INTERGRANULAR CORROSION

A serious type of corrosion is created when the attack is against the grain boundaries. Following the grain boundaries from the metal surface, a crack-like discontinuity develops. The attack is usually related to the segregation of specific elements or the formation of a compound in the boundary. Corrosion then occurs by preferential attack on the grain-boundary phase, or in a zone adjacent to it that has lost an element necessary for adequate corrosion resistance - thus making the grain boundary zone anodic relative to the remainder of the surface. The attack usually progresses along a narrow path along the grain boundary and, in a severe case of grain-boundary corrosion, entire grains may be dislodged due to complete deterioration of their boundaries. In any case the mechanical properties of the structure will be seriously affected. Such cracks can cause material failure under static loading by reduction of load supporting cross-section. In the case of dynamic loading, they are likely to be the beginning source of fatigue failure.

Because those cracks are rarely visually apparent on the surface, NDT may be called upon for their detection.

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.” The problem of knife-line attack can be corrected by reheating the welded metal to allow diffusion to occur.