Mechanism

Preferential local attack at grain boundaries in polycrystalline metals arises due to the higher internal energy of the grain boundary regions. This is enhanced by the segregation of impurities to the boundaries and by the precipitation of second phases which may be more noble and which may also lower the resistance of the surrounding matrix by denudation. The extent of intergranular corrosion will depend on the level of sensitisation and the aggressiveness of the corrosive environment.

In austenitic stainless steel sensitisation due to grain boundary precipitation of Cr carbides can occur on heating in the temperature range 450-900°C, for example during annealing or stress relieving, at service in this range or during welding, when it is called Weld decay (see figure 1).

Figure 1. Schematic views of intergranular corrosion in austenitic stainless steel, for example weld decay
Sensitisation

Sensitisation can be reduced by use of very low carbon grades and by stabilisation by the addition of titanium or niobium. These elements have a greater affinity to form carbides than chromium, hence any carbide precipitation that occurs will not remove Cr from the matrix.