Electrochemical Techniques Applied to Metals Conservation
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ELECTROCHEMICAL TECHNIQUES APPLIED TO METALS CONSERVATION
Virginia Costa Institut de Recherche et Restauration en Archéologie et Paleométallurgie 21 rue des Cordeliers - 60200 Compiègne, France ABSTRACT A review of the applications of electrochemical methods to the conservation of metal artifacts is presented with regards to their use in treatment and research. As treatment techniques electrochemical methods have been used for cleaning, stabilization and consolidation for many years. Presently electrochemical methods are increasingly being used as a research tool, allowing the identification and quantification of products present at the surface of the artifacts as well as evaluation of the compatibility between materials. Examples of application to silver-copper alloys and stainless steels are presented. INTRODUCTION Any method, to be relevant in the treatment of corroded metals, should ideally fulfill some perquisites such as: to be inoffensive to the metal, acting and transforming only the corrosion layer; to replace the expanded and porous crusts of corrosion products by a rigid metallic layer that retains the original shape; to eliminate all harmful compounds that might catalyze further corrosion. In practice, however, these tasks are almost never achieved altogether. Depending on the conservation state of the artifact, priorities have to be established and the treatment will be different if stabilization, cleaning or consolidation is privileged. In this context, the versatility of electrochemical methods is an advantage over chemical or mechanical techniques [1]. Besides presenting some applications of electrochemistry for the treatment of metallic artifacts, this paper present recent research results in the domain considering their technical aspects. ELECTROCHEMICAL TREATMENT The principle of the method is to supply electrons to the corroded artifact so as to reduce compounds existing at the surface. With the object immerged in a conductive solution, electrons can be supplied by different sources, the simplest one being placement in physical, and therefore electrical contact with more active metals, such as aluminum or zinc [2]. The preferential corrosion of these metals will provide the electric charge for the reduction at the artifacts surface. An alternative is using an external power supply, by connecting the artifact to its negative pole and using an inert electrode, such as platinum or stainless steel, to close the circuit. This arrangement allows control of the rate of the reactions taking place at the surface and also avoids contamination of the solution by corrosion products [3]. More recently a reference of potential
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was also included in the solution to work within a three-electrode configuration using a potentiostat instead of a simple power supply. This set up allows accurately selection and control of the nature and the rate of the reactions taking place at the artifact surface. In addition, monitoring the current during the treatment gives information about the completion of
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