Influence of natural seawater variables on the corrosion behaviour of aluminium-magnesium alloy.

Corrosion is an important issue for alloys in natural seawater, where microorganisms can accelerate or mitigate corrosion. Al-Mg alloys are used for marine activities and various associated technologies. Here, the behaviour of AA5083 alloy was investigated in natural seawater with marine exposure lasting up to 50 days and detailing the first 8 days in two experimental series. Experimental work was carried out, including semi-field tests in natural seawater (biotic conditions) compared with abiotic conditions. The open circuit potential (OCP) measurements, during the immersion time, exhibited significantly different behaviours: an OCP downward displacement occurred under abiotic conditions, while, in biotic conditions OCP remained generally stable since the beginning of the immersion, revealing an inhibiting effect of the biological activity on the Al-Mg corrosion. This was accompanied by different surface modifications under biotic conditions: surface and cross-section characterization, performed by scanning electron microscopy with energy dispersive X-ray spectroscopy, showed less corrosion developed on the surface after 8-day immersion and formation of a protective layer during 50-day immersion. The present study shows that marine biological activity positively influences the Al alloy corrosion process, with surface modifications resulting in a protective effect counteracting the aggressiveness of chloride ions.

Membrane electrochemical reactors (MER) for NADH regeneration in HLADH-catalysed synthesis: comparison of effectiveness.

Two membrane electrochemical reactors (MER) were designed and applied to HLADH-catalysed reduction of cyclohexanone to cyclohexanol. The regeneration of the cofactor NADH was ensured electrochemically, using either methyl viologen or a rhodium complex as electrochemical mediator. A semi-permeable membrane (dialysis or ultra-filtration) was integrated in the filter-press electrochemical reactor to confine the enzyme(s) as close as possible to the electrode surface. When methyl viologen was used, the transformation ratio of cyclohexanone varied from 0 to 65% depending on the internal arrangement of the reactor. Matching the reactor configuration to the reaction system was essential in this case. With the rhodium complex, the ultra-filtration MER was tested in continuous and recycling configurations. The best conditions led to 100% transformation of 0.1 L volume of 0.1 M cyclohexanone after 70 h with the recycling mode. Finally, the performances of the reactors are discussed with respect to different evaluations of the production yields.

Glucose oxidase catalysed oxidation of glucose in a dialysis membrane electrochemical reactor (D-MER).

The purpose of this work was to evaluate the effectiveness of a new Membrane Electrochemical Reactor (MER) for the production of gluconic acid by glucose oxidase (GOD) catalysed glucose oxidation. The GOD was confined against the electrode surface with a dialysis membrane. The role of the electrochemical step was to eliminate by oxidation the hydrogen peroxide that appeared as a by-product of the reaction and strongly inhibited and/or inactivated GOD. The dialysis MER gave a transformation ratio of 30% with an initial glucose concentration of around 300 mM. This result is significantly better than the maximum of 10% obtained when hydrogen peroxide was eliminated by addition of a large excess of catalase in solution, as is generally done. The D-MER also revealed unexpected properties of the enzyme kinetics, such as an oscillatory behaviour, which were discussed.

The role of hydrogenases in the anaerobic microbiologically influenced corrosion of steels.

The direct electron transfer between 316 L stainless steel and the NAD-dependent hydrogenase from Ralstonia eutropha was studied by spectroelectrochemistry. The presence of hydrogenase and NAD+ clearly increased the quantity of electricity, which was consumed during the electrolysis performed at potential lower than -0.70 V/SCE. The involvement of hydrogenase in the cathodic depolarisation theory was discussed in the light of these results.

Designing membrane electrochemical reactors for oxidoreductase-catalysed synthesis.

The purpose of this work was to design an electrochemical reactor to enhance the high selectivity of enzyme-catalysed processes. In order to develop economically efficient syntheses, the enzymes must be confined in the strict vicinity of the electrode surface. Here the confinement was achieved with a dialysis membrane in a so-called Dialysis-Membrane Electrochemical Reactor (D-MER). Oxidation of glucose into gluconic acid catalysed by glucose oxidase was a first example. The ADH-catalysed reduction of cyclohexanone into cyclohexanol was also tested in a new type of MER. NADH was electrochemically regenerated thanks to mediator (methyl viologen or rhodium complex). The key point in developing electro-enzymatic process is to ensure the perfect fitting of the reactor design to the reactions that are to be processed.