Electrochemical techniques are mainly known in the field of cultural heritage conservation as a tool for the elimination of corrosion layers or the removal of chlorides. However, these techniques are also a valuable tool for assessing the anti-corrosive efficiency of protective coatings. The aim of this study was to evaluate the performance of different coatings for their use in metallic heritage conservation using polarization resistance (R p) and electrochemical impedance spectroscopy (EIS). Carbon steel samples were prepared to simulate the surface composition and morphology of historic steel artefacts, and coated by a conservator-restorer following the common practices in conservation treatments. Three commercial organic coatings have been studied: a microcrystalline wax (Renaissance TM) and a methyl acrylate/ethyl methacrylate copolymer resin (Paraloid TM B-72) dissolved in acetone-both them commonly used in conservation and restoration treatments-and a ethylene copolymer wax emulsion in water (Poligen TM ES-91009), that has not been used so far for this purposes. Four commercial corrosion inhibitor additives were added to the Paraloid TM B-72 resin and Poligen TM ES-91009 wax. The additives were commercial preparations with the following known active components: a blend of triazoles (M435), an ammonium salt of tricarboxylic acid (M370), a calcium sulphonate (M109), and a bis-oxazoline (Alkaterge-T TM). R p and EIS results showed that the best protection of the steel specimens was afforded by Poligen TM ES-91009 when applied in thick layers. None of the additives have shown a clear improvement of the protection properties of the coatings, and one of them impaired the barrier effect of the coating.

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The focus of this study consists of examining how corrosion potential measurements can contribute in providing information on the effectiveness of storage and stabilization treatments of copper alloys in aqueous solutions. We report on the electrochemical behavior of artificial copper alloy coupons (covered or not with corrosion layers), simulating the behavior of real artifacts, immersed in sodium sesquicarbonate solutions. Particular attention is given to the transformation of the corrosion layer as a function of time. In addition, synchrotron radiation X-ray diffraction measurements are performed before and after the treatment in order to understand the reactions that take place during the immersion processes.Copper alloys recovered from marine environments can be heavily contaminated with chloride ions. While cuprous chloride (nantokite) may lie dormant in the existing corrosion layer, a reaction with moisture and oxygen causes this unstable compound to expand in volume on conversion to one of the copper trihydroxychlorides (atacamite, paratacamite). This creates physical stress within the object affected, resulting in cracking or fragmentation. Ultimately this behavior, often called bronze disease, can reduce an apparently solid object into a heap of powder. For this reason, archaeological copper alloys recovered from wet saline environments should not be exposed directly to the atmosphere, as the metal will then corrode at an accelerated rate in the oxygen-rich air. 1-3 Alternatively, the contamination will have to be treated if the copper alloys are to be stabilized against continuing corrosion.Copper-based objects are therefore usually stored and stabilized (i.e., chloride ions are being leached from the corrosion layer) in a solution using either tap water or diluted sodium sesquicarbonate (equimolar mixture of sodium hydrogen carbonate and carbonate). 2,4-6 Several other methods for stabilizing corroded copper alloys are in practice as well, such as washing the objects in alkaline dithionite, in citric acid inhibited with thiourea, or in aqueous acetonitrile; however, the use of tap water or a sodium sesquicarbonate solution remains the most commonly used, since this procedure proves to be the least damaging to the patina. 2,5 Nevertheless, results often show a certain instability of the artifacts in solution, such as the chemical transformation of the natural patina. Examples include the formation of tenorite (CuO) 7 or chalconatronite (Na 2 Cu(CuO 3 ) 2 ‚3H 2 O). 8 The occurrence of these side effects means that monitoring of the storage and stabilization treatments remains necessary.A literature search shows that storage and stabilization processes are mainly monitored by analyzing the chloride concentration of the solution at regular time intervals. [5][6]9 This is very useful to monitor the chloride removal from the corrosion layer but does not provide information with regard to possible side effects taking place at the metal surface, such as the modification of the natural patina or the develop...