Synergistic role of vanadate pretreatment and electroless Ni–P deposition in mitigating corrosion of AZ91D

Abstract

Magnesium alloy AZ91D is highly prone to rapid degradation in chloride environments due to its high electrochemical activity, limiting its structural applications. In this study, a heat-assisted vanadate conversion pretreatment is developed to engineer the substrate–coating interface and enhance the corrosion resistance of electroless nickel–phosphorus (Ni–P) coatings. The influence of thermal modification on surface passivation and Ni–P nucleation was systematically investigated. Ni–P coatings were deposited under optimized conditions determined via Taguchi design and characterized using electron microscopy, x-ray diffraction, x-ray Photoelectron spectroscopy and electrochemical techniques. The heat-assisted pretreatment produced a dense and oxygen rich chemically stable interfacial layer, promoting uniform nucleation and the formation of a compact coating with a thickness of ∼27–30 µm and a phosphorus content of ∼9.27 wt.%. Electrochemical analysis revealed a significant positive shift in corrosion potential and a reduction in corrosion rate by nearly two orders of magnitude compared to the untreated substrate. Impedance studies further indicated enhanced interfacial resistance and improved barrier properties. The improved performance is attributed to the synergistic effect of the stabilized vanadate layer and the compact, partially amorphous Ni–P coating, which effectively suppress electrolyte penetration and magnesium dissolution.