Electroless nickel-phosphorus plating on AZ31 magnesium alloy pretreated with a chemical conversion coating

In order to further improve the erosion resistance and hardness of conversion coatings on AZ31 samples an electroless nickelphosphorus layer was successfully deposited on the chemical conversion pre-treated AZ31 alloy. The electroless bath contained nickel sulphate and sodium hypophosphite as main salts and the surface of the samples was activated by a 5 g/L AgNO3 solution before the electroless plating process. The conversion coating between the nickel-phosphorus coating and the AZ31 substrate acted as a barrier and prevented corrosion of the alloy. The morphology of the nickel-phosphorus layer was studied using SEM. The crystal structure and the composition were examined by XRD and EDS and the hardness was measured. The corrosion behaviour of the nickel-phosphorus layer deposited on the conversion coating was studied by potentiodynamic polarization in a 0.5M NaCl solution using a saturated standard calomel electrode as a reference electrode. The results show that the nickel-phosphorus coatings had a significantly better corrosion resistance than the AZ31.