corrosion failure

8/7/2019 CORROSION FAILURE

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CORROSIONCORROSION

FAILUREFAILUREPrepared byPrepared by

Avadesh Kumar ShakyaAvadesh Kumar Shakya

M.Tech (M.Tech (Maintenance Engineering and ManagementMaintenance Engineering and Management))IInd SemIInd Sem


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IntroductionIntroduction

Corrosion is chemically induced damage to a materialCorrosion is chemically induced damage to a materialthat results in deterioration of the material and itsthat results in deterioration of the material and itsproperties.properties. This may result in failure of theThis may result in failure of thecomponent.component. Several factors should be consideredSeveral factors should be consideredduring a failure analysis to determine the affectduring a failure analysis to determine the affectcorrosion played in a failurecorrosion played in a failure

example:example:

Type ofcorrosionType ofcorrosion

Corrosion rateCorrosion rateInteraction between corrosion and other failureInteraction between corrosion and other failuremechanismmechanism


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The corrosion failure ofmetallic materials causesThe corrosion failure ofmetallic materials causesbillions ofdollars globally every year. The lackbillions ofdollars globally every year. The lack

ofunderstanding will not only cost a lot ofofunderstanding will not only cost a lot of

money it will also cost human life. Preventingormoney it will also cost human life. Preventingorreducing corrosion processes increases thereducing corrosion processes increases the

productivity and efficiencyofany plant.productivity and efficiencyofany plant.


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Types of corrosionTypes of corrosion

Uniform corrosionUniform corrosion

Galvanic corrosionGalvanic corrosion

Pitting corrosionPitting corrosion

Crevice corrosionCrevice corrosion

Stress corrosion crackingStress corrosion cracking


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Uniform corrosionUniform corrosion

Uniform or general corrosion is typified by the rustingUniform or general corrosion is typified by the rusting

ofsteel. General corrosion is rather predictable.ofsteel. General corrosion is rather predictable. TheThe

life ofcomponents can be estimated based on relativelylife ofcomponents can be estimated based on relatively

simple immersion test results.simple immersion test results. Allowance for generalAllowance for generalcorrosion is relatively simple and commonly employedcorrosion is relatively simple and commonly employed

when designing a component for a knownwhen designing a component for a known

environment.environment.


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Methods used to prevent orMethods used to prevent or

reduce general corrosionreduce general corrosion

CoatingsCoatings

InhibitorsInhibitors

Proper material selectionProper material selection


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Galvanic corrosionGalvanic corrosion

Galvanic corrosion is frequently referred to asGalvanic corrosion is frequently referred to as

dissimilar metal corrosion.dissimilar metal corrosion. Galvanic corrosionGalvanic corrosion

can occur when two dissimilar materials arecan occur when two dissimilar materials are

coupled in a corrosive electrolyte.coupled in a corrosive electrolyte. AnAn

illustration ofgalvanic corrosion would beillustration ofgalvanic corrosion would be

joining two dissimilar metals in electrical contactjoining two dissimilar metals in electrical contact

in seawater.in seawater.


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Pitting corrosionPitting corrosion

Pitting is a localized form ofcorrosive attack.Pitting is a localized form ofcorrosive attack. PittingPittingcorrosion is typified by the formation ofholes or pitscorrosion is typified by the formation ofholes or pitson the metal surface.on the metal surface. Pitting can cause failure due toPitting can cause failure due to

perfo

ratio

n while the to

tal co

rro

sio

n, as measured byperfo

ratio

n while the to

tal co

rro

sio

n, as measured byweight loss, might be rather minimal.weight loss, might be rather minimal. The rate ofThe rate ofpenetration may be 10 to 100 times that by generalpenetration may be 10 to 100 times that by generalcorrosion.corrosion.

Pits may be rather small and diff

icult to

detect.Pits may be rather small and diff

icult to

detect. InInsome cases pits may be masked due to generalsome cases pits may be masked due to generalcorrosion.corrosion. Pitting may take some time to initiate andPitting may take some time to initiate anddevelop to an easily viewable size.develop to an easily viewable size.


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Methods for reducing theeffectsMethods for reducing theeffects

ofpitting corrosionofpitting corrosion

R

educe the aggressivenessof

the enviro

nmentR

educe the aggressivenessof

the enviro

nment Use more pitting resistant materialsUse more pitting resistant materials

Improve the design of the systemImprove the design of the system


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Crevice CorrosionCrevice Corrosion

Crevice corrosion occurs at narrowopenings or spacesCrevice corrosion occurs at narrowopenings or spaces

between two metal surfaces or between metals andbetween two metal surfaces or between metals and

nonmetal surfaces .nonmetal surfaces . A concentration cell forms withA concentration cell forms with

the crevice being depleted ofoxygen.the crevice being depleted ofoxygen. This differentialThis differentialaeration between the crevice (microenvironment) andaeration between the crevice (microenvironment) and

the external surface (bulk environment) gives thethe external surface (bulk environment) gives the

crevice an anodic character.crevice an anodic character. This can contribute to aThis can contribute to a

highly corrosive condition in the crevicehighly corrosive condition in the crevice


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Stress corrosion crackingStress corrosion cracking

Stress corrosion cracking is a failure mechanism that isStress corrosion cracking is a failure mechanism that is

caused by environment, susceptible material, and tensilecaused by environment, susceptible material, and tensile

stress.stress. Temperature is a significant environmentalTemperature is a significant environmental

factor affecting cracking. For stress corrosion crackingfactor affecting cracking. For stress corrosion crackingtooccur all three conditions must be mettooccur all three conditions must be met

simultaneously.simultaneously. The component needs to be in aThe component needs to be in a

particular crack promoting environment, theparticular crack promoting environment, the

component must be made ofa susceptible material, andcomponent must be made ofa susceptible material, andthere must be tensile stresses above some minimumthere must be tensile stresses above some minimum

threshold value.threshold value.


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Factors affecting corrosion failureFactors affecting corrosion failure

Material factorMaterial factor

Environment factorEnvironment factor

Stress factorStress factor

GeometryfactorGeometryfactor

Temperature factorTemperature factor

Time factorTime factor


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Sixteen recognized corrosion experts accepted to complete an opinion pollSixteen recognized corrosion experts accepted to complete an opinion poll

listing the main sublisting the main sub--factors and the common forms of corrosionfactors and the common forms of corrosion


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Some of the factors and information to beSome of the factors and information to be

collected before, during and after the failurecollected before, during and after the failure

Analysis ofmaterial failureAnalysis ofmaterial failure

OnOn--site investigationssite investigations

Welding, soldering and Brazing analysisWelding, soldering and Brazing analysis

Chemical and mechanical testing (ASTM)Chemical and mechanical testing (ASTM)

Metellographic analysis and investigationMetellographic analysis and investigation

Fracture mechanicsFracture mechanics

Engineering calculatio

nsEngineering calculatio

ns Atmospheric testingAtmospheric testing

Accident investigations and Recreation offailureAccident investigations and Recreation offailure


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Plastic and rubber investigationPlastic and rubber investigation Micro and macro photographyMicro and macro photography

Surface analysisSurface analysis

Fractography: fracture mode determinationFractography: fracture mode determination

Engineering mechanics analysisEngineering mechanics analysis

Corrosion testing and investigationsCorrosion testing and investigations

RootRoot--cause failure determinationcause failure determination


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corrosion failure

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