cutting tools before final firing.

Filtered Particle Testing 327

PART 2. Mechanisms of Operation of Filtered
Particle Tests

Selective Fluid Absorption practice, cracks are nearly always wedge
in Porous Test Materials shaped.

Porous materials, in general, will absorb If 1700 mm3 (0.11 in.3) of a fluid are
liquids in proportion to their surface area. placed on the surface containing the
More fluid is absorbed at a discontinuity discontinuity and a uniform entry of
in porous materials than anywhere else, 10 mm (0.4 in.) is accomplished on all
because of the extra absorption area of the surfaces, the manner of penetration by
sidewalls in the discontinuity. The diffusion and capillary action is
absorption is so pronounced that the substantially as shown in Fig. 3. If no
suspended particles are drawn to the site crack were present the volume of liquid
of the discontinuity. Being wider than the absorbed would be proportional to
dimensions of the top edges of the crack, 700 mm3 (0.04 in.3), as indicated by the
they filter out and form a surface portion above line Y in Fig. 3. Because
indication. Because a striking effect is the specimen contains a crack, fluid will
produced readily, it seems reasonable to also be absorbed in the shaded zone
believe that very large differential below line Y. This crack region will
absorption rates are involved. contain about 1000 mm3 (0.06 in.3).

Example of Crack Absorption of Suppose the width of the crack W to be
Liquid Tracer in Porous Material 1 mm (0.04 in.). A slice X of unchecked
porous material of similar dimension will
Figure 2 shows a cross section of a porous possess a total volume of 10 mm3
material containing an idealized crack. (6 × 10–4 in.3). Comparing the absorption
The section is assumed to be 1 mm at point W and point X, it will be noted
(0.04 in.) thick , 70 mm (2.75 in.) wide that 1000 mm3 (0.06 in.3) of liquid are
and 70 mm (2.75 in.) high. Assume that absorbed at point W and 10 mm3
the crack is 50 mm (2.00 in.) deep and (6 × 10–4 in.3) at point X. Thus, there
has a width at the opening of 1 mm exists a differential absorption ratio of
(0.04 in.). The crack is schematically 1000 to 10 or 100 to 1. It is assumed that
drawn in rectangular form although, in the volume of the imaginary crack is
equal to the volume of contained liquid,
FIGURE 2. Schematic cross section of porous which, of course, is never the case.
material containing crack.
The dimensions and figures given are
1 mm only illustrative for the purpose of
(0.04 in.) showing the phenomenon of differential
absorption. In practice, it has been noted
50 mm (2 in.) that the ratio of crack width to depth is
on the order of 1 to 300. Therefore, to be

FIGURE 3. Schematic cross section of porous
material containing crack, showing
penetration of liquid.

WX

Line Y

70 mm
(2.75 in.)

1 mm (0.04 in.)

70 mm (2.75 in.)

328 Liquid Penetrant Testing

more exact, the crack width at point W particle testing technique to locate
may be considered to be 50/300 or about discontinuities reliably. The abilities of
0.15 mm (0.006 in.). Other dimensions various fluids to penetrate porous
remain the same. Thus, with point W at materials depend to a large extent on the
0.15 mm (0.006 in.) and point X at type of material to be tested. Porous
0.15 mm (0.006 in.) width, the concrete can be readily tested with solid
differential absorption ratio would be tracer particles suspended in water
1000 mm3 (0.06 in.3) at point W and containing a wetting agent. The same
1.5 mm3 (9 × 10–5 in.3) at point X, a ratio water base medium used on clay will
of 1000 to 1.5 or about 600 to 1. dissolve it. If the viscosity of the fluid
vehicle is increased, the absorption rate
From this simple example, it can be may be retarded, thus preventing the
seen that high forces are involved in differential forces from operating
producing an indication on a porous efficiently.
material. The example given is idealized: a
variety of factors may prevent such The concentration of solid tracer
differential forces from ever being particles in the test fluid must be kept in
encountered in practice. Under certain certain limits. If the concentration is too
conditions, it may be possible to increase high, excessive backgrounds may form,
or aid those forces by timely water tending to reduce the visibility of the
barriers. discontinuity indication. If the
concentration is too low, the result may
Characteristics of be a weak indication, lacking in
Differential Absorption brightness.

The phenomenon of differential Effects of Substances Absorbed in
absorption depends on a variety of Test Object Pores
factors, including (1) porosity
characteristics of the test object, Water or other substances that may be
(2) substances in the pores of the test present in the pores of the material to be
object and (3) characteristics of the test tested can influence the differential
medium. absorption forces in such a way as to aid
or hinder the technique in its proper
Effect of Size of Porosity of Test operation. For example, unfired or green
Object hotel chinaware may not be porous
enough for testing when the included
If there is excessive porosity relative to water content exceeds 10 percent by
the solid portion of the material to be weight. However laminar discontinuities
tested, the porous body will absorb liquid can be located in green tile bodies with
too rapidly and the differential forces may moisture contents ranging up to
be too low to produce an effective 12 percent.
indication. If the size of the individual
pores is relatively larger than the Classification of Test
discontinuity width, the pores may be Object Porosity
indicated more prominently than the
discontinuity itself. In some cases, the Porosity is relative and the best test for
amount of interconnection of the pores workability of the filtered particle test
influences the absorption characteristics technique is an actual test on the material
of the material considerably. Some types in question. Generally, porous materials
of ceramics have an extensive pore may be divided into five groups, as
structure but each pore may be sealed off described in subsequent paragraphs.
from its neighbors, thus preventing
adequate penetration of the test fluids. Very Porous Material

Effects of Characteristics of Very porous material can be illustrated by
Filtered Particle Test Media the porosity of a fired grinding wheel
made from 250 µm (0.01 in.), or 60 mesh,
The size and shape of the solid tracer particles or by the porosity in fired
particles used in detecting cracks are quite refractories where very large particles are
critical. If the tracer particles are too bonded to still larger particles and the
small, they may enter the discontinuity subtended spaces are left empty. The
rather than filter out. If the tracer filtered particle technique rarely works on
particles are too large, they may lack coarse material of this type. Attempts
mobility and drop out of suspension at have been made, without success, to fill
random rather than filter out selectively the voids temporarily to create an
over the discontinuity. The penetrating artificially good grain structure that will
power of the suspending medium is also a permit detection of discontinuities in this
critical factor in the ability of the filtered type of material.

Filtered Particle Testing 329

Moderately Porous Material differential absorption forces can operate.
For instance, if there were water or wax
Examples of moderately porous materials present in the porous test material, the
are fired grinding wheels made from pores may be filled and the absorption
100 µm (0.004 in.) or 150 mesh particles, forces cannot work as they would
some refractories and carbon materials normally. However, it is frequently
whose range of particle size is smaller possible to remove the space filler before
than in the first classification. Certain testing.
powdered metal compacts and some types
of portland cement concrete belong in Slightly Porous Materials
this category. The filtered particle test
technique begins to operate on materials More difficulty occurs with testing of
of this type. The indications may be slightly porous materials. A structure is
slightly erratic because of cracks running present that may or may not have
through zones of large pores where complete interconnection between bubble
differential absorption forces cannot porosities. A void may or may not be
operate effectively. Figure 4 shows a fired, present at the surface. Examples of this
porous, combustion tube containing a type of material are certain types of fired,
fluorescent indication of a crack. The partially vitrified, unglazed ceramic
background coloration indicates that the materials; some cermets; and certain types
area that does not contain the crack is of carbon and graphite. In general, the
absorbing excessive amounts of fluid and technique will operate on materials of this
that the differential absorption forces are sort, although very poor differential
much reduced. absorption forces are present and the
sensitivity limit is in sight.
Porous Materials for Filtered
Particles Tests Filtered Particle Tests of Porous
Glazed Materials
Examples of medium porosity are many,
for this group includes nearly all types of Examples of porous glazed materials are
unfired dried clayware and certain fired semivitreous wares that have a glazed
porous ceramics. In this group also are surface. If there are cracks in the glazed
certain refractories, both fired and surface that extend through to the porous
unfired; semivitreous materials that are underbody, the filtered particle test
unglazed; unfired and fired cermets; and technique will operate efficiently.
some fired compacts such as carbon and
graphite. The filtered particle technique FIGURE 5. Fluorescent indications of typical laminar cracks in
operates decisively and reliably on this dried clay tile.
group of materials. Figure 5 illustrates
typical indications of laminar
discontinuities in dried clay tile. Cracks of
this type are usually caused by the
entrapment of air at the time of pressing.
The filtered particle technique will operate
on materials of this type, providing

FIGURE 4. Fired, porous combustion tube with fluorescent
indication of crack.

330 Liquid Penetrant Testing

PART 3. Design and Selection of Filtered
Particle Test Media

Selection of Suspending glaze slips are applied onto clayware and
Medium for Filtered other materials after testing, the
Particles suspending medium and particles fire off
with no known effect on the glaze.
The fluid vehicle used for filtered particle
test medium of most porous materials in Types of Fluorescent
industry is a light petroleum distillate. Filtered Particle Test Media
This material is a compromise between
cost, safety, odor and reasonably fast In the manufacture of many clay
drying action. It has been highly products, the existence of a crack in
satisfactory on all types of clayware, the unfired clayware is sufficient justification
solvent residues firing off in kilns with no for scrapping the material. For instance, it
complications. This solvent does not is neither economical nor generally
affect glaze slips applied over it. possible to repair cracks in dishes. The
same thing is true in the testing of tile.
In some cases where a ceramic glaze is On certain unfired porous products such
fired at the same time as the body, as is as insulators and fired ceramic materials,
the case with sanitary ware fired once, it the existence of a crack is cause for
is advisable to let the ware air dry for at rejection. No repair techniques are
least 15 min after testing before glaze is generally feasible. On these materials, it is
applied. Sometimes, if glaze slip is applied sufficient to provide a readily seen surface
to freshly tested ware, the surface pores indication only.
will be plugged with petroleum distillate
and slip will not adhere tightly. This Filtered Particle Test Media for
sometimes causes crawling of the glaze. Location of Surface Cracks Only
This effect is the same as applying a glaze
slip to a piece of clayware that had been The fluorescent test medium
freshly soaked with water. recommended for surface indications only
is a light petroleum distillate base in
For special applications, suspending which are suspended vividly yellow
media other than petroleum distillate may fluorescent particles. The material is
be used. Suspensions have been made shipped ready to use. Figure 6 illustrates
successfully with water, glycols, heavy in schematic form a cross section of an
petroleum distillate and alcohol. In idealized discontinuity in clay containing
general, the suspending medium will be a surface indication only.
light petroleum distillate, now considered
standard. Any suspending medium other FIGURE 6. Schematic cross section of crack in clay with
than the standard distillate increases the surface particle indication of defect.
cost of the basic material, because the
solvents are inherently more expensive. Yellow fluorescent
The one exception is water itself. particles at defect
However, water tends to cause clay to
dissociate and thus is not usable. Random background particles
Suspensions with fluid bases other than
light petroleum distillate must be custom
made for the specific application.

Solid Tracer Particles for Colorless
Test Media penetrating
fluid
In all filtered particle test fluids, the solid
tracer particles are selected for proper size Crack
and shape. The particles are extensively
milled to produce a wide range of particle Porous material
sizes with a predominance of particles of generally dark purple
micrometer size. In all cases, these under ultraviolet
particles will fire off at temperatures of radiation
200 to 315 °C (400 to 600 °F). Where

Filtered Particle Testing 331

Filtered Particle Media for chase out a discontinuity, that is, dig into
Location of Repairable Cracks in the discontinuity, following it to the
Unfired Ware bottom. Figure 7 shows a typical
fluorescent indication of a crack in the
On unfired sanitary ware, high tension foot of a toilet bowl. Figure 8 illustrates
insulators or other cast, assembled or how the inside of the crack is stained with
machined green clay products, the surface fluorescent dyes, so that the discontinuity
indication of a discontinuity is not may be chased out.
necessarily a reason for their rejection.
With green clayware, it is usually possible Figure 9 illustrates in schematic form
to remove or peg the discontinuity or the cross section of a discontinuity where
crack and salvage the piece. For example, the surface as well as the subsurface
if a crack appears in the body of an portion is indicated by fluorescent
extremely large high tension insulator, it materials. Here the dye tracer colors in
probably will be cause for rejection. order of viewing are as follows.
However, if the crack appears in the
petticoat or skirt of the insulator, it may 1. On the surface, yellow fluorescent
be possible to chase out the discontinuities particles are found with a fluorescent
and thus salvage the piece, providing the red yellow background.
shape change does not affect the electrical
characteristics of the completed, fired 2. About halfway down the crack, a red
insulator. fluorescent indication with a
fluorescent yellowish background will
Where salvage of a porous object is be found.
desired, a test medium is used whose
particles indicate the crack at the surface 3. Below this zone, a yellowish blue
and whose staining dyes color the inside fluorescent indication with a purple
of the crack so that an inspector may background will be found.

FIGURE 7. Fluorescent indication of crack in foot of toilet Nonfluorescent Filtered
bowl. Particle Test Media

Colored but nonfluorescent test
suspensions, although as sensitive in
operating ability on porous materials,
cannot be seen so readily as fluorescent
materials on complex shapes such as
insulators, sanitary ware or parts with
hidden curves or recesses. Fluorescent
material is recommended for such
inspection problems. Generally, the
nonfluorescent materials can be used on
simple shapes such as bricks, carbon and
concrete, although fluorescent materials
may be simpler to use and more decisive.

FIGURE 8. Subsurface indication of crack after removal of FIGURE 9. Schematic cross section of defect containing
surface material. subsurface as well as surface fluorescent indication.

Yellow fluorescent
particles at defect

Random background particles

Red fluorescent
particles

Yellow fluorescent
penetrating fluid

Porous material
generally dark purple
under ultraviolet
radiation

332 Liquid Penetrant Testing

PART 4. Filtered Particle Test Equipment

Light Sources for Filtered Equipment for Applying
Particle Testing Filtered Particle
Suspensions
For effective, rapid and reliable testing of
fluorescent filtered particle indications, To get maximum benefits in the testing of
the near ultraviolet radiation output porous products, it is essential that the
should be of high intensity. Ultraviolet test suspension be applied in such a way
radiation sources that meet this as to let the particles move about freely
requirement generally consist of five and penetrate discontinuities. In general,
items: a special bulb; a special filter; a the correct amount of fluid to place on a
fixture or shell, generally portable and piece to be tested is just enough to wet
with a handle; a transformer; and the surface or make it shiny. Any material
interconnecting wires. Generally, 120 V, in excess of this is generally wasted and
60 Hz alternating current is required. anything less than this will not provide
Figure 10 illustrates a typical portable sufficient particle mobility.
ultraviolet lamp used widely in industry.
After application of the fluorescent test Any equipment that disturbs the
materials, the test object is illuminated by mobility of the particles is to be avoided.
the lamp held by the operator. In the past, kerosene has been painted on
Discontinuities indicated by flowing lines clay objects to detect discontinuities. One
readily attract the operator’s eye. should avoid the temptation to use a
brush with a filtered particle medium,
FIGURE 10. Portable ultraviolet lamp used in fluorescent because the bristles of the brush tend to
inspection of high tension bushing. line up the particles in such a manner as
to create false indications. These resemble
true discontinuity indications and may
confuse the observer.

Spray guns or similar equipment that
apply the fluid by an exterior force should
be used with caution, because it is
possible to remove indications by
applying the liquid with too great a force.

Pressure Gun for Spraying Filtered
Particle Suspensions

A portable spray gun has been designed to
spray a narrow, 60 degree spray pattern of
test liquid onto porous objects without
interfering with the formation of
indications. This gun will hold about
0.5 L (0.1 gal) of liquid at an air pressure
of 275 kPa (40 lbf·in. –2), which is placed
on top of the liquid through a special
valve. The air line can be disconnected
and the gun carried about as a portable
instrument. This feature is particularly
valuable when the operator does not wish
to be tied down by a hose connecting
device. If desired, the air hose can be left
connected and the gun filled to its
capacity (1 L or 0.25 gal) and used at a
fixed location.

Filtered Particle Testing 333

Tank Pressure Equipment for
Filtered Particle Tests

Tank equipment is the equivalent of a
pressure gun, except that more than 4 L
(1 gal) of test fluid is held in a tank that
rests on a small dolly. Air can be
pressurized and the hose removed to
allow equipment to be moved about. The
filtered particle test material is sprayed on
the pieces through a 3 m (10 ft) hose by a
gun with a nozzle similar to the pressure
gun mentioned earlier. This equipment is
small enough to be shaken occasionally to
maintain the suspension in proper
condition. Similar equipment is provided
for the spraying of water.

Drum Agitator Equipment for
Filtered Particle Tests

A larger type of equipment consists of a
heavy duty pump and motor that can be
mounted on a rack or a high table. A full
drum of filtered particle suspension can
be placed underneath and specially
designed mixing hoses introduced into
the drum. A long pair of hoses is attached
directly to the pump and the liquid is
continually recirculated in such a manner
as to stir the contents of the drum and
provide a constant concentration of
particles in liquid to the operator. The
fluid is applied with a gun similar to that
used in the pressure tank equipment
mentioned in the preceding paragraph.
This equipment may be piped for
supplying up to ten inspectors operating
off the system.

Other Techniques for Application
of Filtered Particle Test Media

Filtered particle test fluid can be applied
by rather simple measures, depending on
the object to be tested. For instance, some
users have tested pole type electric power
insulators that are about 600 mm (24 in.)
long with a hole 50 mm (2 in.) in
diameter running almost the entire length
of the object. Cracks occasionally occur at
the bottom on the inside. In this case,
regular pressure type equipment may not
be as useful as an ordinary eyedropper,
tablespoon or an oil squirt gun. When
using equipment of this type, it is
essential that the test medium be poured
or squirted as evenly as possible to avoid
drain marks.

A simple pump spray applicator, such
as is sold in supermarkets for household
use with window cleaner etc., will also
work well. These devices have adjustable
nozzles, and the spray may be adjusted to
suit the test. The operator must remember
to shake these from time to time to
ensure complete suspension of particles in
the fluid.

334 Liquid Penetrant Testing

PART 5. Prewetting and Associated Phenomena

Effect of Water on discontinuity and penetrate where no
Fluorescent Particle water barrier is encountered. The dotted
Indications line Y shows the penetration of the fluid
without benefit of water addition.
Many users of the filtered particle test
technique have observed that water plays Differential Absorption Ratios
a very important role in the reproduction with Prewetting
of an indication. As pointed out earlier,
the forces involved in differential If the absorption at point W is compared
absorption are extremely large, running as with that at point X in Fig. 11, it will be
high as 600 to 1. Water can increase this noted that about 1600 mm3 (0.1 in.3) of
differential ratio. Figure 11 shows a cross fluid are absorbed at point W and only
section of a porous material containing a 4 mm3 (2.4 × 10–4 in.3) at point X. By
crack. The dimensions are similar to those comparing amounts of fluid penetrant, it
shown in Fig. 2. If 200 mm3 (0.012 in.3) will be noted that the ratio is 1600 to 4 or
of water are placed on the surface 400 to 1.
containing the discontinuity and it
penetrates uniformly, the manner of Again, as indicated earlier, it has been
penetration is substantially as shown in noted that the ratio of crack width to
Fig. 11. The water forms a band or layer, depth is of the order of 300 to 1.
about 2 mm (0.08 in.) thick and about Therefore, the crack width at point W is
2 mm (0.08 in.) below the surface. A 0.15 mm (0.006 in.); all other dimensions
limited amount of penetration will occur remain the same. Thus, with point W at
in the confines of the crack itself. 0.15 mm (0.006 in.) and point X at
0.15 mm (0.006 in.) width, the
Penetration of Oil Based differential absorption ratio would be
Fluid above Water Barrier 1600 mm3 (0.1 in.3) at point W and
0.6 mm3 (3.7 × 10–5 in.3) at point X. This
If 1700 mm3 (0.11 in.3) of an oil based is a ratio of 1600 to 0.6 or 2700 to 1.
fluid are placed on the surface containing
the discontinuity, a uniform penetration Effects of Prewetting in
will occur as shown in Fig. 11. Because Filtered Particle Testing
the oil is not miscible in the water, it can
only fill the vacant pores about line Z. From this simple idealized example of the
The remainder must enter the effect of water, it can be seen that the
forces involved in producing a filtered
FIGURE 11. Schematic cross section of defect, illustrating particle indication where prewetting
penetration of oil based fluid with water. occurs are much higher than where no
water is used at all. In practice, prewetting
WX has the advantage of aiding in the
production of sharp, clean cut crack
Water Line Z indications with a substantially reduced
Line Y background. Prewetting tends to slow the
absorption rate; however, in most cases
this has been no disadvantage. The value
of prewetting may be nullified if the test
operator allows too much time between
the application of water and application
of test fluid. Too much water applied to
the porous surface of claylike objects may
prevent indications from forming by
clogging pores in cracks.

Indicating Contained Water in
Green Clay Materials

The application of water to clay is usually
discussed in terms of prewetting rather

Filtered Particle Testing 335

than of water already contained in the Cracks Missed in Testing of
clay. If clay is not completely dried, the Clayware
body may contain enough water to
prevent the absorption of any externally In certain instances in the past, there
applied fluid. Prewetting is only have been occasions when the filtered
advantageous when used on dried clay particle test technique seemingly missed
materials. cracks. In almost every case investigated,
it was proved that the filtered particle
In some instances, it is possible to use technique missed cracks in unfired ware
test fluids as a rough indicator of the only because the cracks were not actually
amount of water contained in clay. For present at the time of the testing. It is
example, one filtered particle test medium thought that the cracks were produced
contains, among other things, red after testing.
fluorescent dye. Under certain conditions,
this dye is a soluble in water as well as oil. Cracks Produced by Glaze
In dry clay, the test medium will Application
temporarily stain it a bright pink color. If
the clay is damp, the red dye tends to This strange situation may be explained as
dissolve into the water (contained in the follows: A crack in a porous product is an
clay) and will stain the surface of the clay indication of a stress that was present at
object a very faint pink. that site at one time. The existence of the
crack is evidence that the stress relieved
Applications of Prewetting itself. It is entirely possible that other
Technique stresses locked in clay are not of sufficient
magnitude to produce a crack unless
The addition of water to clay plays a still encouraged to do so. Of all the things that
more important role with test fluids for seem to encourage the releasing of a
detecting discontinuities. In some locked in stress, water seems the most
instances, water is required. On clay likely. In the case mentioned above,
surfaces that have been abraded with steel where cracks seemingly were missed, it
wool, turning tools, emery paper or the has been fairly well established that the
like, it is absolutely essential to prewet the water applied at the time of the glaze
ware before testing. If they are small application to clayware could have
enough, as are some electrical bushings, it released some of the stresses and
is frequently easier to dip test objects very produced cracks that opened in firing.
quickly into a tank of water and test in Thus, where clay products may have
5 min. On sanitary ware or other large locked in stresses that have not produced
objects, the water may be sprayed or cracks, it is possible to release them by
sponged on just before testing. Generally, applying water to the areas in question.
the amount of water required on the This allows the cracks to open up and
surface of the clay is just enough to permits indication of discontinuities by
change the color of the base material. If filtered particle tests.
too much water is used, absorption of the
test fluid is slowed, though not stopped. Detecting Locked In Stress
Practice will determine how much water
is required. One way to determine whether locked in
stresses are present is to use filtered
Exceptions to Prewetting particles to test the ware after drying,
marking and locating any cracks and then
In some cases, water need not be used indicating them on a sketch. The clay
before filtered particle testing. On as-cast object is placed in a dryer or allowed to
clayware that is free of flash and abrasion, dry in air for 24 h and then is retested. If
it is frequently possible to test the ware locked in stresses are present, more cracks
without the prewetting operation. will occur in the clay product the
However, a universal rule is always prewet following day. In some cases it is possible
when in doubt. to develop new cracks every additional
day that the piece of ware is allowed to
Reapplication of Filtered dry. Water tends to encourage the
Particle Test Media swelling and reshrinkage of clay. This
tends to encourage the relief of stresses,
In all cases, regardless of the initial test with the resultant production of cracks
technique, if it is desired to chase down that may be detected with the filtered
the discontinuity and recheck with test particle test fluid.
fluid, it is absolutely essential that the
chased area be prewet before a
reapplication of test fluid. It is impossible
to develop indications on reworked clay
without prewetting.

336 Liquid Penetrant Testing

PART 6. Interpretation of Filtered Particle Test
Indications

Significance of discontinuities in two hotel chinaware
Interpretation plates, one fired and one unfired.

The success of any technique of testing Tightly Closed Cracks
depends to a large extent on the
interpretation of the indications of There have been instances in which
discontinuities. In addition, there is the cracks, reliably established as present in
problem of interpreting false or unfired ware, did not appear to open up
misleading markings that frequently on firing. At one hotel chinaware plant,
resemble true discontinuity indications. cracks were found in two types of plates
before bisque firing. The cracks were
Discontinuities in Unfired S-shaped and occurred in the center of
Clayware both dinner plates and saucers. On firing,
the cracks in the dinner plates invariably
Many kinds of discontinuities occur in opened widely and the cracks in the
unfired clayware before firing. They may saucers did not. It was suspected that the
be contaminants such as iron, copper or crack indications on the unfired saucers
plaster in clayware or they may be cracks had been false. When the fired saucers
and voids. No doubt there are many other were examined closely under a
types as well. One of the most irritating microscope, cracks were found in the
and persistent of all discontinuities same locations as predicted. The cracks
appearing in unfired clayware is were so tightly closed that simple visual
unquestionably the crack. It may have examination could not detect them
been caused by rough handling, mold without filtered particle tests.
sticking, improper forming and shaping
techniques or incorrect drying and Cracks in Fired Clayware
shrinkage. In most cases, the crack is open
to the surface and generally invisible to After several years of experimental work
the unaided eye. When examining wares in potteries, a universal rule has been
after firing, inspectors sometimes confuse established: a crack in unfired ware always
cracks that had been present before firing results in a crack in fired ware. Despite
with cracks caused by the firing itself. this, when a glaze is fired at the same
Figure 12 shows comparable time as the body, the results sometimes
appear to be erratic. A crack in the unfired
FIGURE 12. Unfired (left) and fired (right) hotel chinaware body will always result in an open crack
containing similar defects. after firing; however, its presence may be
disguised by the covering glaze. Predicting
which crack will break through the glaze
after firing is possible only after becoming
completely familiar with the type of
discontinuities that cause trouble in a
particular plant.

Stress Relief Cracking in Firing

Cracks may be considered as an indication
of stress that has partially or completely
relieved itself. Small cracks in unfired
clayware frequently result in very much
larger discontinuities in fired ware. This is
a rough indication that at the time of
firing, the crack, still being under stress,
tends to relieve itself much in the manner
of a run in a stocking. Very large cracks in
unfired clayware nearly always maintain
their size after firing.

Filtered Particle Testing 337

Chasing out Avoid Drain Marks in
Discontinuities in Green Filtered Particle Testing
Clayware
The interpretation of fluorescent
When unfired ware is repaired by chasing indications on porous materials is not
or cutting out an anomalous area, it is particularly difficult, but some instances
considered good practice to recheck the call for experience. In every instance
repaired area by filtered particle testing to when too much test fluid is placed in a
make sure that a new discontinuity has restricted cavity or on a cuplike surface,
not been formed that might be more an excess of fluid will develop in puddles
serious than the original one. It is also that tend to drop out particles in a typical
essential to determine the true length of a ringlike manner. Sometimes these drain
crack and to repair it accordingly. On marks appear to resemble cracks,
sanitary ware, if a small discontinuity is particularly if they appear in the same
chased out but the ends are not direction as genuine discontinuities.
completely removed, these ends may act Many times it is possible to prevent the
as stress raisers during firing and produce formation of ring marks by continuous
relatively large discontinuities after final movement of the part while the test fluid
firing. When discontinuities are patched, is still mobile on the surface of the porous
a rechecking of the area is in order. If the object. This technique, of course, can only
patching tool is used too roughly and the be used on objects that are small enough
clay is too dry, a multitude of cracks will to be manipulated.
form that may open widely enough in
firing to cause rejection. Irrelevant Indications in
Filtered Particle Tests
Interpreting Crack
Indications in Large When test fluids are applied to porous
Structural Ceramic Bodies surfaces repeatedly, an overlapping of the
respective treatments tends to produce
On large structured bodies, cracks due to more marks and rings, which sometimes
the firing procedure may appear as if they lead to confusion. Only experience can
had been unfired cracks. An easy way to assist the viewer in identifying drain
tell the difference is to make a thorough marks, overlaps, scratch marks and true
check of the clayware before firing. Thus, crack indications. Frequently, horizontal
fired ware that contains this type of scratches or grooves have a misleading
discontinuity can be identified as such appearance. True crack indications are
and not confused with true unfired cracks. totally different from those of purely
However, if stresses have been locked in mechanical markings. Crack indications
the unfired ware, as discussed earlier, the generally look like cracks. In other words,
exact identification of a crack can only be they are slightly jagged and invariably
made by the appearance of the internal occur in places where they might be
faces of the fired discontinuity. Figure 13 expected to occur, such as sharp corners
illustrates the fired appearance of typical and joints.
cracks that occurred in an unfired body.
If there is doubt concerning a
FIGURE 13. Fired appearance of cracks in particular mark or indication, it is possible
unfired ware. to wipe away the existing indication and
repeat the application of fluid. If a crack is
present, it will repeat itself. Drain marks
rarely show up in the same manner twice.
When reproducing filtered particle
indications, the initial application of fluid
will have plugged many of the pores, thus
reducing the sensitivity of the technique
slightly. A short wait before reapplication
of test fluid is always advisable.

338 Liquid Penetrant Testing

PART 7. Filtered Particle Testing of Carbon
Matrix Components

Carbon Matrix Material Case Study

Conventional liquid penetrant testing A research program in the United States
does not work well with porous materials. Air Force explored the concept of using
Conventional test methods such as carbon fiber reinforced carbon matrix
radiographic or ultrasonic testing also composite materials in turbine blades and
frequently yield unsatisfactory results on vanes.
these materials. It is in this situation that
filtered particle testing can be used to Test Materials and Conditions
great benefit. The method was developed
for green or lightly fired ceramic Filtered particle solutions were made by
components. Application of filtered adding brightly colored daylight
particle testing to candidate turbine fluorescing pigment particles to an
components should illustrate that the isopropanol carrier fluid. Daylight
method is effective when conventional fluorescing pigments appear to emit more
methods are not. visible light than would be reflected from
them. This occurs because the ultraviolet
Carbon fiber reinforced carbon matrix portion of the radiation illuminating the
composite materials have been considered specimen is down converted to visible
for possible use in turbine blades and light by the pigment. Both yellow and red
vanes. Components made of these pigments were used because they
composites have many advantages, produced the greatest contrast between
including low density, high temperature the light gray background of the silicon
strength and stiffness and high toughness carbide coatings and the discontinuity
or minimal sensitivity to cracklike indications. No attempt was made to
discontinuities. However, these materials optimize the particulate concentration of
do have one significant disadvantage: the solutions or the wetting characteristics
they are combustible and therefore must of the carrier.
be protected from the oxidation
environment of the gas turbine engine. Furthermore, prewetting was not used
For this reason, these materials have only to enhance the sensitivity of the test
been considered for use in nonpiloted air described herein. Specimens were
vehicles or as nonstructural elements in examined with standard white,
piloted air vehicles. fluorescent light sources. The only
enhancement to this setup was to place
Carbon fiber reinforced carbon matrix the light sources on both sides of the
materials are often protected with a specimen to illuminate it slightly off
coating of silicon carbide (SiC). This is a grazing incidence. This produced a useful
stoichiometric carbide that is stable to visual crack enhancement. The fluorescing
very high temperatures and forms a pigment emits light spherically, so the
protective, oxidation resistant silicon discontinuity indications do not have
dioxide (SiO2) film in the operating shadows. As a consequence of this, the
environment of a turbine engine. Because crazing indications appear to be much
the composites are sensitive to oxidation, larger than those produced in a normal
the integrity of this silicon carbide film is filtered particle test.
key to their use in high temperature,
oxidizing environments. Any breach in Discontinuity Indications and
this protective layer could be catastrophic, Interpretation
so the breaks in the silicon carbide film
must be detected with a reliable test The discontinuity indications produced
method. However, silicon carbide coated from the examination of the silicon
carbon matrix material is very porous. carbide coated carbon matrix specimens
Conventional nondestructive testing (Figs. 14 and 15) were very different from
cannot be used to detect discontinuities what might be expected from
in this coating. This situation is perfect conventional test experience. Whereas
for filtered particle testing. one expects a single or at most a small
number of indications, there are many
crack indications on the candidate turbine
vanes. On careful examination up to three

Filtered Particle Testing 339

different levels of crack indications may the component cools below 550 °C
be discerned, each following the (1020 °F). The extensive crazing of these
underlying weave pattern of the carbon components is evident in Figs. 14 and 15.
matrix substrate. This seems reasonable
when one considers the expansion The nondestructive testing engineer
characteristics of the carbon matrix must not only detect and quantify
substrate and the silicon carbide coating. discontinuities but, to surmise their
Silicon carbide is a typical material with a significance, should also be aware of how
positive, linear expansion coefficient. discontinuities might be generated during
Carbon-to-carbon composite materials, on manufacture or service. In this study, the
the other hand, have expansion inspector might have expected to see a
coefficients that change from negative to single crack in the silicon carbide coating
positive above some temperature. The as the dominant discontinuity permitting
expansion coefficient of the carbon oxidation attack of the carbon matrix
matrix used in the vanes shown in Figs. substrate. However, in this case, it is not
14 and 15 is positive above about 550 °C the extensive crazing but rather absence
(1020 °F) and negative below this of crazing that indicates the presence of a
temperature. The silicon carbide oxidation discontinuity. A regular pattern of cracks
resistant coating is applied at a indicates that the tensile stresses that
temperature well above 1000 °C (1800 °F) result from the expansion differences
and then cooled to room temperature between the substrate and coating were
where it was inspected. At the fabrication relieved on cooling from fabrication
temperature the coating and substrate are temperatures. An area of the coating that
in mechanical equilibrium, but on cooling does not contain a regular crazing pattern
stresses build up in the two materials is one in which these thermal stresses
because the expansion coefficients are not were not relieved in a small area. The
equal. This stress buildup is fairly most probable cause of this appearance is
common in many manufacturing the presence of a delamination between
processes involving dissimilar materials. the coating and substrate. The size of the
However, the stress buildup is aggravated cracks at the boundary of a noncrazed
if the substrate were to stop contracting or plate would be several times larger than
— worse — were to begin to expand on those in the uniform cracks associated
cooling as it does with carbon matrix. with the craze pattern. The large crack
Therefore, one would expect the thin, would be the last to close on heating and
polycrystalline silicon carbide coating to would most likely permit atmospheric
craze in a fairly regular pattern upon attack of the susceptible carbon matrix
cooling and for this crazing to increase as substrate. This was partially confirmed by
sectioning and examining several vanes
FIGURE 14. Filtered particle indications of crazing in oxidation after simulated engine testing. There
resistant coating of carbon matrix composite compressor could well be other discontinuities
vane: (a) concave or pressure surface; (b) convex or suction associated with atmospheric attack of
surface. The vane has silicon dioxide coating from high vanes that failed during other tests but an
temperature exposure in simulated jet engine test. Red exhaustive study of coating failures was
filtered particles are daylight fluorescing pigment. not pursued.

(a) Conclusions

The results of this limited study indicate
that filtered particle testing can be used to

FIGURE 15. Filtered particle indications of crazing in oxidation
resistant coating of carbon matrix composite compressor
vane. Vane is as fabricated, not mounted, for simulated jet
engine testing. Yellow filtered particles are daylight
fluorescing pigment.

(b)

340 Liquid Penetrant Testing

detect discontinuities in porous materials
and coatings when conventional methods
fail. An exhaustive study of test
indications and coating discontinuities
was not attempted. However, the filtered
particle test did show that the absence of
a multiple discontinuity pattern can
indicate the presence of a discontinuity.
This situation would be different than one
might expect from previous test
experience.

Additionally, this test offered the
opportunity for an enhancement of the
filtered particle test method. The concept,
in this case, would be to modify the
composition of the particles to provide a
surface treatment at the site of a crack in
addition to providing a crack indication.
Because only the surface of the particle is
involved in the test, there is the
opportunity to alter the chemical
composition of its bulk for various
beneficial treatments at a crack site. Thus
if the materials engineer wished to apply a
surface treatment at the site of a crack,
the filtered particle test provides this
opportunity in addition to a detection
function.

Filtered Particle Testing 341

References

1. DeForest, T. and H.N. Staats.
Section 14, “Principles and Techniques
of Filtered Particle Inspection.”
Nondestructive Testing Handbook,
second edition: Vol. 2, Liquid Penetrant
Tests. Columbus, OH: American
Society for Nondestructive Testing
(1982): p 575-594.

2. DeForest, T. and H.N. Staats. Method of
Detecting Cracks in Porous Surfaces.
United States Patent 2 516 857
(August 1950).

3. DeForest, T. and H.N. Staats. Method of
Detecting Cracks in Porous Surfaces.
United States Patent 2 635 329
(April 1953).

4. DeForest, T. and H.N. Staats. Method of
Detecting Cracks in Porous Surfaces.
United States Patent 2 636 127
(April 1953).

5. Betz, C.E. “Two New Testing Methods
for Ceramic Products.” Nondestructive
Testing. Vol. 7, No. 2. Columbus, OH:
American Society for Nondestructive
Testing (Fall 1948): p 22-26.

6. Staats, H.N. “Which Nondestructive
Test for Finding Defects in Ceramic
Parts.” Materials and Methods. Vol. 36,
No. 3. New York, NY: Reinhold
Publishing Corporation (1952): p 116.

7. Staats, H.N. “Nondestructive Testing of
Green Ware.” American Ceramic Society
Bulletin. Vol. 29, No. 11. Westerville,
OH: American Ceramic Society
(November 1950): p 411-415.

8. Staats, H.N. “The Testing of Ceramics.”
Nondestructive Testing. Vol. 10, No. 3.
Columbus, OH: American Society for
Nondestructive Testing (Winter 1952):
p 23-26.

9. Staats, H.N. “Filtered Particle
Inspection of High Tension
Insulators.” Nondestructive Testing.
Vol. 11, No. 3. Columbus, OH:
American Society for Nondestructive
Testing (January 1953): p 21-24.

342 Liquid Penetrant Testing

12

CHAPTER

Liquid Penetrant Testing in
Primary Metals Production

Frank V. Gricus, Reynolds Metals Company, Richmond,
Virginia
Richard Z. Struk, Shellcast Foundries, Incorporated,
Montreal, Canada