Effect of pH and Heat Treatment on the Characteristics of Ni-P-GO Nanostructured Composite Coating on AZ31D Alloy

In the present study, Ni-P-GO nanocomposite coating with different pH values of path was applied on the AZ31D alloy by electroless. After coating, heat treatment was performed at 400°C for 1 h. The results of microstructural investigations by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) showed that the coating with semi-amorphous structure and cauliflower morphology was formed on the surface of the substrate. According to the EDS results, with the increase in the pH of the electroless bath, the amount of phosphorus coating and absorption of graphene oxide (GO) nanosheets increased. After the heat treatment, maintaining the morphology and growth of colonies, the coating was completely crystallized and nickel phosphide compounds such as Ni2P, Ni3P and Ni5P3 were formed, which increased the hardness. The microhardness test results showed that the hardness and toughness of the coating decreases with the increase of phosphorus. By heat treatment, the maximum hardness reached 1151 H.v. and the toughness decreased to 2.3 (MPa/√m). The results of the polarization test showed that the coating leads to an increase in the corrosion resistance of the substrate and a decrease in the corrosion density up to 0.443 µA/cm². Although heat treatment led to a decrease in corrosion resistance compared to before. Also, increasing the phosphorus of matrix to increase the density of the hypophosphite layer and absorb more GO leads to an increase in corrosion resistance.