Abstract
The nitriding behavior of chromium in nitrogen gas at 773-1473 K and the corrosion resistance of the nitriding products in sulfuric acid solution have been investigated. The main results obtained are summarized as follows.
(1) Nitriding of chromium starts at about 873 K and the surface product was the mixture of α-Cr and Cr2N. The product changed to the mixture of Cr2N and CrN at 973-1273 K. It transformed into the single phase, Cr2N by the dissociation reaction of CrN above 1323 K.
(2) Chromium loses the ability of self-passivation by nitriding. However, surface products at 973-1273 K showed good corrosion resistance which was comparable with passivation films. The products also dissolved as Cr2O72− ions at potential higher than 1.0 V vs. Ag/AgCl.
(3) The mixture of α-Cr and Cr2N, and the single phase of Cr2N were rapidly attacked in 1 kmol·m−3 H2SO4 at 373 K. On the other hand, the mixture of Cr2N and CrN, which contained 33 at% nitrogen or more, showed improved corrosion resistance. This means that the single phase of CrN should have high corrosion resistance.
(4) Cyclic voltammetry showed chromium nitride dissolved mostly as Cr2+ ions. It was presumed that Cr2+ ions were then oxidized to Cr3+ ions on the catalytic platinum surface.
