Comportement anodique des alliages Nb–Co en milieu alcalin

Abstract

Niobium–cobalt alloys are stable to corrosion in an open circuit in the presence of oxygen in 25% KOH solution as long as the weight percentage of cobalt exceeds 50%. Below this value, samples submitted to these conditions disintegrate after 30 days. When these alloys are submitted to anodic polarization an active dissolution is noted followed by passivation and in the presence of cobalt production of oxygen occurs. Electronic Auger Spectroscopic (EAS) analysis of pure niobium and its alloys with 30% and 90% cobalt indicates that the oxide film grows in an irregular manner according to the nature of the surface. Thus, for an identical anodic polarization (0.845 V vs. ENH for 6 hours) the oxide film changes from 4.13 × 10−2 μm for pure Nb to 0.10 μm for the 30% Co–Nb alloy and becomes as thin as 8 × 10−4 μm for the 90% Co–Nb alloy. The concentration profile as shown by EAS indicates a preferential dissolution of Nb over Co in the oxide film. [Journal translation]