Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Controlling current distribution within bath
Patent
1997-08-21
1999-09-28
Gorgos, Kathryn
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Controlling current distribution within bath
204224R, 204279, 204286, 204DIG7, 205136, 205324, C25D 1102
Patent
active
059582065
DESCRIPTION:
BRIEF SUMMARY
This is a national stage application of PCT/EP95/04544, filed Nov. 18, 1995.
FIELD OF THE INVENTION
The invention relates to a method to produce an oxide coating on the metal surface of a workpiece that protects against corrosion and wear and has a selectively reduced coating thickness and a device to carry out the method.
BACKGROUND OF THE INVENTION
It is already known from the technical literature that anodic oxidation may be carried out to produce an oxide coating which protects against corrosion and wear on the metal surface of certain metal alloys. This process has great technical significance for the treatment of metal surfaces which may be coated in this way with an oxide coating which protects against corrosion and wear (for more about this, compare Kraftfahrtechnisches Taschenbuch, Bosch, 21st Edition). Anodic oxidation is utilized, above all, in the surface treatment of an aluminum workpiece to reinforce the natural oxide film and thus to achieve a so-called anodized coating.
An aluminum part is wired as an anode and, together with a counter-electrode, receives a constant or pulsating direct current in an electrolyte. The transformation (oxidation) of metallic aluminum to aluminum oxide forms a coating on the surface, which protects against corrosion and wear. The structure and thickness of the coating are essentially a function of time, temperature and current density.
As a rule, the aluminum oxide coating is very hard and brittle, and therefore it cannot be prevented from chipping, for a corresponding exterior influence of force on the anodized coating, which is undesirable in certain applications.
It is also known that the anodizing of special locations of a component may be prevented or limited to a minimum by means of masking with an insulating lacquer or with protective screens.
However, these methods are very costly for industrial scale manufacture and cannot be automated for the application described, or may be automated only at great cost.
OBJECT OF THE INVENTION
Consequently, the purpose of the invention is to indicate a method and a device of the aforementioned type, which do not have the described disadvantages and which permit, on one hand, an oxide coating which protects against corrosion and wear at the desired locations of the metal surface but, on the other hand, limits or prevents the build-up of an anodized coating at locations of the workpiece where a chipping-off of the anodized coating must be prevented under all circumstances.
SUMMARY OF THE INVENTION
This problem is solved in that an auxiliary anode that is resistant to the electrolyte is attached to the workpiece, to which the current acting upon the workpiece may be partially dissipated. Consequently, it is possible to limit the thickness of the oxide coating in said region, or even to prevent same. Consequently, mechanical stress acting upon the nonanodized locations may be absorbed without danger. A device to carry out the method comprises a cover which is adapted to be placed on the workpiece and to which is attached at least one auxiliary anode, which is connected to a direct-current source in a circuit which is electrically parallel to the workpiece.
Additional objectives, features, advantages and possibilities for use of the present invention are obtained from the following description of an embodiment.
BRIEF DESCRIPTION OF THE DRAWING
In the drawings,
FIG. 1 shows the principle construction to produce an oxide coating to protect against corrosion and wear on a metal surface of a workpiece;
FIG. 2 shows a direct-current source with a separate rectifier to regulate the current in a device according to FIG. 1; and
FIG. 3 shows a direct-current source with a variable resistor to regulate the current in a device according to FIG. 1.
DETAILED DESCRIPTION OF THE DRAWING
FIG. 1 shows the principle construction to produce an oxide coating to protect against corrosion and wear on the metal surface of a workpiece 1, which is dipped in an anodizing electrolyte. The workpiece 1 consists of an aluminum alloy a
REFERENCES:
patent: 3361662 (1968-01-01), Sutch
patent: 3619382 (1971-11-01), Lupinski
patent: 3880725 (1975-04-01), Van Raalte et al.
patent: 4304641 (1981-12-01), Grandia et al.
patent: 5135636 (1992-08-01), Yee et al.
patent: 5173170 (1992-12-01), Brown et al.
Renner Ewald
Rothbauer Horst
Gorgos Kathryn
ITT Manufacturing Enterprises Inc.
Leader William T.
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