Ekcfrochimica Acta, Vol. 38. No. IO, pp. 1471-1479. 1993 0013~4686/93 $6.00 + 003 Printed in Great Britain. Q 1993. Pergamon Press Ltd. zyxwvutsrqp ELECTROCHEMICAL AND PHOTOELECTROCHEMICAL CHARACTERIZATION OF NATURALLY GROWN OXIDE LAYERS ON COPPER IN SODIUM ACETATE SOLUTIONS WITH AND WITHOUT BENZOTRIAZOLE E. M. M. Sum,* C. FIAUD and D. LINCOT Laboratoire d’Eleetroehimie Analytique et AppliquCe,Ecole Nationale Sup&ieure de Chimie de Paris, 11 rue Pierre et Marie Curie, F-75321 Paris Cedex 05, France (Received 18 May 1992; in revisedform 4 January 1993) Abstract-The aim of this work is the characterization of as-grown oxide layers of copper in aerated sodium acetate solutions, in relation to the understanding of the mechanism of protection in the presence of benxotriaxole, which is a well known inhibitor of corrosion. The growth and the characteristics of the film at rest potential are studied using cathodic stripping analysis in a deaerated solution. Its properties are determined by using the photoelectrochemical technique. It is shown that in BTA solution, the pho- toactive layer is the same as without BTA, giving the same optical indirect transitions at 2.6 and 1.9 eV, its thickness being in the range of a few nm. Both cathodic and anodic photocurrents are generated and the particular dependence of the spectral response as a function of potential is evidenced and explained. At the rest potential both photocurrents are zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH sup e rim p o se d . The layer is attributed to cuprous oxide, weakly modified with BTA. In the presence of BTA, changes in the reduction curves are observed and special attention is paid to a cathodic wave in excess, specific to BTA-aerated solutions. The formation of a polymeric copper(I)-BTA outer film is suggested for the formation of a Cu(II)-superoxide complex related to the uptake of molecular dioxygen. Key words: copper, passive film, benxotriaxole, photoelectrochemistry, coulometry. 1. INTRODUCTION Numerous studies have been devoted to the electro- chemistry of copper electrodes in aqueous solutions in relation to the formation and reduction of oxide layers. Two anodic peaks AI, AI1 and two associated cathodic peaks CI and CII are characteristic of the system[l-71, AI and CI correspond to the reaction involving cuprous oxide: 2Cu + H,OoCu,O + 2H+ + 2e- zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE E” = 0.46 VJshe and AI1 and CII correspond to reactions involving cupic oxide : Cu,O + H,Oo2CuO + 2H+ + 2e- E” = 0.67 V/she Cu,O + 3H2002Cu(OH)2 + 2H+ + 2e- E” = 0.73 V/she Whatever the pH, the calculated values are in between the experimental ones due to appreciable kinetic limitations[l]. Modifications of the peak positions have also been related to some rearrange- ment of the films after aging, for instance[8]. The oxide layer has, in general, a duplex structure made up of an inner Cu,O layer followed by CuO and then a Cu(OH), layer, depending on the electrode potential. Strehblow and Titxe[l] found that the l also: Universite de Versailles-St Quentincn-Yvelines 45 avenue des Etats-Uniq F-78000 Versailles, France. inner layer thickness was in the range of lo-15 A and weakly dependent upon the conditions, whereas the CuO/Cu(OH), thickness could increase up to 60 A more or less linearly with the applied potential in the pH range 7-9.2. On the other hand, due to the semiconducting properties of the anodic films on copper, photoelec- trochemical studies have been performed to further characterize the properties of these films[2, 3, 7, 9-133. The films generally behave as p-type semicon- ductors giving rise to a cathodic photocurrent. In some cases anodic photocurrents have been obtained in the anodic polarization range[lO, 111. The analysis of the spectral dependence of the photo- current generation allows us to attribute it mainly to the response of Cu,O films and to determine the values of optical transitions[ 11, 123. The influence of the duplex structure Cu,O/CuO has also been studied[12]; CuO is found to lower the gap values, together with an increase in the flat band potential of the oxide layer[12]. However, the features con- cerning the photoelectrochemical results are not as well resolved as those for the electrochemical behav- iour and some discrepancies appear in the literature. Apart from these studies, mostly carried out in classical electrolytes, the understanding of the physi- cochemical characteristics of the inhibition of copper after the addition of organic additives is an impor- tant field of investigation. BTA, for instance, is a well known inhibitor of copper corrosion[14-18). It was suggested[lS] that benxotriazole forms a [Cu(I) BTA] surface complex and that the presence of a Cu,O layer on the copper surface greatly facilitates 1471