1587884932985_final book 26-04-20

On the other hand, at cathodic areas, reduction reaction (i.e. gain of electrons) takes place.
Usually cathode reactions do not affect the cathode, since most metals cannot be further
reduced. So at cathodic part, dissolved constituents in the conducting medium accepts the
electrons to form some ions (like OH- and O2- ). The metallic ions (at anodic part) and non
metallic ions (formed at cathodic part) diffuse towards each other through the conducting
medium and form a corrosion product somewhere between anode and cathode. The
electrons set free at the anode flow through the metal and are finally consumed in the
cathodic reaction.

Thus we can sum up that electrochemical corrosion involves:

i) The formation of anodic and cathodic areas or parts in contact with each other

ii) Presence of a conducting medium

iii) Corrosion of anodic areas only and

iv) Formation of corrosion product somewhere between anodic and cathodic areas. This
involves flow of electron-current between the anodic and cathodic areas.

At anodic area oxidation reaction takes place (liberation of free electron), so anodic metal is
destroyed by either dissolving or assuming combined state (such as oxide, etc.). Hence
corrosion always occurs at anodic areas.

M (metal) → M n+ + ne-

Mn+ (metal ion) → dissolves in solution → forms compounds such as oxide

At cathodic area, reduction reaction takes place (gain of electrons), usually cathode
reactions do not affect the cathode, since most metals cannot be further reduced. So at
cathodic part, dissolved constituents in the conducting medium accept the electrons to form
some ions like OH and O2-. Cathodic reaction consumes electrons with either by:

(a) evolution of hydrogen or

(b) absorption of oxygen, depending on the nature of the corrosive environment

4.4.1.) Hydorgen Evolution Type:

All metals above hydrogen in the electrochemical series have a tendency to get dissolved in
acidic solution with simultaneous evolution of hydrogen. It occurs in acidic environment.

Consider the example of iron.

At anode: Fe → Fe2+ + 2e-
These electrons flow through the metal, from anode to cathode, where H+ ions of acidic
solution are eliminated as hydrogen gas.

At cathode: 2H+ + 2e- → H2↑
The overall reaction is: Fe + 2H+ → Fe2+ + H2

4.4.2.) Oxygen Absorption Type:

Rusting of iron in neutral aqueous solution of electrolytes (like NaCl solution) in the
presence of atmospheric oxygen is a common example of this type of corrosion. The surface
of iron is usually coated with a thin film of iron oxide. However, if this iron oxide film
develops some cracks, anodic areas are created on the surface; while the well metal parts
acts as cathodes.

At Anode: Metal dissolves as ferrous ions with liberation of electrons.
Fe → Fe2+ + 2e-

At Cathode: The liberated electrons are intercepted by the dissolved oxygen.
½ O2 + H2O + 2e- → 2OH-

The Fe2+ ions and OH- ions diffuse and when they meet, ferrous hydroxide is precipitated.
Fe2+ + 2OH- → Fe(OH)2

(i) If enough oxygen is present, ferrous hydroxide is easily oxidized to ferric hydroxide.

4Fe(OH)2 +O2 + 2H2O → 4Fe(OH)3 (Yellow rust Fe2O3 .H2O)

(ii) If the supply of oxygen is limited, the corrosion product may be even black anhydrous
magnetite, Fe3O4 .

4.4.3 Electrochemical theory of wet corrosion

Electrochemical corrosion involves flow of electron –current between the anodic and
cathodic areas. The anodic reaction (i.e. at anodic area) involves dissolution of metal as
corresponding metallic ions with the liberation of free electrons. As with dry corrosion wet
corrosion reactions are only possible if the free energy of the products of reaction is lower
than the free energy of the reactants. This is the case however for the reaction of nearly all
metals with water and oxygen to give metal hydroxides:

M + H2O (l) → M(OH)2 (s) … (1)
This enables reaction 1 above to proceed through the coupling of two primary corrosion
reactions:

A: M→M2+ + 2e‾ … (2)
(Anode reaction: destroys metal, releases electrons, oxidation)

C: 2e‾ + H2O + ½O2 → _2OH … (3)
(Cathode reaction: consumes electrons; electron sink reaction: reduction).
Electrons liberated in the anode reaction A flow to the site of the cathode
reaction C through the conducting metal. The movement of dissociated ions carries an equal
ionic current in the water.

The distances over which these currents flow can vary from microns to many metres. All wet
corrosion processes can be analysed in terms of anodic and cathodic reactions.
The anodic process is the direct cause of damage to metallic structures but both an anodic
and a cathodic process must occur for a corrosion cell to be formed. The corrosion of metals
by reaction with air and water to form metal hydroxides as shown above is a very important
wet corrosion process, especially in the construction industries. There are other corrosion
reactions resulting from, for example, other cathode reactions (electron-consuming).
The rate of wet corrosion may often be very high compared with dry corrosion on the same
metal at the same temperature. There are two underlying reasons for this:
- the dipolar water molecule stabilizes the free (dissociated) metal ions in solution
- the metallic structure and water in contact with it can both conduct electric current
This enables reaction 1 above to proceed through the coupling of two primary corrosion
reactions:
All wet corrosion processes can be analysed in terms of anodic and cathodic reactions.
The anodic process is the direct cause of damage to metallic structures but both an anodic
and a cathodic process must occur for a corrosion cell to be formed
The corrosion of metals by reaction with air and water to form metal hydroxides as shown
above is a very important wet corrosion process, especially in the construction industries.
There are other corrosion reactions resulting from, for example, other cathode reactions
(electron-consuming)

2e‾ + 2H+ → H2 … (4)
2e‾ + M2+ → M2+ … (5)
or other anode reactions (electron releasing)

M + H2O →MO + 2H+ 2e‾ … (6)
When a metal M is placed in pure water, some ions will immediately pass into solution:

M→M2+ + 2e‾ … (7)
The build-up of negative charge on the metal and the build-up of metal ions in solution

makes possible a back-reaction:
M←M2+ + 2e‾ … (8)

and ultimately an equilibrium is established:
M↔_M2+ + 2e‾ … (9)

At this stage, a steady potential difference now exists between metal and solution. The
magnitude of this potential difference depends on the metal and composition of the
solution. It is not possible to measure this potential difference for a single metal, but the
potential difference (emf) between two metals dipping into a solution can be measured.
Under well-defined conditions, this enables a single potential Em (relative to a common
reference) to be assigned to every metal.

4.5 Corrosion Cell

Corrosion cells are a condition on a metal surface in which a flow of electric current occurs
between the metal surface and an electrolyte with which it is in contact sufficient to cause
the metal to degrade. Corrosion cells have been designed to measure the corrosion
properties of an object immersed in an electrolyte. It is normally used to test the reaction of
metal specimens in a corrosive environment.

Corrosion cells can be created through:

 Electrolysis
 Oxygen concentration cells
 Galvanic action

A corrosion cell consists of four fundamental components:

 Anode
 Cathode
 Conducting environment for ionic movement (electrolyte)
 Electrical connection between the anode and cathode for the flow of electron

current

The driving force behind a corrosion cell is a potential or voltage difference between the
anode and cathode. It is important to know that each of the four elements of the corrosion
cell affect the severity of corrosion.
A corrosion cell can occur on the molecular level. These cells usually are produced by three
factors:

 Irregularities in the metal's surface produced by the original metalworking/forming
or extruding

 Differences in the composition of the metal's surface pressed into the surface by
shaping/rolling or finishing operations

 Stresses induced from forming, welding, etc.
Coating films can be used to control one of these elements - the electrolyte. Applying a
tightly adhered continuous protective film over the surface of the metal and isolating those
points with different potential (anode and cathode) controls corrosion.

4.6 Corrosion Mechanism of Iron oxide (Rust) Formation

Now consider when an iron is immersed in water or sea water which is exposed to the
atmosphere. Now corrosion will occur due to the anodic reaction is:

Fe  Fe+2 + 2e-

And also the medium is exposed to the atmosphere, it contains dissolved oxygen. Both
water and sea water are nearly neutral, thus the cathodic reaction takes place as follows

O2 + 2H2O + 4e  4OH–

Now, remember that the sodium and chloride in the sea water do not participate in the
reaction. The reaction is only between the iron and water. The reaction can be rewritten as
follows

2Fe(OH)2 + H2O + ½ O2  2Fe(OH)2

Now the final product is familiarly called as “Rust”.

Normally the acid solution containing dissolved oxygen will be more corrosive than air free
acids. Oxygen reduction simply provides a new means of “electron disposal”. The same
effect is observed if any oxidizer is present in acid solution.

4.7) Difference between Dry and Wet Corrosion

Dry corrosion Wet corrosion

1 It occurs in dry condition. It occurs in wet condition.

2 If the corrosion takes place due to If the corrosion takes place due to

direct chemical attack (in the absence electrochemical attack in presence of

of moisture), corrosion is known as moisture or a conducting medium

dry corrosion. ,corrosion is known as wet corrosion

3 Explained by absorption mechanism Explained by electrochemical mechanism

4 It occurs on both heterogeneous and It occurs only on heterogeneous metal

homogeneous surfaces. surfaces.

5 Corrosion is uniform. Corrosion is not uniform.

6 It is a slow process. It is a fast process.

7 Corrosion products accumulate at the Corrosion take place at anode but

place where corrosion occurs. products accumulate near the cathode.

Lets Test Your Knowledge

1. Which of the following comes under the wet corrosion?

A] Concentration cell corrosion B] Oxidation corrosion

C] Liquid metal corrosion D]Corrosion by other gases

2. Chemical action of flowing liquid metal at high temperatures is __________

A] Liquid metal corrosion B] Corrosion by other gases

C]Oxidation corrosion D] Wet corrosion

3. Rusting of iron in neutral aqueous solution of electrolyte occurs in the presence of oxygen

with the evolution of ___________

A] Nitrogen B] Chloride

C] Sulphide D] Hydrogen

4. In wet corrosion _________________ are formed at the cathodic areas.

A] Organic compounds B] Metallic ions

C]Non-metallic ions D] Inorganic compounds

5. Which type of reaction occurs in anodic areas?

A] Oxidation B] Reduction

C] Displacement D] Addition

6. Select the incorrect statement about the wet corrosion from the following option.
A] It involves the setting up of large number of galvanic cells
B] It is explained by absorption mechanism
C] It occurs only on heterogeneous metal surface D] It is a fast process

Answers

1 23 4 5 6

AADC AD

4.8 Classifications of Electrochemical corrosion

Even though the fundamental mechanism of corrosion involves creation or existence of
corrosion cells, there are several types or forms of corrosion that can occur. It should
however be borne in mind that for corrosion to occur, there is no need for discrete
(physically independent) anodes and cathodes. Innumerable micro level anodic and cathodic
areas can be generated at the same (single) surface on which anodic (corrosion) and
cathodic (reduction) reactions occur. Each form of corrosion has a specific arrangement of
anodes and cathodes and specific patterns and locations depending on the type can exist.
The most important types are:

 Uniform corrosion
 Galvanic corrosion, concentration cells, water line attack
 Pitting
 Dezincification, Dealloying (selective leaching)
 Atmospheric corrosion
 Erosion corrosion
 Fretting Crevice corrosion; cavitation
 Stress corrosion, intergranular and transgranular corrosion, hydrogen cracking and

embrittlement Corrosion fatigue.

As corrosion most often occurs in aqueous environments, we now explore the
different types of degradation a metal can experience in such conditions:

4.8.1 Pitting Corrosion

Pitting is one of the most destructive types of corrosion, as it can be hard to predict, detect
and characterize. Pitting is a localized form of corrosion, in which either a local anodic
point, or more commonly a cathodic point, forms a small corrosion cell with the
surrounding normal surface. Once a pit has initiated, it grows into a “hole” or “cavity” that
takes on one of a variety of different shapes. Pits typically penetrate from the surface
downward in a vertical direction. Pitting corrosion can be caused by a local break or damage
to the protective oxide film or a protective coating; it can also be caused by non-

uniformities in the metal structure itself. Pitting is dangerous because it can lead to failure
of the structure with a relatively low overall loss of metal.

Mechanism:

The mechanism of this corrosion involves setting up of differential aeration or concentration
cell. Metal area covered by a drop of water, dust, sand, scale etc. is the aeration or
concentration cell. Pitting corrosion is explained by considering a drop of water or brine
solution (aqueous solution of NaCl) on a metal surface, (especially iron). The area covered
by the drop of salt solution as less oxygen and acts as anode. This area suffers corrosion, the
uncovered area acts as cathode due to high oxygen content. It has been found that the rate
of corrosion will be more when the area of cathode is larger and the area of the anode is
smaller. Hence there is more material around the small anodic area results in the formation
hole or pit.

At anode: Fe is oxidized to Fe2+ and releases electrons. Fe Fe2+  +2e-
At cathode: Oxygen is converted to hydroxide ion ½ O2 + H2O + 2e-  2OH
The net reaction is Fe + 2OH-  Fe(OH)2

Preventive measures

 Preparing services with best policy possible finish. Mirror finish dresses fitting best.
 Removing all contaminants, especially free Iron by passivation.
 Designing and fabricating to avoid trapped and pooled liquids.

4.8.2 Intergranular Corrosion

An examination of the microstructure of a metal reveals the grains that form during
solidification of the alloy, as well as the grain boundaries between them. Intergranular
corrosion can be caused by impurities present at these grain boundaries or by the
depletion or enrichment of an alloying element at the grain boundaries. Intergranular
corrosion occurs along or adjacent to these grains, seriously affecting the mechanical
properties of the metal while the bulk of the metal remain intact.
An example of intergranular corrosion is carbide precipitation, a chemical reaction that can
occur when a metal is subjected to very high temperatures (e.g., 800°F - 1650°F) and/or
localized hot work such as welding. In stainless steels, during these reactions, carbon
“consumes” the chromium, forming carbides and causing the level of chromium remaining
in the alloy to drop below the 11% needed to sustain the spontaneously-forming passive
oxide layer.

Mechanism:

(i). The actual mechanism differ from one alloy system to another. Some precipitates form
preferentially at grain boundaries as a result of production, fabrication and welding at
elevated temperature. If these precipitates are rich in alloying elements which are essential
for corrosion resistance the regions adjacent to grain boundaries are depleted of these
elements. Thus the metal is said to be sensitized and it suffers from Intergranular attack in a
corrosion environment.
(ii). Segregation of impurities at grain boundaries may give rise to galvanic corrosion. Now
we consider Austenitic stainless steel of type 304. The universally accepted theory is the
depletion of chromium in the grain boundary areas. Chromium increases the corrosion
resistance of the steel. If chromium is less than 11% then relatively low corrosion resistance
is approached.
(iii). This steel is sensitized to corrosion when heated approximately from 950-1450 ̊F. In this
range chromium carbide is virtually insoluble and precipitates out if carbon content is higher

than o.o2%. So Cr, come out of solid solution results in metal with low chromium contents
in the area adjacent to the grain boundaries. The chromium carbide in the grain boundaries
is not attacked.
(iv). The chromium depleted zone near the grain boundary is corroded because it does not
contain sufficient corrosion resistance to resist attack in many corrosive environment. The
common type 304 usually contains 0.06 t0 0.08% carbon so excess carbon is available for
combining with the chromium to precipitate the carbide.
(v). Carbon diffuse out more readily towards the grain boundaries at sensitizing temperature
but chromium is much less mobile. So chromium carbide is formed on the grain boundaries.
(vi). If the alloy is cut into a thin sheet and cross section of granular boundary area made,
the corroded area would observe as a deep narrow trench when observe at low
magnification.

Preventive measures:

 Low acidity (high pH) will generally reduce the susceptibility to IGC Prevention.
 Weak corrosive conditions do not cause IGC.
 Heat treatment to re-dissolve the carbides (post welding heat treatment).
 Use a stabilized grade of SS, which contain strong carbide-forming elements such as

Nb or Ti and tantalum , which form titanium carbide, niobium carbide and tantalum
carbide preferentially to chromium carbide.
 Use low carbon content grade stainless steel, eg: 316L, 304L ~ 0.03 wt.%, so carbide
formation is minimal.
 By the use of stabilizing heat treatment.
 By the proper choice of welding materials.

4.8.3 Stress Corrosion Cracking

Stress corrosion cracking (SCC) is a result of tensile stress and a corrosive environment,
often at elevated temperatures. Stress corrosion may result from external stress such as
actual tensile loads on the metal or expansion/contraction due to rapid temperature
changes. It may also result from residual stress imparted during the manufacturing process
such as from cold forming, welding, machining, grinding, etc. In stress corrosion, the
majority of the surface usually remains intact; however, fine cracks appear in the
microstructure, making the corrosion hard to detect. The cracks typically have a brittle
appearance and form and spread in a direction perpendicular to the location of the stress.
For example, cold worked brass, which is found in ammunition cartridges, is susceptible to
stress corrosion cracking when exposed to an environment containing ammonia.

Mechanism:

The process of SCC consists of three stages:
• Crack Initiation
• Crack Propagation
• Brittle Fracture

1.) Crack Initiation: SCC is initiated by stress concentrations at defects on the material
surface. The defect may be an existing material defect. The defect may also be a result of
pitting corrosion, crevice corrosion, intergranular corrosion or local galvanic corrosion.

2.)Crack Propagation: The passivation film at the tip of the crack is broken due to plastic
deformation. Pure and normally very active metal is exposed and will be attacked by
corrosion. Grow rate will be a combination of corrosion and cracking. The crack starts to
grow when the stress concentration at the end of the crack exceeds the threshold stress
intensity factor for stress corrosion cracking.

3.)Brittle Fracture: When the stress concentration at the end of the crack exceeds the
critical stress intensity factor (KI > KIC), there will be a rapid, unstable brittle fracture.

Preventive measures:

• Avoid stress concentrators
• Relieve fabrication stresses
• Introduce surface compressive stresses

• Reduce operating stresses
• Change alloy composition
• Change alloy structure
• Use metallic conversion coating
• Add inhibitor
• Modify temperature

4.8.4 Galvanic Corrosion

Galvanic corrosion is the degradation of one metal near a joint or juncture that occurs
when two electrochemically dissimilar metals are in electrical contact in an electrolytic
environment; for example, when copper is in contact with steel in a saltwater environment.
However, even when these three conditions are satisfied, there are many other factors that
affect the potential for, and the amount of, corrosion, such as temperature and surface
finish of the metals. Large engineered systems employing many types of metal in their
construction, including various fastener types and materials, are susceptible to galvanic
corrosion if care is not exercised during the design phase.

Mechanism:

In acidic solution, the corrosion occurs by the hydrogen evolution process; while in neutral
or slightly alkaline solution, oxygen absorption occurs. The electron-current flows from the
anode metal, zinc to the cathode metal, copper.

Zn  Zn2+ + 2e- (Oxidation)
Thus it is evident that the corrosion occurs at the anode metal; while the cathodic part is
protected from the attack.

Other examples of Differential metal corrosion include the following:
1) Buried iron pipe line connected to zinc bar
2) Steel pipe connected to Copper plumbing
3) Steel propeller shaft in bronze bearing
4) Zinc coating on mild steel
5) Lead tin solder around copper wires

Preventive measures

 Placing a thin layer of an insulator between two metals or materials
 Selecting the materials having very less potential difference

4.8.5 Concentration Cell Corrosion:

It occurs due to electrochemical attack on the metal surface, exposed to an electrolyte of
varying concentrations or of varying aeration. It occurs when one part of metal is exposed
to a different air concentration from the other part. This causes a difference in potential
between differently aerated areas. It has been found experimentally that poor-oxygenated
parts are anodic.
Examples: i) The metal part immersed in water or in a conducting liquid is called water line
corrosion.
ii) The metal part partially buried in soil.

Thus, this type of corrosion can be mainly divided into two variations:
(i) an electrolyte of different concentration
(ii) differential aeration

Mechanisms:

(i) Concentration cell corrosion due to an electrolyte of different concentrations:
Consider a metal bathed in an electrolyte containing its own ions of different concentration
at different regions of the metal surface. The basic corrosion reaction is where a metal atom
in direct contact with dilute electrolyte acts as an anode and undergoes corrosion.
Since the electrolyte as an ion can proceed both forward and backwards, and will eventually
reach equilibrium. If a region of the electrolyte (adjacent to the metal) were to exhibit a
decreased concentration of metal ions, this region would become anodic to the other
portions of the metal surface. As a result, this portion of the metal would corrode faster in
order to increase the local ion concentration.
The net effect is that local corrosion rates are modulated in order to homogenize reduction
ion concentrations within the electrolyte.

Preventive measures:

(i) Sealing the faying surfaces in a manner to exclude moisture.
(ii) Proper protective coating application with inorganic zinc primers is also effective in
reducing faying surface corrosion.

Mechanisms:

(ii) Concentration cell corrosion due to differential aeration:
If a metal is partially immersed in a conducting solution the metal part above the solution is
more aerated and becomes cathodic. The metal part inside the solution is less aerated and
thus becomes anodic and suffers corrosion.

At anode: Corrosion occurs (less aerated) M  M2+ + 2e
At cathode: OH- ions are produced (more aerated) ½ O2 + H2O +2e-  2OH-

Preventive measures:

(i) Sealing
(ii) Maintaining surfaces clean
(iii) Avoiding the use of materials that permit wicking of moisture between faying surfaces.

4.8.6 Crevice Corrosion

Crevice corrosion is also a localized form of corrosion and usually results from a stagnant
microenvironment in which there is a difference in the concentration of ions between two
areas of a metal. Crevice corrosion occurs in shielded areas such as those under washers,

bolt heads, gaskets, etc. where oxygen is restricted. These smaller areas allow for a
corrosive agent to enter but do not allow enough circulation within, depleting the oxygen
content, which prevents re-passivation. As a stagnant solution builds, pH shifts away from
neutral. This growing imbalance between the crevice (microenvironment) and the external
surface (bulk environment) contributes to higher rates of corrosion. Crevice corrosion can
often occur at lower temperatures than pitting. Proper joint design helps to minimize
crevice corrosion.

Mechanism:

In general corrosion occurs rapidly in the area with less oxygen full stop increase in acidity
and chloride content of crevice solution initiates passive film breakdown process and
thereby enhances the rate of corrosion reaction. Finally corrosion propagation continues
leading to further deterioration of the material.

Stage 1: Corrosion normally occurs both inside and outside the queries with the following
reactions:
Anode reaction: M  M+ + ne-
Cathode reaction: 2O2 + 2H2O + 4e  4OH-
The positively charged metallic ions are electrostatically counterbalanced by hydroxyl ions.
Stage 2: The cathode reaction inside the crevice consumes most of the available oxygen

Stage 3: The Chloride and hydroxyl ions diffuse into the device to maintain minimum
potential energy does forming the metal chloride. Hydrolysis of metal chloride lowers pH,
and: MCL + nH2O -----> M(OH)n + nHCl.
Stage 4: More Mn+ ions attack more Cl- ions leading to lower pH inside the crevice. This
accelerates The dissolution of metal and more Mn+ ions will be produced that will lower the
pH.

Preventive measures

 Increasing resistance by using alloys that have high percentage of chromium,
molybdenum and/or nitrogen.

 Design and fabricate to avoid crevices.
 Frequently inspecting for inadvertent crevices and closing devices buy continuous

welding caukling or soldering.

4.8.7 Microbiological corrosion

This corrosion involves degradation of materials by bacteria, moulds and fungi or their by
products. Some types of bacteria consume Oxygen and cause differential aeration type of
system which results in corrosion. The corrosion occurs at the zones poor in oxygen
concentration. For example, the bacillus, algae the atoms. This type of corrosion can occur
by a range of actions such as:
1) attack of the metal or protective coating by acid by products sulphur, hydrogen sulphide
or ammonia
2) direct interaction between the microbes and metal that sustains attack.

For example, corrosion of steel pipes in waterlogged neutral ground containing little oxygen
is often much more severe than might be expected. Cast iron pipes of 6mm wall may
perforate in less than 5 years.

Mechanism:

Sulphate reducing bacteria such as desulphovibrio, which flourish in oxygen free conditions,
assist the corrosion of iron apparently by consuming hydrogen produced in the cathode
reaction does enhancing the rate of cathodic reaction.

H++ e-  H

Bacteria make use of H+ to reduce sulphate concentration. The overall corrosion cell
reaction is as follows:

8H+ + 8e-  8H

SO4 2- + H+ S2- + 4H2O

The corrosion product FeS is formed as a result of the above reactions.

Preventive measures

 Selecting resistant materials
 Ensuring frequent cleaning
 Controlling chemistry of surrounding media and removal of nutrients
 Using of biocides
 Applying cathodic protection

4.8.8 Uniform Corrosion

Uniform corrosion is considered an even attack across the surface of a material and is the
most common type of corrosion. It is also the most benign as the extent of the attack is
relatively easily judged, and the resulting impact on material performance is fairly easily
evaluated due to the ability to consistently reproduce and test the phenomenon. This type
of corrosion typically occurs over relatively large areas of a material’s surface.

4.8.9 Dealloying

Dealloying is a rare form of corrosion found in copper alloys, gray cast iron, and some
other alloys. Dealloying occurs when the alloy loses the active component of the metal and
retains the more corrosion resistant component in a porous "sponge" on the metal surface.
It can also occur by redeposition of the noble component of the alloy on the metal surface.
Control is by the use of more resistant alloys-inhibited brasses and malleable or nodular cast
iron.

The brass on the left dezincified leaving a porous copper plug on the surface. The gray cast
iron water pipe shown on the right photo has graphitized and left graphitic surface plugs
which can be seen on the cut surface. The rust tubercules or bubbles are also an indication
of pitting corrosion.
The bottom photo shows a layer of copper on the surface of a dealloyed 70% copper-30%
nickel cupronickel heat exchanger tube removed from a ship. Stagnant seawater is so

corrosive that even this normally corrosion-resistant alloy has corroded. Virtually all copper
alloys are subject to dealloying in some environments.

4.8.10 Fretting Corrosion

The rapid corrosion that occurs at the interface between contacting, highly loaded metal
surfaces when subjected to slight vibratory motions is known as fretting corrosion.

The photo above(left) shows fretting corrosion of a fence post and wires which swing in the
wind and wear against the post. Both the fence post and the connecting wires are
experiencing fretting corrosion.
This type of corrosion is most common in bearing surfaces in machinery, such as connecting
rods, splinted shafts, and bearing supports, and often causes a fatigue failure. It can occur in
structural members such as trusses where highly loaded bolts are used and some relative
motion occurs between the bolted members. Fretting corrosion is greatly retarded when the
contacting surfaces can be well lubricated as in machinery-bearing surfaces so as to exclude
direct contact with air.
The bearing race above(right) is a classic example of fretting corrosion. This is greatly
retarded when the contacting surfaces can be well lubricated as in machinery-bearing
surfaces so as to exclude direct contact with air.

Let’s test your Knowledge

1) The corrosion which forms sponge like pores is
A] Fretting corrosion B] Delloying Corrosion
C] Pitting corrosion D] Uniform corrosion

2) The corrosion which is formed when subjected to physical vibrations is

A] Microbiological Corrosion B] Delloying Corrosion

C] Pitting corrosion D] Fretting corrosion

3) What is the main reason for galvanic corrosion?

A] Improper design B] Dissimilar metal

C] vibration D] Bacteria

4) The intergranular corrosion can be prevented using ______
A] stabilized grade of stainless steel containing titanium and niobium as an alloying

element
B] low carbon grade of stainless steel
C]. both a. and b. D]. none of the above

5) Two dissimilar metals of equal area that are in contact are to be coated but enough
coating is on hand with half of the total area. For maximum corrosion protection, which one
of the following must be coated?

A] Half Anode and half cathode B] Only Cathode

C] Only anode D] Both C and B

Answers
12 345
BDBCC

4.9 Factors that affect the rate of corrosion

There are two factors that influence the rate of corrosion. Hence a knowledge of these
factors and the mechanism with which they affect the corrosion rate is essential because
the rate of corrosion is different in different atmosphere.

1. Nature of the metal 2. Nature of the corroding environment

4.9.1 Nature of the metal:

a) Physical state: The rate of corrosion is influenced by physical state of the metal (such

as grain size, orientation of crystals, stress, etc). The smaller the grain size of the metal or
alloy, the greater will be its solubility and hence greater will be its corrosion. Moreover,

areas under stress, even in a pure metal, tend to be anodic and corrosion takes place at
these areas.

b) Purity of metal: Impurities in a metal cause heterogeneity and form minute/tiny

electrochemical cells (at the exposed parts), and the anodic parts get corroded. The cent
percent pure metal will not undergo any type of corrosion. For example, the rate of
corrosion of aluminium in hydrochloric acid with increase in the percentage impurity is
noted.

% purity of 99.99 99.97 99.2
aluminium 1 1000 30000
Relative rate of
corrosion

c) Over voltage: The over voltage of a metal in a corrosive environment is inversely

proportional to corrosion rate. For example, the over voltage of hydrogen is 0.7 v when zinc
metal is placed in 1 M sulphuric acid and the rate of corrosion is low. When we add small
amount of copper sulphate to dilute sulphuric acid, the hydrogen over voltage is reduced to
0.33 V. This results in the increased rate of corrosion of zinc metal.

d) Nature of surface film: In aerated atmosphere, practically all metals get covered with

a thin surface film (thickness=a few angstroms) of metal oxide. The ratio of the volumes of
the metal oxide to the metal is known as a specific volume ratio. Greater the specific volume
ratio, lesser is the oxidation corrosion rate. The specific volume ratios of Ni, Cr and W are
1.6, 2.0 and 3.6 respectively. Consequently the rate of oxidation of tungsten is least, even at
elevated temperatures.

e) Relative areas of the anodic and cathodic parts: When two dissimilar metals or

alloys are in contact, the corrosion of the anodic part is directly proportional to the ratio of
areas of the cathodic part and the anodic part. Corrosion is more rapid and severe, and
highly localized, if the anodic area is small (eg: a small steel pipe fitted in a large copper
tank), because the current density at a smaller anodic area is much greater and the demand
for electrons can be met by smaller anodic areas only by undergoing corrosion more briskly.

f) Position in galvanic series: When two different metals or alloys are in electrical

contact in the presence of an electrolyte, the more active metal (higher in the galvanic
series) suffers corrosion. The rate and severity of the corrosion depends upon the difference
in the positions of the metals. The greater the difference, the faster is the rate of corrosion
of the anodic metal/alloy.

g) Passive character of metal: Metals like Tl, Al, Cr, Mg, Ni and Co are passive and they

exhibit much higher corrosion resistance then expected from their positions in galvanic
series, due to the formation of highly protective, but very thin film(of oxide) on the metal or
alloy surface. Moreover, the film is of such a “self healing” nature, once broken, repairs

itself, on re-exposure to oxidising conditions. Thus, corrosion resistance of stainless steel is
due to the passivating character of chromium present in it.

h) Solubility of corrosion products: In electrochemical corrosion, if the corrosion

product is soluble in the corroding medium, then corrosion proceeds at a faster rate. On the
contrary, if the corrosion product is insoluble in the medium, or reacts with the medium to
form another insoluble product (eg.: PbSO4 formation in case of Pb in H2SO4 medium), then
the corrosion product functions as a physical barrier, thereby suppressing further corrosion.

i) Volatility of corrosion products: If the corrosion product is volatile, it volatizes as

soon as it is formed, thereby leaving the underlying metal surface exposed for further
attack. This causes rapid and continuous corrosion leading to excessive corrosion. For
example, molybdenum oxide (MoO3, the oxidation corrosion product of molybdenum, is
volatile).

4.8.2 Nature of the corroding environment

a) Temperature: The rate of corrosion is directly proportional to temperature, i.e., rise in

temperature increases the rate of corrosion. This is because the rate of diffusion of ions
increases with rise in temperature.

b) Humidity of air: The rate of corrosion will be more when the relative humidity of the

environment is high. The moisture acts as a solvent for oxygen, carbon dioxide, sulphur
dioxide etc. in the air to produce the electrolyte which is required for setting up a corrosion
cell. “Critical humidity” is defined as the relative humidity above which the atmospheric
corrosion rate of the metal increases sharply. If the corrosion product possesses the ability
to absorb moisture, then in presence of a humid atmosphere all necessary required for
electrochemical type corrosion exist, hence corrosion rate is enhanced. Rusting of iron
increases appreciably when the critical humidity of air reaches 60% to 80% and rusted spots
act as corrosion centres. Similarly, particles of dust, soot, fly ash and charcoal in presence of
moisture may act as corrosion centres.

c) Presence of impurities in atmosphere: Atmosphere in industrial areas contains

corrosive gases like CO2 , H2S, SO2 ,and fumes of HCl, H2SO4 etc. In presence of these
gases, the acidity of the liquid adjacent to the metal surfaces increases and its electrical
conductivity also increases, thereby the rate of corrosion increases, and electrical
conductivity also increases. Similarly, in marine atmosphere, the presence of sodium and
other chlorides (in the sea water) leads to increased conductivity of the liquid layer in
contact with the metal surface, thereby corrosion is speeded up.

d) Presence of suspended particles in atmosphere: In case of atmospheric

corrosion:

(i) if the suspended particles are chemically active in nature (like NaCl, Ammonium
sulphate), they absorb moisture and act as strong electrolytes, thereby causing enhanced
corrosion;
(ii) if the suspended particles are chemically inactive in nature (eg., charcoal), they absorb
both sulphur gases and moisture and slowly enhance corrosion rate.

e) Influence of pH: Generally acidic media (ie., pH<7) are more corrosive than alkaline

and neutral media. However atmospheric metals (like aluminium, zinc, lead, etc) dissolve in
alkaline solutions as complex ions. The corrosion rate of iron in oxygen-free water is slow
until the pH is below 5. The corresponding corrosion rate in presence of oxygen, is much
higher. Consequently, corrosion of metals, readily attacked by acid, can be reduced by
increasing the pH of the attacking environment. Example: zinc (which is rapidly corroded,
even weakly acidic solutions such as carbonic acid) suffers minimum corrosion at pH 11.

f) Nature of ions present: Presence of anions like silicate in the medium leads to the

formation of insoluble reaction products (eg. Silica Gel), which inhibits further corrosion. On
the other hand, chloride ions is present in the medium destroys the protective and passive
surface film, thereby exposing the metal or alloy surface for fresh corrosion. Many metals
including iron undergo corrosion rapidly if the corroding medium contains ammonium salts.
Presence of even traces of copper or any noble metal in mine waters accelerates the
corrosion of the iron pipes used for carrying such water.

g) Conductance of the corroding medium: The conductance of the corroding

medium is of profound importance in the corrosion of underground or submerged
structures because the corrosion current depends on this factor. Conductance of clay and
mineral based soils is higher than that of dry sandy soils. Hence, stray currents from power
leakages will damage the metallic structures to a higher extent which are buried under
clayey and mineralised soils, compared to the structures in dry sandy soil.

h) Formation of oxygen concentration cell: With the increase in supply of oxygen/air

to the moist-metal surface, the corrosion is promoted. Less oxygen concentration (eg.
Oxide-coated part or less exposed part) parts become anodic; while the more oxygenated
regions or parts more exposed oxygen become cathodic, thereby leading to the formation
of “oxygen concentration cell” in which the anodic part suffers corrosion.

At the cathode surface electrons are consumed. Thus:
2 H2O + O2 + 4e-  4OH-

Supply of electrons (for oxidation) takes place in the anodic parts. Thus:
Fe Fe2+ + 2e-i

Thus, oxidation concentration cell promotes corrosion, but it occurs where the oxygen
concentration is lower. Water Line corrosion of buried pipelines and cables, passing from
one type of soil to another, and crevice corrosion are due to the formation differential-
oxygen concentration cells

j) Polarization of electrodes: Potential difference between the anode and Cathode is

the driving force of an electrochemical corrosion process of the corrosion rate is controlled
by the current flowing in the circuit. The extension of corrosion can be reduced by eating
certain inorganic or organic substance is called innovators to the corroding environment. In
the context of corrosion, polarization refers to the potential shift away from the open circuit
potential (free corroding potential) of a corroding system. If the potential shifts in the
"positive" direction (above Ecorr ), it is called "anodic polarization". If the potential shifts in
the "negative" direction (below Ecorr ), it is called "cathodic polarization".

For all metals and alloys in any aqueous environment, cathodic polarization always reduce
the corrosion rate. Cathodic protection is essentially the application of a cathodic
polarization to a corroding system.

For a non-passive system (e.g. steel in seawater), anodic polarization always increases the
corrosion rate. For systems showing active-to-passive transition, anodic polarization will
increase the corrosion rate initially and then cause a drastic reduction in the corrosion rate.
Anodic protection is essentially the application of anodic polarization to a corroding system.

4.10 Methods of corrosion control

Corrosion can be controlled or prevented by following method:
3) Improvement of design
4) Proper selection of materials
5) Cathodic protection
6) Anodic protection
7) Modifying the environment
8) Use of protective coatings

4.10.1 Improvement of Design

Design modifications can help reduce corrosion and improve the durability of any existing
protective anti-corrosive coatings.

1 - METALLIC CONTACTS

Bimetallic corrosion is serious and it occurs when two materials differing in
electrochemical potential are joints together, Corrosion of coupling will occur if a
galvanic cell is formed. Corrosion control is achieved by Avoid by direct contact
between two metals by means insulating or by applying protective coating.

2-DEPOSIT AND IMPURITIES

Deposit and impurities must be not allowed because they are caused formation of
differential aeration cells and allows the adsorption of moisture from air which lead to
corrosion, the passive surface of steel may be destroyed by such deposit , the sites under
the deposit become anode and the lead to pitting.

3-CREVICES

Any point at which two metals surfaces are separated by a narrow gap is a possible cell
,moisture enters the gap often in by capillary action , when the liquid is in contact with air ,
the oxygen is replenished but the centre of the water film becomes impoverished in
oxygen and corrosion occur at that point. Crevices are formed behind spot-welded overlays
or bolt joints under rim of sheet metal which has been folded to give a smooth outer
edge , at bolted or riveted joints and at shined or overplayed plates. To minimize
crevice corrosion:
1.Use welded joints in preference to bolted or riveted
2.Avoid sharp corner, edges, and packets
3.Use fillers and mastics to fill any gaps

4-INADEQUATE DRAINAGE AND VENTILATION

If light rain or spray falls on a bare steel surface, rings of rust will be found after
the water has evaporated. Each droplet acts as a differential-aeration cell and the
rust rings develop where iron ions from the anode meet the hydroxyl ions generated on
cathode. If the surface is free draining or there is adequate ventilation to dry the
water droplets rapidly, the corrosion damages will be limited. Even on paint surface,
there will be damage if the droplets persist for long time

5- FLOWING WATER SYSTEM

The majority of corrosion problems in flowing water system are caused by obstruction to
smooth flow.
The pipe should be design for a smooth flow and all valves, flanges and other fittings should
be installed in accordance with design specifications to allow minimum disturbance to
smooth flow.
Attack occurs in condenser tubes handling sea water which circulates at high velocity with
turbulent flow .The problem can overcome by decreasing the velocity and streamlining the
design of pipelines .sharp changes in flow direction must be not allowed. The use of
sacrificial baffle plates is effective in minimizing corrosion.
Increasing the pipe diameter is another way of reducing velocity and minimizing
corrosion in flowing water system.

6-DESIGNS FOR LIQUID CONTAINERS

A good design for liquid container must offer the following:
1)Freedom from sharp corners and edges.
2)Smooth flow of liquid from the container.
3)Complete drainage from the corners without any water traps. The elimination
of water traps is essential to minimize the formation of differential oxygen cells which
lead to corrosion.
4)Complete internal and external coating of the containers, if cost effective Design should
consider mechanical and strength requirement together with an allowance for corrosion .

4.10.2 Proper Selection of Materials:

Corrosion may be minimized by employed an appropriate design as discussed , on
the other hand the selection of appropriate materials in a given environments is a
key for corrosion control strategy the the contribution of corrosion resistance to be the
process of material selection

(1) Composition of the corrosive medium.
(2) Physical and external factors affecting the medium, such as pH, conductivity,
temperature and velocity affect the magnitude of corrosion induced by the medium.
(3) Presence of dissolved gases, such as oxygen, carbon dioxide, hydrogen sulfide in
fluids promotes the corrosivity of the medium.
(4) Presence of organic matter and bacteria promote the corrosivity of the
environment. For instance, sulfide reducing bacteria, desulfovibrio and Clostridium,
induce corrosion by producing hydrogen sulfide, a serious corrodant for steel. Algae, yeasts
and molds also contribute to corrosion.
(5) Pure metal shows higher corrosion resistance that impure metal.
(6) Dissimilar materials are to be avoided and if it is required then use a proper insulation so
that the 2 metala don’t come is contact.
(7) The anodic metal should have larger area than that of cathodic area.

4.10.3 Cathodic Protection

Cathodic Protection is one of the most effective methods for preventing most types of
corrosion on a metal surface. In some cases, Cathodic Protection can even stop corrosion
damage from occurring. Metals, especially ferrous metals, corrode in the presence of
oxygen, water, and other impurities such as sulfur. Without Cathodic Protection , metals act
as the anode and easily lose their electrons and thus, the metal becomes oxidized and
corroded. Cathodic Protection simply supplies the metal with electrons from an external
source, making it a cathode.The principle in this method is to force the metal to be
protected to behave like a cathode thereby corrosion does not occur. There are 2 types of
cathode protection:

(1) Sacrificial Anode Protection(Galvanic Cathode Protection)

(2) Impressed current cathodic protection

1. Galvanic Cathodic Protection

Galvanic cathodic protection involves protecting a metal surface of a piece of equipment
using another metal that is more reactive. The latter metal, usually called the galvanic or
sacrificial anode, has a less negative electrochemical potential compared to the metal
component being protected. Therefore, the sacrificial anode undergoes oxidation rather
than the operating equipment. This technique is illustrated in Figure below for an offshore
platform with a steel pipe submerged into seawater. The sacrificial anode is an aluminum
anode in this example.
Sometimes, steels are galvanized rather than connected to galvanic anodes. Galvanized
steels are steels that are coated with a protective zinc layer. The zinc layer acts to
cathodically protect steel against corrosion in most underground and marine environments.

2. Impressed Current Cathodic Protection

Impressed Current Cathodic Protection is a more economical method of cathodic protection
when underground pipelines are long or offshore equipment is too large to protect via one
or few galvanic anodes. In Impressed Current Cathodic Protection electrons are supplied to
the cathodic structure using an external DC power source (also called a rectifier). The steel
component is connected to the negative terminal of the power source and the impressed

current anodes are connected to the positive terminal of the power source. For simplicity,
the figure below shows one cathode and one anode connected by a rectifier. In application,
multiple anodes are connected to the positive terminal of the power source.

Comparison of sacrificial anode method and impressed current cathodic
method

No Sacrificial Anode Method Impressed Current Method

1 External power supply is not required. External power supply is required.

2 Investment is low. Investment is high.

3 This requires periodic replacement of Replacement is not required as anodes

sacrificial anode. are stable.

4 Soil and microbiological corrosion Soil and microbiological corrosion effects
effects are not considered. are into account.

5 This is the most economical method This is well suited for large structures are
especially when short term protection long term operations.
is required.

6 This is a suitable method when the This is a suitable method even when the

current requirement and the resistivity current requirement and the resistivity of

of the electrolytes are relatively low. the electrolytes are high.

4.10.4 Anodic Protection

Anodic protection is the method or technique adopted to reduce the corrosion of the
surface of a metal by connecting it as an anode with respect to an inert cathode in the cell
formed due to an electrochemical reaction in the corrosive environment, and ensuring that
the electrode potential is controlled to keep the metal in a passive state.

This is an electrochemical method of corrosion control in which an external potential
control system, called potentiostat, is used to produce and maintain a thin non corroding,
passive film on a metal or an alloy. The use of potentiostat is to shift corrosion potential into
passive potential so that the corrosion of the metal is stopped. The potential of the object
(say acid storage tank) to be protected is controlled by potential controller (potentiostat) so
that under certain potential range, the object becomes passive and prevents further
corrosion. This potential range depends upon the relationship between the metal and the
environment.

Anodic protection involves coating the iron alloy steel with a less active metal, such as tin.
Tin will not corrode, so the steel will be protected as long as the tin coating is in place. This
method is known as anodic protection because it makes the steel the anode of an
electrochemical cell. Anodic protection is often applied to carbon steel storage tanks used
to store sulfuric acid and 50% caustic soda. It is used in acid coolers in dilute sulphuric acid
plants or also used in chromium in contact with hydrofluoric acid.In these environments
cathodic protection is not suitable due to extremely high current requirements.

Limitations:

1. This method cannot be applied in the case of corrosive medium containing aggressive
chloride.

2. This cannot be applied if protection breaks down at any point, it is difficult to reestablish.

Comparision between Anodic and Cathodic Protection

No Cathodic Protection Anodic Protection

1 This method is applicable to all This method is applicable to only those
metals. metals which show active passive behaviour.

2 Used where there is no source of More aggressive corrosive environments can
powder by employing sacrificial be handled.
anodes.

3 Lower installation costs but higher Higher installation costs but lower operating

operating costs. costs.

4 Standard and well established Better corrosion protection can be achieved
method. with fewer electrodes.

4.10.5 Modifying the environment

Environment plays a major role in the corrosion of metals. Hence, we can prevent corrosion
to a great extent by modifying the environment. Some of the methods are

i) Deaeration:

Fresh water contains dissolved oxygen. The presence of increased amount of oxygen is
harmful and increases the corrosion rate. Deaeration involves the removal of dissolved
oxygen by increase of temperature together with mechanical agitation. It also removes
dissolved carbon dioxide in water .

ii) By using inhibitors:

Inhibitors are organic or inorganic substances which decrease the rate of corrosion. Usually
the inhibitors are added in small quantities to the corrosive medium. Inhibitors are classified
into

1) Anodic inhibitors (chemical passivators)

2) Cathodic inhibitors (adsorption inhibitors)
3) Vapour phase inhibitors (volatile corrosion inhibitors)

1) Anodic Inhibitors:

Inhibitors which retard the corrosion of metals by forming a sparingly soluble
compound with a newly produced metal cations. This compound will then adsorb on
the corroding metal surface forming a passive film or barrier. Anodic inhibitors are
used to repair a) the crack of the oxide film over the metal surface b) the pitting
corrosion c) the porous oxide film formed on the metal surface. Examples:
Chromate, phosphate, tungstate, nitrate, molybdate etc.

2) Cathodic Inhibitors:

Depending on the nature of the cathodic reaction in an electrochemical corrosion,
cathodic inhibitors are classified into :

(a) In an acidic solution:
The main cathodic reaction is the liberation of hydrogen gas, the
corrosion can be controlled by slowing down the diffusion of H+ ions
through the cathode. Eg., Amines, Mercaptans, Thiourea etc

2 H+ + 2 e- = H2

(b) In a neutral solution:

In a neutral solution, the cathodic reaction is the adsorption of oxygen or
formation of hydroxyl ions. The corrosion is therefore controlled either by
eliminating oxygen from the corroding medium or by retarding its
diffusion to the cathodic area. The dissolved oxygen can be eliminated by

adding reducing agents like Na2SO3 . The diffusion of oxygen can be
controlled by adding inhibitors like Mg, Zn or Ni salts. Eg., Na2SO3 , N2H4 ,
Salts of Mg, Zn or Ni.

½ O2 + H2O + 2e- = 2OH-

3) Vapour phase inhibitors:
These are organic inhibitors which are readily vapourised and form a protective layer
on the metal surface. These are conveniently used to prevent corrosion in closed
spaces, storage containers, packing materials, sophisticated equipments etc.
Examples are Dicyclohexylammonium nitrate, dicyclohexyl ammonium chromate,
benzotriazole, phenylthiourea etc.

4.11 Use of Protective Coatings

Protective
coatings

Inorganic Organic

metallic chemical ceramic organic lining organic
(phosphate) (aluminium) (rubber and coating
plastic lining)

Anodic cathodic emulsion
(galvanizing) (tinning) paint

enamel varnish paint lacquet

oil varnish spirit varnish

The corrosion can be prevented by adding an extra layer of coat on the metals. Mainly there
are 2 types of coatings :

1) Inorganic
2) Organic

4.11.1 Inorganic Protective coating:

Inorganic coatings can be produced by chemical action, with or without electrical
assistance. The treatments change the immediate surface layer of metal into a film of
metallic oxide or compound which has better corrosion resistance than the natural oxide
film and provides an effective base or key for supplementary protection such as paints. In
some instances, these treatments can also be a preparatory step prior to painting.

They can be classified as

A] Metallic

B] Chemical

C] Ceramic

A] Metallic Coatings:

A metallic coating forms a corrosion resistant protective layer that can withstand
harsh environmental conditions by changing the surface properties of the material on which
it is applied. Metallic coatings contain a metallic element or alloy. Metallic coatings can be
applied by using a sprayer, electrochemically, chemically or mechanically. These coatings are
applied on equipment requiring a shiny or glossy appearance and protection from sunlight,
corrosion and oxidation.

i) Anodic Coating

An anodic coating is a type of coating material that utilizes anodizing to provide
increased thickness, color and protection to aluminum or any type of substrate.
This coating consists of the oxide film that is created on metal through
electrolysis, with the metal acting as an anode.

Eg: The natural coating that exists in aluminum as well as other ductile and soft
metal can be very thin, making it easily damaged. Thus, building up the anodic
coating offers very beneficial properties to the surface of the aluminum. An
anodic coating is produced through the process of anodizing, or reversed
electroplating. In anodizing, the surface serves as the anode, or positive
electrode, within the electrolytic cell. The aluminum is submerged in a water
solution or acid electrolyte. The consistency of the coating will depend on the
controlled temperature.

The anodic coating can be very permeable and will trap or accept almost all
materials that pass through its pores. This can be either advantageous or
disadvantageous to the existing properties. In order to prevent this, the coatings
or pores are sealed through hydrolyzing, or the addition of water to the oxide.

The resultant coating will become hard, smooth, transparent and homogeneous.

ii) Cathodic Coating

Cathodic coatings involve coating metal, which is cathodic with respect to the
substrate in an electrochemical cell.

Cathodic coatings are also known as electrophoretic deposition (EPD), e-coating,
electro coating, cathodic electro deposition, electrophoretic coating, anodic
electro deposition and electrophoretic painting.

The cathodic coating process involves submerging a part into a container which
holds the coating bath or solution and applying direct current through the
cathodic electro deposition bath using electrodes. Typically voltages of 25-400
volts are used in electro coating. The object to be coated is one of the electrodes
(cathode), and a set of counter electrodes are used to complete the circuit.

After deposition the object is rinsed to remove the undeposited bath. The
coating is finally subjected to a curing process in which reduction of porosity and
cross linking of coating material takes place. This makes the surface smooth and
continuous.

No Anodic Coating Cathodic Coating

1 Coating of less Noble metal on base Coating of more Noble metal or base

metal with respect to electrochemical metal with respect to electrochemical

series. series.

2 It protects the base metal sacrificially. It protects the base metal due to Noble
nature.

3 It continues to protect the metal by It protects the metal till the coating is

galvanic action, even if the coating is perfect. a break in the coating causes

broken. rapid corrosion.

4 Example : galvanizing, coating of zinc Example: tinning, coating of the Sn on

on iron iron.

5 It cannot be used for storing food It can be used for storing acidic food
foodstuffs full stop stuffs

(ps the Methods of application is explained later in this chp)

B] Chemical Conversion coating

A chemical conversion coating is a coating that is produced by electrochemical or
chemical reaction of metals, giving a superficial layer which contains the metal compound. A
chemical conversion coating is also known as chromating, chromate conversion and
alodining.

Unlike anodizing, chemical conversion coatings do not need electricity, making production
more cost effective. It can be colored or clear, depending on preference.

Conversion coatings are applied on metal parts for corrosion protection. These are acidic in
order to transform a metal substrate to a zinc phosphate or iron surface. It is the chemical
reaction that makes the metal surface improve field performance and paint adhesion.

Chemical conversion coatings can go through either electro-chemical or chemical processes,
which may include any of the following: Chromate conversion - Mainly utilized on aluminum
surfaces OR Zinc and iron phosphate conversion - Mainly applied on steel substrates

This type of coating is utilized to provide a surface for paint to adhere to throughout the
process of curing. Without it, metal and other surfaces would have paint that only sits on
top of the surface rather than being bonded to it mechanically.

C] Ceramic Coating

Ceramics are inorganic, non-metallic substances generated by heating clay to a
very high temperature so that it hardens. Ceramics are more resistant to corrosion than
metals or alloys.

Ceramics are sometimes adhered to metallic surfaces to prevent corrosion and material
degradation. The incorporation of ceramics (known as a cermet) can modify a material's
properties such as density, conductivity, brittleness, insulation and magnetism.
Advanced ceramics have become more common as their development has progressed. Such
ceramics are not purely clay but are generated using oxides or non-oxides. The oxides used
include alumina and zirconia, while non-oxides include silicon carbide (SiC) and
molybdenum disilicide.

4.11.2 Methods of application of METALLIC COATINGS

1) Hot dipping:

The metal to be coated is dipped in the molten bath of coating metal for sufficient time.
Then it is polished. For good at the base of surface metal must be very clean. Eg zinc,
lead, tin , aluminium on iron ,steel ,copper. which have relatively higher melting point
than that of the zinc lead and tin to widely used methods of hot dipping are
a) galvanising zinc over iron
b) tin over iron

a) Galvanising:
Galvanizing is the process of coating iron on or steel sheets with a

coat or zinc to prevent them from rusting the process is carried out as
follows.
The iron or steel article ( for example sheet, pipe wire) is first cleaned by
pickling with dilute sulfuric acid solution for 15-20 minutes at 60 to 90 degree
Celsius. This treatment also removes any scale, rust(oxide layer) and
impurities.

The article is then well was and dried. It is then dipped in a bath of molten
zinc, at 425 to 430 degree Celsius the surface of the Earth is kept covered
with a flux ammonium chloride to prevent oxide formation. When the article
is taken out, it is found to have been coated with a thin layer of zinc.

It is then passed through a pair of hot rollers. This process remove any
superfluous excess of zinc and produces a thin film of uniform thickness.
Then it is annealed temperature of 650 degree Celsius and finally, cooled
slowly .
Uses: It is most widely used for protection of iron from atmospheric
corrosion in the form of roofing sheets wires coma pipes nails comma bolts,
screws comma buckets tubes, etc. It may be pointed here that zinc gets
dissolved in dilute acids to form highly toxic or poisonous compounds. Hence
galvanized utensils cannot be used for preparing and storing food stuffs,
especially acidic once.

b) Tinning:
Tinning is a coating tin over the iron or steel articles. The process

consists in first treating the steel sheet in dilute sulphuric acid ( pickling) to
remove any oxide film.
After this, it is passed through a part of zinc chloride flux. The flux helps the
molten metal to adhere to the metal sheet.

Next, the sheet passes through a tank of molten tin and finally through a
series of rulers from underneath the surface of a layer of palm oil. The palm
oil protect the hot tin coated surface against oxidation. The rollers remove
any excess of tin and produce a thin film of uniform thickness on steel sheet.

Uses : Tin possesses considerable resistance against atmospheric corrosion.
Moreover, because of Non- toxic nature of tin, tinning is widely used for
coating steel, copper and brass sheets, used for manufacturing containers for
storing food stuffs, ghee, oils, kerosene and packing food materials. Tinned
copper sheets are employed for making cooking utensils and refrigeration
equipment.

2) Metal Cladding:

Metal cladding is a type of protective coating, where the protective material such
as metal powder or foil is bonded to a substrate by applying heat and/or pressure.

The study of metal cladding is significant because this method of corrosion protection and
wear protection is generally very reliable and cost-effective. In addition, the process
parameters can be optimized for different metals and composites in various critical
applications.

Metal cladding is a method of protecting one metal (or composite) by forming a layer of a
second metal to its surface by using techniques such as diffusion, deformation and lasers.

The advantage of cladding is that the process as well as the material can be chosen as per
the needs of the application and the bond strength required. With metal cladding, the
surfaces can develop adequate wear resistance. The method is suitable for complex shapes
as well.

3) Electroplating:

Electroplating is the process of coating a metal with a thin layer of another
metal by electrolysis to improve the metal's corrosion resistance.

The metals most commonly used in plating are: Copper, Nickel, Gold, Silver, Chrome, Zinc,
Tin

Electroplating is also known as electrode position and electroplated coating.
In the process of electroplating the anode is connected to the positive terminal, and the
cathode (metal to be plated) is connected to the negative terminal. Both are immersed in a
solution that contains an electrolyte and then connected to an external supply of direct
current. When DC power is applied, the anode is oxidized—its metal atoms dissolve in the
electrolyte solution. These dissolved metal ions are reduced at the cathode and form a
coating. The current through the circuit is adjusted so that the rate at which the anode is
dissolved equals the rate at which the cathode is plated.
Different metals can be coated using the electroplating process. Formulating the right
electrolyte is important for the quality of plating.
Properties of the electrolyte that must be considered when making a selection are:

 Corrosivity
 Resistance
 Brightness or reflectivity
 Hardness
 Mechanical strength
 Ductility
 Wear resistance

4) Metal Spraying:

Metal spraying is a process for covering a surface with a metallic coating using a spray of
molten particles.
These processes can also be labeled with a more general term, thermal spraying. However,
the general term includes coatings created with not just metallic materials, but also oxides
and ceramics.
Metal spraying works by first subjecting the source material to a high degree of heat to
achieve a molten state. The molten material is then atomized into small particles and

sprayed outwards onto a surface. The molten particles do not heat the surface because the
heat of a particle is proportional to its size. On contact, the particle flattens out and adheres
to the surface as it hardens.

The deposition rate of the surface is typically faster than other coating processes such as
chemical vapor deposition and electroplating. Coatings created by metal spraying range in
thickness from 20 µm to several mm, depending on the conditions and methodology. Layers
created by metal spraying may have the following characteristics:

 Increased durability
 Increased hardness
 Increased or decreased friction
 Increased or decreased corrosion protection
 Increased wear resistance
 Modified electrical properties
 Additional protection to damaged materials

With a variety of metal spraying methods, the choice of method depends on the particular
application. Deposit efficiency, bond strength, ease of operation, safety, changeover time,
maintenance time and costs, appearance of the coating finish and the ability to automate
the coating application affect the choice of spraying methodology. For corrosion protection,
aluminum, zinc and alloys of the two are typically used.

Lets Test your Knowledge

1) __________ is used for producing a coating of low melting metal such as Zn, Sn, Pb, Al

on Fe, steel and Cu.

A]Hot dipping B]Anodic coating

C]Cathodic coating D] Galvanizing

2) _________ is the process of coating Fe or steel with a zinc coating.

A]Tinning B]Hot dipping

C]Galvanizing D]None of above

3) ______ is the process of coating of tin over Fe or steel.

A]Tinning B]Galvanizing

C]Metal cladding D] Sheardizing

4) _______ coating is non toxic in nature.

A] Sn B] Zn

C] Fe D] Cu

5) Destruction of metal starts ________.
A] At the bottom B]Just on layer below from surface
C]In the middle D] At the surface

Answers
123 45
AC A A D

4.12 The following tables provides a gist of what the various
characteristics of water provide to the corrosion process:

Water properties Corrosivity

Hardness Source of scaling that promotes corrosion
Alkalinity Produces foam and motion of solids
pH Corrosion depends on its value
Sulphates Produces scaling
Chloride Increases water corrosivity
Silica Generates scaling in hot water. Condensers and
steam turbines
Total Dissolved Solids Increases electrical conductivity and corrosivity
(TDS)
Temperature Elevated temperatures increases corrosion rates

4.13 Worldwide Impact of Corrosion (Statistics)

 India with a GDP of around $2 trillion loses as much as $100 billion (more than Rs 6
lakh crore) every year on account of corrosion, which can be checked by using zinc to
galvanise steel structures. "India loses around 4-5 per cent of GDP annually on
account of corrosion losses," Hindustan Zinc Ltd (HZL) CEO Sunil Duggal. (2016)

 NACE International today released the "International Measures of Prevention,
Application and Economics of Corrosion Technology (IMPACT)" study, in which it
estimates the global cost of corrosion to be US$2.5 trillion, equivalent to roughly 3.4
percent of the global Gross Domestic Product (GDP). (2016)

 Statistic chart by NACE International region and country wise (as of 2016):

4.14 Summary

Corrosion can be defined as a phenomenon of spontaneous deterioration of metals its alloys
through chemical or electrochemical reactions when exposed to the environment
Consequences of corrosion include formation of corrosion product over the machinery
whereby the efficiency of the machine gets lost. Corrosion also causes contamination
releases toxic products is health hazard excetra.
Classification of corrosion- based on the environment to which the metal is exposed
corrosion is divided into chemical or dry corrosion and electrochemical or wet corrosion.
Chemical or dry corrosion can be classified into three types oxidation corrosion corrosion by
other gases like Hydrogen chlorine excetra and liquid metal corrosion.
Types of electrochemical corrosion- galvanic cell corrosion and differential aeration
corrosion.
Factors influencing the rate of corrosion nature of metal and nature of the environment.
Corrosion control- some of the corrosion control methods are proper selection of Metals,
use of pure metals, use of Metal alloys, cathodic protection, sacrificial anode method,
impressed current cathodic protection, use of inhibitors and auditor Nobita's cathodic in
Nobita's and changing the environment.
Application of protective coatings- the main types of coatings are metallic coatings, chemical
conversion coatings, organic coatings, organic Linings and ceramic protective materials.

4.15 Model Questions

4.15.1 MCQ Type

1. Volatile oxidation corrosion product of a metal is
A. Fe2O3 B. MoO3
C. Fe3O4 D. FeO

2. Lower is PH , corrosion is,
A. Greater B. Lower
C. Constant D. None of above

3. Electrochemical corrosion takes place on,
A. Anodic area B. Cathodic area
C. Near cathode D. Near anode

4. Chemical formula of Rust is,
A. Fe2O3 B. FeO
C. Fe3O4 D. Fe2O3.XH2O

5. Which of following metals could provide cathodic protection to Fe?
A. Al & Cu B. Al & Zn
C. Zn & Cu D. Al & Ni

6. Smaller the grain size, corrosion is,
A. Greater B. Lower
C. Constant D. Doesn’t affected

7. Process of corrosion enhanced by,
A. AIR & Moisture B. Electrolytes in water
C. Metallic impurities D. Gases like CO2 & SO2
E. All of above.

8. Standard electrode potential of hydrogen is,
A. 1.00 V B. 0.00 V
C. 0.01 V D. 0.001 V

9. Ratio of volumes of metal oxides to metal is known as,

A. Specific mass ratio B. Volume ratio

C. Specific ratio D. Specific volume ratio

10. Coating of Zn, Al and Cd on steel are _________ , because their electrode potentials are
lower.

A. Cathodic B. Anodic

C. Not affecting D. None of above

11. _________ is the process of coating Fe or steel with a zinc coating.

A. Tinning B. Hot dipping

C. Galvanizing D. None of above

12. Fe or steel is ____ with respect to copper.

A. Anodic B. Cathodic

C. Corrosive D. Non corrosive

13. Al is _________ than Zn.
A. Less anodic B. More anodic
C. Less Cathodic D. More Cathodic
84. Zn is more ________ than Fe.
A. Electronegative B. Corrosive
C. Electropositive D. None of above

14. Coating applied must be chemically __________ to the environment.
A. Inert B. Reactive
C. Soluble D. Non reactive

15. Evolution of hydrogen type corrosion occurs in _______ environment.
A. Acidic B. Neutral
C. Basic D. Alkaline

16. Anodic reaction involves dissolution of metal as corresponding metallic ions with
liberation of _______.

A. Pair of electron B. Free electron

C. Ions D. Current in electrolytic solution.

17. Corrosion is a process reverse of ______ of metal.

A. Destruction B. Extraction

C. Rusting D. Galvanizing

18. Reddish scale of iron oxide has molecular formula __________.
A. Fe(OH)3 B. Fe2O3
C. Fe3O4 D. FeO

4.15.2 Short Answer Type

1.) Search the open literature for methods that have been proposed to visualize corrosion
processes on various metals and alloys.

2.) Explain what are the main differences between direct and indirect costs associated to
corrosion damage. Provide some examples from your own experience.

3.) Describe the principles of a Daniell cell and elaborate on the usefulness of the Daniell cell
when it was introduced.

4.) Write a short-hand description of the reactions involved in the corrosion of zinc and iron.

5.) Why are there always a minimum of two electrochemical reactions to explain even the
simplest corrosion reaction?

6.) What is an anodic process in a corrosion reaction? Provide some examples.
What is a cathodic process in a corrosion reaction? Provide some examples.

7.) Corrosion problems can rarely be attributed to single forms of corrosion. Provide some
examples to illustrate that statement.

8.) Provide some examples of uniform corrosion and general description.

9.) Where would uniform corrosion be a concern? Provide examples and explanation.

10.) Why would pitting corrosion be much more prone to provoke a catastrophic failure
than uniform corrosion generally does?

11.) Explain in your own words the role played by dissolved oxygen in the general
mechanism proposed to explain the various steps in crevice corrosion.

12.) Provide some examples of crevice corrosion different from those described in the book.

12.) Extend the crevice corrosion scenario to situations encountered with non-metallic
materials.

13.) Why would it be misleading to call a galvanic corrosion process 'electrolysis', as many
still refers to when discussing galvanic corrosion.

14.) Explain why intergranular corrosion is often considered to be a localized form of
galvanic corrosion.

15.) Will fatigue corrosion affect metals and alloys that have usually a good endurance limit?
Explain your answer.

16.) What is a corrosion cell and what are its main components?

17.) Can crevice corrosion degenerate in other forms of corrosion? Provide some examples.

4.15.3 Long Anwers

1) What is corrosion of metals? Explain the mechanism of oxidation corrosion.
2) What are the factors that affect electrochemical corrosion rate? Discuss.
3) Differentiate chemical and electrochemical corrosion. Mention any four factors that
affect electrochemical corrosion.
4) Describe the mechanism of electrochemical corrosion by hydrogen evolution and oxygen
adsorption. 5) Explain water line corrosion.
6) How is galvanic corrosion occur>
7) Deposition of oil or dust on metal surfaces for a long period is undesirable. Give reasons.
8) Describe the mechanism of differential aeration corrosion taking pitting as example.
9) Explain the electrochemical theory of corrosion with suitable example.
10)Discuss the mechanism of chemical and electrochemical corrosion.
11) Explain the following:

i) hydrogen embrittlement ii) liquid metal corrosion iii) pitting corrosion 28

iv) crevice corrosion v) pipeline corrosion

References

 (Ref: Sully J R, Taylor D. W, Electrochemical Methods of Corrosion Testing, Metals Hand Book. Vol 13,
1987.)

 Roberge PR. Handbook of Corrosion Engineering. New York: McGraw-Hill, 1999.
 TotalMateria.com
 Gibson Stainless Inc. Catalogues
 NPTEL courses
 Engineering Chemistry - Wiley India
 Corrosion Engineering publication by McGraw Hill
 CorrosionPedia
 uobabylon.edu (University of Babylon)
 Jain and Jain, Engineering Chemistry, 15th Edition, Dhanpat Rai Publishing Co., New Delhi.
 S.S. Dara, Engineering Chemistry, 1st Edition, S. Chand & Co, New Delhi.
 corrosion.ksc.nasa.gov
 slideshare.com
 https://www.corrosionclinic.com/corrosion_A-Z/polarization.htm
 corrosionpedia.com/2/1551/corrosion/how-to-control-corrosion-by-improving-design
 chrome-

extension://oemmndcbldboiebfnladdacbdfmadadm/http://www.uobabylon.edu.iq/eprints/
publication_12_18945_228.pdf
 https://inspectioneering.com/tag/cathodic+protection
 https://www.researchgate.net/publication/279181019_ANODIC_PROTECTION_AGAINST_CO
RROSION_AND_CRACKING_OF_DIGESTER_VESSELS
 https://corrosion-doctors.org/MetalCoatings/Inorganic.htm
 https://www.corrosionpedia.com