Non-chromate chemical treatments used on magnesium alloys

Details Non-chromate chemical treatments used on magnesium alloys Non-chromate chemical treatments used on magnesium alloys

2000-01-20

2001-11-13

Koehler, Robert R. (Department: 1775)

Stock material or miscellaneous articles

Composite

Of metal

C148S273000, C148S275000, C148S283000, C148S285000, C428S628000, C428S629000, C510S254000, C510S257000, C510S258000

Reexamination Certificate

active

06316115

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to finishing operations for products made from magnesium alloys, and particularly to non-chromate chemical treatments which form a protective film to protect the product from surface corrosion.
2. Description of the Prior Art
Magnesium alloys have many advantages, such as low density, high thermal conductivity and high conductivity of the surface thereof. Because magnesium alloys can comply with the trend forward lighter, thinner, shorter, and smaller fittings having better heat dissipation and improved EMI shielding characteristics for electronic apparatus, magnesium alloys is replacing plastics and become the newest popular material of mobile computer, consumer and mobile communication electronic products. However, magnesium alloys are vulnerable to oxidation in air, so the surfaces of the alloys need a protective film for inhibiting corrosion.
Chromate is normally used to chemically modify surfaces of products made from magnesium alloys. Such as the chemical treatment operations for magnesium alloys proposed by Dow Chemical Company and U.S. Pat. No. 4,676,842 disclose the use of chromate to chemically modify metallic materials, which forms a compact protective film inhibiting the corrosion on surfaces of magnesium alloys. Chromic ions (Cr
3+
) in the film and the acid pickle are poisonous and dangerous pollutant to the environment and result in recycling difficulties of magnesium alloys and increased cost of manufacture and waste handling. Since chromic ions (Cr
3+
) are to be banned from use in the near future, a reliable operation without using chromate is desired.
SUMMARY OF THE INVENTION
An object of the present invention is to overcome the shortcomings of the prior art by providing a non-chromate chemical conversion coating to the surfaces of magnesium alloys to inhibit corrosion on the surfaces.
Another object of the present invention is to provide a non-chromate operation to reduce environmental pollution.
Another object of the present invention is to provide a process using non-chromate formulations, which operates at a lower temperature and reduces cost.
Two formulations in accordance with the present invention comprise an acid pickle prepared mainly from hydrofluoric acid and a binary alcohol, and a conversion solution prepared mainly from nitric acid, permanganate, and ammonium acid difluoride.
A process in accordance with the present invention which uses the new formulations follows. A specimen composed of a magnesium alloy is initially immersed into an alkaline solution at 333.15~343.15 degrees Kelvin for 3~5 minutes for degreasing, and is then rinsed twice with purified water at ambient temperature. The rinsed specimen is then dipped in the prepared acid pickle at ambient temperature for 0.5~1.5 minutes. After acid pickling, the specimen is rinsed twice with purified water at ambient temperature. Following the fourth rinsing the specimen is immersed into the prepared conversion solution at 293.15~333.15 degrees Kelvin for 0.5~1.5 minutes to form a compact protective film on the surface of the specimen. The modified specimen is rinsed for a fifth and sixth times using purified water at ambient temperature. Following the final rinse, the specimen is dried at 333.15~343.15 degrees Kelvin for 50 minutes. Except for the drying step, at least one of the above steps includes an environment with ultrasonic waves. Each of the rinsing steps consumes 10~20 seconds.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description of the preferred embodiment.


REFERENCES:
patent: 2313753 (1943-03-01), Loose
patent: 3010854 (1961-11-01), Satterfield
patent: 57-41376-A (1982-03-01), None
patent: 6-29235-A (1994-02-01), None

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