Ageing of passivated materials in seawater : study of the oxygen reduction reaction C. COMPERE 2* , N. LE BOZEC 1 , M. L'HER 1 , A. LAOUENAN 1 , D. COSTA 3 and P. MARCUS 3 1 UMR 6521, Faculté des Sciences, BP 809, 29285 BREST CEDEX, France 2 Service Matériaux et Structures, IFREMER, BP 70, 29280 PLOUZANE, France 3 Laboratoire de Physico-Chimie des Surfaces, Ecole Nationale Supérieure de Chimie de Paris, 11 rue P. et M. Curie, 75 005 PARIS, France ABSTRACT A comparative study of stainless steel (SS) behaviour as a function of immersion time in natural and artificial seawater is presented. It concerns the evolution of : the open circuit potential, the oxygen reduction reaction and the passive layers composition (XPS). The results reported in this paper show that a combination of parameters is probably involved in the mechanisms of SS ennoblement in natural seawater. Introduction Materials such as stainless steels (SS) immersed in natural seawater are rapidly covered with a biofilm [1-3]. Simultaneously, their electrochemical behaviour is modified. Numerous works are reported in the literature [4-6] but the understanding of these complex mechanisms is still subject to investigations. Several hypotheses have been proposed to explain the ennoblement of the free corrosion potential and the reasons for the modifications in cathodic activity [7]. Among them, Chandrasekaran and Dexter [8] have suggested that hydrogen peroxide could be produced, in the biofilm, by some of the living organisms. This idea was then adopted by Dupont and co-workers [9, 10] : They proposed an enzymatic activity of aerobic organisms producing hydrogen peroxide (H 2 O 2 ) with a catalytic effect on the oxygen reduction reaction. However, no electrochemical study of the oxygen reduction reaction has been so far simultaneously carried out in artificial and natural seawaters. The present paper gives further results with information in different media such as flowing natural and artificial seawaters and warns against hasty conclusion. Special attention must be paid to the fact that the oxygen reduction reaction may be described as following a 2 or 4 electrons transfer with H 2 O 2 as an intermediate or final product. The effects of H 2 O 2 on the cathodic activity will also be reported. Since the biofilm growth seems to have an important role in the modifications of the SS electrochemical parameters, the effects of composition, thickness and ageing of passive films in seawaters must not be neglected.