Characterization of surface of amorphous Ni-Nb-Ta-P alloys passivated in a 12 kmol/m3 HCl solution

Abstract

The melt-spun amorphous (Ni60Nb40-xTax)0.95P5 alloys containing 15 at% or more tantalum are spontaneously passivated in a 12 kmol/m3 HCl solution open to air at 303 K. Their open circuit potentials are nobler than that of pure tantalum metal. The amorphous alloys with 10 at% or less tantalum, however, show the active-passive transition. XPS analysis revealed that the spontaneous passive films are rich in tantalum and niobium cations and deficient in nickel cation. Thickness of the spontaneously passivated film is much thinner than that of less protective film on low tantalum-bearing alloys for prolonged immersion, indicating the passive film is highly dense and stable. Thus the high corrosion resistance of the alloys with high tantalum content is attributed to the formation of highly stable and protective passive film enriched mainly with tantalum cation.