The Effect P Content of High and Low Temperature Heat Treatment on the Properties of Ni-P-GO Electroless Nanocomposite Coating on Magnesium Alloy

In this study, Ni-P-GO nanocomposite coating was applied on the AZ31D surface by the electroless method. After coating, the crystallization process was carried out at 250 °C and 500 °C temperatures for one hour. As a result, microstructural studies by X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed that a coating with the semi-amorphous cauliflower morphology formed on the surface. Moreover, after the crystallization process not only the morphology was preserved and the coating colony experienced growth, but also the intermediate phase of nickel phosphide was formed. Therefore, the results showed that as P content increases, the tendency to absorb GO nanoparticles increases. At low temperatures, the Ni2P phase was initially formed. However, as the P content and the crystallization temperature increased Ni2P transformed to Ni3P. As a consequence, this improved the hardness due to both precipitation of the intermediate phases and the crystallization process. As the P increased at 500 °C, the energy needed for forming unstable Ni5p < sub>2 and Ni8p < sub>3 phases was provided. The results of the polarization test showed that applying the coating in comparison with the substrate leads to an increase in corrosion resistance. Besides, increasing P in the field increases the density of the hypophosphite layer which leads to an improvement in corrosion resistance. Corrosion resistance diminished as heat treatment was conducted. Also, as the temperature of the heat treatment increased, 0.424 A/cm2µ passed the surface, which is still lower than 1.328 A/cm2µ, this is specific to the uncoated substrate