Process of Ni - P Double - Layer Electroless Plating on Magnesium Alloy and Properties of Coating

Abstract

In order to improve the hardness and corrosion resistance of AZ91D magnesium alloy, a double - layer Ni - P coating was prepared on its surface by electroless plating without hydrofluoric acid, and the optimal double - layer process was determined by characterizing the microstructure, composition, microhardness and corrosion resistance of the coating through the utilization of metallographic microscope, scanning electron microscope (SEM), X - ray diffractometer (XRD), Energy dispersive spectrometer (EDS), microhardness tester and electrochemical workstation. Results showed that the coating obtained by first alkaline and then acid double - layer electroless plating process adhered well to the substrate, and possessed a thickness of 26.28 μm, a uniform and dense surface, and a linear roughness Ra of 0.623 μm. The obtained coating hardness by this process was 550.54 HV, which was 7.9 times higher than that of the substrate. Moreover, the inner layer with a P content of 3.209% had a crystal structure and crack holes, and the outer layer with a P content of 9.713% had few amorphous structure defects, which could cover the inner defects to improve corrosion resistance. Besides, compared with the substrate, the corrosion potential of the coating prepared using this process shifted positively by 719 mV and the self - corrosion current density was reduced by 2 orders of magnitude.

Keywords

• az91d magnesium alloy|ni - p coating|double - layer electroless plating|microstructure and properties