Fabrication of low-cost Ni-P composite coating on Mg alloys with a significant improvement of corrosion resistance: Critical role of mitigating the galvanic contact between the substrate and the coating

Highlights: • A low-cost Ag activation process was proposed. • The intermediate layer was fabricated to mitigate the galvanic contact. • A pre-plating in alkaline bath and a secondary plating in acidic bath were proposed. • The corrosion resistance and adhesion strength were both significantly improved. High galvanic corrosion susceptibility and expensive Pd-activation process are two critical obstacles for the application of electroless nickel (EN) plating on Mg alloy. In this work, the plasma electrolytic oxidation (PEO) coating was sealed by chemical conversion post-treatment to obtain an intermediate layer with high electrical resistance, aiming to minimize the susceptibility of galvanic corrosion. A low-cost Ag-activation technique was proposed to catalyze the EN plating process. Results indicate a dramatic improvement of corrosion resistance of EN composite coating. In addition, the composite coating also exhibits high adhesion, hardness, electrical conductivity and low magnetism.