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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.