Intergranular corrosion of non-sensitized austenitic stainless steels in nitric acid

Intergranular corrosion of non-sensitized austenitic stainless steels in nitric acid. Austenitic stainless steels can suffer intergranular corrosion (IGC) in nitric acid even though they have a low carbon content and they are not sensitized (by welding, for example). Under high oxidizing conditions (high temperature, high nitric acid concentration and with the presence of other oxidizing species) the corrosion potential of stainless steels is increased to the transpassive zone. In this domain the grain boundaries are preferentially attacked. Grooves are formed between the grain and their progression leads to grains drops. Because IGC is of heterogeneous nature the calculation of the steel degradation kinetics is not easy. For this particular form of IGC a model is proposed to estimate the life duration of these materials. The IGC is described with two different corrosion rates: V(j), the local corrosion rate at the grain boundaries, and V(s), the corrosion rate on the grain surface. V(s) and V(j) are bounded with the morphology of the grooves (angle and deepness). Supposing that V(j) and V(s) are not time-dependent the model reproduces the evolution of the IGC morphology and of some parameters such as the thickness affected by the IGC, the mass loss (expressed in terms of equivalent homogeneous thickness loss) and the relative surface in contact with the corrosive medium. The methodology is as follows. The corrosion rates V(j) and V(s) are estimated with a short corrosion test by characterizing the groove morphology. Then the model can extrapolate the long-term behavior of the IGC. The calculations of the model were successfully compared to some long-term corrosion experiments.