Determination of the long-term intergranular corrosion rate of stainless steel in concentrated nitric acid

Stainless steels with low carbon content and free from any precipitation undergo intergranular attack in hot nitric acid. The corrosion rate measured by weight loss requires prolonged immersion testing to reach the apparent steady state corrosion, which coincides with the onset of grain dropping. A more appropriate method for predicting the long-term penetration rate is described in this study. A close observation and a statistical analysis of the attack grooves were firstly undertaken using immersion testing. The major findings are an outstanding morphology of the grooves with flat planes and preserved angle even after the onset of grain dropping, as well as a constant rate of the penetration into the surface. The formation of the grooves could then be represented by a geometrical model put forward by Beaunier and co-workers. Consequently, the method proposed for predicting the penetration rate consists in measuring the depth and the angle of the grooves obtained in short time immersion testing. Multiplying the penetration rate calculated from the previous data by the ratio between the penetration depth and the length of the grain boundary path does give an accurate long-term penetration rate. The method has been shown to apply successfully to AISI 304L stainless steel in several nitric solutions.