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Comment on "High nickel release from 1- and 2-euro coins," Nature, Vol. 419, 12 September 2002

by Roger Hooper, BMet, PhD, FIM, FICorr, Ceng and Ivor Kirman, BMet, PhD


September 16, 2002 -- The brief communication "High nickel release from 1- and 2-euro coins" contained a number of points that appear to be either inaccurate or misleading. We are bringing these points to your attention in view of the wide media reporting of this article.

 

  1. Coinage was excluded from the EU Nickel Directive as, in normal handling and use, coins are not in direct and prolonged contact with the skin. The tests used by the authors are designed to be used on articles, to ensure compliance to the Nickel Directive, and are, therefore, not appropriate for testing coinage alloys.

     

  2. The maximum acceptable nickel release rate in the directive is 0.5 µg/cm2/week but, because of the inaccuracy of the test procedure, the actual test result is multiplied by 0.1, as specified in EN 1811:1998. Therefore, an article with a test result of 5 µg/cm2/week is acceptable. If coins were being tested for compliance with the Nickel Directive, the results shown would be 24 or 32 times the acceptable level for direct and prolonged skin contact, not the reported 240 and 320 fold.

     

  3. The high corrosion rate of the extracted centre of the 1-euro coin is misleading. The communication states that the 'pill' of the 1-euro coin is cupronickel (75%Cu, 25%Ni). The 'pill' is actually composed of a sandwich of pure nickel between two layers of cupronickel. The sandwich structure is intended to make the coins difficult to counterfeit and easily recognisable by coin machines. The inner nickel layer is contained within the coin in normal use and thus would not take part in any corrosion testing of the coin itself. However, extracting the 'pill' would expose this core and would create the possibility of a galvanic effect that would never occur in normal usage. The very high nickel release rate quoted for the 'pill' could itself be due to such an effect.

     

     

  4. Discussion of the "corrosion tests" is unsatisfactory. No explanation is offered of the cause of the 1-euro coin turning "brown" nor of the apparent lack of corrosion of the cupronickel Swiss I franc coin. Both observations are surprising. Many tests have shown that cupronickel coins corrode in artificial sweat at the rate of 16-40 �g/cm2/week. (see reference)

     

     

  5. This same reference also reports round robin testing (by government laboratories) that shows that the 2-euro coin has an average nickel release rate of 34 �g/cm2/week and pure nickel has an average release rate of only 4 µg/cm2/week. The figures reported in the Nature article are up to six times these values, which appears surprising. The EN 1811:1998 test has been shown to give very variable results when conducted by inexperienced laboratories and this is one reason for considering a review of the Nickel Directive. In view of the differences noted between the results shown in Figure 1 and earlier work reported in the literature, it is regrettable that the communication did not give more detail about the testing procedure ' especially the number of coins tested, the variation of release rates from coin to coin, origin of the "pure nickel" referred to in the Figure.

     

  6. For galvanic effects to be significant would require the surface and junction to be covered with a film of sweat for a prolonged period. These conditions are unlikely to be met in normal handling and use of coins.

     

In summary, the communication is an interesting exercise but it has little relevance to determining the suitability of materials for coinage. The widespread reporting of this information might raise unnecessary alarm.

 

 

Reference:

Liden,C. & Carter,S. Contact Dermatitis 44, 160-165 (2001).

Affiliations:
Dr Hooper is an independent metallurgical consultant. Among his clients is the Nickel Development Institute, with whom he works on various matters relating to metallurgy and corrosion of nickel-containing alloys. Dr Hooper is a member the EU Committee CEN/TC 283 WG 4, which is responsible for developing the standards used to ensure compliance with the Nickel Directive. email: rogerhooper@btinternet.com

Dr Kirman is President, Nickel Development Institute. email: ikirman@nidi.org.

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