Alain Bergel

5 – Local analysis by SVET of the involvement of biological systems in aerobic biocorrosion

When a material is used in a non-sterile natural environment, microorganisms colonize its surface and grow to form a biofilm. The local conditions beneath the biofilm may induce severe biodeterioration and biocorrosion phenomena. Mechanisms in aerobic natural waters are still discussed although it is commonly agreed that the biofilm catalyses the reduction of oxygen. Several hypotheses have been proposed to explain this catalysis: the formation of hydrogen peroxide, the modification of the oxides on the material surface and the presence of biological molecules (enzymes as peroxidase, catalase …) that contribute to the different steps of oxygen reduction. In this work, this last hypothesis has been investigated. Generally studies in biocorrosion are based on the measure of global parameters (corrosion potential versus time, current under polarisation …). On the contrary, the aim of the work presented here was to develop a local approach to study material/biofilm interfaces. The SVET was used to put in light coupling between physico-chemical conditions, local mass transfers, surface state and electrochemical behaviour, which led to aerobic biocorrosion. The purpose of this paper is to demonstrate the ability of the SVET to probe catalysis phenomena induced by biofilm. In order to simplify the studied system, the activity of natural biofilm on surface was simulated by immobilising hemic proteins (hemin or myoglobin) on the material. The hemic protein was chosen as model of the enzymes that are able to catalyse oxygen reduction. Two systems were tested: • Adsorption of hemin from DMSO on a stainless steel surface • Successive deposits of myoglobin and Poly(EthyleneImine) on a graphite surface. To perform SVET analysis, galvanic coupling was prepared on sample surface: it was obtained by immobilising the catalyst only on a defined area of the sample. The first results open new routes in understanding biocorrosion and defining strategies to avoid it. They clearly show that the SVET should be a well-adapted technique to probe electrochemical activity of the biofilm.