Studies of Several Pickling and Activation Processes for Electroless Ni-P Plating on AZ31 Magnesium Alloy

Several chromium-free pickling and activation processes were investigated in order to develop an eco-friendly pretreatment for electroless Ni-P plating on magnesium alloy. The coatings obtained from etching only in H3PO4 solution exhibit poor adhesion due to the existance of an interlayer. Meanwhile, a corrosion model of Mg alloy in H3PO4 solution was proposed. When fluoride was added into the pickling solution, the adhesion of the coatings was also unsatisfied because of the bad mechanical interlocking action. By comparison, the coatings obtained from pickling in H3PO4 solution followed by NH4HF2 activation displayed wonderful properties in both adhesion and corrosion resistance.