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Methods for corrosion protection of magnesium alloy castings

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Update time : 2023-01-18 14:40:03

Magnesium and magnesium alloys are the most difficult metals to plate for the following reasons:

(1) Magnesium oxide is easily formed on the surface of magnesium alloy casting, which is not easy to be cleaned and seriously affects the coating binding force;

(2) The electrochemical activity of magnesium is too high, all the acid plating solution will cause rapid corrosion of magnesium matrix, or the replacement reaction with other metal ions is very strong, the replacement coating is very loose;

(3) The second phase (such as rare earth phase, y is equal) has different electrochemical characteristics, which may lead to uneven deposition;

(4) The standard potential of the coating is much higher than that of the magnesium alloy substrate, and any through hole will increase the corrosion current, causing serious electrochemical corrosion. However, the electrode potential of magnesium is very negative, so it is difficult to avoid the H2 evolution of pinholes during plating.

(5) The density of magnesium alloy castings is not very high, and there are impurities on the surface, which may become the source of coating pores.

Therefore, the chemical conversion film method is generally used to first dip zinc or manganese, then copper plating, and then other electroplating or chemical plating treatment, in order to increase the binding force of the coating. Magnesium alloy electrodeposits include Zn, Ni, Cu-Ni-Cr, Zn-Ni coatings, and chemical deposits are mainly Ni-P and Ni-W-P coatings.

Single - electroless nickel plating is sometimes insufficient to protect magnesium alloys well. By combining electroless Ni layer with alkaline electroplating Zn-Ni coating, the coating with thickness of about 35um can withstand the neutral salt spray corrosion for 800-1000h after passivation. Some people also use electroless nickel plating as the bottom layer, and then DC plating nickel can get microcrystalline nickel coating, the average crystalline particle size is 4Onm, because of the grain refinement and the coating porosity is greatly reduced, the structure is more dense.

Electroplating or electroless plating is a surface treatment method for obtaining superior corrosion resistance and electrical, electromagnetic and decorative properties at the same time. The disadvantage is that Cr, F and plating solution in pretreatment have serious environmental pollution; Most of the coatings contain heavy metal elements, which increases the difficulty and cost of recovery. Due to the characteristics of magnesium matrix, the binding force needs to be improved.

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