Role of the reversible electrochemical deprotonation of phosphate species in anaerobic biocorrosion of steels

Abstract

Sulphate reducing bacteria are known to play a major role in anaerobic microbiological influenced corrosion of steels, but mechanisms behind their influence are still source of debates as certain phenomena remain unexplained. Some experiments have shown that hydrogen consumption by SRB or hydrogenase increased the corrosion rate of mild steel. This was observed only in the presence of phosphate species. Here the cathodic behaviour of phosphate species on steel was studied to elucidate the role of phosphate in anaerobic corrosion of steel. Results showed: a linear correlation between reduction waves in linear voltammetry and phosphate concentration at a constant pH value; that phosphate ions induced considerable anaerobic corrosion of mild steel, which was sensitive to hydrogen concentration in the solution; and that the corrosion potential of stainless steel in presence of phosphate was shifted to more negative values as molecular hydrogen was added to the atmosphere in the reaction vessel. Phosphate species, and possibly other weak acids present in biofilms, are suggested to play an important role in the anaerobic corrosion of steels via a reversible mechanism of electrochemical deprotonation that may be accelerated by hydrogen removal.

Microbial electrolysis cell (MEC): Strengths, weaknesses and research needs from electrochemical engineering standpoint

“…Less energy is required to extract the hydrogen atom from the acid molecule than from the water molecule. This catalytic pathway is particularly effective on mild steel [168] and stainless steel [166,167] surfaces. It decreases the overpotential necessary for hydrogen evolution, working around neutral pH, using cathodes made of steel, which can be less expensive than nickel alloys.…”

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

“…Although the detailed mechanisms by which phosphate species lead to the formation of protective layers and the composition of the deposit obtained in phosphate solutions are still research topics, the main point is to have the right amount of Fe II (compared to Fe III) on the material surface that, in contact with the phosphate in the medium, leads to the formation of a vivianite deposit. This is the case in abiotic conditions when using chelating agent for instance [49]; the vivianite layer deposit also depends on phosphate concentration [50,8], on how the preceding oxide layer forms, which is linked to the experimental conditions (electrode potential [51] and the presence or not of oxygen [47,52]). In biotic conditions, other mechanisms are suggested: oxygen consumption by the biofilm as the driving force to form vivianite [53], acceleration of Fe (III) reduction to Fe (II) in presence of microorganisms (such as Geobacter sulfurreducens [54]).…”

“…presents the phosphate species as an efficient homogeneous catalyst for the reduction of proton/water [8]. The cathodic deprotonation of phosphate species (Reaction (4)) is relatively fast on steel surfaces and not strictly limited by the forward electron uptake (as is Eq.…”

“…Sulphate-reducing bacteria and thiosulphate-reducing bacteria (SRB/TRB) are the most clearly identified causes of anaerobic microbially influenced corrosion (MIC) of steels in natural environments [1][2][3][4][5][6][7]. Several mechanisms have been proposed to explain anaerobic MIC by SRB and TRB [8][9][10][11][12][13][14][15]. As far as SRB are concerned, the most often evoked mechanism is based on the production of sulphide ions by the metabolic reduction of sulphates.…”

Exacerbation of the mild steel corrosion process by direct electron transfer between [Fe-Fe]-hydrogenase and material surface

“…Two mechanisms of hydrogenase action on corrosion have been established [5,6,18]. The first involves a synergetic effect between hydrogenase and phosphates (or weak acids) in presence of a redox mediator [18][19][20]. The second mechanism proposed does not require a redox mediator and hydrogenase catalyses the reduction of protons or water by direct electronic transfer [5,6].…”