Le Maroc utilise depuis des décennies des pièces dʼun dirham en nickel pur ou en cupronickel. Les résultats de pesées sur un échantillon de 401 pièces confirment que le cupronickel sʼuse plus vite et révèlent lʼimportance particulière du frai du dirham. Des études au microscope électronique à balayage et des analyses quantitatives par ICP montrent que le métal labile est principalement formé de copeaux, le plus souvent en cupronickel, produits par le frottement des monnaies entre elles. Secondairement les pièces présentent des résidus de sueur avec du cuivre dissout. La quantité de nickel labile sur une pièce, entre 4,8 et 33 μg, tout comme la quantité de nickel transférée aux doigts par la manipulation dʼune pièce, entre 1,7 à 5,4 μg, est fortement corrélée au degré dʼusure. On montre que la mesure du nickel labile est un test simple et fiable de la contamination par les monnaies. Pour citer cet article : P.-G. Fournier et al., C. R. Physique 7 (2006).

When the euro was introduced, the fact that some coins contain nickel, which is known to be an allergen, gave rise to controversy. More generally, this raises the question of metal transfer from coins to skin. Morocco has used for decades one-dirham coins made of pure or alloyed nickel. Studying their wear, the labile metal on their surface and the transfer to fingers in handling may therefore be especially instructive. Weighing statistics for a sample of 401 coins confirm that cupronickel coins wear out more quickly than pure nickel coins and reveal that the dirham suffers a much stronger wear than other currencies for which wear statistics are available. SEM studies supplemented by ICP quantitative analyses show that the labile metal is mainly made up of chips, even after many handlings. These chips are often cupronickel, even on pure nickel coins, which shows that they are produced by the friction of coins against one another. Secondly, the surface of coins presents sweat residue with an important proportion of copper and a little nickel, which confirms that sweat dissolves surface copper. Depending on the alloy and date, coins have between 20 and 140 μg of labile copper and nickel, with a content of one quarter of nickel on cupronickel coins and about one half on pure nickel coins. The most worn cupronickel coins are the coins that present the largest amount of labile metal, and even labile nickel. In our experiments, the metal transfer to fingers when a cupronickel coin is handled for the first time represents between 4 and 9% of the labile metal and 0.05% of the annual wear. A simple and reliable test of nickel contamination consists in measuring the labile nickel. To cite this article: P.-G. Fournier et al., C. R. Physique 7 (2006).