Corrosion of iron by sulfate-reducing bacteria: new views of an old problem

Abstract

About a century ago, researchers first recognized a connection between the activity of environmental microorganisms and cases of anaerobic iron corrosion. Since then, such microbially influenced corrosion (MIC) has gained prominence and its technical and economic implications are now widely recognized. Under anoxic conditions (e.g., in oil and gas pipelines), sulfate-reducing bacteria (SRB) are commonly considered the main culprits of MIC. This perception largely stems from three recurrent observations. First, anoxic sulfate-rich environments (e.g., anoxic seawater) are particularly corrosive. Second, SRB and their characteristic corrosion product iron sulfide are ubiquitously associated with anaerobic corrosion damage, and third, no other physiological group produces comparably severe corrosion damage in laboratory-grown pure cultures. However, there remain many open questions as to the underlying mechanisms and their relative contributions to corrosion. On the one hand, SRB damage iron constructions indirectly through a corrosive chemical agent, hydrogen sulfide, formed by the organisms as a dissimilatory product from sulfate reduction with organic compounds or hydrogen ("chemical microbially influenced corrosion"; CMIC). On the other hand, certain SRB can also attack iron via withdrawal of electrons ("electrical microbially influenced corrosion"; EMIC), viz., directly by metabolic coupling. Corrosion of iron by SRB is typically associated with the formation of iron sulfides (FeS) which, paradoxically, may reduce corrosion in some cases while they increase it in others. This brief review traces the historical twists in the perception of SRB-induced corrosion, considering the presently most plausible explanations as well as possible early misconceptions in the understanding of severe corrosion in anoxic, sulfate-rich environments.

FIG 1

External corrosion on buried gas transmission pipeline in bog soil of Germany. (A) Trench with coated carbon steel gas pipeline in water-logged, anoxic soil (1.4 mM sulfate, 17 mM dissolved inorganic carbon [DIC]). External corrosion has occurred under disbonded coating at welding sites (arrow). (B) Welding site with corrosion pits. Disbonded asphalt coating and corrosion products (FeS/FeCO₃) were removed. Numbers indicate pit depth in millimeters. Bar, 20 cm. (C) Higher magnification of corrosion pits from a different site at the same pipeline. Bar, 2 cm.

FIG 2

Corrosion of an iron key in the presence of Desulfovibrio ferrophilus strain IS5 (A to C) and corrosion under sterile (control) conditions (D to F). Both incubations were performed in artificial seawater medium at pH 7.3 and without addition of organic substrates (lithotrophic medium). (A to C) Electrical microbially influenced corrosion (EMIC) of the first key (A) led to substantial buildup of biogenic corrosion crust (B) and metal destruction (C) during 9 months. (D to F) Abiotic corrosion of another key (D) in sterile medium during 27 months formed minimal corrosion products (E) and led to negligible metal loss (F). Bar, 1 cm. (A) Iron key before incubation with D. ferrophilus strain IS5. (B) Iron key with biogenic corrosion crust after 9 months of incubation with pure culture of strain IS5. (C) Residual iron after removal of the crust (B) with inactivated acid (10% hexamine in 2 M HCl) revealed 80.3% (2.7 g) iron weight loss due to corrosive activity of strain IS5. Hexamine-HCl did not dissolve Fe⁰. (D) Iron key before sterile incubation. (E) Iron key incubated in sterile artificial seawater medium. Corrosion is much less pronounced despite 27 months of incubation. (F) Residual iron after removal of corrosion products with inactivated acid (10% hexamine in 2 M HCl) revealed 2.9% (0.09 g) iron weight loss due to abiotic corrosion. Hexamine-HCl did not dissolve Fe⁰.

FIG 3

Phylogenetic tree constructed from full-length 16S rRNA gene sequences of cultivated sulfate-reducing bacteria within the Deltaproteobacteria. The tree shows SRB isolates capable of direct electron uptake (EMIC; orange and ★) and hydrogenotrophic SRB that cannot corrode iron by the EMIC mechanism (blue and *). Other SRB (black) were not tested on Fe⁰. All depicted SRB corrode iron via the CMIC mechanism in the presence of suitable electron donors and sulfate. The tree does not include all cultivated SRB. I, Desulfobulbaceae; II, Desulfobacteraceae; III, Desulfovibrionaceae. The tree was calculated based on maximum likelihood with the ARB software package and SILVA database (126, 127). Branching with bootstrap values below 75 is not depicted. The scale bar represents a 10% difference in sequence similarity. “Mic” isolates are from Mori et al. (2010) (47). The figure was adapted from Enning (2012) (61).

FIG 4

Schematic illustration of different types of iron corrosion by sulfate-reducing bacteria (SRB) at circumneutral pH. Biotic and abiotic reactions are shown. Depicted biotic reactions tend to be much faster than abiotic corrosion reactions. SRB attack iron via electrical microbially influenced corrosion (EMIC) or chemical microbially influenced corrosion (CMIC). Stoichiometry of the illustrated reactions is given in the lower panel of this figure. Please note that all depicted processes may occur simultaneously on corroding metal surfaces but differ in rates and relative contributions to corrosion. (A) Specially adapted lithotrophic SRB withdraw electrons from iron via electroconductive iron sulfides (EMIC). Excess of accepted electrons may be released as H₂ (via hydrogenase enzyme). Participation of possibly buried (encrusted) SRB in sulfate reduction and hydrogen release is currently unknown. (B) Biogenic, dissolved hydrogen sulfide reacts with metallic iron. (C) Overall representation of CMIC. Organotrophic SRB produce hydrogen sulfide which reacts with metallic iron. (D) Sulfide stress cracking (SSC) of iron due to biogenic hydrogen sulfide. (E) Catalytic iron sulfides may accelerate reduction of H⁺ ions to H₂. (F) Slow, kinetically impeded reduction of H⁺ ions to H₂ at iron surfaces. (G) Consumption of H₂ from reaction E or F by SRB does not accelerate the rate of H₂ formation (no “cathodic depolarization”; see the text). Note that CMIC quantitatively depends on the availability of biodegradable organic matter (here schematically shown as carbon with the oxidation state of zero, CH₂O).