The effect of phosphorus addition on the corrosion behavior of arc-melted Ni-10Ta-P alloys in 12 M HCl

The corrosion resistance of arc-melted Ni-10Ta-P alloys containing 0, 10 and 20 at% phosphorus in 12 M HCl solution at 30°C was investigated. The alloys containing 0 and 10 at% phosphorus suffer severe corrosion. The addition of 20 at% phosphorus to crystalline Ni-10Ta alloy results in a three-orders-of-magnitude decrease in the corrosion rate. The open circuit potentials of the Ni-10Ta alloys containing O and 10 at% phosphorus stay almost constant in the active region of nickel, while the open circuit potential of the Ni-10Ta-20P alloy increases almost linearly in the initial 2 h. The Ni-10Ta alloy consists of intermetallic Ni₈Ta and immersion in 12 M HCl results in faceting dissolution. Ni-10Ta-10P alloy is composed of major Ni₈Ta and Ni₃P phases and minor Ni₂Ta and Ni₂P phases. Immersion of Ni-10Ta-10P alloy leads to preferential dissolution of the Ni₈Ta phase and to continuous thickening of the corrosion product film consisting mostly of tantalum as cations. Ni-10Ta-20P alloy consists of Ni₂Ta, Ni₃P, Ni₂P and NiP phases. Immersion of Ni-10Ta-20P alloy gives rise to initial increase in elemental phosphorus on the surface as a result of selective dissolution of nickel and selective oxidation of tantalum. The formation of elemental phosphorus with a high cathodic activity is responsible for the initial ennoblement of the open circuit potential and for the formation of the passive film in which tantalum is highly concentrated. The higher corrosion resistance of Ni-10Ta-20P alloy than Ni-10Ta-10P alloy is attributable to the formation of the Ni₂Ta phase with a higher tantalum content than the Ni₈Ta phase which is the readily corroded major intermetallic phase in the Ni-10Ta-10P alloy.