7/21/2019 Different Forms of Corrosion_uniform Corrosion_general Corrosion

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UniformGalvanicCrevicePittingIntergranularMICSCCHBHEHICFatigueErosionFrettingStrayCurrentIndex

DifferentTypesofCorrosion Recognition,Mechanisms&Prevention

UniformCorrosion(GeneralCorrosion)

Recognition

What isuniformcorrosion?Uniformcorrosionorgeneralcorrosion,as sometimescalled, isdefinedasa typeof

corrosionattack(deterioration)thatismoreorlessuniformlydistributedovertheentireexposedsurfaceofametal

(seeillustration

below).

Uniform

corrosion

also

refers

to

the

corrosion

that

proceeds

at

approximately

the

same

rate

overtheexposedmetalsurface.

Cast irons and steels corrode uniformlywhen exposed toopen atmospheres, soils

andnaturalwaters,leadingtotherustyappearance.

Thephotoontherightshoweduniformcorrosion(rusting)ofapairofsteelnutsused

to fasten a galvanized steel clamp on a street lamp post. In sharp contrast, the

galvanized steel clamp did not show any signs of corrosion but its surface was

discoloredbytherust. It isalso interestingtonote that thesurfaceof the topbolt

looked

like

galvanize

but

the

surface

of

the

bolt

below

was

completely

rusted

(just

likethenut).

Thephoto (above, left) shows a steel coupon corroded (rusted)uniformlyover its

entire surface after immersion in oxygen aeratedwaterwhile the same batch of

coupons exposed to deaerated water (above, right) retained their metallic

appearancewithnovisiblecorrosion(rust).

Innaturalenvironment,oxygen is theprimarycauseofuniformcorrosionofsteels

andothermetalsandalloys.

Mechanisms

1http://www.corrosionclinic.com/types_of_corrosion/uniform_corrosion.htm

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Whatcausesuniformcorrosion?Theanodicreactioninthecorrosionprocessisalwaystheoxidationreaction:

M=M++e (1)

Inacidicenvironments,i.e.,pH7, reductionofdissolvedoxygen is thepredominantcathodic

processthatcausesuniformcorrosion:

O2+2H

2O+4e=4OH (3)

Withuniformdistributionofcathodicreactantsovertheentireexposedmetalsurface,reactions(2)and/or(3)take

place in a "uniform"manner and there is no preferential site or location for cathodic or anodic reaction. The

cathodesandanodesarelocatedrandomlyandalternatingwithtime.Theendresultisamoreorlessuniformlossof

dimension.

Prevention

Howtopreventuniformcorrosion?Uniformcorrosionorgeneralcorrosioncanbepreventedthroughanumberof

methods:

Usethickermaterialsforcorrosionallowance

Usepaintsormetalliccoatingssuchasplating,galvanizingoranodizing

UseCorrosioninhibitorsormodifyingtheenvironment

Cathodicprotection

(Sacrificial

Anode

or

Impressed

Current

ICCP)

and

Anodic

Protection

Formoredetails

MoredetailsonUniformCorrosionorGeneralCorrosionareincludedinthefollowingcorrosioncourseswhichyou

cantakeasinhousetrainingcourses,onlinecoursesordistancelearningcourses:

CorrosionandItsPrevention(5daymodule)

CorrosionandItsPrevention(2daymodule)

Corrosion,Metallurgy,FailureAnalysisandPrevention(3days)

MarineCorrosion,CausesandPrevention(2days)

MaterialsSelection

and

Corrosion

(2

days)

StainlessSteelsandAlloys:WhyTheyResistCorrosionandHowTheyFail(2days)

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2http://www.corrosionclinic.com/types of corrosion/uniform corrosion.htm