Nowadays, magnesium alloys are extensively used in several areas such as aviation and automotive industry because of their low density, high strength/weight ratio and good castability. But the poor corrosion resistance of magnesium alloys severely limits their potential wide applications in more civil areas. Although zinc electroplating is an oldest way for metal’s protection, it is a relative new method for protecting magnesium alloys.
Magnesium alloys have very high chemical reactivity and they are prone to form a passive oxide layer quickly in the air. Thus, a special pre-treatment is necessary before electroplating because the quality of the electroplating layer is determined by the quality of the pre-treatment. The mechanism of zinc immersion pre-treatment process on magnesium alloy is deeply investigated, and the experimental results are as follows:
A new kind of activating solution which is more efficient and environmental than the traditional one was developed. And the change of the surface morphologies of the α phase and the β phase in the AZ91D magnesium alloy in the activating process was also investigated. During the activating process, a kind a phosphate film which contains numerous micro-cracks formed on both surface of the α phase and the β phase. This kind of phosphate film could not only prevent the further oxidation of the magnesium alloy, but could also be dissolved quickly in the coming zinc immersion process.
The zinc immersion process could be divided into three distinct stages. In the first stage, the phosphate film formed in the activating process dissolved quickly and some zinc particles deposited on the surface of the alloy randomly; in the second stage, zinc particles which seemed much orderly than these random ones deposited on the phase boundary between the α phase and the β phase firstly and then on the β phase; in the third stage, zinc particles grew spirally on the uncovered α phase. There are also three stages which corresponded to the surface morphologies on the V-t curve of the zinc immersion process. Additionally, a kind of surfactant which could smoothes the zinc immersion layer was selected through numerous experiments.
The zinc electroplating processes on AZ91D magnesium alloy were also investigate systematically. Firstly, we selected the alkaline cyanide-free zinc plating process from numerous zinc plating processes and applied it to electroplating of magnesium alloy successfully. Secondly, a new kind of electroplating process which is designed for magnesium alloy was developed and the optimum parameters were also achieved with orthogonal experiment.
Additionally, the corrosion behavior of WE54 magnesium alloy was investigated in 3.5%NaCl aqueous solution. The electrochemical study showed that the value of corrosion potential of WE54 magnesium was –1.8V (Vsce). Through the comparison of corrosion behavior between WE54 and AZ91D, it was showed that the value of corrosion potential of WE54 was approximately 200mV lower than that of AZ91D, and the corrosion rate of WE54 was one order smaller than that of AZ91D. After 72 hours’ constant immersion and corrosion products removal, deeper grooves along grain boundaries was revealed but lighter corrosion was apparent on the matrix of WE54 alloy. As far as AZ91D alloy was concerned, severe localized corrosion was dominant and network-bone-like structure was remained.
修改评论