Anodic polarization experiments in H2SO4 and normalHCl 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 0.2N normalHCl , in which crystalline Ni pits readily, Ni‐20P resists pitting and passivates as effectively as in 0.2N H2SO4 . 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 normalNiO 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.

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