Monitoring the conservation of metal objects: evaluation of a new approach

The need to conserve metal artifacts in museum collections frequently leaves the museum professional with the choice between attempting to remove layers of corrosion with the attendant loss of material and exposure of a fresh unprotected surface, or using a treatment such as electrochemical reduction and passivation which may restore the surface metal. In case of the latter, electrochemistry provides many powerful methods and can be applied on a scale from submerged shipwrecks to the smallest metal artefacts. In general, all of these electrochemical approaches require evaluation. A powerful way to implement the evaluation is to use synchrotron X-ray diffraction (SR-XRD) and other X-ray methods to monitor, for example, an electrochemical process in real time, and an alternative technique to examine the reactivity of the resulting surface. Ultra-low energy dynamic SIMS (uleSIMS) is a new technique in this application, with the virtue that exceptionally small amounts of corrosion can be detected (e.g. incomplete nm-scale layers), so that the surface chemistry of a metal can be studied after treatment and brief exposure to the museum environment, without the need for accelerated corrosion. In this study we show how uleSIMS and SR-XRD can obtain complimentary information on surfaces produced in a new electrochemical cell for use in-situ in a synchrotron beam line, and compare results from silver and copper. We show that, on the one hand, in-situ SR-XRD can follow the surface modification due to electrochemical treatment in real time, whereas ule SIMS can be used subsequently to detect the early presence of tarnish and corrosion on unprotected and treated surfaces, providing a benchmark against which to evaluate conservation methods