An innovative procedure of electroless nickel plating in fluoride-free bath used for AZ91D magnesium alloy

• Double-layer coating on AZ91D deposited from alkaline and acidic baths.
• Formed structure of low phosphorus crystal and high phosphorus amorphous.
• Higher corrosion resistance of double-layer coating than single Ni–P coating.
• Realized electroless Ni–P plating in fluoride-free baths.

An innovative and environmentally friendly procedure for electroless nickel plating on AZ91D magnesium alloy was studied in the present paper. The composition of the alkaline and acidic fluoride-free baths and their plating process were investigated and discussed in detail. A double-layer coating was formed on AZ91D magnesium alloy by alkaline and subsequent acidic plating. The inner alkaline layer was composed of Ni–P coating with low P content, but the outer acidic layer consisted of Ni–P coating with high P content. The morphologies, composition and phase structure of the double-layer coating were characterized with Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), and X-ray diffraction (XRD), respectively. The corrosion resistance of Ni–P coatings on the AZ91D magnesium alloy was evaluated by neutral salt spray and electrochemical polarization tests. The corrosion potential of AZ91D magnesium alloy with the double-layer coating was elevated by 1.14 V to Mg substrate. The salt spray test demonstrated that the corrosion resistance of AZ91D alloy with the Ni–P double-layer coating was greatly improved. A novel process of electroless nickel plating was successfully developed in fluoride-free baths. The environmental pollution could be significantly reduced due to discharge of fluoride.