Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Treating substrate prior to coating
Reexamination Certificate
1998-07-17
2001-08-28
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Treating substrate prior to coating
C205S321000
Reexamination Certificate
active
06280598
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for the anodization of magnesium and magnesium based alloys and products produced by that method.
DESCRIPTION OF THE PRIOR ART
A major component of the building industry and, in particular, although not solely, the metal joinery industry has been aluminum based products. Although the price of aluminum has increased in recent years, it is still the principal material of many components due to its strength, weight and the finishes available to aluminum.
By contrast, magnesium prices has remained relatively stable and is not a serious competitor to aluminum. It exhibits similar properties in terms of strength and weight. In the case of both aluminum and magnesium, these materials require some form of corrosion resistant and wear resistant coatings. Both materials easily discolor upon exposure to the atmosphere through oxidization.
The anodization of aluminum is a relatively easy procedure compared with the equivalent anodization of magnesium. It is for this reason that the aluminum has been preferred despite the rising price. Therefore, an advantage exists for magnesium should the anodization process be simplified to allow this material to compete equally with aluminum in a number of applications.
Previous attempts to anodize magnesium have involved the use of base solutions of concentrated alkaline hydroxides. These usually take the form of sodium or potassium hydroxides in a concentrated solution. This anodization process is generally provided through the supply of a DC current at a range of 50 volts to 150 volts. Some methods have suggested the use of AC current as well.
A coating is formed on the magnesium through the formation of sparks within the bath containing the sodium or potassium hydroxide. The tracking of the sparks across the surface of the magnesium element slowly places the coating onto the magnesium. The use of sparks throughout the process leads to a relatively high current usage and to significant heat absorption by the bath itself. Therefore, any commercial anodization plant requires substantial cooling equipment to reduce the temperature of the bath through the use of this process.
The coating formed by this anodization process is an opaque coating with a white or gray color. However, the coating is not a direct visual comparison with anodized aluminum and, therefore, has a problem matching other components made from anodized aluminum. This leads most manufacturers only to use aluminum throughout their manufacture.
Some prior art processes use hydrofluoric acid or acid fluoride salts in which magnesium is not attacked because of the formation of a protective layer of magnesium fluoride on the metal surface. This protective layer is not soluble in water and thus prevents further attack.
A further method of anodizing magnesium or alloys of magnesium relies on this property to create a rough, very porous layer which forms an excellent base for paint or other surface coatings to be applied afterwards. Commonly, such an anodic film may be formed in an electrolyte of very high pH, containing alkali hydroxides. The process proceeds by means of sparking which sparking forms a sintered ceramic oxide film as the metal substrate is coated.
A number of proprietary methods for anodization of magnesium or alloys of magnesium exist which seek to avoid this problem and create a uniform film. This can only be done by incorporating other species into the film as it is formed. Some processes use silicates. Others use various ceramic materials. Some of these processes involve the use of hydrofluoric acid or acid fluoride salts, eg; ammonium bifluoride. These are extremely hazardous materials causing fume and safety problems to the plant operators, and disposal problems. The process may be carried out on a magnesium based material which preferably contains magnesium in the range of 70% to 100% by weight.
OBJECT OF THE INVENTION
Therefore, it is an object of the present invention to provide a method for the anodization of magnesium or magnesium alloys which will provide a coating similar to anodized aluminum, add corrosion resistance and/or overcome some of the disadvantages of the prior art and/or at least provide the public with a useful choice.
SUMMARY OF THE INVENTION
The invention may broadly be said to consist in a method for the anodization of magnesium based materials comprising:
providing an electrolytic solution containing ammonia and/or an amine;
providing a cathode in said solution;
placing magnesium based material as an anode in said solution; and
passing a current between the anode and cathode through said solution so that a coating is formed on said material.
Another aspect of the invention consists of a material containing magnesium, anodized by the method previously defined.
Further aspects of this invention may become apparent to those skilled in the art to which the invention relates upon reading the following description.
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H. K. DeLong, “Practical Finishes for Magnesium”,Metal Progress, 6/1970, vol. 97, No. 6, pp. 105-108.
Derwent Abstracts Accession No. 85-313716/50.
F.A. Lowenheim.Electroplating, McGraw-Hill Book Co., New York, pp 135, 1978 Month of publication not available.
Barton Thomas Francis
Macculloch John Arnold
Ross Philip Nicholas
Brooks & Kushman P.C.
Gorgos Kathryn
Leader William T.
Magnesium Technology Limited
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