Deposition Process and Properties of Electroless Ni-P-Al₂O₃ Composite Coatings on Magnesium Alloy

To improve the corrosion resistance and wear resistance of electroless nickel-phosphorus (Ni-P) coating on magnesium (Mg) alloy. Ni-P-Al₂O₃ coatings were produced on Mg alloy from a composite plating bath. The optimum Al₂O₃ concentration was determined by the properties of plating bath and coatings. Morphology growth evolution of Ni-P-Al₂O₃ composite coatings at different times was observed by using a scanning electronic microscope (SEM). The results show that nano-Al₂O₃ particles may slow down the replacement reaction of Mg and Ni²⁺ in the early stage of the deposition process, but it has almost no effect on the rate of Ni-P auto-catalytic reduction process. The anti-corrosion and micro-hardness tests of coatings reveal that the Ni-P-Al₂O₃ composite coatings exhibit better performance compared with Ni-P coating owing to more appropriate crystal plane spacing and grain size of Ni-P-Al₂O₃ coatings. Thermal shock test indicates that the Al₂O₃ particles have no effect on the adhesion of coatings. In addition, the service life of composite plating bath is 4.2 metal turnover, suggesting it has potential application in the field of magnesium alloy.