CHAPTER 7

CORROSION OF METALS

DJJ 30113 MATERIALS SCIENCE AND ENGINEERING

7.1 Demonstrate the electrochemical 7.3 Correlate the forms of
considerations corrosion with the prevention
a. Electrochemical reactions
b. Electrochemical cell methods
c. Passivity a. Inhibitors
d. Oxidation b. Design changes
7.2 Examine the forms of corrosion in metals
a. Uniform attack c. Coatings
b. Galvanic d. Cathodic protection
c. Crevice
d. Pitting
e. Intergranular
f. Stress corrosion cracking

7.1 Demonstrate the electrochemical considerations

WHAT IS CORROSION????

DEFINITION

the deterioration the breaking down of 3
of a material essential properties in a
resulting from
material due to chemical
chemical attack by reactions with its
its environment. surroundings.

A loss of electrons of metals reacting with
water and oxygen.

HOW Oxygen Rust Water Air
high (oxygen)
CORROSION
HAPPEN? Cathode Oxygen low
(+ve) Anode (-ve)

Electron IRON

movement

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7.1 a) ELECTROCHEMICAL REACTION

The anodic reaction, consisting of the oxidation of the metal, can be given as Fe → Fe2+ + 2e–. The fundamental components in an
electrochemical corrosion cell are :
Oxygen( O2),which is highly soluble in the aqueous layer, is a possible electron acceptor. Oxygen 1) Anodic site
reduction in neutral or basic media takes place according to the reaction O2 + 2H2O + 4e– → 2) Cathodic site
4OH–. 3) Electron path (metallic connection)
4) Ionic path (electrolyte)

All four components must be present for corrosion
to occur.

Metal at the anode is oxidised causing it to lose
mass, or corrode.

The electrons produced due to this reaction travel
to the cathode through the metallic path, consume
in the reaction and prevent the cathodic area from
corroding.

Most common electrolyte mediums are water, soil,
and chloride contaminated concrete. The electron
path can be internal to the metal itself or external
such as a wire, direct connection or other
interconnecting metallic components

→ + + − + + − → M

Periodic table

LEO = loss electron oxidation
GER = gain electron reduction

Number of electron

7.1 b) ELECTROCHEMICAL CELL

Electrochemical Cell: a system consisting of
electrodes that dip into an electrolyte and in which
a chemical reaction either uses or generates an
electric current

The two half-cell reactions are:

Zn (s) → Zn2+ (aq) + 2e–
(oxidation)

Cu2+ (aq) + 2e– → Cu (s)
(reduction)

Anode: the electrode at which oxidation
occurs. Electrons are given up by the
anode and flow to the cathode.
Represented with a negative sign to show
that electrons flow from it.

Cathode: the electrode at which
reduction occurs. Electrons flow from
the anode to the cathode. Represented
with a positive sign.

7.1 c) PASSIVITY

Passivity

• Under particular environmental conditions, some normally active metals and alloys
lose their chemical reactivity and become extremely inert (having little or no ability to
react)

• Example : chromium, iron, nickel, titanium, and many of their alloys
• Passive behavior results from the formation of a highly adherent and very thin oxide

film on the metal surface, which serves as a protective barrier to further corrosion

Stainless steels • are highly resistant to corrosion in a rather wide variety of
atmospheres as a result of passivation.

• They contain at least 11% chromium, which as a solid solution
alloying element in iron, minimizes the formation of rust;
instead, a protective surface film forms in oxidizing

Aluminum • highly corrosion resistant in many environments because it
also passivates.

• If damaged, the protective film normally re-forms very
rapidly.

• However, a change in the character of the environment may

cause a passivated material to revert to an active state.



7.1 d) OXIDATION / DRY CORROSION

• Dry corrosion or oxidation occurs when oxygen in the air reacts with metal without the presence of a liquid.

• Typically, dry corrosion is not as detrimental as wet corrosion, but it is very sensitive to temperature.If you hold a
piece of clean iron in a flame, you will soon see the formation of an oxide layer!

• Most engineering metals have a slow oxidation rate in the atmosphere at ambient temperature.

• The differences in the rate of dry corrosion vary from metal to metal as a result of the mechanisms involved and dry
corrosion the oxygen has to be able to make contact with the metal surface.

• Initially this is not a problem, but as soon as corrosion starts to occur the oxide layer,that forms on the metal surface,
will limit the amount of oxygen that can further react with the metal

5.47.2CEOxaRmRiOneStIhOeNforms of corrosion in metals

TYPES OF CORROSION

UNIFORM GALVANIC

CREVICE PITTING

INTERGRANU STRESS CORROSION
LAR CRACKING

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5.4 CORROSIOis tNhe attack of a metal at essentially the

same at all exposed areas of its surface.

UNIFORM • Most metal corrode because of
CORROSION chemical reaction with oxygen in
atmosphere (air and moister).

• Produce oxide layer on the
surface – corrode the surface.

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5.4 CORROSION
Galvanic corrosion is an electrochemical process in which one
metal corrodes preferentially when it is in contact with a different
type of metal and both metals are in an electrolyte as discussed in
GALVANIC Wet Corrosion / Electrochemical Corrosion.
CORROSION

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5.4 CORROSIOisNaocfocurmr inofclroecvaicliezseadnedleucntrdoecrhsehmieilcdaeldcosurrrofascioens that can
where

stagnant solutions can exist.

CREVICE It’s usually occurs between
CORROSION cracked parts or in the
crevice of same or different
metal surfaces that Solution
attached together weather trapped
the crevice of a metal and
a metal or between a
metal and a non-metal.

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5.4 CORROSIOisNa form of extremely localized attack that results in
holes in the metal.

PITTING The pit acts as the anode while
CORROSION the undamaged part of the
metal is the cathode.It begins
with a chemical breakdown in
the form of a scratch or spot.

Solution
trapped

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is localized corrosion attack at and / or adjacent to the
grain boundaries of an alloy due to welding, stress, heat

treating or improper service.

INTERGRANULAR It is occur when different
potential between atoms at
CORROSION
the grain-boundaries and
create the boundaries of Intergranular
anode and cathode. corrosion

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5.4 CORROSION is a complex form of corrosion which arises due to
stress and corrosive environment

The stress is developed in the material due to bending or
stretching of the material.

STRESS

CORROSION
CRACKING

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7.3 Correlate the forms of corrosion with the prevention
methods

PREVENTION METHODS

INHIBITORS DESIGN
COATING CHANGES

CATHODIC
PROTECTION

INHIBITORS Corrosion of metallic surfaces can be reduced or
controlled by the addition of a small amount of

chemical compounds to the corrosive environment

• The addiction of the Inhibitors will reduced the corrosion rate of the metal either

by retarding the anodic process or cathodic reaction or both.
• The specific inhibitor depends on both the alloy and the corrosive environment.
• Other inhibitor attach themselves to the corroding surface and interfere with

either the oxidation or the reduction reaction or form a very thin protective

coating.
• Inhibitors are normally used in closed systems such as automobile radiators and

steam boilers.
• The inhibitor is chemically adsorbed on the surface of the metal and forms a

protective thin film with inhibitor effect or by combination between inhibitor ions

and metallic surface.

Examples of Application of Inhibitors

Petroleum Industry.
Corrosion phenomena in the petroleum industry occur in a two-phase medium of water and hydrocarbon. It is the
presence of a thin layer of water which leads to corrosion, and rigorous elimination of water reduces the corrosion
rate to a negligible value. The inhibitors used in petroleum industry, both in production and refining are either oil
soluble–water insoluble types or oil soluble–water dispersible compounds.

Packaging Industry.
For transportation of machinery, components and equipment by sea, vapor phase cyclohexylamine and hexa-
methylamine are used.

Sour Gas Systems.
A major problem is encountered in steel pipelines in various sour gas environments. Chemical inhibition is one of the
effective methods used to mitigate sulfide induced corrosion. Inhibitors containing alkyl ammonium ions are found to
suppress corrosion effectively.

Potable Water Systems.
Corrosion is experienced in potable water transportation pipes of steels and cast iron. Inhibitors, such as Ca(HCO3)2
and polyphosphates are commonly used to combat corrosion.

Engine Coolants.
Inhibitors, such as NaCrO4 (sodium chromate), borates and nitrites (NaNO2) and mercaptabenzothia-zole are
widely used for protection of auto-mobile engines. Chromates are a health hazard.

Examples of Application of Inhibitors

COATING Coating a metal with a layer/ material can prevent /
minimize the corrosion attack.

METALLIC NON -
METALLIC

NOBLE SACRIFICIAL ORGANIC INORGANIC

NOBLE ** A noble metal is a metal or alloy characterized by it
lack of chemical reactivity.

METALLIC • is a protective coating in the form of a thin metallic
COATING
layer that is applied to a base metal to prevent
corrosion.

• The thin coating layer is more noble (cathodic / less
reactive) than the base metal.

• The noble coating protects the base metal from harsh
environments where corrosion is possible.

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NOBLE Easily corrode

Not easily 27
corrode

METALLIC
COATING

NOBLE NOBLE ( CATHODIC / COATING /
LESS REACTIVE METAL) THIN
METALLIC
COATING METALLIC
LAYER

IRON

BASE
METAL

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METALLIC NOBLE
COATING
• Only effective if it is undamaged. If the noble coating layer
becomes damaged, the metal being coated will undergo more
rapid localized corrosion due to galvanic reactions.

• Common noble coatings include nickel, tin and chromium,
although these coatings are only considered noble coatings if
they are applied to a metal that is less noble than they are.

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METALLIC SACRIFICIAL
COATING
• is a coating in which a less noble material is applied as protective
layer to a more noble base metal.

• The most common example of sacrificial coating is galvanized
steel (steel coated with zinc).

• One of the benefits of a sacrificial coating over a noble coating is
that localized damage will not necessarily cause an intense
increase in localized corrosion.

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SACRIFICIAL BASE
METAL
METALLIC
COATING IRON

ANODIC / COATING /
MORE THIN

REACTIVE METALLIC
METAL) LAYER

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NON - ORGANIC & INORGANIC
METALLIC
COATING • Organic coatings then are those that have an organic
binder, often epoxy.

• Inorganic coatings are those that have an inorganic
coating such as a silicate.

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DESIGN Selection of design for building and bridge
CHANGES

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

• Ideally, designs should avoid
trapping dust and water,
encourage movement of air, and
avoid open crevices.

• Ensuring the metal is accessible
for regular maintenance will also
increase longevity







5.4 PCRCOAOTTHERCORTDOIIOCSNION protection of a metal by connecting it to a
sacrificial anode or by impressing a direct

current voltage to make it a cathode.

Cathodic protection is commonly used to protect numerous

structures against corrosion, such as ships, offshore floaters, subsea

equipment, harbours, pipelines, tanks; basically all submerged or
buried metal structures.

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CORRODE AND Base metal : Base metal:
DAMAGE IRON COPPER

STEEL HULL PROPELLER
Anode (BRONZE)

Cathode

SHAFT

More
reactive

ELECTRON
MOVEMENT

SEA WATER

Least
reactive

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STEEL HULL PROPELLER (BRONZE)
Anode Cathode

SHAFT ATTACH MORE ANODIC
ELEMENT THAN STEEL TO
ZINC PROTECT THE STEEL HULL

ELECTRON
MOVEMENT

SEA WATER

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5.4 CORROSION

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