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

Abstract

A chromium-free pickling process of magnesium alloys in H 3PO 4 + Na 2MoO 4 solution for electroless Ni–P plating was described. The dosage of Na 2MoO 4 was established by detecting adhesion and corrosion resistance of chemical nickel coatings. Electrochemical behaviors of pickling solution of H 3PO 4 + Na 2MoO 4 and NH 4HF 2 activation solution were investigated with the open circuit potential curves and the polarization curves. The results show that the Na 2MoO 4 has strong inhibition ability. Na 2MoO 4 in H 3PO 4 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 cm 3 dm −3 85% H 3PO 4 + 5 g dm −3 Na 2MoO 4 pickling bath and activation in 200 g dm −3 NH 4HF 2 solution. This procedure of surface pretreatment before electroless nickel plating can replace the existing acid pickling containing chromium and HF activation.