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INTERGRANULAR

CORROSION

NAME : NIDHI S. SHAH

ROLL NO : 922



Intergranular Corrosion

INTRODUCTION: Just as most engineeringmetals are mixtures of one or more metals, they

consist of large numbers of individual metalcrystals called grains that are joined together attheir surfaces or grain boundaries. As there canbe differences in composition at or adjacent tothese grain boundaries, selective corrosion canoccur at these sites. Definition. Intergranular corrosion is a selective attack of a metal at or adjacent tograin boundaries.

Mechanism. There are three mechanisms that have beenidentified as causing intergranular corrosion invarious situations. 1. The first mechanism is the selective attackof grain boundary material due to its high energycontent. Metal crystals form in an ordered

arrangement of atoms because this orderedarrangement has a lower energy content than a



disordered arrangement. Grain boundaries arehighly disordered as they are at the boundaries of crystals which, although they are internally

ordered, have random orientation with respect to

each other. The disordered grain boundary isoften 10 to 100 atoms wide and these atomshave a higher energy than the surroundingatoms. Higher energy material can be morechemically active than lower energy material andthus, the grain boundary material can be anodicwith respect to the surrounding grains. When thisoccurs, the anodic area is small and the cathodic

area is large, thus, rapid attack can occur. Theresult is that the individual grains are no longeroined with the strong grain boundary “glue” anddisintegrate leaving a powdery residue and roughgrainy surface. 2. A second mechanism is selective attack of grain boundary material that has a differentcomposition from the surrounding grains. When

metals crystallize from the molten state, thecrystals tend to be more pure than the moltenmaterial. This is because the pure metal crystalsare more ordered and have a lower energycontent than if they contained large amounts of impurities. In some cases, most of the impuritiesare concentrated at the grain boundaries. Whenthe composition of this impure material causes itto be more anodic than the surrounding grains,

rapid attack can occur with results similar tothose described above. When the composition of the impure grain boundary material causes it tobe more cathodic than the surrounding grains,the favorable anode/cathode area ratio makesthis situation relatively innocuous. Contamination

of grain boundaries can sometimes also occurafter manufacture. Mercury on aluminum canpenetrate and contaminate the grain boundaries

and cause subsequent intergranular attack. This



is why mercury and mercury compounds areprohibited aboard aluminum ships or on aircraft. 3. A third mechanism is selective attack

adjacent to the grain boundaries due to the localdepletion of an alloying element. This form of attack can occur in many stainless steels. It iscalled sensitization. Many stainless steels rely ona combination of nickel and chromium for theircorrosion resistance. As both nickel andchromium are expensive, they are added only inamounts necessary to obtain the necessary

corrosion resistance. Another element, which iscommonly present in ail steels, is carbon. Instainless steels, carbon atoms tend toconcentrate at the grain boundaries as animpurity during solidification. Chromium carbidescan form adjacent to the grain boundaries duringwelding and heat treatment. When thesecompounds form, the chromium is removed fromthe alloy adjacent to the grain boundaries and

the resulting alloy does not have enoughchromium content to remain passive. Again,there is a very unfavourable anode/cathode arearatio and rapid attack can occur. Three differentmethods are used to avoid this type of attack instainless steels during welding or other heating. a. The first method to avoid sensitization isthrough heat treatment. At high temperatures

(above 1,800°F), chromium carbides are unstableand will redissolve if they have formed. At lowtemperatures, (below 1,000°F) the chromiumand carbon atoms cannot move and formation of chromium carbides is prevented. Formation of the chromium carbides is a problem primarily inthe ranges of 1,100 to 1,600°F. When weldingstainless steel, some area adjacent to the weld islikely to reach this temperature range long

enough to form amounts of chromium carbides.

http://www.corrosionist.com/Materials_Stainless_Steel_Welding.htm







When this occurs, or when the alloy is otherwisesensitized, it should be heated to temperaturesabove 1,800°F to redissolve the carbides, then

rapidly cooled to below 1,000°F to avoid carbide

formation. b. The second method used to avoidsensitization in stainless steels is to reduce thecarbon content of the alloy to very low levels.These low carbon grades (such as 304 L and 316L; L stands for low carbon) do not have enoughcarbon to form carbides and is thus resistant to

sensitization during welding. Care must be taken, however, to not introduce additional carbonduring welding from contamination, such as canbe caused by oil or grease. c. The third method used to avoid sensitizationin the stainless steels is to intentionally add anelement that will combine with the carbon but isnot required for passivity of the alloy. Titanium

and niobium have a greater affinity for carbonthan chromium. They are added to the alloy

during manufacture in amounts to combine withall of the carbon present in the alloy and thusinhibit sensitization. Type 321 stainless steelcontains titanium and Type 347 stainless steelcontains niobium. These alloys, or the low carbongrades, should be used when welding withoutheat treatment is required. Examples. Aluminum alloys are susceptible to intergranularattack, usually the type that is caused bysegregation of impurities at the grain boundaries.In addition to the stainless steels, some nickelalloys are also subject to sensitization andsubsequent intergranular attack.

http://www.corrosionist.com/Materials_Stainless_Steel_Intro.htm


http://www.corrosionist.com/Materials_Aluminium_Alloys_Intro.htm





http://www.corrosionist.com/Materials_Nickel_Alloys_Intro.htm











Appearance. Intergranular attack caused by high grainboundary energies or impurities at the grain

boundaries results in attack with a grainy residueand rough surface. Under high magnification, theindividual grains are often visible. Intergranularattack of aluminum alloys is associated withpitting or other localized attack. Sensitization instainless steels has a similar grainy appearance.When caused by welding it is often localized innarrow bands adjacent to the weld and is

sometimes called “knife line attack.”

Significant Measurements. Microscopic examination of sectioned samples isoften required to verify that intergranular attackhas occurred. There are several standardizedmethods for determining the resistance of

stainless steels to sensitization.

SOME PICTURES OF INTERGRANULAR CORROSION.

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