Evaluation of Geobacter sulfurreducens influence on the anaerobic biocorrosion of steels.

Abstract

Corrosion costs 4% of GDP of industrialised countries out of which 20% are due to Microbially Influenced Corrosion (MIC) (Dupon-Morral, 2004, Flemming). While the implication of Sulfato Reducing Bacteria is now well agreed (Réferences), few work studied the implication of other microorganisms in anaerobic corrosion. On behalf of that, direct electron transfer to solid electrodes by some bacteria has been recently discovered and implemented in fuel cells. Among those microorganisms, Geobacteraceae are the most widespread microorganisms in soils and sediments in which microbial reduction of Fe(III) is an important process, either in the natural degradation of organic compounds or in their bioremediation. Geobacter species have been shown to be predominant microorganisms on electrodes harvesting electricity from the sediments. They have the capability to switch from a soluble electron donor or acceptor to an insoluble one. Consequently, they oxidize organic electron donor to carbon dioxide transferring the electron directly to graphite electrodes (Esteve-Núñez) []. On the other side, the ability of Geobacter sulfurreducens to reduce nitrate to nitrite or fumarate to succinate with a graphite electrode serving as an electron donor has also been demonstrated in the field of microbial fuel cells (Holmes) (Gregory) []. 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 (Holmes) []. The aim of this study was to assess the influence of G. sulfurreducens on the occurrence of corrosion testing different kind of steels. Experiments were performed with pure cultures of G. sulfurreducens on mild steel (XC45) and three different kinds of stainless steels (ferritic steel, 304L, 316L). In all cases, the free potential EOC increased by 400 mV, 3 hours following the injection of the bacteria and continued to increase until it stabilised at t = 100h. On the contrary, 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 significant ennoblement of EOC. The presence of the micoorganisms was consequently directly responsible for the potential increase. Moreover, cathodic polarisation curves performed at the end of the experiments at a scan rate of 0.5 mV.s-1 showed that the pitting potential Epit depended on the concentration of the electron donor (acetate) present in the medium. When enough acetate was present in the medium, the pitting potential in the presence of the bacteria was 340 mV higher than in its absence as if Geobacter protected the steel from corrosion, yet the passivation currents increased up to six fold. Whereas when the medium lacked with acetate, Epit in the presence of the bacteria, was lower than Epit of the control in some cases and higher in others because it was more difficult for the bacteria to survive in this poor medium. The surface of the electrodes was then analysed by SEM and epifluorescence microscopy. The morphology and the amount of pits were discussed with respect to the potential increase and the composition of the medium. Figure 1 shows the distribution of G. sulfurreducens at the surface of a 304L SS electrode after immersion during 10 days in a Geobacter medium containing 5 mM acetate and 5% (v/v) bacteria. G. sulfurreducens tend to accumulate around the pits forming a dense biofilm (Fig. 1a) whereas less bacteria, distributed randomly, were present on the parts of the electrode that were pits free (Fig. 1b). Consequently, G. sulfurreducens tend to accumulate preferentially in the zones of the metal where there are some structural defects. During the same experiment, the steel behaved as an electron donor but also as an electron acceptor: - Steel is generally an electron supplier in biocorrosion: G. sulfurreducens acquired energy for growth by obtaining electrons through direct contact with the steel. - Enzymatic dosage of acetate at the end of the experiment confirmed that G. sulfurreducens oxidized partially electron donor (acetate) into carbon dioxide by using the electrode as an electron acceptor which explains Eoc ennoblement. In the presence of G. sulfurreducens, the pitting potential was shifted toward the positive values as if the bacteria were capable of protecting the steel. In fact, G. sulfurreducens might act as a “stocking reserve” for electrons. These electrons are released during the potential scan delaying the removal of electrons from the material. This ability of G. sulfurreducens to behave as a “stocking reserve” explains why the steel repassivated quicker. In conclusion, G. sulfurreducens created a new cathodic reaction and amplified the reduction kinetic. This might highly increase the corrosion risk especially in the presence of sulfur into the solution. This is the first time that the new mechanism of microbial direct electron transfer is clearly demonstrated into the framework of biocorrosion.