Régine Basséguy

Checking graphite and stainless anodes with an experimental model of marine microbial fuel cell.

A procedure was proposed to mimic marine microbial fuel cell (MFC) in liquid phase. A graphite anode and a stainless steel cathode which have been proven, separately, to be efficient in MFC were investigated. A closed anodic compartment was inoculated with sediments, filled with deoxygenated seawater and fed with milk to recover the sediments sulphide concentration. A stainless steel cathode, immersed in aerated seawater, used the marine biofilm formed on its surface to catalyze oxygen reduction. The cell implemented with a 0.02m(2)-graphite anode supplied around 0.10W/m(2) for 45 days. A power of 0.02W/m(2) was obtained after the anode replacement by a 0.06m(2)-stainless steel electrode. The cell lost its capacity to make a motor turn after one day of operation, but recovered its full efficiency after a few days in open circuit. The evolution of the kinetic properties of stainless steel was identified as responsible for the power limitation.

Corrosion of carbon steel by bacteria from North Sea offshore seawater injection systems: Laboratory investigation

Influence of sulfidogenic bacteria, from a North Sea seawater injection system, on the corrosion of S235JR carbon steel was studied in a flow bioreactor; operating anaerobically for 100days with either inoculated or filtrated seawater. Deposits formed on steel placed in reactors contained magnesium and calcium minerals plus iron sulfide. The dominant biofilm-forming organism was an anaerobic bacterium, genus Caminicella, known to produce hydrogen sulfide and carbon dioxide. Open Circuit Potentials (OCP) of steel in the reactors was, for nearly the entire test duration, in the range -800<E(OCP)/mV (vs. SCE)<-700. Generally, the overall corrosion rate, expressed as 1/(Rp/Ω), was lower in the inoculated seawater though they varied significantly on both reactors. Initial and final corrosion rates were virtually identical, namely initial 1/(Rp/Ω)=2×10(-6)±5×10(-7) and final 1/(Rp/Ω)=1.1×10(-5)±2.5×10(-6). Measured data, including electrochemical noise transients and statistical parameters (0.05<Localized Index<1; -5<Skewness<-5; Kurtosis>45), suggested pitting on steel samples within the inoculated environment. However, the actual degree of corrosion could neither be directly correlated with the electrochemical data and nor with the steel corrosion in the filtrated seawater environment. Further laboratory tests are thought to clarify the noticed apparent discrepancies.

Impact of the chemicals, essential for the purification process of strict Fe-hydrogenase, on the corrosion of mild steel.

The influence of additional chemical molecules, necessary for the purification process of [Fe]-hydrogenase from Clostridium acetobutylicum, was studied on the anaerobic corrosion of mild steel. At the end of the purification process, the pure [Fe-Fe]-hydrogenase was recovered in a Tris-HCl medium containing three other chemicals at low concentration: DTT, dithionite and desthiobiotin. Firstly, mild steel coupons were exposed in parallel to a 0.1 M pH7 Tris-HCl medium with or without pure hydrogenase. The results showed that hydrogenase and the additional molecules were in competition, and the electrochemical response could not be attributed solely to hydrogenase. Then, solutions with additional chemicals of different compositions were studied electrochemically. DTT polluted the electrochemical signal by increasing the Eoc by 35 mV 24 h after the injection of 300 μL of control solutions with DTT, whereas it drastically decreased the corrosion rate by increasing the charge transfer resistance (Rct 10 times the initial value). Thus, DTT was shown to have a strong antagonistic effect on corrosion and was removed from the purification process. An optimal composition of the medium was selected (0.5 mM dithionite, 7.5 mM desthiobiotin) that simultaneously allowed a high activity of hydrogenase and a lower impact on the electrochemical response for corrosion tests.

Discerning different and opposite effects of hydrogenase on the corrosion of mild steel in the presence of phosphate species

Mild steel coupons were exposed to hydrogenase in a 10 mM phosphate solution. Control coupons were covered by a layer of vivianite. The injection of hydrogenase caused a fast increase in the open circuit potential; this increase depended on the amount of hydrogenase injected and increased from 8 mV for 30 μL hydrogenase to 63 mV for 80 μL. The presence of enzyme resulted in a thicker deposit: high amounts induced the accumulation of corrosion products. Hydrogenase that was deactivated by air revealed a protective effect: non-degradation was observed. In contrast, hydrogenase that was denatured by heat provoked an important deposit of corrosion products with a heterogeneous, cracked structure. The study showed that the action of hydrogenase is not linked to its regular enzymatic activity but to a balance between the protective effect of its protein shell and the electrochemical action of its iron-sulphur clusters. Depending on the operating conditions, hydrogenase can either enhance or mitigate the formation of a corrosion layer on mild steel.