Electrochimica Acta. Vol. 38, No. 7, pp. 981-987, 1993 Printed zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA in Great Britain. 0013-4686/93 56.00 + 0.00 0 1993. Fwgamon Prrss zyxwvutsrqp Ltd. ELECTROCHEMICAL BEHAVIOUR OF COPPER ELECTRODE IN CONCENTRATED SULFURIC ACID SOLUTIONS A. H. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA MOREIRA,* A. V. BENEDETTI,* P. L. CAB~T~ and P. T. A. SUMODIO$ *Institute de Quimica da Universidade Estadual Paulista, CP. 355,14800-Araraquara, Brazil tDepartamento de Quimica Fisica, Universidad de Barcelona, Av. Diagonal 647, 0802&Barcelona, Spain $Instituto de Quimica da Universidade de SHo Paulo, CP. 20780,01498_SH0 Paulo, Brazil (Received 28 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF M ay 1992; in revisedform 8 October 1992) Abstract-The electrochemical behaviour of copper in 6.0moll-’ sulfuric acid at 3o”C, was studied by means of the potentiodynamic method. At low potential sweep rates, u < ZOOmVs-‘, the data reveal that the anodic process is basically constituted of copper dissolution and a film formation which inhibits further metal oxidation and which may undergo further dissolution. For higher potential sweep rates, a modification in the passivation region of the voltammogram is observed. It can be ascribed to a change in the passivation mechanism which possibly involves different surface species. The kinetic relationships derived from the poten- tiodynamic l/E curves obtained at low u suggest a film formation via a dissolution/precipitation mecha- nism. Key words: copper, sulfuric acid, potentiodynamic studies, cyclic voltammetry, dissolution/precipitation process. INTRODUCTION The electrochemical behaviour of copper in the dif- ferent aqueous media has been extensively studied since the beginning of this century; a comprehensive review was made by Bertocci and Turner[l J. Corro- sion characteristics in acidic media have been recent- ly reviewed by Smyrl[2]. At low concentrations of the acid it is already well established that copper dissolution is the main anodic process. The mechanism proposed by Bockris and co-workersp, 41 is generally accepted[2], although other mechanisms were also proposed[& 63. However, it is also well known that at more con- centrated solutions passivation may occur[7]. Differ- ent authors have observed the formation of a cuprous oxide layer on the surface[8-13-J. Elias[ll] has admitted that during passivation there is forma- tion of a Cu,O layer followed by another of CuO. The solubilization of this latter oxide exposes the former layer, which passivates the metal. On the other hand, experiments using stationary methods and rde and rrde techniques allowed the proposal of a more general mechanism of the dissolution- precipitation process[12]. These authors point out that the cuprous and cupric oxides formed during the anodic polarization, in contact with H,SO, should undergo a transformation into Cu, Cu,SO, and CuSO,. Other authors suggest that passivation occurs via formation and growth of a CuSO, film instead of the corresponding cuprous oxide[14-161. Therefore, although studied considerably, the subject still suggests aspects to be examined further. In con- centrated sulfuric acid, it is mostly due to the con- flicting conclusions regarding either the proposal of mechanisms or the nature of the species involved in the passivating process. Most of the related studies were performed at con- stant current conditions, basically because the process of electrochemical machining is carried out this way[I, 173. Very few studies were performed using other techniques such as forced convection[12], alternating current[13] or quasi- stationary potential sweep[9]. Almost no studies were made using the potentiodynamic method, a powerful tool for the understanding of surface pro- cesses, such as the study of the electrochemical behaviour of copper in alkaline media[18-241. Therefore, in order to contribute to the under- standing of the electrochemistry of copper in acidic media, the present work discusses the poten- tiodynamic E/I behaviour of copper in concentrated sulfuric acid solution. EXPERIMENTAL DETAILS All the electrochemical studies were performed in a conventional three electrode cell. The working electrode was made by encapsulating Cu 99.9% cylinders in pipe shaped glass holders to expose a disc with a diameter of 0.8mm to the solution. The exposed base of the cylinder was oriented vertically in the cell to permit the elimination of gas bubbles. The potential of the working electrode was measured against a reversible hydrogen electrode in the same solution (rhe), connected to the cell via a Luggin cap- illary. The auxiliary electrode was a large area plat- inized Pt wire. All the experiments were carried out at 30°C with 50cm3 of deaerated 6.0 M H$O., solu- tion, prepared from Merck p.a. H,SO, and tri- distilled water. 981