Sulfate-reducing bacteria lower sulfur-mediated pitting corrosion under conditions of oxygen ingress

The effect of oxygen ingress into sour water containing dissolved sulfide on the production of sulfur and polysulfide (S-PS) and associated iron corrosion was investigated. Biotic (active SRB present), abiotic (autoclaved SRB present), and chemical (no bacteria present) conditions were compared. Under biotic conditions formation of S-PS was only seen at a high ratio of oxygen to sulfide (R(OS)) of 1 to 2.4. General corrosion rates increased 10-fold to 0.10 mm/yr under these conditions. Under abiotic and chemical conditions S-PS formation increased over the entire range of R(OS) with general corrosion rates reaching 0.06 mm/yr. Although general corrosion rates were thus highest under biotic conditions, biotically corroded coupons showed much less pitting corrosion. Maximum pit depth increased to 40-80 microm with increasing R(OS) for coupons incubated for 1 month under abiotic or chemical conditions but not for biotically incubated coupons (10 microm). This appeared to be related to the properties and size of the sulfur formed, which was hydrophobic and in excess of 10 microm under chemical or abiotic conditions and hydrophilic and 0.5 to 1 microm under biotic conditions. Hence, perhaps contrary to expectation, SRB lowered pitting corrosion rates under conditions of oxygen ingress due to their ability to respire oxygen and produce a less aggressive form of sulfur. Microbial control, which is usually required in sour systems, may be counterproductive under these conditions.