Researches On Processes Of Electroplating Zinc And Cu/Ni/Cr On AZ91D Magnesium Alloy

Magnesium alloys are regarded as green engineering materials in 21st century. However, poor corrosion resistance has often prevented it from further exploitation. Electroplating on magnesium alloy can get a quite bright alloy-layer which has better antiseptic ability and conductive performance, the alloy-layer raises hardness, anti-corrosion and wearability of the surface of the magnesium alloy. 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, effect of bath compositions and operation conditions such as temperature, currenty and time on compactness of the zinc and Cu/Ni/Cr coatings have been investigated. The relationship of operation conditions to thickness of the coatings was studied by weight method. The specimens have been pickled in NaCl or sulfate acid solution to evaluate the porosity of the coating. The coating adhesion was detected by scribe-grid test and thermal shock test. The morphology of the coating was charactered by metallographic microscopy and SEM. The research results of the experiments show that:1. The pretreatment of acid pickling in oxalic acid, basic activation and zinc immersion in potassium pyrophosphate bath can obtain good zinc coating and the high quality of post-plating.2. When electroplating zinc, zincate zinc plating was carried out at 30℃by 3.0 A.dm^-2 for 20 min, highly compact zinc coating can be obtained, but the additives play an important role. Prophosphate zinc plating was carried out by 1.0³.0 A.dm^-2 for more 10 min, the coating obtained has good corrosion resistance. The coatings obtained by zinc immersion had higher corrosion resistance than without zinc immersion. The phytic acid has the function of complexing with metal. It easily forms a protection film of single molecular on the metal surface to obscure the contact of oxygen with the metal and prevent magnesium alloy from corrosion in the process of electroplating. The good adhesion of zinc coating can be obtained.3. Pyrophosphate copper electroplating is conducted by ammonium citrate as a auxiliary complexing agent at around 40℃, the narrow range of current density can...