The austenitic stainless steels when exposed to a range of boiling 2 Normal sulfuric-nitric acid solutions pass through a passive zone of negligible corrosion, a transition zone of irregular corrosion and two active zones of substantial corrosion. The corrosion rates in the passive and active zones were as anticipated while the rates in the transition zone were entirely unexpected. A plot of the corrosion rates in the transition zone revealed an unusual hump in the curve whose maximum occurred at a critical concentration.
From spectrographic analyses the corrosion rates or iron, chromium and nickel also were determined. A plot of these rates indicated that the hump in the transition zone of the corrosion curve was attributed to excessive corrosion of chromium in this zone. The percentage of the total corrosion attributable to iron was extremely high and above 99 percent in all the mixed acid solutions. In the transition zone there was an increase of chromium and nickel corrosion with a corresponding decrease of iron corrosion. 6.2.5
