First time assessment of Geobacter sulfurreducens role in the microbially influenced corrosion of steels

Losses due to corrosion are evaluated at 4% of the GDP of industrialised countries and Microbially Influenced Corrosion (MIC) may be responsible for 10% of these costs [1]. Whereas the general mechanism of anaerobic biocorrosion involving sulphate reducing bacteria (SRB) seems now well documented but the implication of other types of bacteria is often suspected. Geobacter species have the capability to oxidize organic electron donor to carbon dioxide transferring the electron directly to electrodes [2]. On the other side, the ability of Geobacter sulfurreducens to reduce nitrate to nitrite or fumarate to succinate with a graphite electrode serving as electron donor has also been demonstrated recently in the field of microbial fuell cell design [3]. Direct electron transfer to solid electrodes is achieved through periplasmic and outer membrane c-type cytochromes. Outer membranes proteins and even some kind of conductive pili that serve as biological nanowires are also involved in the electron transfer chains, mainly to Fe(III) and Mn(IV) oxides [4]. The ability of G. Sulfurreducens to use the electrode as electron donor has led us to ask whether G. Sulfurreducens could create a new cathodic reaction in the MIC process. The aim of this work was to study the possible influence of G. sulfurreducens on the occurrence of corrosion of steels. Experiments were performed in anaerobic conditions with pure cultures of G. sulfurreducens on four different kinds of steels. In each case the free potential increased by 300 mV after the injection of the bacteria whereas control experiments performed with the injection of the sterile medium or the bacteria suspension after filtration on a 0,2 µm filter did not induce any variation in the free potential. The presence of G. sulfurreducens was directly responsible for the potential increase and consequently the increase of corrosion risk. The steels were analysed using SEM and EDS techniques as well as confocal laser microscopy. Fig. 1 shows that almost no corroded zones were noticed in the presence of the filtrate (micrograph a) whereas with G. sulfurreducens, large and deep corroded areas grouped by zones were observed (micrograph b). It was also demonstrated that the potential step is controlled by the amount of bacteria contained in the inoculum and by the initial concentration of the electron donor (acetate). Depletion of electron donor forces G. Sulfurreducens to use the steel as electron source, but small concentration is necessary These preliminary results may introduce for the first time into the framework of biocorrosion the new mechanism of microbial direct electron transfer with Geobacter species.