Structure And Corrosion Resistance Of Nickel Plated Coating On Surface Of MAO Coated AZ31Magnesium Alloy

In order to improve the comprehensive properties of MAO (Micro-arc Oxidation) coated AZ31magnesium alloy, nickel plated coating was used as the top layer by electroless plating technique. Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), energy dispersed X-ray spectroscope analyses (EDS), electrochemical techniques and immersion test were used for the analyses of surface morphology, structure, chemical compositions and corrosion resistance of each coating system.The new process of electroless nickel plating on the surface of MAO coated sample was investigated. The pre-activation process was carried out by immersion of the substrate in the γ-APS solution. The silane was linked substrate alloy by Si-O-Mg and the activation process was achieved by the chemisorption of-NH2to Pd2+. Then electroless plating was carried out after reduction without commonly sensitization process using the toxic Sn2+ion, which made the process become more greener.The surface morphology, structure and corrosion resistance of nickel plated samples before and after sealing treatment by BTESPT were studied. Silane post-treatment could improve its corrosion resistance markedly. Compared to the sample before sealing treatment, the corrosion current density was dropped by50%, the corrosion potential was promoted50mV and the value of RP was increased by100%.The surface and cross-section morphologies, corrosion resistance of sample with MAO/epoxy resin/Ni-P coating were investigated. The epoxy coating was used as interlayer between MAO coating and nickel plated coating. Compared to the sample with MAO/epoxy resin coating, after electroless nickel plating, the corrosion potential was941mV higher than the later and its RP also increased greatly. The results of long term corrosion testing in3.5wt%NaCl solution suggested that the immersion time reached to1368h. Compared to the sample without interlayer, there was a little change in the value of corrosion current density and corrosion potential. But the immersion time of the later in3.5wt%NaCl solution was only about10h, suggesting that the corrosion resistance was improved a lot by the interlayer.The failure on mechanism of the MAO/epoxy resin/Ni-P coating was studied. At the initial stage of immersion, the corrosive solution could penetrate into the nickel plated coating and epoxy resin very slowly, which reason was that the nickel plated coating and epoxy coating could seal the micro-defects to each other. With the immersion time prolonging, there were more and more corrosive solution penetrating into this coating system and the MAO coating began to be corroded after the corrosion channels formed. At the later stage of immersion, the MAO coating was destroyed little by little and the substrate began to be corroded. The corrosion rate became faster and faster while the corrosion product accumulating on the surface of substrate alloy became more and more and lifted up the coating system at last.