Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
1999-10-22
2002-01-01
Copenheaver, Blaine (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C428S472200, C428S472300, C205S316000, C205S318000, C205S321000, C205S333000
Reexamination Certificate
active
06335099
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a corrosion resistant article of a magnesium material (in this description, both magnesium and magnesium alloys will hereinafter be comprehensively referred to as “magnesium material”) having metallic substrate gloss and a method for preparing the article and more specifically to a corrosion resistant article of a magnesium material having metallic substrate gloss, which has an anodic oxide film on the outer surface of an article of magnesium material and has a colorless or colored transparent electrodeposition coating film on the anodic oxide film as well as a method for preparing a corrosion resistant article of a magnesium material having metallic substrate gloss which comprises the steps of forming an anodic oxide film on a substrate of a magnesium material by anodization using an electrolyte having a novel composition and then forming, on the anodic film, a colorless or colored transparent electrodeposition coating film by electrodeposition technique.
BACKGROUND ART
Among the practically used metals, the magnesium materials are the lightest ones and also have high specific strength and therefore, there have been tried to apply them in various fields such as interior and exterior parts for motor cars and two-wheeled vehicles, parts for household appliances, containers for storage such as bags and suitcases, goods for sports, parts for optical machinery and tools, sticks and further new fields in electronic industries such as computers and acoustics or they have been put into practical use. However, the magnesium materials, among practically used metals, are the most active metal materials and accordingly, it has been difficult to use them per se without any treatment because of their low corrosion resistance.
As surface-treating methods for improving the corrosion resistance of the magnesium materials, there have conventionally been used, for instance, chemical conversion treatments, anodization treatments and coating and plating techniques. In particular, a relatively uniform film is formed on a substrate surface, if the surface is first anodized to form an anodic oxide film and then subjected to a sealing treatment, and thus such treatments have been used for a rust-proofing treatment or a surface treatment for coating.
When imparting decorative properties to articles of magnesium materials while making the most use of the gloss and color tone of the metal substrate surface thereof, the surface of the articles cannot be subjected to coating. However, the surface of the magnesium material is highly susceptible to oxidation and therefore, the article should be subjected to any surface treatment to maintain the initial gloss and color tone of the metal substrate surface thereof.
The film obtained through the conventional chemical conversion treatment or anodization treatment using chromic acid or a bichromate gets colored white or brown or black or further green. Moreover, even a film obtained by an anodization treatment without using any chromic acid or bichromate may ensure corrosion resistance. In this case, however, the resulting anodic oxide film should have a thickness of not less than several micrometers to ensure the desired corrosion resistance and for this reason, the surface obtained after the anodization treatment is inevitably dimmed or colored.
For instance, Japanese Un-Examined Patent Publication No. Hei 9-176894 discloses a surface-treating method comprising the step of forming an anodic oxide film using, as an electrolyte, an aqueous solution which comprises at least one member selected from the group consisting of hydroxides, carbonates and bicarbonates of alkali metals or alkaline earth metals and to which a film-forming stabilizer is added and the patent also describes that the color tone of the anodic oxide film sometimes provides the color of the substrate surface per se. In the method for forming an anodic oxide film disclosed in this patent, however, the anodic film gets colored when it is formed in a thickness required for achieving desired corrosion resistance.
As an alternative surface-treating method capable of maintaining the gloss and color tone of the substrate surface of the magnesium material, there has also been known a method in which an organic clear coating is electrodeposition-coated. However, the formation of an organic coating film inevitably causes changes in the gloss and color tone of the metal substrate and the surface thereof often becomes dim.
Accordingly, it is an object of the present invention to provide an article of a magnesium material which can eliminate the problems associated with the conventional anodic oxide film and clear coating film or which has the gloss and color tone of the metal substrate surface of the magnesium material or which has the gloss of the metal substrate while the color tone thereof is slightly changed and which is excellent in corrosion resistance.
It is another object of the present invention to provide a method for preparing a corrosion-resistant article of a magnesium material having the gloss of the metal substrate surface, which comprises the steps of forming an anodic oxide film on the surface of an article of a magnesium material and then forming, on the anodic film, a colorless or colored transparent electrodeposition coating film.
DISCLOSURE OF THE INVENTION
The inventors of this invention have found that a highly corrosion-resistant, thin and colorless transparent anodic oxide film which have never been achieved can be obtained by anodization of an article of a magnesium material using an electrolyte which makes use of a combination of a phosphate, conventionally used as a film-forming stabilizer when anodizing a metal, and an aluminate and that an article of a magnesium material whose corrosion resistance is further improved can be obtained by forming a colorless or colored transparent electrodeposition coating film on the anodic film and have thus completed the present invention based on these findings.
Accordingly, the corrosion-resistant article of a magnesium material having the gloss of the metal substrate surface, according to the present invention, comprises an anodic oxide film formed on the external surface of an article of a magnesium material, which never changes the gloss of the metal substrate and a colorless or colored transparent electrodeposition coating film on the anodic film.
Moreover, the method for preparing a corrosion-resistant article of a magnesium material having the gloss of the metal substrate surface, according to the present invention, comprises the steps of immersing an article of a magnesium material in an electrolyte containing a phosphate and an aluminate and an optional bath stabilizer consisting of a hydroxyl group-containing organic compound to thus form an anodic oxide film through anodization of the surface of the article, thereafter optionally immersing the anodized article in hot water to subject it to a sealing treatment and forming a colorless or colored transparent electrodeposition coating film on the anodic film through electrodeposition.
THE BEST MODE FOR CARRYING OUT THE INVENTION
The article of a magnesium material used in the present invention may be made of any magnesium material. Examples of such materials are magnesium alloys such as Mg—Al alloys, Mg—Al—Zn alloys, Mg—Al—Mn alloys, Mg—Zn—Zr alloys, Mg-rare earth element alloys and Mg—Zn-rare earth element alloys; and elemental magnesium. In the present invention, the article of a magnesium material may have any surface condition prior to the anodization treatment and thus have such surface conditions as a mirror-finished surface through polishing and a diecast metal surface.
When an anodic oxide film is formed on an article of a magnesium material according to the present invention, the article is subjected to a pre-treatment before the anodization treatment. For the article having such a diecast metal surface, this pre-treatment may be a variety of known treatments, which have conventionally been used prior to the anodization treatment
Dobashi Makoto
Higuchi Tsutomu
Suzuki Mitsuo
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