Electroplated aluminum parts and process for production

Details Electroplated aluminum parts and process for production Electroplated aluminum parts and process for production

2001-08-09

2004-02-17

Wong, Edna (Department: 1753)

Electrolysis: processes, compositions used therein, and methods

Electrolytic coating

Coating moving substrate

C205S141000, C205S185000, C205S213000, C205S223000, C427S328000, C427S436000, C427S406000, C427S242000

Reexamination Certificate

active

06692630

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to process for the electroplating of aluminum parts, including the electroplating of coinage blanks. The invention also extends to electroplated aluminum parts, including coinage products.
BACKGROUND OF THE INVENTION
Electroplating of aluminum or aluminum alloy substrates is more difficult than on many other materials because an oxide film coats aluminum immediately when exposed to air or water. This oxide film results in uneven deposition of electroplates, and poor adhesion of the plate. Several approaches exist for the pretreatment of aluminum and aluminum alloys substrates for electroplating. Those include a) etching, in which the substrate is pitted with an attacking solution, b) anodizing, in which an oxide film is thickened by anodizing and then etched to roughen the surface; c) electroless nickel plating, in which nickel is deposited from solution without the use of an applied current, and d) precoating, in which the oxide film is first removed with cleaners or acid, and then immediately coated with tin or zinc, more typically zinc, by immersion deposition. When zinc is used, this precoating process is known as zincating, the immersion solution is termed a zincate or zincating solution, and the coating is often termed a zincate coating or zincate layer.
Kodak developed and patented zincating solutions in about 1927. It was a simple solution of sodium hydroxide and zinc chloride. Later, in 1953, W. G. Zelley proposed three zincating solutions that are referred to as “simple” zincating solutions. The three “simple” zincating solutions, together with typical substrate cleaning, conditioning, and post-zincating strike layers, are discussed in ASTM B253-87 “Preparation of Aluminum Alloys for Electroplating.” The drawbacks of the simple zincating solutions were that they had to be operated differently for different aluminum alloys and that the adhesion of the electroplated layer to the aluminum was inconsistent. Subsequent improvements to zincating aluminum included using zincate solutions containing elements such as copper, nickel and iron, with complexing agents such as cyanide and tartrate, to keep the metals in solution, and double dipping in which a first zincate immersion coating was stripped off in a suitable acid prior to forming a second zincate immersion coating.
In the 1960's, W. Canning Ltd. developed a Modified Alloy Zincate (MAZ) solution. This solution was designed to generate improved adhesion over the simple zincating solutions, to eliminate the need for depositing intermediate strike layers of metals such as copper, brass or nickel prior to electroplating, and to produce more consistent process results. Included in the preferred MAZ solution besides zinc, were the additional metals of copper, nickel and iron. This work is referenced in Great Britain Patent 1,007,252, granted in 1965.
In spite of many advances made in the electroplating of aluminum and its alloys, adhesion of the electroplate to the substrate still continues to be a problem. While a weakly adherent electroplated layer may suffice for applications in which the final product is primarily aesthetic, many practical applications demand good adhesion of the electroplated layers to the underlying aluminum substrate.
A particularly difficult environment for electroplated products is circulation coinage. Today, many countries of the world rely on plated coinage in which coinage metals, such as nickel, copper, bronze or brass overlayers are electroplated onto cores of coinage metals such as zinc, steel, or nickel. Processes of electroplating such coinage cores have been developed to ensure that a highly-adherent electroplated layer is formed which can withstand a bend test. The bend test is one indication of whether the plated coinage product can withstand the rigors of a deforming process, that is a minting step, without delamination of the electroplated layers from the substrate. While bend tests may vary, in general, to pass such a test for circulation coinage, the plated coin blank is bent through a 90° angle and the plated layer must not be removable with a sharp instrument such as a file or knife. Although aluminum and its alloys have been used in coins, to the inventors' knowledge, no electroplated circulation coinage products with aluminum or aluminum alloy cores exist in the world today. Efforts by the inventors to apply a simple zincating solution, or an MAZ solution to aluminum substrates, as set out in the Examples of this application, failed to produce adequate adhesion to pass a bend test.
Japanese Patent Application JP 19910146184, published as JP 4369793 on Dec. 22, 1992, to Yagiken, K K and others, describes gaming tokens produced from aluminum or its alloys to include a colored anodized layer and zinc nickeling or zinc-nickel-chrome plating. Japanese Patent Application JP19910187628, published as JP 535963 on Feb. 12, 1993, to Yagiken, K K and others, also discusses game machine tokens and their manufacture. This latter reference uses a zincating procedure to coat aluminum blanks that are used for game machines. The zincate referred to in this patent is Substar™ ZN-111 manufactured by Okuno Reagent Industry of Japan. There is no indication in the reference that the tokens are minted after plating. Efforts by the inventors to duplicate the process of this Japanese reference, as set out in Example 8 of this application, failed to produce a coinage product with sufficient adhesion of the plate to function as a circulation coin.
There is still a need for an effective aluminum pretreatment process for the electroplating of aluminum parts, which results in a plate with sufficient adhesion to withstand the rigors of a deformation process. There is a particular need, for coinage purposes, of an aluminum pretreatment and electroplating process which will produce a plated coinage product which can withstand a bend test without causing delamination of the electroplated layers from the underlying substrate.
SUMMARY OF THE INVENTION
The present invention provides both an improved zincating and an improved copper strike process for the pretreatment of substrates of aluminum and its alloys, such that subsequent electroplating layers are sufficiently adherent so as to withstand a deformation process without causing delamination of the electroplated layers from the substrate. In a preferred embodiment, the pretreatment processes of this invention are capable of producing electroplated products which meet the rigorous adhesion requirements of the circulation coinage industry and allow for the mass production of small barrel electroplated parts such as coinage blanks. The process has been demonstrated to produce electroplated coin blanks with very good adhesion of several different electroplated layers to the aluminum substrate, and to allow a strike of the zincated aluminum coin blanks at practical current densities for barrel electroplating.
The improved copper strike process of this invention has the advantage of operating at realistic and efficient current densities for barrel plating. Standard electroplating barrels are limited to currents of about 1000 Amps, and a typical operational current density in manufacturing is approximately 0.25 A/dm
2
, based on total area of the charge. The literature relating to plating aluminum refers to current densities from 2.5 A/dm
2
-40 A/dm
2
. As the standard electroplating barrel establishes a total current limitation of about 1000 Amps, the only method of increasing the current density is by reducing the area of the quantity of parts that are in the barrel. Reducing the loading of the barrel translates into a loss of manufacturing productivity in barrel electroplating.
In developing the process of this invention, the inventors determined that simple zincating solutions, together wilt those developed as MAZ and Substar™ (as referred to above), were inadequate to meet the manufacturing and quality requirements for electroplated coinage. In particular these prior art zincating processes did not produce a p

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