Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly single metal coating
Reexamination Certificate
2000-02-18
2001-03-06
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly single metal coating
Reexamination Certificate
active
06197183
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the electrodeposition of chromium onto zinc or zinc alloys.
A great number of articles, such as machine parts, are fabricated from zinc base metal compositions such as zinc and zinc alloy die castings. Such articles usually are provided with surface finishes which inhibit, reduce, or eliminate corrosion associated with outdoor exposure. A commonly utilized surface finish for these purposes is a composite coating of copper, nickel, and chromium which is applied by first polishing, degreasing and cleaning the surface of the article and thereafter sequentially electrodepositing thereon layers of copper, nickel, and chromium. However, upon prolonged exposure to the elements the electrodeposited surface finishes begin to blister and peel off.
Die casting of zinc base metal is an extremely versatile method of fabricating complex metal shapes with close dimensional tolerances and at a relatively low cost. However, because of susceptibility to corrosion, a protective coating is usually required. Typical such protective coatings are applied by electrodeposition by first applying a copper strike, and then one or more coating of nickel followed by a coating of chromium. While die cast parts produced from zinc base metals provide the basic advantages of cost and weight, corrosion nevertheless is a problem even with the protective coatings. Besides, the wear resistance of such parts is notoriously inadequate for applications involving friction contact between moving surfaces.
The usual commercial chromium plating electrolytes utilized for protective coatings consist of aqueous chromic anhydride (CrO
3
) solutions, also commonly referred to as chromic acid solutions, which contain certain catalysts which enable the chromium contained in the solution to be electrodeposited. These catalysts usually are sulfate (SO
4
═
) and silicofluoride or fluosilicate (SiF
6
═
) ions. In order to optimize the chromium electrodeposition conditions these catalysts must be present in certain specific relative amounts based on the concentration of chromic acid present in the electrodeposition bath. In the so-called self regulating electrodeposition baths concentrations of the cooperating catalyst ions are controlled automatically by means of the solubility characteristics of the compounds that are used to supply these ions to the bath solution. Illustrative self-regulating chromium electrodeposition baths are disclosed in U.S. Pat. No. 2,640,022 to Stareck and in U.S. Pat. No. 2,686,756 to Stareck et al.
U.S. Pat. Nos. 4,095,014 and No. 4,156,634 to losso describe an effective surface treatment of zinc alloys to provide a wear resistant surface. Recent advances in zinc alloy technology, however, such as the high performance ternary zinc-copper-aluminum alloys, require adjustments in electrodeposition techniques and electrodeposition baths for the development of optimum surface properties. To that end, the present invention provides an improved electrodeposition bath composition well suited for the enhancement of the surface hardness of relatively, high copper, low aluminum zinc alloys.
SUMMARY OF THE INVENTION
An electrodeposition bath suitable for enhancing surface hardness of relatively high copper, low aluminum zinc alloys comprises an aqueous chromic acid solution containing sulfate ions in an amount so that the weight ratio of chromic acid to the sulfate ions is about 75:1 to about 125:1, respectively, and further containing fluoride ion in an amount of about 0.2 to about 0.8 grams per liter of the solution, boric acid in an amount of about 0.1 to about 0.8 grams per liter of the solution, and an alkali metal carbonate in an amount of about 0.4 to about 0.6 grams per liter of said solution.
To provide a fabricated zinc base metal article with the wear-resistant skin and the chromium-enriched sub-surface layer, the fabricated article is precleaned and then immersed in the electrodeposition bath and connected into the electrodeposition circuit as the cathode thereof. Electrodeposition is carried out by passing direct current from a submerged anode to the cathode through the bath and is commenced with an initial strike of relatively short duration (less than about one minute) at an elevated voltage of about 7.5V to about 12.5V. Thereafter chromium from the bath is deposited on the cathode at a relatively lower voltage (at least about 20 percent lower than strike voltage) for a time period of about one minute to about 45 minutes. Chromium electrodeposition takes place at a relatively high current density and at a relatively low bath temperature, the electrodeposition bath temperature can vary from about 90° F. to about 135° F. (about 32° C. to about 57° C.), and preferably is in the range of about 100° F. to about 130° F. (about 38° C. to about 54° C.). The cathode current density is at least about 3.5 amperes per square inch and preferably about 4 to about 5 amperes per square inch. The preferred cathode current density varies to some extent with the configuration of the workpiece, the bath temperature and, at a given current efficiency, generally increases with increasing bath temperature.
As used herein and in the appended claims, by the expression “zinc base metal” is meant zinc or a zinc alloy normally used for the manufacture of die cast parts and containing varying amounts of aluminum, magnesium, copper and similar alloying elements.
REFERENCES:
patent: 2640022 (1953-05-01), Stareck
patent: 2686756 (1954-08-01), Stareck et al.
patent: 3654101 (1972-04-01), Aoun
patent: 4095014 (1978-06-01), Iosso
patent: 4156634 (1979-05-01), Iosso
patent: 4472249 (1984-09-01), Chessin
Gorgos Kathryn
Leader William T.
Olson & Hierl Ltd.
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