A SIMPLE APPROACH TO ENRICH THE CHROMIUM-FREE CONVERSION COATING SYSTEM ON MAGNESIUM ALLOY FOR CORROSION RESISTANCE

摘要

This present research aimed to use safer hydrogen halide (HI and HBr) based-treatments on magnesium alloy. This work demonstrates the possibility to use these halides to prepare the MgO-MgI_2/MgBr_2 layers on AZ91D Mg alloy in alkaline solution. However, water-soluble metal salts were not responsible for the corrosion protection, but metal oxides (MgO) were found to be responsible for the corrosion protection; whereas Mg (OH)2 formed first and then successively converted to MgO by heat-treatments. The electrochemical results demonstrated that the introduction of MgO could mitigate the corrosion process and an improvement in corrosion resistance was obtained. This treatment was then used as a pre-treatment for the subsequent electroless nickel (EN) coating on the substrate and it showed the good corrosion current density and better corrosion potential on the magnesium alloy. This new treatment and EN layers were characterized by XPS, X-RD and SEM. The electrochemical behaviour of this new-treatment and EN was evaluated in 3.5 wt% NaCl. All the EN samples exhibited active-passive potentiodynamic polarization behaviour. The zero current potential (ZCP), passive current density and break-down potential generally were improved and the corrosion rate was reduced for the treated and EN coated samples. The complete experimental details, results and discussion will be presented in this paper.