Effects of nanocrystalline heterogeneity on the corrosion behavior of sputter-deposited chromium-niobium alloys

The nanocrystalline heterogeneity has been introduced into sputter-deposited Cr-Nb alloys by heat treatment in order to clarify the effect of alloy heterogeneity on the corrosion resistance of the alloys. The as-deposited amorphous Cr-Nb alloys are spontaneously passive in 6 and 12 M HCl and show higher corrosion resistance than chromium and niobium. The heat treatment leads to the formation of two phase mixtures of bcc Cr and Cr₂Nb, Cr₂Nb and niobium-rich amorphous phase, Cr₂Nb and bcc Nb, and chromium-rich amorphous phase and bcc Nb, depending on the alloy composition and the heat treatment conditions. It has been found that the Cr₂Nb phase has high corrosion resistance, comparable to that of the amorphous alloys. The formation of nanocrystalline bcc Cr or bcc Nb phases results in the increase in the initial passive current density. However, the long time potentiostatic polarization has revealed that the steady state anodic current density of the heat-treated alloys is similar to that of the as-deposited alloys as a consequence of preferential dissolution of less corrosion-resistant phases. Accordingly, the high corrosion resistance of the amorphous Cr-Nb alloys is sustained even after the introduction of nanocrystalline heterogeneity. (C) 2000 Elsevier Science Ltd. All rights reserved.