The effect of phosphorus on the passivation behavior of Ni-10Ta-P alloys in 12 M HCl

The corrosion behavior of Ni-10Ta-P alloys in 12 M HCl solution was investigated. An increase in the alloy phosphorus content improves the corrosion resistance of the Ni- 10Ta-P alloys. Ni-10Ta-15P and Ni-10Ta-20P alloys with sufficient phosphorus additions are spontaneously passivated due to the formation of a stable passive film in which tantalum ions are accumulated, comprising more than 70% of cations just after immersion. The composition and thickness of the film on these alloys are almost unchanged during prolonged immersion. By contrast, Ni-10Ta-5P and Ni-10Ta-10P alloys with insufficient phosphorus additions are not passivated spontaneously and continue to dissolve actively. Thus, a tantalum-enriched film cannot be formed for initial short periods of immersion. A large amount of dissolution of Ni-10Ta-5P and Ni-10Ta-10P alloys results in accumulation of tantalum ions on the alloy surface, and tantalum species precipitate and are concentrated in corrosion products on the alloy surface. Accordingly, the concentration of tantalum ions in the corrosion product film is increased slowly to more than 90% of cations, which is significantly higher than that in the stable passive film formed on the alloys with high phosphorus additions.