Influence of Cu addition on the microstructure, and corrosion behavior of electroless Ni-Cu-P coating on squeeze-cast Al-Cu-Mg alloy

Abstract

Electroless nickel‑phosphorus based (ENP) coating on Al alloys offers superior mechanical, and tribological properties conferring resistance against corrosion. Further, the inclusion of copper in the ENP coating can improve the thermal stability and surface passivation. The present work explores the effect of a gradual increase in cupric sulfate (0.1, 0.2, and 0.5 g L⁻¹) concentration on the microstructural evolution and corrosion behavior of squeeze-cast Al-Cu-Mg alloy in NaCl solution while maintaining the nickel-phosphorous bath parameters viz., pH, temperature, and deposition time unchanged. The phase, microstructure, and tribological analysis show that 0.1 gL⁻¹ CuSO₄ addition is optimal, providing a few microns thick, smooth, and compact coating of nodular particulates demonstrating the highest microhardness (∼314 ± 7 HV_0.05) and scratch resistance (H_s ∼930 MPa). However, further increasing the Cu²⁺-ions concentration beyond 0.1 g L⁻¹ changes the morphology to a cauliflower-like nodular structure consisting of surface heterogeneities with low microhardness and high surface roughness owing to the softening of deposited Cu particles. Potentiodynamic measurements and electrochemical impedance spectroscopy results show that Ni-0.1Cu-P specimen exhibits lowest corrosion current density (i_corr ∼0.53 μA cm⁻²), and the highest charge-transfer resistance (R_ct ∼5.64 × 10⁴ Ω.cm²), among entire specimens owing to the formation of dense, granular structure with high P (∼12.34 wt%) and low Cu (∼1.5 wt%) content. The Mott-Schottky analysis of Ni-0.1Cu-P specimen confirms the formation of p-n type bipolar semiconductor passive films blocking the movement of anions with lower donor carrier density (N_d ∼1.46 × 10¹⁹) preventing the passive films breakdown and providing better corrosion resistance.