Studies On Economy-Practising And Green Electroplating On Magnesium Alloys

Magnesium alloys are regarded as green engineering materials in 21st century. Magnesium is the lightest metal structural material in engineering application. It will be widely used in aerospace, automobile and electronics industry for its outstanding characteristics, such as high strength-to-weight ratio, excellent cast ability, electromagnetic shielding property, machinability, damping property, and recycle characteristics. However, their applications are limited severely for their low corrosion resistance.Electroplating is an excellent surface treatment, the plating layer has many advantages such as good corrosion and wear resistance, deposit uniformity, good electrical and thermal conductivity and solder ability etc. The electroplating coating as the preconditioning can carry on regular shelter and decorating, so electroplating is a good method to obtain highly anti-corrosion metallic coating.The magnesium alloy characteristic and application, the research progress of corrosion and protections of magnesium alloy were reviewed in this thesis. The pretreatment of magnesium alloy electroplating was studied, and achieved the goal of non-chromium and cost reduction. Solution performance of electroplating Cu and Zn was studied by Hull Cell and polarization curve, also the formula has been adjusted and optimized gradually. Leveling ability of vanillin and phytic acid in both solution was studied with cathodic polarization, rotating disk electrode. The result shows that vanillin exhibits a good leveling ability in both solution, however phytic acid shows opposite. The effects of three current wave forms (smoothed, half-wave rectified and full-wave rectified DC) on electroplating of Cu and Zn on AZ91D Mg alloy were studied by SEM, neutral salt spray (NSS) test and adhesion strength test. Considering of Cu deposits, the results showed that micromorphologies of the deposits prepared at full- and half-wave rectified DC are similar, featuring uniform grain size and regular and compact crystal arrangement. The deposit prepared at smoothed DC features larger grain size, wider crack between grains, and less compact crystal arrangement. The corrosion resistances and adhesion strengths of the deposits prepared at three current wave forms are good. The performances of the deposit prepared at full-wave rectified DC are optimal. Considering of Zn deposits, results showed that the coating morphology of direct wave is regular, close. Grain size is uniform; Size of half wave is not uniform, arranged in disorder. Although full wave is similar with direct current, there are more small hole on it. NSS testing showed that the corrosion resistance of direct current is best of all. Nick testing showed that the adhesion strength of each coating is good. So the coating performances of direct wave zinc plating layer on AZ91D magnesium alloy is the best.