Effect of Additives and Heat Treatment on the Formation and Performance of Electroless Nickel-Boron Plating on AZ91D Mg Alloy

The effect of thiourea (TU) and saccharin (SAC) on the deposition rate, surface morphology, boron (B) content, micro-hardness, and corrosion resistance of an electroless Ni-B plating layer on AZ91D magnesium alloy was investigated. The electroless Ni-B plating layer was characterized by scanning electron microscopy (SEM), electron probe micro-analysis (EPMA), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), Vickers hardness and electrochemical techniques in 3.5 wt% NaCl aqueous solution. The results showed that TU and SAC had different effects on the morphology and a similar influence on the particle size of the plating. TU slightly decreased the B content in the coating, while SAC had no effect. The two additives reduced the Ni-B plating rate and enhanced the corrosion resistance of the coating. Appropriate amounts of the additives improved higher micro-hardness of the plating. Elements diffusion occurred between substrate and coating during heat-treatment. After heat-treatment the deposited coating layer transformed from an amorphous structure to crystalline.