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PREVENTIONSFOR

INTERGRANULARCORROSION

: BY:NAME: INAMDAR CHIRAYU PRADIP

ROLL NO: 940 (B.E.-IV)

YEAR: 2011-2012


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WHAT IS INTERGRANUAL CORROSION???

Intergranular corrosion refers to preferential (localized)corrosion along the grain boundaries.

It is the selective dissolution of the grain boundaries from

the microstructure of metals or alloys .

This selective dissolution may lead to the disintegrationof the grains.

Intergranular corrosion in sensitizedstainless steelsand exfoliationin aluminum alloys represent industriallysignificant examples of this form of damage.


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PRINCIPLE

It involves the creation of galvanic corrosion cells withinthe microstructure of an alloy
http://en.wikipedia.org/wiki/Galvanic_corrosionhttp://en.wikipedia.org/wiki/Microstructurehttp://en.wikipedia.org/wiki/Alloyhttp://en.wikipedia.org/wiki/Alloyhttp://en.wikipedia.org/wiki/Microstructurehttp://en.wikipedia.org/wiki/Galvanic_corrosion


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EXFOLIATION

It is a form of intergranular corrosion that manifests itself bylifting up the surface grains of a metal by the force ofexpanding corrosion products occurring at the grainboundaries just below the surface.
http://en.wikipedia.org/wiki/Intergranular_corrosionhttp://en.wikipedia.org/wiki/Corrosionhttp://en.wikipedia.org/wiki/Crystallitehttp://en.wikipedia.org/wiki/Crystallitehttp://en.wikipedia.org/wiki/Crystallitehttp://en.wikipedia.org/wiki/Crystallitehttp://en.wikipedia.org/wiki/Corrosionhttp://en.wikipedia.org/wiki/Intergranular_corrosion


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PREVENTION FOR EXFOLIATION CORROSION

The use of coatings

Selecting a more exfoliation resistant alloys

Using heat treatment to control precipitate distribution.


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SENSITIZATION


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In the case of the austeniticstainless steels, when thesesteels are sensitized by being heated in the temperaturerange of about 500 to 800C, depletion of chromium inthe grain boundary region occurs, resulting insusceptibility to intergranular corrosion. This effect is

known assensitization

of steels.

Such sensitization of austenitic stainless steels canreadily occur because of temperature servicerequirements, as in steam generators, or as a result of

subsequent welding of the formed structure.
http://en.wikipedia.org/wiki/Austenitichttp://en.wikipedia.org/wiki/Stainless_steelhttp://en.wikipedia.org/wiki/Steam_generatorhttp://en.wikipedia.org/wiki/Weldinghttp://en.wikipedia.org/wiki/Weldinghttp://en.wikipedia.org/wiki/Steam_generatorhttp://en.wikipedia.org/wiki/Stainless_steelhttp://en.wikipedia.org/wiki/Austenitic


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CAUSES & PREVENTIONS

CAUSES:

Certain alloys when exposed to a sensitizingtemperature they become particularly susceptibleto intergranular corrosion.

In a corrosive atmosphere, the grain interfaces of these

sensitized alloys become very reactive and intergranularcorrosion results. This is characterized by a localizedattack at an adjacent to grain boundaries with relativelylittle corrosion of the grains themselves. The alloydisintegrates and/or loses its strength.
http://en.wikipedia.org/wiki/Grain_boundaryhttp://en.wikipedia.org/wiki/Corrosionhttp://en.wikipedia.org/wiki/Corrosionhttp://en.wikipedia.org/wiki/Grain_boundary


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In certain aluminum alloys, small amounts of iron havebeen shown to segregate in the grain boundaries and

cause intergranular corrosion

It has been shown that the zinc content of a brass is

higher at the grain boundaries and subject to suchcorrosion

High-strength aluminum alloys such as the Duralumin-type alloys (Al-Cu) which depend upon precipitatedphases for strengthening are susceptible to intergranularcorrosion following sensitization at temperatures of about120C
http://en.wikipedia.org/wiki/Aluminium_alloyhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Brasshttp://en.wikipedia.org/wiki/Duraluminhttp://en.wikipedia.org/wiki/Duraluminhttp://en.wikipedia.org/wiki/Brasshttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Aluminium_alloy


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Nickel-rich alloys such as Inconel 600 and Incoloy 800show similar susceptibility.

Die-castzinc alloys containing aluminum exhibitintergranular corrosion by steam in a marine atmosphere.

Cr-Mn and Cr-Mn-Ni steels are also susceptible tointergranular corrosion following sensitization in thetemperature range of 400-850C.
http://en.wikipedia.org/wiki/Nickelhttp://en.wikipedia.org/wiki/Inconelhttp://en.wikipedia.org/wiki/Incoloyhttp://en.wikipedia.org/wiki/Die_castinghttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Steelshttp://en.wikipedia.org/wiki/Steelshttp://en.wikipedia.org/wiki/Steamhttp://en.wikipedia.org/wiki/Zinchttp://en.wikipedia.org/wiki/Die_castinghttp://en.wikipedia.org/wiki/Die_castinghttp://en.wikipedia.org/wiki/Die_castinghttp://en.wikipedia.org/wiki/Incoloyhttp://en.wikipedia.org/wiki/Inconelhttp://en.wikipedia.org/wiki/Nickel


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PREVENTIONS

Use low carbon (e.g. 304L, 316L) grade of stainlesssteels. Lower the C content to below 0.03%, so that thecarbides are not stable.

Use stabilized grades alloyed with titanium (for exampletype 321) or niobium (for example type 347). Titaniumand niobium are strong carbide- formers. They react withthe carbon to form the corresponding carbides therebypreventing chromium depletion.

Use high temperature solution heat treatment to dissolvethe precipitates. (Post welds heat treatment of sensitizedsteel).


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HIGH-TEMPERATURE SOLUTION HEAT TREATMENT

The alloy is heated to a temperature of about 1,060 to 1,120Cand then water quenched.

Chromium carbide is dissolved at these temperatures, and a

more homogeneous alloy is obtained.

Unsuitable for treating large assemblies, and also ineffectivewhere welding is subsequently used for making repairs or forattaching other structures.

The equipment must be quench-annealed to eliminatesusceptibility to weld decay. Rapid cooling from the solutiontemperature is very important
http://en.wikipedia.org/wiki/Heat_treatmenthttp://en.wikipedia.org/wiki/Heat_treatment


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LOWERING THE CARBON CONTENT.

Lowering the carbon to below 0.03% (type 304L) does notpermit sufficient carbide.

These is also called the extra- low-carbon (ELC) steels.

The situation is same as above except that here weld decay isabsent in the low carbon plate


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THE STRONG CARBIDE FORMERS OR STABILIZERS

columbium (or columbium plus tantalum) and titanium, areused to produce types 347 & 321 stainless steels,

respectively

These elements have a much grater affinity for carbon than

does chromium

The stabilized steels eliminate the economic and otherobjections of solution- quenching the unstabilized steels after

fabrication or weld repair.


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THANK

YOU

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