Microstructures and corrosion resistance of conversion film-electroless Ni-W-P ternary coatings on AZ91D magnesium alloy

Ni-W-P ternary coating was synthesised on AZ91D magnesium alloy substrates by an integrated method. The corrosion resistance mechanism of conversion film-electroless Ni-W-P coatings on a substrate has been examined. Results revealed that the corrosion resistance of Ni-W-P coating with a phytic acid conversion film was better than that with a stannate film. The surface morphologies of Ni-W-P coatings were of an amorphous cellular structure, uniform and dense. Moreover, it was observed that the corrosion rate of Ni-W-P coating with a phytic acid conversion film heat-treated at 200°C reached the minimum of 0.0345 g m−2·h−1, the self-corrosion potential was −0.335 V, and the capacitive arc radius from electrochemical impedance spectroscopy testing was the largest, which revealed that the corrosion resistance of these Ni-W-P coatings was the best. The results were ascribed to the formation of a stable chelate on the substrates after phytic acid pretreatment, which improved the bonding strength between the Ni-W-P coatings and substrates.