Effect of Surface Nitriding on Corrosion Resistance of Chromium in Sulfuric Acid Solution

Abstract

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 α-Cr and hexagonal Cr2N. The product changes to a mixture of Cr2N and cubic CrN at 973∼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∼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 α-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.