FORMS OF CORROSIONFORMS OF CORROSION

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

Consultant-Metallurgy & Corrosion,Consultant-Metallurgy & Corrosion,Tiruvalla, KeralaTiruvalla, Kerala

FORMS OF CORROSIONFORMS OF CORROSIONCorrosion may Corrosion may be classified in be classified in different waysdifferent waysWet / Aqueous Wet / Aqueous corrosion & Dry corrosion & Dry CorrosionCorrosionRoom Room Temperature/ Temperature/ High High Temperature Temperature CorrosionCorrosion

CORROSION

WET CORROSION DRY CORROSION

CORROSION

ROOM TEMPERATURECORROSION

HIGH TEMPERATURECORROSION

WET & DRY CORROSIONWET & DRY CORROSION

Wet / aqueous corrosionWet / aqueous corrosion is the is the major form of corrosion which occurs major form of corrosion which occurs at or near room temperature and in at or near room temperature and in the presence of waterthe presence of water

Dry / gaseous corrosionDry / gaseous corrosion is is significant mainly at high significant mainly at high temperaturestemperatures

WET / AQUEOUS CORROSIONWET / AQUEOUS CORROSION Based on the appearance of the corroded Based on the appearance of the corroded

metal, wet corrosion may be classified asmetal, wet corrosion may be classified asUniform or GeneralUniform or GeneralGalvanic or Two-metalGalvanic or Two-metalCreviceCrevicePittingPittingDealloyingDealloyingIntergranularIntergranularVelocity-assistedVelocity-assistedEnvironment-assisted crackingEnvironment-assisted cracking

UNIFORM CORROSIONUNIFORM CORROSIONCorrosion over the Corrosion over the entire exposed entire exposed surface at a surface at a uniform rate. e.g.. uniform rate. e.g.. Atmospheric Atmospheric corrosion.corrosion.

Maximum metal Maximum metal loss by this form.loss by this form.

Not dangerous. Not dangerous. Rate can be Rate can be measured in the measured in the laboratorylaboratory

GALVANIC CORROSIONGALVANIC CORROSIONWhen two dissimilar When two dissimilar metals are joined metals are joined together and exposed, together and exposed, the more active of the the more active of the two metals corrode two metals corrode faster and the nobler faster and the nobler metal is protected. This metal is protected. This excess corrosion is due excess corrosion is due to the galvanic current to the galvanic current generated at the generated at the junctionjunctiona) Steel plates with copper a) Steel plates with copper rivets in seawater 15 rivets in seawater 15 months.months.b) ) Copper plates with b) ) Copper plates with steel rivets, same steel rivets, same environmental conditionsenvironmental conditions

CREVICE CORROSIONCREVICE CORROSIONIntensive localized Intensive localized corrosion within corrosion within crevices & shielded crevices & shielded areas on metal areas on metal surfacessurfaces

Small volumes of Small volumes of stagnant corrosive stagnant corrosive caused by holes, caused by holes, gaskets, surface gaskets, surface deposits, lap jointsdeposits, lap joints

DEALLOYINGDEALLOYINGAlloys exposed to Alloys exposed to corrosives corrosives experience selective experience selective leaching out of the leaching out of the more active more active constituent. e.g. constituent. e.g. Dezincification of Dezincification of brass.brass.

Loss of structural Loss of structural stability and stability and mechanical strengthmechanical strength

PITTINGPITTINGA form of extremely A form of extremely localized attack localized attack causing holes in the causing holes in the metalmetal

Most destructive Most destructive formform

Autocatalytic Autocatalytic naturenature

Difficult to detect Difficult to detect and measureand measure

INTERGRANULAR CORROSIONINTERGRANULAR CORROSIONThe grain boundaries in The grain boundaries in metals are more active metals are more active than the grains than the grains because of segregation because of segregation of impurities and of impurities and depletion of protective depletion of protective elements. So elements. So preferential attack preferential attack along grain boundaries along grain boundaries occurs. e.g. weld decay occurs. e.g. weld decay in stainless steelsin stainless steels

VELOCITY ASSISTED VELOCITY ASSISTED CORROSIONCORROSION

Fast moving Fast moving corrosives cause corrosives cause

a) Erosion-a) Erosion-Corrosion, Corrosion,

b) Impingement b) Impingement attack , and attack , and

c) Cavitation c) Cavitation damage in metalsdamage in metals

CAVITATION DAMAGECAVITATION DAMAGECavitation is a special Cavitation is a special case of Erosion-case of Erosion-corrosion.corrosion.

In high velocity In high velocity systems, local systems, local pressure reductions pressure reductions create water vapour create water vapour bubbles which get bubbles which get attached to the metal attached to the metal surface and burst at surface and burst at increased pressure, increased pressure, causing metal damagecausing metal damage

ENVIRONMENT ASSISTED ENVIRONMENT ASSISTED CRACKINGCRACKING

When a metal is subjected to a tensile When a metal is subjected to a tensile stress and a corrosive medium, it may stress and a corrosive medium, it may experience Environment Assisted experience Environment Assisted Cracking. Four types:Cracking. Four types:

Stress Corrosion CrackingStress Corrosion Cracking

Hydrogen EmbrittlementHydrogen Embrittlement

Liquid Metal EmbrittlementLiquid Metal Embrittlement

Corrosion FatigueCorrosion Fatigue

STRESS CORROSION STRESS CORROSION CRACKINGCRACKING

Static tensile stress Static tensile stress and specific and specific environments environments produce crackingproduce crackingExamples:Examples:1) Stainless steels 1) Stainless steels in hot chloridein hot chloride2) Ti alloys in 2) Ti alloys in nitrogen tetroxidenitrogen tetroxide3) Brass in 3) Brass in ammoniaammonia

HYDROGEN EMBRITTLEMENTHYDROGEN EMBRITTLEMENTHigh strength High strength materials stressed materials stressed in presence of in presence of hydrogen crack at hydrogen crack at reduced stress reduced stress levels.levels.Hydrogen may be Hydrogen may be dissolved in the dissolved in the metal or present as metal or present as a gas outside.a gas outside.Only ppm levels of Only ppm levels of H neededH needed

LIQUID METAL EMBRITTLEMENTLIQUID METAL EMBRITTLEMENTCertain metals like Certain metals like Al and stainless Al and stainless steels undergo steels undergo brittle failure when brittle failure when stressed in contact stressed in contact with liquid metals with liquid metals like Hg, Zn, Sn, Pb like Hg, Zn, Sn, Pb Cd etc.Cd etc.Molten metal atoms Molten metal atoms penetrate the grain penetrate the grain boundaries and boundaries and fracture the metalfracture the metalFig. Shows brittle IG Fig. Shows brittle IG fracture in Al alloy fracture in Al alloy by Pbby Pb

CORROSION FATIGUE CORROSION FATIGUE S-N S-N DIAGRAMDIAGRAM

Synergistic action Synergistic action of corrosion & of corrosion & cyclic stress. Both cyclic stress. Both crack nucleation crack nucleation and propagation and propagation are accelerated by are accelerated by corrodent and the corrodent and the S-N diagram is S-N diagram is shifted to the leftshifted to the left

AirAir

CorrosionCorrosion

log (cycles to failure, N f)

Stre

ss A

mplitu

de

CORROSION FATIGUE, CORROSION FATIGUE, CRACK PROPAGATIONCRACK PROPAGATION

Crack propagation Crack propagation rate is increased by rate is increased by the corrosive actionthe corrosive action

Log (Stress Intensity Factor Range, K

log

(Cra

ck G

row

th R

ate,

da/

dN

)

HIGH TEMPERATURE(DRY) HIGH TEMPERATURE(DRY) CORROSIONCORROSION

Exposure to high temperatures in air Exposure to high temperatures in air or other gaseous environments or other gaseous environments causes sever material damage. The causes sever material damage. The major corrosion processes are:major corrosion processes are:

OxidationOxidation

Corrosion in Sulfur environmentsCorrosion in Sulfur environments

Hydrogen attackHydrogen attack

OXIDATIONOXIDATIONOxidation, Scaling, Tarnishing refer to the Oxidation, Scaling, Tarnishing refer to the reaction between air or oxygen in the reaction between air or oxygen in the absence of water.absence of water.Pilling-Bedworth ratio (R = Md / nmD Pilling-Bedworth ratio (R = Md / nmD where M is the molecular weight of the where M is the molecular weight of the scale, D is the density of the scale, m is scale, D is the density of the scale, m is the atomic weight of the metal, d is the the atomic weight of the metal, d is the density of the metal, and n is the number density of the metal, and n is the number of metal atoms in a molecular formula of of metal atoms in a molecular formula of the scale) indicates the oxidation the scale) indicates the oxidation resistance of a metal. For best resistance, resistance of a metal. For best resistance, R should be near unity.R should be near unity.

OXIDATION-ELECTROCHEMICAL OXIDATION-ELECTROCHEMICAL REACTIONREACTION

Oxidation, like aqueous corrosion is an Oxidation, like aqueous corrosion is an electrochemical process, consisting of two partial electrochemical process, consisting of two partial processes,processes,

M → M M → M +2+2 + 2 e + 2 e-- ----------- Metal oxidation at metal- ----------- Metal oxidation at metal-scale interfacescale interface½ O½ O22 + 2 e + 2 e-- → O → O22 --------- Oxygen reduction at scale- --------- Oxygen reduction at scale-gas interface.gas interface.--------------------------------------------M + ½ OM + ½ O22 → MO --------------------Overall reaction → MO --------------------Overall reactionThe oxide scale acts as the electrolyte through which The oxide scale acts as the electrolyte through which ions and electrons move to make the above reactions ions and electrons move to make the above reactions possible. The electronic and ionic conductivities of possible. The electronic and ionic conductivities of the scale thus determine the rate of oxidation of the the scale thus determine the rate of oxidation of the metal.metal.

KINETICS OF OXIDATIONKINETICS OF OXIDATIONOxidation leads to scale formation Oxidation leads to scale formation

and weight gain. The reaction and weight gain. The reaction follows several rate laws. follows several rate laws. If Y is If Y is the thickness of the oxide scale the thickness of the oxide scale and t the time of exposure, and t the time of exposure, thenthen

1.1. Linear: dy/dt = KLinear: dy/dt = KLL or Y = K or Y = KLLt + t + constant where Kconstant where KLL is the linear is the linear rate constant. e.g. Na, K.rate constant. e.g. Na, K.

2.2. Parabolic: dy/dt = KParabolic: dy/dt = KPP /Y or Y /Y or Y22 = = 2K2KPPt + constant. Kt + constant. KPP is parabolic is parabolic rate constant. e.g. Cu, Ni, Fe, Cr rate constant. e.g. Cu, Ni, Fe, Cr and Co at elevated and Co at elevated temperatures.temperatures.

3.3. Cubic: YCubic: Y33 = K = KCCt + constant. t + constant. KKCC is is cubic rate constant. e.g. Zr and cubic rate constant. e.g. Zr and Hf exposed for short durations.Hf exposed for short durations.Logarithmic: Y = KLogarithmic: Y = Kll log(Ct + A). log(Ct + A). KKll is logarithmic rate constant, is logarithmic rate constant, and C and A are constants. e.g. and C and A are constants. e.g. Initial oxidation rate of many Initial oxidation rate of many metals at low temperaturesmetals at low temperatures

Oxidation-resistant alloysOxidation-resistant alloysThe oxide characteristics determine The oxide characteristics determine the oxidation resistance of an alloythe oxidation resistance of an alloy Most oxides are non-stoichiometric Most oxides are non-stoichiometric compounds with structural defects. compounds with structural defects. They may be n-type or p-type They may be n-type or p-type semiconductors whose conductivities semiconductors whose conductivities could be altered by alloy additions. could be altered by alloy additions. This principle is used in developing This principle is used in developing high temperature oxidation resistant high temperature oxidation resistant alloys like Fe-Cr, Fe-Cr-Al, and Ni-alloys like Fe-Cr, Fe-Cr-Al, and Ni-base alloys.base alloys.

CORROSION IN OTHER CORROSION IN OTHER GASEOUS ENVIRONMENTSGASEOUS ENVIRONMENTS

Sulfur compounds: High temperature Sulfur compounds: High temperature degradation of metals when exposed to sulfur degradation of metals when exposed to sulfur compounds like H2S, SO2 and vaporized compounds like H2S, SO2 and vaporized sulfur is referred to as sulfidation.sulfur is referred to as sulfidation. Rapid Rapid scaling and internal precipitation of stable scaling and internal precipitation of stable sulfides lead to degradation in mechanical sulfides lead to degradation in mechanical properties of high temperature alloysproperties of high temperature alloysDecarburization and hydrogen attack: When Decarburization and hydrogen attack: When steels are exposed to hydrogen at high steels are exposed to hydrogen at high temperatures, Decarburization occurs by the temperatures, Decarburization occurs by the reaction, reaction, C (Fe) + 4 H→CHC (Fe) + 4 H→CH44

Hot Corrosion: accelerated high temperature Hot Corrosion: accelerated high temperature corrosion of materials under sulfur gaseous corrosion of materials under sulfur gaseous atmospheres and the presence of fused atmospheres and the presence of fused sulphate compounds on the metal surface.sulphate compounds on the metal surface.