A study of chromium-free pickling process before electroless Ni–P plating on magnesium alloys

A chromium-free pickling process of magnesium alloys in H3PO4 + Na2MoO4 solution for electroless Ni–P plating was described. The dosage of Na2MoO4 was established by detecting adhesion and corrosion resistance of chemical nickel coatings. Electrochemical behaviors of pickling solution of H3PO4 + Na2MoO4 and NH4HF2 activation solution were investigated with the open circuit potential curves and the polarization curves. The results show that the Na2MoO4 has strong inhibition ability. Na2MoO4 in H3PO4 solution can reduce active sites of microcathodic and microanodic zones in the corrosion cells of the substrate surface of magnesium alloys and plays an inhibition role. The activation film with some oxides and fluorides can prevent the substrate magnesium from the fierce displacement and corrosion reaction of electroless plating bath. The chemical Ni–P coating with good adhesion and corrosion resistance was obtained by the pretreatment of 200 cm3 dm−3 85% H3PO4 + 5 g dm−3 Na2MoO4 pickling bath and activation in 200 g dm−3 NH4HF2 solution. This procedure of surface pretreatment before electroless nickel plating can replace the existing acid pickling containing chromium and HF activation.