Surface treatment technology of magnesium alloy plate

1. Chemical conversion treatment

The chemical conversion coating of magnesium alloy can be divided into chromate series, organic acid series, phosphate series, KMnO4 series, rare earth element series and stannate series according to the solution.

The traditional chromate film has a dense structure with Cr as the framework, and the Cr containing structured water has a good self-repair function and strong corrosion resistance. However, Cr is highly toxic and the cost of wastewater treatment is relatively high. Therefore, it is imperative to develop chromium-free conversion treatment. The magnesium alloy is treated in KMnO4 solution to obtain a chemical conversion coating of amorphous structure, which is equivalent to chromate film in corrosion resistance. The chemical conversion treatment of alkaline stannate can be used as a pretreatment for electroless nickel plating of magnesium alloys, replacing traditional processes containing harmful ions such as Cr, F or CN. The porous structure of the chemical conversion coating shows good adsorption during the activation before plating, and can change the adhesion and corrosion resistance of the nickel plating layer.

The conversion coating obtained by organic acid treatment can simultaneously possess comprehensive properties such as corrosion protection, optics, and electronics, and it occupies a very important position in the new development of chemical conversion treatment.

The chemical conversion film is thin, soft, and weak in protection, and is generally only used as an intermediate layer for decoration or protection.

2 . Anodizing

Anodizing can obtain a better wear-resistant and corrosion-resistant coating base coating than chemical conversion, and has good bonding force, electrical insulation and thermal shock resistance. It is one of the commonly used surface treatment technologies for magnesium plates. .

The electrolyte of traditional magnesium plate anodizing generally contains chromium, fluorine, phosphorus and other elements, which not only pollutes the environment, but also harms human health. In recent years, the corrosion resistance of the oxide film obtained by the environmentally-friendly process researched and developed is greatly improved compared with the classic process Dow17 and HAE. The excellent corrosion resistance comes from the uniform distribution of Al, Si and other elements on the surface after anodic oxidation, so that the formed oxide film has good compactness and integrity.

It is generally believed that the pores in the oxide film are the main factor affecting the corrosion resistance of magnesium plates. The study found that by adding an appropriate amount of silicon-aluminum sol to the anodizing solution, the thickness and density of the oxide film layer can be improved to a certain extent, and the porosity can be reduced. In addition, the sol component will cause the film formation rate to increase rapidly and slowly in stages, but basically does not affect the X-ray diffraction phase structure of the film.

However, the anodic oxide film is brittle and porous, and it is difficult to obtain a uniform oxide film layer on complex workpieces.