Chapter 1 Basics of Corrosion

8/13/2019 Chapter 1 Basics of Corrosion

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Corrosion is defined as the chemical or electrochemicalreaction between a metal and its environment resulting

in the loss of the material and its properties.Corrosion Reactions in the presence of water and Oxygen:

Anodic Reactions:

2Fe 2Fe2++ 4e-

Cathodic Reactions:

O2+ 2H2O + 4e-4OH-

Net Reactions:

2Fe2++ 4OH-2Fe(OH)22Fe(OH)2Fe2O3+ H2O

2Fe +02+ H

2O Fe

2O

3


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FORMS OF CORROSION

No AttackSome metals and alloys are essentially

unaffected by corrosion in certain environments.This may be either because they are more

stable in their metallic forms than in a combined

form or because they forms natural protective

films on their Surface that provide completely

effective passivity.e.g Gold, Platinum.

Uniform CorrosionUniform corrosion is a form of corrosion

in which a metal is attacked at about the

same rate over the entire exposed surface.

When a metal is attacked by uniform

corrosion, the location of anodic and cathodic

areas shifts from time to time.

Uniform Corrosion, where each

location behaves as anode &

cathode from time to time.

Metal/Cross section magnified


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Selective leaching or De-alloyingIn certain alloys under certain

environment selective removal of one metal

can occur resulting localized (perforation

type) or more uniform (layer type) loss. The

most prevalent form of de-alloying is

selective removal of zinc from brass, which

is also known as de-zincification.

Inter-granular attackAlloys such as austenitic stainless steel &

some alloys, when inadequately heat-treated

during welding results chromium carbideprecipitation over grain boundaries.

Concentration variation between grain vicinity

& grain boundary creates potential difference,

which initiate corrosion process, resulting

loss of mechanical properties.

In graphitic corrosion, the graphite

acts as a cathode, dissolving the iron

and leaving Graphite frame.


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Stress corrosion crackingThe phenomenon of stress corrosion

cracking is defined as the occurrence of

microscopic brittle fracture of a normally

ductile metal due to the combined action

of tensile stress (residual or applied)

and some specific environment.

Microbial Induced CorrosionMIC occurs at whole range of

temperature & wherever nutrients andphysical condition allows growing

microorganism. Microorganism can be

bacteria, fungi etc.


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Fretting CorrosionFretting or fretting corrosion may be

defined as that form of damage, which

occurs at the interface of two closelyfitting surfaces when they are subject to

slight relative oscillatory slip.The surfaces

are often badly pitted and finely divided

oxide detritus is formed. Crevice Corrosion

It occurs mainly in the crevices at the

junction of two metals exposed in

aggressive environment. Acceleratedattack occurs because of a differential in

oxygen concentration. Oxygen deficit

portion inside the crevices act as anode &

oxygen rich area outside the crevices act

as cathode.


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Fili-form (under film) corrosionCorrosion that occurs under a coating

in form of randomly distributed thread

like filaments. For this reason it is alsocalled as under film corrosion. It doesn't

destroy or weaken metallic components

but only affects surface appearance.

Hydrogen embrittlementThe loss of ductility of metal resulting

from absorption of hydrogen. High

strength steels are susceptible to

embrittlement as a result of dissolved

hydrogen and a majority of stress

corrosion cracking failures of these

materials is attributed to hydrogen

embrittlement.


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Liquid Metal Embrittlement (LME)Liquid Metal Embrittlement is the

decrease in ductility of a metal caused by

contact with liquid metals such as Zn, Cd,

Hg, Pb etc., very small amounts of liquid

metal are sufficient to result in

embrittlement. Some events that may

permit LME under appropriate

circumstances are welding, soldering,

heat treatment, hot working etc. Liquid

metal not only reduces the ductility but

also reduces the tensile strength and

failures can occur at loads below yield

stress.

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Chapter 1 Basics of Corrosion