4.7.1 DEFINITION

It has been defined commonly as a form of localized attack on the grain boundaries of a metal or alloy in corrosive media, which results in the loss of strength and ductility. The localized attack may lead to dislodgment of the grain. It works inwards between the grains and causes more loss of strength than the same total destruction of metal uniformly distributed over the whole surface. The attack is distributed over all the grain boundaries cutting the surface. Intergranular corrosion is less dangerous than stress corrosion, which occurs when stress acts continuously or cyclically, in a corrosive environment, producing cracks following mostly intergranular paths. The difference between the two, i.e. intergranular corrosion and stress corrosion cracking is important. There are many materials which are susceptible to intergranular corrosion and not to stress corrosion cracking. The attack is very common on stainless steel, nickel and aluminum alloys.

Metals are crystalline materials. They consist of grains. As corrosion proceeds along the grain boundaries, the grains become weaker particularly at the grain boundaries and they eventually disintegrate (Fig. 4.26).

Figure 4.26: SEM micrograph of grain boundary corrosion in HAZ of 316L SS Weldment ( 900)

All metals and alloys are joined together by grain boundaries. The intergranular corrosion...