Nidhi Shah Ppt

8/3/2019 Nidhi Shah Ppt

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NAME : NIDHI S. SHAH

ROLL NO : 922


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INTRODUCTION:Just as most engineering metalsare mixtures of one or more metals, they consist oflarge numbers of individual metal crystals calledgrains that are joined together at their surfaces or grain

boundaries. As there can be differences in compositionat or adjacent to these grain boundaries, selectivecorrosion can occur at these sites.

Definition:Intergranular corrosion is a selectiveattack of a metal at or adjacent to grain boundaries.


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Mechanism.

There are three mechanisms.1. The first mechanism is the selective attack of grain

boundary material due to its high energy content.

Metal crystals form in an ordered arrangement of atoms

because this ordered arrangement has a lower energycontent than a disordered arrangement.

Grain boundaries are highly disordered as they are at theboundaries of crystals which, although they are internally

ordered, have random orientation with respect to eachother.

The disordered grain boundary is often 10 to 100 atomswide and these atoms have a higher energy than the

surrounding atoms.


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Higher energy material can be more chemically active

than lower energy material and thus, the grain boundarymaterial can be anodic with respect to the surroundinggrains.

When this occurs, the anodic area is small and the

cathodic area is large, thus, rapid attack can occur. Theresult is that the individual grains are no longer joinedwith the strong grain boundary glue and disintegrateleaving a powdery residue and rough grainy surface.

2. A second mechanism is selective attack of grainboundary material that has a different composition fromthe surrounding grains.

When metals crystallize from the molten state, the

crystals tend to be more pure than the molten material.


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This is because the pure metal crystals are more orderedand have a lower energy content than if they contained

large amounts of impurities .

In some cases, most of the impurities are concentrated atthe grain boundaries.

When the composition of this impure material causes itto be more anodic than the surrounding grains, rapidattack can occur with results similar to those describedabove.

When the composition of the impure grain boundarymaterial causes it to be more cathodic than thesurrounding grains, the favourable anode/cathode area

ratio makes this situation relatively innocuous.


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Contamination of grain boundaries can sometimes also

occur after manufacture.

Mercury on aluminium can penetrate and contaminate thegrain boundaries and cause subsequent Intergranular

attack. This is why mercury and mercury compounds areprohibited aboard aluminium ships or on aircraft.

3. A third mechanism is selective attack adjacent to the

grain boundaries due to the local depletion of an alloyingelement.

This form of attack can occur in many stainless steels. It is

called sensitization.


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Many stainless steels rely on a combination of nickel

and chromium for their corrosion resistance. As bothnickel and chromium are expensive, they are addedonly in amounts necessary to obtain the necessarycorrosion resistance.

Another element, which is commonly present in ailsteels, is carbon.

In stainless steels, carbon atoms tend to concentrate atthe grain boundaries as an impurity duringsolidification.


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Chromium carbides can form adjacent to the grainboundaries during welding and heat treatment.

When these compounds form, the chromium isremoved from the alloy adjacent to the grainboundaries and the resulting alloy does not haveenough chromium content to remain passive.

Again, there is a very unfavourable anode/cathode arearatio and rapid attack can occur.

Three different methods are used to avoid this type ofattack in stainless steels during welding or otherheating


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a. The first methodto avoid sensitization is through

heat treatment.

At high temperatures (above 1,800F), chromiumcarbides are unstable and will redissolve if they haveformed.

At low temperatures, (below 1,000F) the chromium

and carbon atoms cannot move and formation ofchromium carbides is prevented.


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Formation of the chromium carbides is a problem

primarily in the ranges of 1,100 to 1,600F. Whenwelding stainless steel, some area adjacent to the weldis likely to reach this temperature range long enoughto form amounts of chromium carbides.

When this occurs, or when the alloy is otherwisesensitized, it should be heated to temperatures above1,800F to redissolve the carbides, then rapidly cooledto below 1,000F to avoid carbide formation.

b. The second methodused to avoid sensitization instainless steels is to reduce the carbon content of thealloy to very low levels.
http://www.corrosionist.com/Materials_Stainless_Steel_Welding.htmhttp://www.corrosionist.com/Materials_Stainless_Steel_Intro.htmhttp://www.corrosionist.com/Materials_Stainless_Steel_Intro.htmhttp://www.corrosionist.com/Materials_Stainless_Steel_Welding.htm


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These low carbon grades (such as 304 L and 316 L; L

stands for low carbon) do not have enough carbon toform carbides and is thus resistant to sensitizationduring welding.

Care must be taken, however, to not introduceadditional carbon during welding from contamination,such as can be caused by oil or grease.

c. The third methodused to avoid sensitization in thestainless steels is to intentionally add an element thatwill combine with the carbon but is not required forpassivity of the alloy.


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Titanium and niobium have a greater affinity forcarbon than chromium.

They are added to the alloy during manufacture in

amounts to combine with all of the carbon present inthe alloy and thus inhibit sensitization.

Type 321 stainless steel contains titanium and Type 347stainless steel contains niobium. These alloys, or thelow carbon grades, should be used when weldingwithout heat treatment is required.


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Examples.

Aluminum alloys are susceptible to Intergranular

attack , usually the type that is caused by segregation ofimpurities at the grain boundaries.

In addition to the stainless steels, some nickel alloys arealso subject to sensitization and subsequent Intergranularattack.

Appearance.

Intergranular attack caused by high grain boundaryenergies or impurities at the grain boundaries results inattack with a grainy residue and rough surface.
http://www.corrosionist.com/Materials_Aluminium_Alloys_Intro.htmhttp://www.corrosionist.com/Materials_Stainless_Steel_Intro.htmhttp://www.corrosionist.com/Materials_Nickel_Alloys_Intro.htmhttp://www.corrosionist.com/Materials_Nickel_Alloys_Intro.htmhttp://www.corrosionist.com/Materials_Stainless_Steel_Intro.htmhttp://www.corrosionist.com/Materials_Aluminium_Alloys_Intro.htmhttp://www.corrosionist.com/Materials_Aluminium_Alloys_Intro.htm


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Under high magnification, the individual grains are oftenvisible. Intergranular attack of aluminium alloys isassociated with pitting or other localized attack.

Sensitization in stainless steels has a similar grainyappearance.

When caused by welding it is often localized in narrow

bands adjacent to the weld and is sometimes called knifeline attack.

Significant Measurements.

Microscopic examination of sectioned samples is oftenrequired to verify that Intergranular attack has occurred.There are several standardized methods for determiningthe resistance of stainless steels to sensitization.


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SOME PICTURES OF

INTERGRANULAR CORROSION

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Nidhi Shah Ppt