An XPS Investigation into the Passivity of an Amorphous Ni‐20P Alloy

Anodic polarization experiments in and electrolytes show that amorphous Ni‐20P alloy passivates at potentials between −0.3 and about 0.2V (SCE) and dissolves transpassively above 0.2V (SCE). In , in which crystalline Ni pits readily, Ni‐20P resists pitting and passivates as effectively as in . Analysis by x‐ray photoelectron spectroscopy of Ni‐20P surfaces after polarization indicates an enrichment of elemental P on the alloy surface, the presence of hypophosphite anions and the absence of oxidized nickel species. Thus, the Ni‐20P alloy does not passivate by development of a three‐dimensional oxide film, such as the film that forms on nickel. A chemical passivation process is proposed that is controlled by the formation and surface retention of hypophosphite anion, which acts as a barrier between the alloy and the electrolyte.