Magnesium alloy chromium-free fluorine-free phosphating solution and phosphating method
Magnesium alloy known as "the 21st century the most development potential and promising green engineering material", small density (about 1.8g / cm3), high specific strength, modulus of elasticity, eliminate shock resistance, ability to withstand impact loads than the largest aluminum alloy, used in the field of aviation, telecommunications, automotive and other fields more and more widely. However, due to the active chemical nature of magnesium, the electrode potential is very low (-2.34 KV), which results in poor corrosion resistance of magnesium alloy, which seriously hinders its promotion and application. To improve the corrosion resistance of magnesium alloys, we can start from controlling the content of alloy impurities, developing new corrosion-resistant alloys, surface modification and surface coating, and for large-scale industrial production, using corrosion-resistant protective film and coating protection. Processing is the most economical and easy way.
Currently, the preparation of the magnesium alloy corrosion protective film is often used chromate treatment, such as the famous DOW7 process employs sodium chromate and magnesium fluoride, the protective film is prepared with a certain corrosion resistance. However, the hexavalent chromium produced by this process is highly toxic, seriously hazardous to the environment and human health.
In the chromium-free phosphorous chemical industry, the main problems are: the phosphating treatment technology contains a large amount of heavy metals such as nickel , lead , nitrite, fluorine and carcinogens, which are not in line with the national environmental protection requirements for the coating industry; There are many components (more than four kinds) in the formulation, and the influencing factors are complicated. Compared with the chromate treatment, the corrosion resistance of the film layer is poor, and the corrosion resistance of the fluoride-free material is worse. The Chinese patent with the publication number CN101096761A mentions the formulation of the phosphating solution on the surface of the magnesium alloy, containing manganese , zinc , fluorine and the like, using potassium permanganate and zinc dihydrogen phosphate, but the fluoride resistance is not high; The Chinese patent with the publication number CN1598055A refers to a magnesium alloy phosphating solution formulation containing a corrosion inhibitor, sodium fluoride, which is prone to harmful substances to pollute the environment. The literature "Study on the phosphating process of AZ31 magnesium alloy" (Gao Huanfang et al., Surface Technology, 2008, 37(4): 37-39) prepared a corrosion resistant coating on the surface of magnesium alloy AZ31, but the phosphating formula contained nickel and fluorine. In addition to nitrite substances, there are many components (9 components), and the corrosion resistance of the film layer is inferior to that of chromate treatment.
In response to the above problems, Cui Xuejun, Wang Xiuchun, Lu Junfeng, etc. of the Institute of New Materials of Shandong Academy of Sciences studied a magnesium alloy chromium-free and fluorine-free phosphating solution and a phosphating method. The magnesium alloy has a chromium-free and fluorine-free phosphating solution per liter. Containing 1-50 g of dihydrogen phosphate, 0-30 g of nitrate, 0.1-5 g of additive; immersing the treated magnesium alloy workpiece in the above phosphating solution with pH of 2.0-5.0 with phosphoric acid and ammonia water Phosphating treatment is carried out at 30 to 100 ° C for 1 to 60 minutes to obtain a phosphate film. The phosphating treatment of the magnesium alloy by the phosphating solution and the process of the invention can obtain a phosphate conversion film with good corrosion resistance and fine uniformity on the surface of the magnesium alloy, and the phosphating film can effectively improve the subsequent coating coating. Adhesion and protective properties. The phosphating solution on the surface of the magnesium alloy of the invention has few components and is simple to prepare; the phosphating method of the phosphating solution is easy to control, the process is stable, the cost is low, and the sediment is less.
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