Electrochemical monitoring of the storage or stabilization of archaeological copper based artefacts in sodium sesquicarbonate solutions

Archaeological copper-based artifacts recovered from wet and salty environments are often stored or stabilized in sodium sesquicarbonate solutions. Modification of the natural patina and development of active corrosion can occur during these processes, which implies the need for monitoring storage/stabilization processes. The focus of the study consists of examining how corrosion potential (Ecorr) measurements and voltammetric curves can contribute in providing information on the effectiveness of storage and stabilization treatments. Particular attention is given to side effects such as the transformation of the corrosion layers

Electrochemistry reveals archaeological materials

The characterization of materials constituting cultural artefacts is a challenging step in their conservation, due to the object’s uniqueness and the reduced number of conservation institutes able to supply non-destructive analysis. We propose an alternative analytical tool, which combines accessibility (low cost and portable) and high sensitivity, based on electrochemical linear sweep voltammetry (LSV) with paraffin impregnated graphite electrode (PIGE). To investigate the composition of “white alloys” that certainly have been used as decoration on copper-based Roman fibulae, sampling was done very locally by gently rubbing the selected areas with the PIGE. LSV results evidence the presence of silver, lead, and tin, supporting the argument provided by typological analysis that these metals were used for decoration

Study of Corrosion Potential Measurements as a Means to Monitor the Storage and Stabalization Processes of archaeological Copper Artefacts

Archaeological copper artefacts recovered from wet saline environments are often stored in tap water and stabilized in sodium sesquicarbonate solutions. Modification of the natural patina and development of active corrosion can occur during these processes. This implies that monitoring of storage/stabilisation processes is necessary. The focus of the study consists of examining how corrosion potential (Ecorr) measurements can contribute in providing information on the effectiveness of storage and stabilisation treatments. This paper reports on the Ecorr versus time plots of artificially prepared copper coupons (covered or not with corrosion layers) immersed in tap water and a sodium sesquicarbonate solution. Synchrotron radiation XRD was performed in parallel to understand the reactions that take place during the immersion processes

Engineering a highly defective stable UiO-66 with tunable Lewis-Brønsted acidity : the role of the hemilabile linker

The stability of metal-organic frameworks (MOFs) typically decreases with an increasing number of defects, limiting the number of defects that can be created and limiting catalytic and other applications. Herein, we use a hemilabile (Hl) linker to create up to a maximum of six defects per cluster in UiO-66. We synthesized hemilabile UiO-66 (Hl-UiO-66) using benzene dicarboxylate (BDC) as linker and 4-sulfonatobenzoate (PSBA) as the hemilabile linker. The PSBA acts not only as a modulator to create defects but also as a coligand that enhances the stability of the resulting defective framework. Furthermore, upon a postsynthetic treatment in H2SO4, the average number of defects increases to the optimum of six missing BDC linkers per cluster (three per formula unit), leaving the Zr-nodes on average sixfold coordinated. Remarkably, the thermal stability of the materials further increases upon this treatment. Periodic density functional theory calculations confirm that the hemilabile ligands strengthen this highly defective structure by several stabilizing interactions. Finally, the catalytic activity of the obtained materials is evaluated in the acid-catalyzed isomerization of a-pinene oxide. This reaction is particularly sensitive to the Bronsted or Lewis acid sites in the catalyst. In comparison to the pristine UiO-66, which mainly possesses Bronsted acid sites, the Hl-UiO-66 and the postsynthetically treated Hl-UiO-66 structures exhibited a higher Lewis acidity and an enhanced activity and selectivity. This is further explored by CD3CN spectroscopic sorption experiments. We have shown that by tuning the number of defects in UiO-66 using PSBA as the hemilabile linker, one can achieve highly defective and stable MOFs and easily control the Bronsted to Lewis acid ratio in the materials and thus their catalytic activity and selectivity

Insights into electrolytic stabilization with weak polarization as treatment for archaeological copper objects

Immersion of corroded copper artefacts in dilute sodium sesquicarbonate solution is a well-recognized stabilization technique-especially in the conservation of objects recovered from marine environments and therefore saturated with chlorides. Here we describe three linked experiments performed to investigate a variation on this treatment, involving the application of a low potential to the artefact in order to drive the chloride extraction process. This includes a new spectroelectrochemical approach which allows 2-D pseudorandom X-ray reflection diffraction patterns to be obtained without interrupting the reaction in solution. Experiments were carried out on synthetically produced chloride layers on copper (nantokite and atacamite). We show that a thick chloride layer is, in general, replaced by a thin cuprite layer through a mechanism which involves detachment of the chloride crystallites from the surface prior to dissolution

An in-situ X-ray absorption spectroelectrochemistry study of the response of artificial chloride corrosion layers on copper to remedial treatment

In the experiments described in this study, we make use of X-ray absorption spectroscopy (XAS) at the K-edge of copper for monitoring the surface–solution interface of corroded metals in-situ in a sodium sesquicarbonate solution. We show conclusively that, unlike previously published XRD measurements, the XAS spectra are almost wholly characteristic of the contaminated solution and not the corroded surface, in cases where the sesquicarbonate is effective at removing chlorides. This is true even when there is only 125 µm of fluid over the sample, which demonstrates that there is very effective absorption of the Cu Kα radiation from the surface by copper in solution. As a function of time we observe a rising intensity in the XANES and EXAFS regions which is accompanied by a proportional increase in the degree of EXAFS modulation. The latter is due to the fact that a significant proportion of the corrosion product goes into solution, rather than being converted to a stable compound on the surface. Moreover, the spectra show that the local atomic environment of the copper in solution is extremely similar to that in the surface. Overall from both XAS and previously performed XRD data we can conclude that the chlorine containing corrosion products become detached from the surface and go partly into solution, while a thin cuprite layer forms (or is already present at) at the original metal surface