The corrosion resistance of electroless deposited nano-crystalline Ni–P alloys

Coatings of Ni–P alloys are increasingly used for their shiny appearance, the high degree of hardness and very good corrosion resistance. Electroless coatings are preferred in industry compared to the electroplated ones because they adhere well to different materials and form a homogeneous coating even on complicated geometries. Electroless Ni–P alloys produced as a coating on technical iron in a commercial autocatalytic hypophosphite-type plating bath were studied, with regard to their corrosion behaviour upon immersion in near neutral sulphate and chloride electrolytes. The anodic polarization curves of the alloys showed a current plateau at potentials E < +0.2 V SCE confirming their high corrosion resistance. The dynamic cathodic polarization curves in deaerated solutions showed a Tafel behaviour with a slope of ca. −0.4 V, indicating inhibition of oxygen reduction on “as received” and on mechanically polished surfaces. Corrosion rates of ca. 0.5–0.7 μA/cm2 were found during prolonged immersion in near neutral solutions open to air. Potentiostatic polarization at selected potentials in the range of the current plateau (−0.1 V SCE, +0.1 V SCE) showed a steady decrease of the current density following a power law with exponent −0.5, thus a diffusion controlled process. These results show that the suppression of the anodic dissolution and the low corrosion rates of Ni–P alloys cannot be associated with the oxide-film passivity. Despite this fact, black spots identified as localized corrosion appeared on mechanically polished and “as plated” electroless Ni–P deposits after prolonged potentiostatic polarization at potentials in the range of dissolution suppression.