Evaluation of Corrosion Potential Measurements as a Means To Monitor the Storage and Stabilization Processes of Archaeological Copper-Based Artifacts

Abstract

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...

Tracking the progression of bronze disease – A synchrotron X-ray diffraction study of nantokite hydrolysis

“…Corrosion starts from an irregularity where the coupon was cut from a sheet. Sodium sesquicarbonate is in fact used as an active conservation treatment for chloride-infected metals [18,19]. It transforms nantokite to cuprite, therefore 'removing' chlorides from the copper artefact.…”

Time resolved spectroelectrochemistry studies for protection of heritage metals

“…In addition the copper samples were corroded artificially to obtain different chloride containing corrosion products, including nantokite (CuCl) and atacamite (Cu 2 (OH) 3 Cl). Details on these corrosion protocols can be found in previous papers by Leyssens et al 18,19 X-ray absorption spectroscopy (XAS) experiments were performed at DUBBLE (station BM26A, European Synchrotron Radiation Facility, Grenoble). Cu K edge (8?979 keV) XAS spectra were recorded as a function of energy defined by the stepping of an Si (1 1 1) double crystal monochromator.…”

Non-destructive analysis and testing of museum objects: An overview of 5 years of research

“…Therefore storage and stabilization treatments of the objects are usually carried out in aqueous solutions. Leyssens et al [91,92] have investigated whether corrosion potential measurements (Ecorr) can provide useful information on the effectiveness of the currently used stabilization and storage treatments. Results in the past have shown certain instability of the artifacts during their storage, including the transformation of natural patina and the development of active corrosion [93,94].…”