Role of Elemental Sulfur and Polysulfide in Corrosion Mediated by Sulfate Reducing Bacteria

Abstract

Sulfate reducing bacteria (SRB), such as Desulfovibrio vulgaris sp. Hildenborough, are strict anaerobes which reduce sulfate to sulfide, which is toxic and leads to reservoir souring and pipeline corrosion. SRB can be controlled through nitrate injection. Nitrate stimulates the nitrate reducing, sulfide oxidizing bacteria (NR-SOB) which reduce nitrate to nitrite. Nitrite is inhibitory to SRB, as it blocks a key sulfate reduction enzyme. Nitrite and oxygen (from periods of oxygen ingress) can also react chemically with sulfide to produce elemental sulfur (S) or polysulfide (PS), both of which are also inhibitory to SRB. In this way, nitrite and oxygen seem to dually inhibit SRB. However, both S and PS are very corrosive species, and their formation in pipelines is undesirable. Recent data suggests that sulfur formation during nitrate injection may occur, which may increase corrosion risk. SRB can overcome nitrite and oxygen stress, as well as S and PS stress. This raises the question: what role are SRB playing in pipeline corrosion? Oxygen and nitrite were added to live and autoclaved D. vulgaris cells, and it was found that D. vulgaris could remove some polysulfide through an electron transport chain involving the hybrid cluster proteins Hcp1 and Hcp2. While making an hcp1 mutant, a genomic island was found, which improves oxygen tolerance at the expense of growth rate. Subsequent studies demonstrated that live D. vulgaris is able to prevent the formation of S and PS up to an oxygen: sulfide ratio of 1:1, although this did not translate to a decrease in weight loss corrosion. S and PS were formed with oxygen under biotic (live cells), abiotic (autoclaved cells) and chemical (high sulfide media) conditions. Corresponding pitting corrosion increased as biotic<abiotic<chemical. Properties of the sulfur formed in each condition were examined, and it was found that the presence of D. vulgaris changed the properties of the sulfur, seemingly decreasing the severity of pitting. These findings indicate that biocide programs should be carefully considered before administration, and that once started, they should only be used under oxygen free conditions. Also, these results present the possibility of using biosurfactants as corrosion inhibitors.