ROLE OF PHOSPHORUS IN THE ELECTROCHEMICAL-BEHAVIOR OF ELECTROLESS NI-P ALLOYS IN 3.5 WT.PERCENT 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. elect rochemical impedance spectroscopy techniques to characterize the effec t of phosphorus on the corrosion behavior of electroless nickel-plated mild steel in neutral and alkaline 3.5 wt.% NaCl solutions. The anodi c polarization behavior of the Ni-P alloys in NaCl electrolyte was ver y similar for all deposits except Ni-4.8P. According to the electroche mical impedance data, the charge transfer resistance R(ct) and film re sistance R(f) of these deposits have subsequently been studied as func tions of the phosphorus content. It was shown that the R(ct) and R(f) values increased with increasing phosphorus content of the electroless nickel coating. The Ni-P deposits remained bright in appearance durin g the periods of electrochemical experiment, and no evidence of pittin g was observed after testing. Based on X-ray photoelectron spectroscop y analysis, the passivation of Ni-P alloys was attributed to the forma tion and adsorption of a layer of hypophosphite.