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Just as most engineering metals are mixtures of one or more metals, they consist of large numbers of individual metal crystals called grains that are joined together at their surfaces or grain boundaries. As there can be differences in composition at or adjacent to these grain boundaries, selective corrosion can occur at these sites.
Intergranular corrosion is a selective attack of a metal at or adjacent to grain boundaries.
There are three mechanisms that have been identified as causing intergranular corrosion in various situations.
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 energy content than a disordered arrangement. Grain boundaries are highly 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 is often 10 to 100 atoms wide and these atoms have a higher energy than the surrounding atoms. Higher energy material can be more chemically active than lower energy material and thus, the grain boundary material can be anodic with respect to the surrounding grains. When this occurs, the anodic area is small and the cathodic area is large, thus, rapid attack can occur. The result is that the individual grains are no longer joined with the strong grain boundary “glue” and disintegrate leaving a powdery residue and rough grainy surface.
2. A second mechanism is selective attack of grain boundary material that has a different composition from the surrounding grains. When metals crystallize from the molten state, the crystals tend to be more pure than the molten material. This is because the pure metal crystals are more ordered and have a lower energy content than if they contained large amounts of impurities. In some cases, most of the impurities are concentrated at the grain boundaries. When the composition of this impure material causes it to be more anodic than the surrounding grains, rapid attack can occur with results similar to those described above. When the composition of the impure grain boundary material causes it to be more cathodic than the surrounding grains, the favorable anode/cathode area ratio makes this situation relatively innocuous. Contamination of grain boundaries can sometimes also occur after manufacture. Mercury on aluminum can penetrate and contaminate the grain boundaries and cause subsequent intergranular attack. This is why mercury and mercury compounds are prohibited aboard aluminum ships or on aircraft.
3. A third mechanism is selective attack adjacent to the grain boundaries due to the local depletion of an alloying element. This form of attack can occur in many stainless steels. It is called sensitization. Many stainless steels rely on a combination of nickel and chromium for their corrosion resistance. As both nickel and chromium are expensive, they are added only in amounts necessary to obtain the necessary corrosion resistance. Another element, which is commonly present in ail steels, is carbon. In stainless steels, carbon atoms tend to concentrate at the grain boundaries as an impurity during solidification. Chromium carbides can form adjacent to the grain boundaries during welding and heat treatment. When these compounds form, the chromium is removed from the alloy adjacent to the grain boundaries and the resulting alloy does not have enough chromium content to remain passive. Again, there is a very unfavourable anode/cathode area ratio and rapid attack can occur. Three different methods are used to avoid this type of attack in stainless steels during welding or other heating.
a. The first method to avoid sensitization is through heat treatment. At high temperatures (above 1,800°F), chromium carbides are unstable and will redissolve if they have formed. At low temperatures, (below 1,000°F) the chromium and carbon atoms cannot move and formation of chromium carbides is prevented. Formation of the chromium carbides is a problem primarily in the ranges of 1,100 to 1,600°F. When welding stainless steel, some area adjacent to the weld is likely to reach this temperature range long enough to form amounts of chromium carbides. When this occurs, or when the alloy is otherwise sensitized, it should be heated to temperatures above 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 avoid sensitization in stainless steels is to reduce the carbon content of the alloy to very low levels. These low carbon grades (such as 304 L and 316 L; L stands for low carbon) do not have enough carbon to form carbides and is thus resistant to sensitization during welding. Care must be taken, however, to not introduce additional carbon during welding from contamination, such as can be caused by oil or grease.
c. The third method used to avoid sensitization in the stainless steels is to intentionally add an element that will combine with the carbon but is not required for passivity of the alloy. Titanium and niobium have a greater affinity for carbon than chromium. They are added to the alloy during manufacture in amounts to combine with all of the carbon present in the alloy and thus inhibit sensitization. Type 321 stainless steel contains titanium and Type 347 stainless steel contains niobium. These alloys, or the low carbon grades, should be used when welding without heat treatment is required.
Aluminum alloys are susceptible to intergranular attack, usually the type that is caused by segregation of impurities at the grain boundaries. In addition to the stainless steels, some nickel alloys are also subject to sensitization and subsequent intergranular attack.
Intergranular attack caused by high grain boundary energies or impurities at the grain boundaries results in attack with a grainy residue and rough surface. Under high magnification, the individual grains are often visible. Intergranular attack of aluminum alloys is associated with pitting or other localized attack. Sensitization in stainless steels has a similar grainy appearance. When caused by welding it is often localized in narrow bands adjacent to the weld and is sometimes called “knife line attack.”
Microscopic examination of sectioned samples is often required to verify that intergranular attack has occurred. There are several standardized methods for determining the resistance of stainless steels to sensitization.
Source : "Corrosion Control" NAVFAC MO-307 September 1992
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