Characterization of archaeological bronze and evaluation of the benzotriazole efficiency in alkali medium H. Hassairi, L. Bousselmi * , S. Khosrof and E. Triki An analytic study permits us to characterize the altered surface of an archaeological bronze coin and to determine the structure and the composition of the patina covering the whole surface of this artefact. This patina could be interpreted at first sight as a type I patina, with copper product deposits on it, with some punctual and enlarged localized type II corrosion. To achieve the purpose of this work, we investigate the beha- viour of an archaeological bronze in the presence of benzotriazole (BTA) in alkali medium with the intention of getting a better passivity while favouring the formation of a polymeric film on the surface of the working electrode. The behaviour of the con- sidered interface is investigated by electrochemical impedance spectroscopy, in the presence and absence of an oxide layer, according to the immersion time. At pH ¼ 9, in the presence of 15 mmol/l of BTA, the optimum percentage of inhibition efficiency (IE%) is 67% obtained after 30 min of immersion. A pre-polarization of the bronze working electrode is realized in order to accommodate the preservation technique used in museums and to improve the formation of the Cu(I)-BTA polymeric film. The use of a pre-polarized electrode for 30 min at 35 mV/SCE carries an enhancement of the protection versus the non-polarized electrode. While comparing the result of our investigation with that obtained using the traditional preserva- tion method, we can establish that using a concentration of BTA 15 times lower, important inhibitor efficiencies (%) of 92 and 97.4%, respectively, for 30 min and 96 h of immersion are reached. 1 Introduction An archaeological object is a technological, historic and sometimes artistic document. Information that it transmits concerns several scientific domains. It applies all as much to the metallic materials as to the corroded surfaces. Archaeological bronze artefacts have the particularity to be able to form, in many natural corrosive mediums, a pleasant protective layer sign of antiquity. This layer is called patina and appears to be generally brownish-green or greenish-blue. Patinas are chemically and metallurgically complex structures. It is essentially due to the natural reaction of copper content in bronze with the dioxide of carbon, the oxygen, the sulphuric acid and the humidity of air or soil [1–7]. A large number of studies on ancient and historical bronzes have tried to establish the chemical characteristics and structure of natural patinas grown on artefacts exposed to soil [2–6]. Previous investigations concerning the classical structure of corrosion products of copper alloys by Organ [5] reveal that the main constituents of the surface layer of the patina are green-coloured copper (II) compounds covering a red cuprous oxide layer in contact with the metal core of the alloy. Depending on the environment these copper (II) salts could be malachite Cu 2 (CO 3 )(OH) 2 formed in soil, brochantite CuSO 4 3Cu(OH) 3 in the atmosphere and atacamite CuCl 2 3Cu(OH) 2 in the seawater. The corrosion mechanism of Cu–Sn alloys is today explained by applying the copper model [2,3,8,9]. A mechanism of formation of the tin-enriched corrosion layer has been proposed, involving a decuprification process through the layer, and the migration of environmental anions through it. After the excavation of metallic artefacts, environment modifications can ensure the resurgence of corrosion which leads to the destruction of the materials. Thus, the stabilization of actively corroding archaeological bronzes remains a difficult problem for conservators [1,10]. However, a deep understanding of the decay processes is necessary for restoration purposes. Moreover, in the present time the use of benzotriazole (BTA) has become a standard element in the conservation of cuprous-based metals. In a pioneer work, Dugdale and Cotton [4] proposed in 1963 an electrochemical investigation of the BTAH behaviour of copper in an NaCl solution. Since that time, many efforts have been made to elucidate the mechanism of action of BTA and the mode of fixation of the molecule at a copper (oxide) surface. The treatment with BTA does not remove the cuprous chloride from the artefact; rather, it forms a barrier between the cuprous chloride and moisture of the atmosphere [7]. In this method of protection, the BTA forms an insoluble, complex compound with cupric ions. The 32 DOI: 10.1002/maco.200704064 Materials and Corrosion 2008, 59, No. 1  L. Bousselmi, H. Hassairi, E. Triki Unite ´ de Recherche ‘‘Corrosion et Protection des Me ´talliques’’, Ecole Nationale d’Inge ´nieurs de Tunis, 1002 Tunis (Tunisia) E-mail: latifa.bousselmi@certe.rnrt.tn H. Hassairi Laboratoire de Traitement et Recyclage des Eaux Use ´es, Centre de Recherche et des Technologies des Eaux, Route Touristique de Soliman, 8020 Soliman (Tunisia) S. Khosrof Laboratoire de Conservation-Restauration, Institut National du Patrimoine, 2000 Bardo (Tunisia) www.wiley-vch.de/home/wuk ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim