The Anodic Behaviours of Stainless Steels in Nitric Acid Solution and the Effects of Addition of Halogen Ions to It.

Abstract

Studies were carried out on the anodic behaviours of stainless steels in HNO₃ containing halogen ions.
The results obtained were as follows:
(1) In HNO₃ solution containing halogen ions, the critical current density in active state was larger than in H₂SO₄ solution.
(2) Of the halogen ions added to HNO₃ solution, the one that had the strongest action in preventing the passivation of metallic Ni was found to be Br^- ion, while for Fe, Fe-Cr, Fe-Ni and Fe-Cr-Ni alloys it was Cl^- ion.
(3) The oxidation action of HNO₃ made Ni and Fe-Ni alloys depolarize, and, thereby, the anodic dissolution of Ni and Fe-Ni was made much easier than that in H₂SO₄ solution. But Fe-Cr alloys which become passive easily were found to become passive as soon as they are set as alloy anodes.
While in the passive state, the oxidation action of HNO₃ had the two effects: one makes the passive film steady and the other inhibits the indirect dissolution reaction as Cr⁶⁺→Cr³⁺+3e^-by Cl^- ion.