Magnesium alloys have been paid more and more attention by industry because of their unique properties. Especially, AZ91D magnesium alloy has been used more and more because of its good castability, high mechanical strength and good ductility. However, at present, the research, development and application of magnesium alloy surface treatment technology are relatively backward. So far, a single coating technology that can fully protect magnesium from corrosion under harsh use conditions has not been developed. Organic coatings and conversion organic coatings are mature, but they are far from meeting the industrial requirements. Compared with other surface treatment technologies, electroplating and electroless plating of magnesium alloys require less investment, and can obtain multifunctional coatings with various morphologies, which can meet the requirements in all aspects. However, for electroless plating, waste disposal is a serious problem. Most electric bathtubs are short-lived. Therefore, waste is easy to produce. In addition, the use of toxic chemicals in pretreatment and electroplating bath, such as chromium compounds, cyanide compounds and fluoride compounds, requires further research on the development of "green" electroplating technology. However, for electroplating, it is difficult to obtain uniform coatings with complex shapes due to the uneven emission power of current required for metal deposition.
Because magnesium alloy has unique electrochemical reactivity, it is extremely difficult to electroplate the substrate surface, so pretreatment is very important for high quality deposition. Although zincate and electroless nickel plating process have been applied to some extent, many problems still need to be solved. The first is the adhesion strength of the coating. On the premise of ensuring good adhesion strength of the coating, electroplating or electroless plating process should be developed to obtain better anti-corrosion protection layer. The key to research and development lies in the different pretreatment requirements of different magnesium alloys. Practice shows that the appropriate process is selected according to the actual application. At present, the reported electroplating and electroless plating of magnesium alloys can only be applied to some electronic components and automobile parts, and there is still a long way to go before application, such as the application in automobile wheels. In addition, a lot of chromium trioxide, hydrofluoric acid and even highly toxic cyanide are used in the pretreatment of magnesium alloys, which pollute the environment. The surface of magnesium alloy is prone to pitting corrosion in acidic solution, and developing a pretreatment method with no acidity, strong adaptability, long service life and low environmental load is the future research direction.
