| Electroless nickel (EN) deposition is a viable option to improve the corrosion and wear characteristics of AZ91D Mg alloy, but challenges exist in the deposition process due to the corrosion of Mg in the EN bath, a process which can also reduce the stability of the bath solution. For this reason, stabilizers are normally required to extend the bath life.; A novel multilayer protective coating process, including immersion copper (Cu) and EN deposition, has been developed for the AZ91D Mg alloy to avoid these difficulties. An optimized Cu immersion coating was obtained by using a 10 min Cu sono-immersion plating in a bath containing 3.3 M HF. The substrate was pretreated using a chromium oxide etch followed by an alkaline activation.; Electrochemical investigations using electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) measurements indicated that the formation of a fluoride-containing film on the Mg AZ91D alloy surface inhibited the Cu deposition kinetics. The application of sonication during Cu immersion deposition increased the anodic reaction rates; i.e., the Mg dissolution rates. This resulted in the enhanced Cu deposition on the Mg AZ91D alloy.; The effect of stabilizer (thiourea and maleic acid) concentration on EN bath stability was assessed using a palladium-accelerated stability test. Accordingly, the various EN deposits formed with various concentrations of different stabilizers were characterized; significant effects on the morphology and grain size of the deposits were observed.; Corrosion of the EN deposits in an oxygenated 5% NaCl solution was investigated by electrochemical methods. The corrosion potential (Ecorr) for the EN deposited Mg AZ91D alloy was about 1.0 V higher than that of AZ91D Mg alloy. The EIS results indicated a higher corrosion rate for the EN deposit prepared with thiourea compared to those prepared with maleic acid or with no stabilizer. Ex-situ surface analyses also showed distinct effects of stabilizers on the corrosion process of an EN deposit.; Successfully applied multilayer protective coatings enhanced the corrosion resistance of the Mg AZ91D alloy. The addition of the stabilizer during EN deposition affected the surface morphology, corrosion mechanism and, hence, corrosion rates of the EN deposits.; Keywords. Mg AZ91D alloy, immersion plating, electroless plating, Ni-P binary alloy, sono-immersion, stabilizer, bath stability, trace impurity, phosphorus enrichment, grain boundary, corrosion, morphology, microstructure, composition, surface/corrosion film. |
