EUROCORR 2018: applied science with constant awareness: part 4*

Abstract

Sessions on this topic took place on the Tuesday, chaired by D. Neff (Laboratoire Archéomatériaux et Prévision de l’Altération, Gif-sur-Yvette, France), S. Grassini (Polytecnico di Torino, Italy) and D. Watkinson (Cardiff University, UK). There were 16 oral presentations (six full papers), and four posters (abstract-only) on this topic. Four full papers and one paper abstract are summarised herein. ‘Corrosion inhibition of bronze alloy by aloe vera extract in neutral media for application on archaeological bronze artefacts’ was described by A. Abdelwahab (Grand Egyptian Museum Conservation Centre, Mansoura, Egypt). Various chemical components of aloe vera extract, especially the tannins, can be adsorbed onto the metal surface and block the active sites, thereby reducing the corrosion rate. In this work, the inhibition efficiency for binary Cu-Sn alloy coupons (simulating archaeological bronze alloys) in 3.5% NaCl was studied by electrochemical impedance spectroscopy (EIS), potentiodynamic polarisation, colourimetry and a salt spray test. The results demonstrated effective inhibition by aloe vera extract. The efficiency increased with increased concentration of inhibitor, but decreased with increased temperature. The maximum efficiency of 85.04% was obtained with 160 ppm of extract. D. Mills (University of Northampton, UK) spoke on ‘Insitu evaluation of the protectivity of coatings applied to metal cultural artefacts using non-destructive electrochemical measurements’ (Figure 1). A laboratory investigation was conducted using the DC resistance method to evaluate the protection afforded by typical coatings used at the National Museum of Gdansk on copper and steel Q panels. Coatings on actual archaeological artefacts on display inside the museum building were then examined in situ. The technique of choice for the latter was the less intrusive electrochemical noise measurement (ENM), in which cells are mounted on one piece of substrate, the current and potential noise between the substrate and two SCEs is recorded and the resistance noise (Rn) is calculated. Results from a range of items recovered from shipwrecks in the Baltic sea provided some evidence of correlation between coating thickness and the level of protectivity. K. Strachotova (University of Chemistry and Technology, Prague, Czech Republic) described the ‘Consolidative electrochemical reduction of carbonate-based lead corrosion products’. Long-term exposure of lead to corrosive organic acid vapours leads to formation of insoluble lead carbonates. The attack is followed by periodic liberation of the acid, thus re-initiating corrosion in an auto-catalytic process. Hence, removal of corrosion products is a promising measure for preventing further degradation. The present work compared potentiostatic and galvanostatic methods for the consolidation of corrosion products on artificially corroded lead samples in 0.5 mol/l sulphuric acid, 0.5 mol/l sodium sulphate or 3% disodium edetate. Although cathodic polarisation prevented formation of new lead sulphate corrosion products, sodium sulphate seemed very ineffective for the transformation of carbonate-based corrosion products. Meanwhile, disodium edetate showed extreme corrosivity towards lead in the absence of cathodic polarisation. V. Gouda (National Research Centre, Dokki, Egypt) spoke on the ‘Characterisation of a collection of ancient Egyptian metallic artefacts from the museum’ (Figure 2). Artefacts made from copper base alloys during specific periods (new kingdom and late period) before the Greco Roman Period and stored in the basement of the Egyptian Museum were examined. The elemental compositions were determined by non-destructive X-ray fluorescence (XRF) technique to indicate a mainly ternary system of Cu Pb Fe alloys for three items, but a mainly quaternary system of Cu Pb Fe Sn alloys for 14 items. While the percentage of iron varied from 0.5– 20%, themajority of objects showed a range of 2–4%. This contrasts to the literature, where only small concentrations of iron <0.1% have been found in copper alloys of that period – suggestive of a smelting impurity rather than deliberate inclusion. ‘Evaluation of the laser cleaning mechanism on Cu-based corrosion products’ (abstract only) was described by E. Di Francia (Politecnico di Torino, Italy). Laser cleaning is favoured in cultural heritage because the ablation process can selectively remove thin layers in a highly precise and controlled way without causing damage. To examine whether compounds found after laser cleaning are due to re-oxidation of ablated materials by the atmosphere, the present study performed laser cleaning tests in the presence of O-enriched air. The levels of O in the laser-treated samples relative to the normal atmospheric levels were measured by time-offlight secondary ion mass spectrometry (ToF-SIMS). In this way, any re-oxidation and re-deposition of the etched material would be detectable. While initial results showed that re-oxidation and re-deposition happen during highenergy ablation, further study is needed to verify a correlation with the power used.