The electrochemical behavior of electroless plated Ni-P alloys in concentrated NaOH solution

Electroless Ni-P deposits with phosphorus content ranging from 4.8-12.8 wt.% were examined using potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) techniques to characterize the effect of P on the corrosion behavior of electroless nickel-plated mild steel in deaerated 40 wt.% NaOH solution. Anodic polarization of the electroless Ni-P alloys in caustic NaOH solution shows that the passive current density decreases with increasing P content in the deposits. At an applied potential of -1.2 V vs. saturated calomel electrode (V sub SCE ) (close to their E sub corr ), EIS data indicate that the R sub ct for Ni-P alloys in NaOH solution increases with increasing P content. X-ray photoelectron spectroscopy (XPS) results suggest that the primary constituent formed on the Ni-P surface after EIS measurement in 40 wt.% NaOH solution at an applied potential of -0.4 V sub SCE (in the passive region) is Ni(OH sub 2 ), which is responsible for the passivity of the Ni-P alloys. The polarization resistance of Ni-P alloys in NaOH solution at -0.4 V sub SCE also increases with increasing P content.