クロムの硫酸溶液中での耐食性に及ぼす表面窒化の影響

抄録

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.