Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2000-08-30
2002-06-25
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S275000, C148S279000
Reexamination Certificate
active
06409844
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a surface-treated article of magnesium or magnesium alloys, a method of the surface preparation of an article of magnesium or magnesium alloys, and a method of the coating of such an article. In particular, the present invention relates to a method of the surface preparation of the article of magnesium or magnesium alloys (hereinafter referred to as “magnesium article”) which can perform the degreasing and chemical conversion treatment of the magnesium article in one step, and a method of the coating of a magnesium article, which is performed in combination with the above surface preparation.
PRIOR ART
Housings of CRTs used in home appliances such as TVs, word processors, personal computers, etc. are made of plastics from the viewpoint of strength, and easiness of production. However, with the rise of concern about environmental quality, there arise some trends towards the replacement of plastics with materials which can be easily recycled.
In the case of plasma displays which are vigorously developed as displays, it is desired to produce their housings with materials having high electromagnetic shielding effects.
Thus, it may be contemplated to produce housings with metals. Metal housings should satisfy various requirements, for example, light weight, sufficient strength, causing no environmental problems, good corrosion resistance, etc. Metals which satisfy such requirements include magnesium and its alloys.
When magnesium and its alloys are used as the materials of housings, the housings are coated to improve their appearances. However, it is necessary to subject the housings to surface preparation (pre-treatment) to improve the adhesion of coatings and corrosion resistance.
Hitherto, the surface preparation of magnesium articles has been carried out by a JIS method or a DOW method, which uses chromium compounds. Since the chromium compounds are toxic compounds, a waste water containing chromium compounds cannot be drained into sewers without sophisticated post-treatment.
JP-A-6-330341 discloses, as a surface preparation method of articles of magnesium or its alloys using no chromium compounds, a method using a zinc phosphate-treatment liquid containing zinc ions, manganese ions, phosphate ions, a fluorine-containing compound and an accelerator for the chemical conversion of a film.
However, this method is not satisfactorily harmless to the environment since the treatment liquid contains heavy metals such as zinc and manganese, and also fluoride ions which make it difficult to treat a waste water.
JP-A-6-116740 discloses a surface-treating method comprising treating an article of a magnesium alloy containing aluminum with a pyrophosphate salt solution and then with the solution of an alkali metal hydroxide.
This method is good since it uses neither toxic materials nor heavy metals, but is not an economically advantageous method since it comprises a number of process steps.
JP-A-5-51679 discloses a method for the formation of a protective coating by anodization comprising the steps of dipping the article of magnesium or its alloy in a low-alkalinity aqueous electrolytic solution containing borate or sulfate anions, phosphoric acid and fluoride or chloride ions and depositing magnesium phosphate and magnesium fluoride or chloride on the surface of the article of magnesium or its alloy while applying a direct current through the article.
This method is not satisfactorily harmless to the environment since it contains fluoride ions which make it difficult to treat a waste water. Furthermore, this method is not advantageous from the viewpoint of costs since it requires apparatuses such as a rectifier, electrodes, etc. to apply a direct current.
The surfaces of magnesium articles, which are produced by die casting or thixomolding, are contaminated with releasing agents, oxide films, greases, etc. Thus, it is necessary to remove such contaminants to adjust the surface conditions. To this end, the surfaces are degreased, pickled, or subjected to the surface adjustment, prior to chemical conversion.
These treating methods have drawbacks such that the number of treating steps is large, the number of items for controlling liquids is large, the amount of waste water is large, and so on. Furthermore, the chemical conversion liquids contain many materials which are controlled from the viewpoint of environmental protections, and thus the load on waste water disposal and influences on the environment are concerned about.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a surface-treated magnesium article having good corrosion resistance, on which coatings can be formed with good adhesion, Another object of the present invention is to provide a method for the surface preparation of a magnesium article, and also a method for the coating of a magnesium article, which can reduce the number of process steps, improve the corrosion resistance of the magnesium article, and have less influences on the environment.
The above and other objects can be accomplished by
(1) an article of magnesium or its alloy having a surface layer which contains magnesium condensed phosphate and magnesium phosphate,
(2) a method for the surface preparation of an article of magnesium or its alloy comprising treating the article with a treating liquid which contains
1,000 to 20,000 ppm of alkaline metal ions,
1,000 to 50,000 ppm of condensed phosphate ions, and
100 to 20,000 ppm of borate ions,
and has pH of at least 8, and
(3) a method for coating an article of magnesium or its alloy comprising the steps of:
treating the article with a treating liquid which contains
1,000 to 20,000 ppm of alkaline metal ions,
1,000 to 50,000 ppm of condensed phosphate ions, and
100 to 20,000 ppm of borate ions,
and has pH of at least 8,
removing the article from the treating liquid,
washing the article with water,
drying the washed article, and
coating the dried article.
DETAILED DESCRIPTION OF THE INVENTION
Articles to be treated by the method of the present invention are those made of magnesium or its alloys. In particular, such articles are housings of CRT displays for TVs, word processors, personal computers, but are not limited to them.
Such articles can be produced by any conventional methods, for example, sheeting, die casting, and the like.
The compositions of magnesium alloys are not limited, and any magnesium alloys, which are industrially used, may be used in the present invention.
Preferred examples of magnesium alloys include magnesium alloys containing aluminum (e.g. AZ-91A, B and D, etc.).
Examples of condensed phosphate salts, which are contained in a treating liquid used in the method of the present invention, include salts of polyphosphoric acid, metaphosphoric acid, and ultraphosphoric acid. Among them, polyphosphoric acid of the formula: M
n+2
P
n
O
3n+1
in which n=2, 3, 4, . . . , in particular, polyphosphoric of this formula in which n is 3 is preferable.
The dispersibility of polyphosphate salts in water decreases as the molecular weight of polyphosphoric acid increases. hen the molecular weight of polyphosphoric acid is too low, the unctions of condensed phosphate which the present invention uses, for example, sequestering properties, buffering properties, dispersing properties, etc. may deteriorate.
Condensed phosphoric acid tends to be hydrolyzed in an aqueous medium in a neutral or acidic range to form orthophosphoric acid through tripolyphosphoric acid and pyrophosphoric acid. The rate of hydrolysis depends on pH and a temperature. That is, the rate of hydrolysis increases as pH decreases or as a temperature increases. Thus, a treating liquid used in the present invention has pH of at least 8, preferably in the range between 8 and 11, and a treating temperature is preferably 70° C. or less.
Magnesium articles can be mildly etched when the sequestering properties are used among. the various properties of condensed phosphoric acid. The surfaces of magnesium articles can be cleaned by etching with condensed phosph
Fujiwaki Takeshi
Kondou Yoshiaki
Nishikawa Yukio
Nishizawa Yoshihiko
Sakamoto Yoshinori
Oltmans Andrew L.
Sheehan John
Wenderoth Lind & Ponack LLP
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