The Role of P in the Anodic Inhibition of an Amorphous Co-20p Alloy in Acidic Electrolytes

Anodic polarization of an amorphous Co-20P alloy in deaerated 0.2N H₂SO₄ or 0.2N HC1 shows alloy dissolution to be inhibited at potentials between approximately -0.3 and 0.2 V (SCE), while pure crystalline Co dissolves readily and does not passivate at anodic potentials in either electrolyte. Analysis of Co-20P surfaces by x-ray photoelectron spectroscopy after anodic polarization reveals an enrichment of elemental P in the near-surface region, the presence of approximately a monolayer of hypophosphite and phosphate ions on the surface, and a virtual absence of oxidized cobalt species. Thus, the Co-20P alloy does not inhibit dissolution by the development of a three-dimensional cobalt oxide film. A chemical process that is controlled by the formation and surface retention of hypophosphite anions is proposed. An identical passivation mechanism has been previously observed with an amorphous Ni-20P alloy.