Corrosion and wear behavior of electroless Ni-P-Cu coatings containing 8 and 18 wt% Cu

Electroless Ni-P-Cu nanocrystalline coatings, which contained 8 and 18 wt% Cu along with approximately 11 wt% P content, were deposited onto a mild steel substrate. This deposition process involved adjusting the Ni/Cu ratio in a constant Cu content bath. The bath included NiSO₄·6H₂O (12.5 or 6.25 g/L), CuSO₄ (0.05 g/L), and NaPO₂H₂·H₂O (20 g/L) as the sources of Ni, Cu, and reducing agent, respectively. The deposition occurred at pH 7.5 and a temperature of 80 °C with slight agitation. The deposition rate of Ni-8 wt% Cu-10.5 wt% P and Ni-18 wt% Cu-11.5 wt% P coatings was measured to be approximately 16 μm/h and 26 μm/h, respectively. The investigation showed that the coatings exhibited a nodular morphology with nearly similar nodule size and crystallite size. The microhardness of the coatings decreased from 408 ± 10 HV to 374 ± 8 HV with increasing Cu content while both coatings exhibited higher hardness than the bare substrate (140 ± 3 HV). Corrosion investigations using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods were conducted in a naturally aerated 3.5 wt% NaCl solution at ambient temperature under stagnant conditions. The results demonstrated that the coatings exhibited better corrosion resistance compared to the substrate. The corrosion current density of the coatings increased from 0.65 μA.cm−2 to 1.4 μA.cm−2 with increasing the Cu content of the coatings from 8 to 18 wt%. Deposition of Ni-Cu-P coatings significantly improved the wear resistance of mild steel substrate and the coating containing 8 wt% Cu showed a lower wear rate, attributed to its higher hardness.