EVIDENCE OF CHEMICAL PASSIVITY IN AMORPHOUS Ni-20P ALLOY.

Anodic polarization experiments in H//2SO//4 and HCl electrolytes show that amorphous Ni-20P alloy passivates at potentials around 0 V(SCE) and dissolves transpassively above 0. 2 V(SCE). In 0. 2N HCl, in which crystalline Ni pits readily, Ni-20P resists pitting and passivates as effectively as in 0. 2N H//2SO//4. Analysis by x-ray photoelectron spectroscopy of Ni-20P surfaces after polarization indicates the presence of hypophosphite ions and the absence of oxidized nickel species. Thus, Ni-20P alloy does not passivate by development of a three-dimensional oxide film, such as the NiO film that forms on nickel. A chemical passivation process is proposed that is controlled by the formation and surface retention of hypophosphite anion.