EFFECT OF SURFACE NITRIDING ON CORROSION-RESISTANCE OF CHROMIUM IN SULFURIC-ACID-SOLUTION (REPRINTED FROM J JAPAN INST METALS, VOL 55, PG 204, 1991)

The nitriding behavior of chromium as a function of temperature and the corrosion resistance of the nitriding products in a sulfuric acid solution were investigated. Nitriding of chromium starts at about 873 K, and the surface product is a mixture of alpha-Cr and hexagonal Cr2N. The product changes to a mixture of Cr2N and cubic CrN at 973 approximately 1273 K, and it transforms into the single phase Cr2N by the dissociation of CrN above 1323 K. Chromium loses the ability of self-passivation with nitriding, but the surface product at 973 approximately 1273 K exhibits a low current density in the anodic polarization measurement, which is comparable with the passive current density of chromium. The mixture of Cr2N and CrN containing 33 at% nitrogen or more shows improved corrosion resistance in 1 kmol.m-3 H2SO4 at 373 K, although the mixture of alpha-Cr and Cr2N, and the single phase Cr2N are rapidly attacked. This means that the single phase CrN should have a high corrosion resistance. Cyclic voltammetry reveals that chromium nitrides dissolve mostly as Cr2+ ions. The Cr2+ ions are presumed to be oxidized to Cr3+ ions on platinum surface by the catalytic action.