Effect of the Potential of Etching Treatment and Passivation Treatment on the Stability of Passive Stainless Steels

Stability of the passive 18-8 steel in acid solution is found to depend on the potential during the etching treatment as well as the passivation treatment. The potential of the steel during the passivation treatment is controlled by changing the concentration of nitric acid solution or by using a potentiostat. Also a constant potential of the steel during the etching treatment is attained in a dilute nitric acid solution or in a concentrated sulphuric acid solution with or without applying the external polarization. Stability which is decided by measuring the self-activation time in oxygen-free sulphuric acid solution increases with increasing the potential of the passivation treatment, and a critical potential, at which the stability changes abruptly, is admitted irrespective of the method of the passivation treatment, i. e., chemical passivation or potentiostatically controlled passivation. Structural changes of the passive film are concluded to take place at this critical potential of 0.4 volt (vs. SCE). Etching potential, also, changes the self-activation time of the passive steel treated at a constant passivation condition. The maximum stability is obtained at -0.32V of the etching potential. This fact is explained by assuming the selective enrichment of chromium on the surface bofore passivation.