ã€China Aluminum Industry Networkã€‘ There are four main methods for corrosion protection of magnesium alloys: chemical conversion treatment, anodizing, metal coating, and laser disposal.
Chemical conversion treatment
Magnesium alloy chemical conversion membrane can be divided into: chromate system, organic acid system, phosphate system, KMnO4 system, rare earth element system, and stannate system.
The structure of the traditional chromate film with Cr as skeleton is very fine, and the Cr with structure water has excellent self-correction function, and the corrosion resistance is very strong. However, Cr has a relatively high toxicity and waste disposal costs are relatively high. It is imperative to develop chromium-free conversion and disposal. Magnesium alloys can be disposed of in KMnO4 solution to obtain an amorphous chemical conversion coating. Corrosion resistance is comparable to that of chromate films. The chemical conversion treatment of alkaline stannates can be used as a pre-treatment for electroless nickel plating of magnesium alloys, replacing the traditional harmful ions containing Cr, F or CN. The porous structure of the chemical conversion film exhibits excellent adsorption in the activation before plating, and can change the adhesion and corrosion resistance of the nickel layer.
The conversion film obtained by the organic acid treatment can simultaneously have the comprehensive functions of corrosion protection, optics and electronics, and it occupies an important position in the new development of chemical conversion and disposal.
The chemical conversion film is thin, soft, and weak in protection, and is usually used only as a middle layer of a decoration or a protective layer.
Anodizing provides a wear-resistant, corrosion-resistant paint basecoat that is superior to chemical conversion, and combines outstanding bonding, electrical insulation, and thermal shock resistance. It is one of the common surface treatment techniques for magnesium alloys. .
Anodized electrolytes of conventional magnesium alloys usually contain elements such as chromium, fluorine, and phosphorus, which not only pollute the environment but also endanger human health. The functions of the oxide film, such as anti-corrosion, obtained by the environmentally-friendly skills developed in these years have greatly improved over the classic Dow17 and HAE. The corrosion resistance comes from the uniform distribution of Al, Si and other elements on the surface after anodization, resulting in excellent fineness and integrity of the oxide film formed.
It is generally believed that the presence of pores in the oxide film is a major factor affecting the corrosion resistance of the magnesium alloy. It was found that after participating in the appropriate anodizing solution in the silicon-aluminum sol component, to a certain extent, the oxide film thickness, fine density, and porosity decreased. In addition, the sol component causes the filming speed to show a rapid and gradual increase, but does not substantially affect the X-ray diffraction phase structure of the film layer.
However, the anodic oxidation film has a large brittleness and is porous, and it is difficult to obtain a uniform oxide film layer on a disordered workpiece.
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