Why is magnesium alloy rod the most difficult metal to plate?

Magnesium alloy rod metal coating:

Magnesium alloy rods are the most difficult metal to plate for the following reasons:

(1) Magnesium oxide is easily formed on the surface of magnesium alloy rods, which is difficult to remove and seriously affects the adhesion of the coating;

(2) The second phase (such as rare earth phase γ Equal) has different electrochemical characteristics, which may lead to uneven deposition;

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

(4) The electrochemical activity of magnesium is too high, and all acid plating solutions will cause rapid corrosion of magnesium matrix, or the single displacement reaction with other metal ions is very strong, and the combination of the replaced coating is very loose;

(5) 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, which makes it difficult to avoid hydrogen evolution from the pinholes during plating;

Therefore, the chemical conversion coating method is generally used to first immerse zinc or manganese, and then copper plating, followed by other electroplating or chemical plating treatments to increase the adhesion of the coating. Magnesium alloy rod electroplated coatings include Zn, Ni, Cu Ni Cr, Zn Ni, etc., while chemical coatings mainly include Ni P, Ni W-P, etc.

A single chemical nickel plating layer is sometimes not sufficient to effectively protect magnesium alloys. Research has shown that by combining an electroless Ni coating with an alkaline Zn Ni coating, approximately 35 μ The m thick coating can withstand neutral salt spray corrosion for 800-1000h after passivation. Some people also use chemical nickel plating as the bottom layer, and then use direct current nickel plating to obtain microcrystalline nickel coatings, with an average crystal particle size of 40nm. Due to the refinement of the grains, the porosity of the coating is greatly reduced and the structure is more dense.

Electroplating or electroless plating is a surface treatment method to obtain excellent corrosion resistance and electrical, electromagnetism and decorative properties at the same time. The disadvantage is that the Cr, F and plating solution in the pre-treatment cause serious environmental pollution; Most of the plating contains heavy metal elements, which increases the difficulty and cost of recovery. Due to the characteristics of the magnesium matrix, there is still a need to improve the bonding strength.