Examination of the Electrochemical Compatibility of a Selection of Bimetallic Coins Using 3D-Skp and 3D-Svet

Coins as a means of exchange or barter have existed for over 2000 years. Early examples from the Bronze Age were stamped from crudely shaped discs of metal and this principle of manufacture has continued through to the present day. With the face value of modern coins usually being significantly higher than the material value and production cost, they can be prone to forgery, with a reported 3% of certain denominations in the UK being counterfeit¹. In an effort to combat this and to add distinguishing features to the different denominations, an increasing number of coins around the world consist of two or more alloys. These are usually arranged as an inner and outer piece, to form two concentric rings. With two different alloys in electrical contact there exists the possibility for bimetallic corrosion to occur; hence it is important that the two alloys are potentiometrically matched.       In this study, a selection of bimetallic coins has been examined via 3D-SKP and 3D-SVET. The SKP technique is used to examine the surface potential differences between the two alloys. The SKP scans have been performed using height scanning mode in order to compensate for the topographical features present and enable a sufficiently close scan distance to elucidate good potential measurements. Subsequently the coins have then been subjected to 3D SVET scans while immersed in an electrolyte, in order to assess if the potential differences identified during the SKP assessment result in measureable galvanic corrosion.

            The SKP was able to determine a Volta potential difference of between 50 – 100mV for the paired alloys. When investigated via the SVET, this relatively small potential difference was sufficient to generate measureable galvanic corrosion currents when immersed in weak electrolyte and scanned using the 3D-SVET. The anodic and cathodic sites detected via the SVET corresponded to the relative Volta potentials as detected with the SKP.

                         

1. Osbourne, G. (2014), Chancellor George Osborne's Budget 2014 speech, Available: https://www.gov.uk/government/speeches/chancellor-george-osbornes-budget-2014-speech  [20 March 2014].