Method for surface treatment of magnesium or magnesium alloy by anodization

Abstract

Disclosed herein is a method for the surface treatment of magnesium or a magnesium alloy by anodization to form an anodized oxide coating on the magnesium or magnesium alloy. The method comprises: removing impurities and an oxide layer present on the surface of magnesium or a magnesium alloy using a strongly alkaline aqueous solution (pretreatment); and immersing the pretreated magnesium or magnesium alloy in an alkaline electrolyte and applying a direct current having a current density of 3 A / dm2 or less to the electrolyte to form a magnesium oxide coating (microarc plasma anodization).

Description

BACKGROUND[0001]1. Technical Field[0002]The present invention relates to a method for the surface treatment of magnesium or a magnesium alloy by anodization to improve the corrosion resistance of the magnesium or magnesium alloy. More specifically, the present invention relates to a method for the surface treatment of magnesium or a magnesium alloy using an environmentally friendly electrolyte without the need to use environmental contaminants, such as chromium and manganese, to basically prevent the occurrence of toxic waste water and improve the corrosion resistance and coating adhesiveness of the magnesium or magnesium alloy as an undercoat.[0003]2. Description of the Related Art[0004]It is known that magnesium is the sixth most abundant metal element and has about two thirds the specific gravity of aluminum. Magnesium is the lightest metal of currently available metals and has advantages in terms of specific strength (i.e., strength / specific gravity), durability, electromagnetic...

AI Technical Summary

Benefits of technology

[0014]It is another object of the present invention to provide a technique for forming an anodized coating with good characteristics by improving the pretreatment processing to solve the problems of poor degreasing and greatly decrease the amount of organic waste water and by microarc plasma anodization wherein a low voltage and a direct current are applied in an environmentally friendly electrolyte containing no chromium, manganese and Si4+ ions.

Problems solved by technology

Despite the above advantages, magnesium alloys are sensitive to both alkalis and acids owing to their high activation tendency.

Accordingly, magnesium or a magnesium alloy suffers from severe galvanic corrosion when exposed to corrosive environments.

The dry coating process has many limitations in that deposition plating under vacuum is difficult to perform owing to high vapor pressure, the working space is limited and considerable production costs are incurred.

This substitution promotes the degree of aging of the electrolyte and the galvanic corrosion cause poor corrosion resistance of the plated surface of the magnesium alloy, leading to low value-added production.

The conversion coating processes do not give satisfactory corrosion and wear resistance of coatings, limiting their application to the pretreatment of coatings.

The conversion coating processes use chromium (Cr) or manganese (Mn), which is known to cause serious pollution problems, for the formation of coating.

Thus, the conversion coating processes have a decisive disadvantage in that there is a high possibility of regulating the production of coatings in the near future.

The anodization causes pollution problems because it uses chromium or manganese, likewise other coating processes. FIG. 1 is a flow chart illustrating a prior art surface treatment method for forming an anodized coating on the surface of magnesium or a magnesium alloy.

However, since the high-voltage anodization requires the use of a rectifier having a very high capacity, it is not appropriate for the mass production of magnesium products.

Further, many cracks are formed in the oxide coatings made of hard and inflexible ceramic.

Further, the oxide coatings show poor adhesion to coatings formed in the subsequent step to improve the corrosion resistance.

However, the commercial effects are not great despite the use of high-priced rectifiers.

In this case, the removal of impurities (such as oils) present on the surface of magnesium is not sufficient, which may induce defects of the anodized coating.

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