Effect of NOx gases on corrosion of stainless steel in hot nitric acid solutions

Abstract

The effect of NOx purging on the corrosion behaviour of stainless steel in hot 8M nitric acid has been investigated. Gravimetric corrosion rate measurements and electrochemical potential and polarisation data made on nitric acid grade (NAG) 18/10L stainless steel are related to the concentration of nitrous acid produced by NOx purging, which in turn is related to the solution redox potential measured using clean Pt. Purging with NOx is shown to reduce the redox potential of nitric acid solution, but in the absence of other dissolved species acts as an oxidising agent to stainless steel and increases the corrosion rate. The effect of NOx purging saturates at relatively low flow rate owing to the fact that the solution becomes saturated with nitrous acid. However, in 8M nitric acid containing 3 g L-1 Fe and 0·7 g L-1 Ru at 100°C, NOx purging has no effect on the corrosion rate of NAG 18/10L. This is considered to indicate that Fe and Ru at these concentrations polarise stainless steel to the same corrosion potential as NOx is capable of achieving.