Characterization and corrosion resistance of duplex electroless Ni-P composite coatings on magnesium alloy

Three different types of electroless nickel (EN)-phosphorus plating were applied on AZ31 wrought magnesium alloy. The first type was plain mid-phosphorus coating, the second one was duplex, with the first layer having a mid-phosphorus content and the upper one consisting of high-phosphorus whereas the third type of coating was also duplex with a first mid-phosphorus layer and a second high-phosphorus one with incorporated ceramic TiO2 and ZrO2 microparticles. Characterization of deposits by means of scanning electron microscopy and X-ray diffraction analysis proves the production of adherent, defect free coatings with differences in crystallinity depending on P content. Surface roughness was maintained at acceptable levels while a great increase in microhardness was observed that was further enhanced by ceramic microparticles incorporation. Electrochemical testing, that was performed by Tafel polarization in 3.5% NaCl solution, show that all the coatings effectively protect the Mg alloy substrate. Additionally, the composite coatings exceptionally enhance the protective capability of the system after heat treatment at 200 °C.