Electroless Ni–P layer with a chromium-free pretreatment on AZ91D magnesium alloy

A phosphate-manganese conversion film was proposed as the pretreatment layer between Ni–P coating and AZ91D magnesium alloy substrate, to replace the traditional chromium oxide plus HF pretreatment. The subsequent Ni–P deposited on the layer was also characterized by its structure, morphology, microhardness and corrosion-resistance. The pretreatment layer on the substrate not only reduces the corrosion of magnesium during Ni–P plating process, but also reduces the potential difference between the matrix and the second phase. Thus, a Ni–P coating with fine and dense structure was obtained on the AZ91D magnesium alloy, which shows better corrosion resistance than the Ni–P with chromium oxide plus HF as pretreatment.