different forms of corrosion

8/14/2019 Different forms of corrosion

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FORMS OF CORROSION

Prof T. K. G. Namboodhiri(Retd.), I. T. , Banaras Hindu University)

Consultant-Metallurgy & Corrosion,Tiruvalla, Kerala


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FORMS OF CORROSION

Corrosion maybe classified indifferent ways

Wet / Aqueouscorrosion & DryCorrosion

Room

Temperature/HighTemperatureCorrosion

CORROSION

WET CORROSION DRY CORROSION

CORROSION

ROOM TEMPERATURECORROSION

HIGH TEMPERATURECORROSION


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WET & DRY CORROSION

Wet / aqueous corrosionis themajor form of corrosion which occursat or near room temperature and in

the presence of water

Dry / gaseous corrosionissignificant mainly at high

temperatures


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WET / AQUEOUS CORROSION

Based on the appearance of the corrodedmetal, wet corrosion may be classified asUniform or GeneralGalvanic or Two-metalCrevicePittingDealloying

IntergranularVelocity-assistedEnvironment-assisted cracking


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UNIFORM CORROSION

Corrosion over theentire exposedsurface at auniform rate. e.g..

Atmosphericcorrosion.

Maximum metalloss by this form.

Not dangerous.Rate can bemeasured in thelaboratory


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GALVANIC CORROSIONWhen two dissimilarmetals are joinedtogether and exposed,the more active of thetwo metals corrodefaster and the nobler

metal is protected. Thisexcess corrosion is dueto the galvanic currentgenerated at the

junctiona) Steel plates with copper

rivets in seawater 15months.b) ) Copper plates withsteel rivets, sameenvironmental conditions


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CREVICE CORROSION

Intensive localizedcorrosion withincrevices & shieldedareas on metal

surfaces

Small volumes ofstagnant corrosivecaused by holes,

gaskets, surfacedeposits, lap joints


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DEALLOYING

Alloys exposed tocorrosivesexperience

selective leachingout of the moreactive constituent.e.g. Dezincification

of brass.Loss of structuralstability andmechanicalstrength


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PITTING

A form of extremelylocalized attackcausing holes in themetal

Most destructiveform

Autocatalytic nature

Difficult to detectand measure


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INTERGRANULAR CORROSION

The grain boundariesin metals are moreactive than the grainsbecause of

segregation ofimpurities anddepletion of protectiveelements. So

preferential attackalong grainboundaries occurs.e.g. weld decay instainless steels


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VELOCITY ASSISTED

CORROSION

Fast movingcorrosives cause

a) Erosion-

Corrosion,b) Impingementattack , and

c) Cavitationdamage in metals


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CAVITATION DAMAGE

Cavitation is a specialcase of Erosion-corrosion.

In high velocitysystems, localpressure reductionscreate water vapourbubbles which get

attached to the metalsurface and burst atincreased pressure,causing metal damage


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ENVIRONMENT ASSISTED

CRACKING

When a metal is subjected to atensile stress and a corrosivemedium, it may experience

Environment Assisted Cracking. Fourtypes:

Stress Corrosion Cracking

Hydrogen Embrittlement

Liquid Metal Embrittlement

Corrosion Fatigue


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STRESS CORROSION

CRACKING

Static tensile stressand specificenvironmentsproduce cracking

Examples:

1) Stainless steelsin hot chloride

2) Ti alloys innitrogen tetroxide

3) Brass inammonia


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HYDROGEN EMBRITTLEMENT

High strengthmaterials stressedin presence ofhydrogen crack atreduced stresslevels.

Hydrogen may bedissolved in themetal or present asa gas outside.

Only ppm levels ofH needed


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LIQUID METAL EMBRITTLEMENT

Certain metals likeAl and stainlesssteels undergobrittle failure whenstressed in contact

with liquid metalslike Hg, Zn, Sn, PbCd etc.Molten metal atomspenetrate the grain

boundaries andfracture the metalFig. Shows brittle IGfracture in Al alloyby Pb


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CORROSION FATIGUE

S-N DIAGRAMSynergistic actionof corrosion &

cyclic stress. Bothcrack nucleationand propagationare accelerated by

corrodent and theS-N diagram isshifted to the left

AirAir

CorrosionCorrosion

log (cycles to failure, Nf)

StressAmplitude


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CORROSION FATIGUE,

CRACK PROPAGATION

Crack propagation

rate is increased bythe corrosive action

Log (Stress Intensity Factor Range,

K

log(Crack

GrowthRate,d

a/dN)


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HIGH TEMPERATURE(DRY)

CORROSION

Exposure to high temperatures in airor other gaseous environmentscauses sever material damage. The

major corrosion processes are:

Oxidation

Corrosion in Sulfur environments

Hydrogen attack


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OXIDATION

Oxidation, Scaling, Tarnishing refer to thereaction between air or oxygen in theabsence of water.Pilling-Bedworth ratio (R = Md / nmD

where M is the molecular weight of thescale, D is the density of the scale, m isthe atomic weight of the metal, d is thedensity of the metal, and n is the numberof metal atoms in a molecular formula ofthe scale) indicates the oxidationresistance of a metal. For best resistance,R should be near unity.


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OXIDATION-ELECTROCHEMICAL

REACTION

Oxidation, like aqueous corrosion is anelectrochemical process, consisting of two partialprocesses,

M

M

+2

+ 2 e

-

----------- Metal oxidation atmetal-scale interface O2+ 2 e

-O2 --------- Oxygen reduction atscale-gas interface.----------------------

M + O2

MO --------------------OverallreactionThe oxide scale acts as the electrolyte throughwhich ions and electrons move to make theabove reactions possible. The electronic and ionic

conductivities of the scale thus determine therate of oxidation of the metal.


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KINETICS OF OXIDATIONOxidation leads to scale

formation and weight gain.The reaction follows severalrate laws. If Y is the thicknessof the oxide scale and t thetime of exposure, then

1. Linear: dy/dt = KLor Y = KLt +

constant where KLis thelinear rate constant. e.g. Na,K.

2. Parabolic: dy/dt = KP/Y or Y2

= 2KPt + constant. KPisparabolic rate constant. e.g.Cu, Ni, Fe, Cr and Co atelevated temperatures.

3. Cubic: Y3

= KCt + constant. KCis cubic rate constant. e.g. Zrand Hf exposed for shortdurations.Logarithmic: Y = Kllog(Ct +A). Klis logarithmic rateconstant, and C and A areconstants. e.g. Initial

oxidation rate of many metalsat low temperatures


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Oxidation-resistant alloys

The oxide characteristics determinethe oxidation resistance of an alloy

Most oxides are non-stoichiometric

compounds with structural defects.They may be n-type or p-typesemiconductors whose conductivitiescould be altered by alloy additions.

This principle is used in developinghigh temperature oxidation resistantalloys like Fe-Cr, Fe-Cr-Al, and Ni-base alloys.


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CORROSION IN OTHER

GASEOUS ENVIRONMENTS

Sulfur compounds: High temperaturedegradation of metals when exposed tosulfur compounds like H2S, SO2 andvaporized sulfur is referred to as sulfidation.Rapid scaling and internal precipitation of

stable sulfides lead to degradation inmechanical properties of high temperaturealloysDecarburization and hydrogen attack: Whensteels are exposed to hydrogen at high

temperatures, Decarburization occurs by thereaction, C (Fe) + 4 HCH4Hot Corrosion: accelerated high temperaturecorrosion of materials under sulfur gaseousatmospheres and the presence of fused

sulphate compounds on the metal surface.

different forms of corrosion

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