A novel palladium-free surface activation process for electroless nickel deposition on micro-arc oxidation film of AZ91D Mg alloy

In this paper, a novel palladium-free and environmentally friendly surface activation process for electroless Ni plating on micro-arc oxidation (MAO) film of AZ91D Mg alloy was studied. A tight, dense and continuous structure of Ni-B/Ni-P coating was obtained on micro-arc oxidation film of AZ91D Mg alloy followed by an immersion into silver ion ink activation solution. Compared with bare AZ91D magnesium alloy, the corrosion potential of the composite coating increased by 1.2 V, while the corrosion current density decreased by two orders of magnitude. Electrochemical impedance spectroscopy (EIS) results showed that the corrosion resistance of AZ91D magnesium alloy was significantly improved by the composite coating. The MAO film as an intermediate layer could reduce the galvanic corrosion between Ni and Mg substrate in 3.5 wt.% NaCl solution. This novel method is a simple, acid-free and cost effective activation process for a replacement of Pd activation method in electroless plating Ni process.