A new method for electroless Ni–P plating on AZ31 magnesium alloy

Coating Ni–P films on AZ31 magnesium alloys via electroless plating and organic coatings (organsilicon heat-resisting varnish), was studied. An organic coating was proposed as the interlayer between Ni–P coating and AZ31magnesium alloy substrate, to replace the traditional chromium oxide plus HF pretreatment. The Ni–P deposited on the interlayer was also characterized by its structure, morphology and corrosion-resistance. The interlayer 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. The result of the cross-cut test indicates the adhesion between the substrate and the interlayer is good enough. A Ni–P film with fine and dense structure was obtained on the AZ31 magnesium alloy. The electrochemical measurements show that the sample with Ni–P film exhibits lower corrosion current density and more positive corrosion potential than the substrate. Furthermore, the Ni–P coating on the AZ31 magnesium alloy exhibits high corrosion resistance in the rapid corrosion test illustrated in this paper. The method proposed in this work is environmentally friendly: no fluoride or hexacalent chromium compounds are used. In addition, it provides a new concept for plating the metals, which are considered difficult to plate due to high reactivity.