APPLICATIONS

BACKGROUND

The most widely-used plated silvery- white coin configuration worldwide is nickel-plated- steel (NPS), with the Royal Canadian Mint's patented multi-ply nickel/copper/nickel-plated steel product having made major inroads in the market in recent years. This product is described in U.S. Patent 5,151,167 which is incorporated herein by reference. Despite the well-deserved popularity of these products, they are still cost-prohibitive for many applications, particularly when there is a "spike" in the price of nickel. In addition to the price of nickel, some other cost drawbacks include the following:

1. The tendency of steel to corrode (rust) requires a significant plating thickness (typically 25 μηι in the center of the coin, and 2-3 times that at the rim)

2. Steel coinage products must be annealed after blanking, to soften the steel for coining and to relieve stresses induced by the electroplating process. In some instances, this requires two separate annealing cycles.

3. After annealing, the plated coinage blanks must be burnished to produce a bright, lustrous finish prior to coining.

This invention addresses all of these concerns in a cost-effective manner, without sacrificing the performance and service life of the finished coin.

The US 1-cent coin has been produced from copper-plated zinc (CPZ) blanks since 1982. Nearly 300 billion coins have been produced in this configuration. A copper plating thickness of only 7-8 μπι has proven to be sufficient to protect the zinc blanks from corrosion in service. Annealing and burnishing are not required; as-plated blanks are shipped directly to US Mint facilities for coining. Coining die life is far superior to that obtained with plated steel blanks. Although zinc is more expensive than low-carbon steel, these processing advantages continue to make copper-plated zinc an economically sound choice for low-denomination coins.

As in the US, many countries use "red" (copper-colored) coins for their lowest- denominations circulation coinage. Then the coins move up to a "white" finish (silver-colored) for intermediate denominations, and a "yellow" finish (gold-colored, such as brass or bronze) for the highest denominations.

Since the advent of the CPZ penny, a number of finishes to promote zinc as a suitable substrate for higher-denomination coinage have been investigated. At this time, both white and yellow options are available. Both of the finishes are bronzes, and both require the use of a copper plating layer beneath the bronze layer. Both of these bronze plating processes are complicated to control, requiring frequent plating bath analysis and monitoring. Both processes also require burnishing after plating to achieve a bright appearance prior to coining. While they are viable finishes for medium-denomination coins, they are too labor-intensive for the lowest denominations.

SUMMARY

The process and plated products described below were conceived while attempting to remove an "immersion tin" deposit from some bronze-plated coinage blanks (planchets). This coating sometimes forms spontaneously in bronze plating baths, and is undesirable. Immersion tin layers are typically only a few atomic layers thick, and it was reasoned that these tin layers should be fairly easy to remove. The blanks were burnished in a centrifugal disc machine with stainless steel media for 30 minutes to remove to the tin. The tin layer not only withstood the mechanical action of the burnishing operation, but it came out looking very attractive - bright and shiny.

DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

The surprising results of the burnished immersion tin deposits noted above led to an attempt to set up an immersion tin process to purposely produce such a finish over copper-plated coinage blanks. However, it proved difficult to produce a consistent immersion tin finish on blanks in a plating barrel. It was decided that it would be better to use a tin electroplating process, so that the plating thickness and appearance could be better controlled. The next step was to determine the most cost-effective way to proceed. The following were deemed to be the most important considerations: • Allow the use of existing plating lines, presently used to produce CPZ blanks;

• Keep the tin plating thickness to a minimum;

• Produce a bright as-plated finish that requires no burnishing (ready to coin);

• Demonstrate wear and corrosion resistance at least as good as that of the US Penny.

To retain the proven service life of CPZ, all testing was conducted with a target copper thickness of 7-8 μπι applied to a metal substrate in a conventional cyanide copper plating process.

Except for the earliest trials, the tin plating was deposited using an acid-based system. This was chosen for two reasons:

• Acid tin is plated from the +2 valence, versus the +4 valence in alkaline tin baths.

Therefore, at an equivalent current, alkaline tin takes approximately twice as long to plate as acid tin;

• Acid tin can be plated with a bright finish without polishing or burnishing, while alkaline tin has a matte appearance. Matte tin would require burnishing to achieve a bright finish. Acid tin plating can be used with brighteners which makes the tin deposit even shinier, as desirable for coinage appiclations.

The initial trials with acid tin centered upon plating various thicknesses on top of CPZ blanks. Alkaline cyanide copper plating is a preferred type of plating to serve as a base for tin plating when plating over zinc. A laboratory plating barrel load of coin blanks (approximately 8 pounds), was tin-plated for three-minute intervals. Approximately 50 blanks were removed from the barrel at the end of each interval. A total of nine 3 -minute cycles resulted in average tin plating thicknesses (measured in the center of the blank) ranging from 0.4 to 3.8 μιη. As noted above, the underlying copper plating on all blanks measured 7-8 μηι.

Samples from each of the nine sets were marked to identify their thickness level. These blanks were then placed in a centrifugal disk burnishing machine, using stainless steel media and an aqueous burnishing compound. A total of six burnishing cycles were run, and blanks from each of the nine sets were examined, both with and without magnification. There was no evidence of exposed copper on any of the samples after the first cycle. After two cycles, only Set # 1 (0.4 μηι) showed evidence of exposed copper, and only at 10X magnification. After four cycles, there was only slight visual evidence for Set # 1, and Sets # 2 (0.8 μπι) and # 3 (1.2 μηι) showed evidence of exposed copper only under magnification. After six cycles, Sets # 2, # 3, and # 4 (1.5 μηι) showed evidence of exposed copper, but only under magnification.

The wear resistance of the finish on coined tokens was then assessed at three different levels of tin thickness: 0.8, 1.2, and 2.0 μηι. The intention was to keep the tin thickness to a practical minimum. Details of the wear test are as follows:

1. Eight tokens of each type were weighed using an analytical balance.

2. Test media was prepared and assembled for each set of tokens:

• 13 grams of leather strips, 38 mm x 3mm x 1.5 mm

• 7 grams of cotton/polyester cloth strips, 100 mm x 20 mm

• 7 grams of size 0000 cork stoppers.

3. Artificial sweat solution was prepared:

• 20 g/1 sodium chloride (NaCl)

• 2.5 g/1 dibasic sodium phosphate (Na ₂ HP0 ₄ )

• 2 ml lactic acid (C ₃ H ₆ 0 ₃ )

• Distilled water to volume of 1 liter.

4. Leather strips were soaked in sweat solution for 30 minutes.

5. Sweat solution was drained from the leather strips.

6. Leather strips, cotton strips, corks, and tokens were placed into a 500-ml rectangular high-density polyethylene (HDPE) bottle, and the cap was securely fastened.

7. Filled bottles were inserted into round PVC sleeves. The sleeves were fitted with stops to prevent the bottles from spinning within the sleeves.

8. Sleeves were placed onto rollers driven by an electric motor. The rotation speed of the sleeves is 37 revolutions per minute.

9. The test was stopped every 160 hours to allow tokens to be removed. Tokens were weighed and inspected for condition after each cycle.

10. After the first 160 hours, an additional 5 ml of sweat solution was added to each HDPE bottle before the test was resumed. This was the only time that additional sweat solution was added.

11. The test was continued for a total of 640 hours (4 cycles). Upon completion, the tokens were once again weighed and inspected for condition. Figure 1 shows the results of this weight loss test for tokens. All three sets of samples were reasonably close in wear performance, with the 1.2 μιη tokens somewhat better than the other two. Over the past years, this same test has been performed on many different coinage materials, both plated and solid alloys. The performance of these tokens was surprisingly strong in comparison to many other common materials.

A second wear test was run with two objectives in mind:

1. Confirm that the initial set of samples could be reproduced successfully;

2. Conduct a direct comparison to CPZ and nickel-plated steel tokens.

A target tin plating thickness of 1 μηι was chosen for this test, over the usual 7-8 μπι of copper plating.

Results of this test are shown in Figure 2. It can be seen that the addition of the thin tin layer made a dramatic improvement in the wear resistance of the tokens. There was a 58% reduction in weight loss compared to CPZ, and the results compared quite favorably to the more costly nickel-plated steel, with three times the plating thickness.

Additional tests have been run on sample tokens, including exposure to high humidity and to steam, as well as salt water immersion. In all cases, their performance has been equal or superior to that of CPZ. Given that CPZ pennies have been in circulation in the US for 30 years, and in Canada for 15 years (co-mingled with a copper-plated steel version), it is evident that this plating approach is viable for long-term circulation.

To confirm that the process used to produce coin blanks is suitable for production use, a full-scale test was conducted. In all, 23 trial loads were plated, with load sizes ranging from 80- 1 10 pounds of zinc blanks. The finished product had the same appearance and properties as samples that had been produced on a laboratory scale. Despite the original emphasis upon low cost, the excellent performance of this process and the resulting electroplated products makes it a strong candidate for higher-denomination coinage as well.

Figure 3 shows that two embodiments of this disclosure actually outperformed solid cupronickel, in the form of US 5-cent coins, in a wear test. The zinc samples were the same size as a 5-cent blank (planchet), but weren't coined. It should be noted that this test revealed that a 5 μηι copper plating thickness is insufficient to protect the base zinc alloy from the acid tin plating bath. There was evidence of zinc attack in the form of pits. This is the reason why the zinc samples with only 1 μιη of tin actually outperformed those with 3 μηι of tin in this test.

Subsequent to the wear tests performed above, the opportunity arose to have tests conducted in conjunction with two national mints. In both instances, planchets were supplied to the mints in various sizes and were coined using production-scale equipment. Both mints utilized tumbling procedures similar to that outlined above, but each with significant differences. In the first, a larger container was used. Although the container was round, it had interior baffles to keep the sample coins moving throughout the test. Given the greater diameter of the container, the coins would have presumably been subjected to more forceful impacts than in the test detailed above. In the second test, there was also a round, baffled jar, but no simulated sweat or other media. Thus, the sole mode of wear was part-on-part. An equivalent quantity of nickel- plated coins in the same sizes was run in the container alongside the coins produced in accordance with this disclosure.

Based upon the results of these tests, it was determined that the original 1 μπι tin thickness was insufficient for circulation coins, due to the underlying copper plating "showing through" on the high-wear areas of the coins, primarily at the rim. While this condition is not harmful, it would likely be deemed unsatisfactory for aesthetic reasons by many mints. However, this original embodiment would be suitable for low-wear applications like commemorative tokens and even for mints seeking a cheap coin, unconcerned about the potential for copper to show thorough.

Additional wear tests were run at both facilities, with tin plating layers ranging from 3 μηι to as high as 10 μπι. In all instances, the coinability and appearance remained excellent. As would be expected, the time to copper exposure on the sample coins was directly proportional to the tin plating thickness. Based upon test results of the national mints, samples with 3 and 5 μηι of tin would likely be considered acceptable for circulation, with a preference for 5 μπι and a recommendation of greater thickness for higher-denomination coins.

Another feature of this electroplating process that suggests its use in higher-denomination coinage is its ability to be tailored to produce a range of electromagnetic signatures (EMS). By varying the relative thicknesses of the two plating layers, one can produce a range of distinctive EMS characteristics for vending, sorting, and counting mechanisms. The copper layer is the more influential of the two, due to its high conductivity (~ 100% IACS) in comparison to tin (~ 15% IACS). The effect of copper plating thickness over a zinc substrate on conductivity is demonstrated in Table 1. As the measurement frequency is increased, the effect of the copper plating thickness becomes more pronounced.

Table 1. Effect of Plating Thickness upon Conductivity Test Results (Zinc Blanks)

The EMS effect was further verified by running the two sets of zinc samples described in Figure 3 through a Scan Coin 4000 coin sorting machine. Despite a difference of only 2 μπι between the copper and tin thicknesses, two of the parameters used for sorting showed a noticeable difference in their values. These were high-frequency conductivity (as would be expected), as well as magnetic permeability. It is evident that greater differences between plating thicknesses would enable the production of a series of coins in accordance with this disclosure, each with its own distinct EMS.

It should be noted that this plating process is also applicable to copper-plated steel coinage. A thin layer of tin plating over the copper plating would create the same silvery-white appearance, but the total plating thickness would need to be sufficient to protect the steel, approximately 25 μπι. Such blanks would have to be annealed prior to plating, because the low- melting tin deposit would not be able to withstand the required temperatures. The additional copper plating thickness, the required annealing, and the reduction in coining die life are all drawbacks compared to the zinc-based configuration. Nonetheless, many countries continue to use copper-plated steel, and the addition of the tin layer would provide a low-cost silvery-white coin for those not comfortable with switching to zinc-based coinage.

The plating process described herein builds upon proven technology for low- denomination coinage (copper-plated zinc or steel) by adding a thin layer of tin. Tin is not normally considered for coinage applications, due to its softness, but a thin layer over copper plating shows excellent wear and corrosion resistance. This enables the production of silvery- white coins at a price point far below that of nickel-plated steel, and with the inherent advantages of improved die life at the mint. It has also been demonstrated that the applicability of this process goes beyond the lowest denominations. Performance enhancements are available by increasing the copper and/or tin plating thicknesses, to improve wear and corrosion characteristics and to produce unique EMS properties.

Another concern that is addressed by this process is the issue of nickel allergies and the potential classification of nickel as a carcinogen. The European Union recently passed legislation that restricts the use of various nickel compounds. The plating process described herein is entirely nickel-free.

Examples

1. For circulation coinage and heavy-use tokens: Zinc substrate, 7-10 μιη copper plating, 3- 7 μπι tin plating.

2. For higher-denomination coinage: Zinc substrate, 10-13 μπι copper plating, 7-10 μηι tin plating.

3. For lowest-cost coinage and tokens (low wear or few aesthetic concerns): Zinc substrate, 7-8 μηι copper plating, 1 -2 μηι tin plating.

Steel-based alternatives to each:

Circulation coinage: Steel substrate, 15-20 μπι copper plating, 3-7 μηι tin plating.

Higher-denomination coinage: Steel substrate, 18-25 μιη copper plating, 7-10 μπι tin plating.

Lowest Cost coinage: Steel substrate, 15-20 μηι copper plating, 1-2 μπι tin plating.

It will be appreciated by those skilled in the art that the above silvery-white material for use in coinage and token applications is merely representative of the many possible embodiments of the invention and that the scope of the invention should not be limited thereto, but instead should only be limited according to the following claims.