Role of phosphorus in the electrochemical behavior of electroless Ni-P alloys in 3.5 wt.% NaCl solutions

Electroless Ni-P deposits with phosphorus contents ranging from 4.8 to 12.8 wt.% were examined using d.c. polarization curves and a.c. electrochemical impedance spectroscopy techniques to characterize the effect of phosphorus on the corrosion behavior of electroless nickel-plated mild steel in neutral and alkaline 3.5 wt.% NaCl solutions. The anodic polarization behavior of the Ni-P alloys in NaCl electrolyte was very similar for all deposits except Ni-4.8P. According to the electrochemical impedance data, the charge transfer resistance Rct and film resistance Rf of these deposits have subsequently been studied as functions of the phosphorus content. It was shown that the Rct and Rf values increased with increasing phosphorus content of the electroless nickel coating. The Ni-P deposits remained bright in appearance during the periods of electrochemical experiment, and no evidence of pitting was observed after testing. Based on X-ray photoelectron spectroscopy analysis, the passivation of Ni-P alloys was attributed to the formation and adsorption of a layer of hypophosphite.