Marine sulfate‐reducing bacteria cause serious corrosion of iron under electroconductive biogenic mineral crust

Abstract

Iron (Fe0) corrosion in anoxic environments (e.g. inside pipelines), a process entailing considerable economic costs, is largely influenced by microorganisms, in particular sulfate-reducing bacteria (SRB). The process is characterized by formation of black crusts and metal pitting. The mechanism is usually explained by the corrosiveness of formed H2S, and scavenge of ‘cathodic’ H2 from chemical reaction of Fe0 with H2O. Here we studied peculiar marine SRB that grew lithotrophically with metallic iron as the only electron donor. They degraded up to 72% of iron coupons (10 mm × 10 mm × 1 mm) within five months, which is a technologically highly relevant corrosion rate (0.7 mm Fe0 year−1), while conventional H2-scavenging control strains were not corrosive. The black, hard mineral crust (FeS, FeCO3, Mg/CaCO3) deposited on the corroding metal exhibited electrical conductivity (50 S m−1). This was sufficient to explain the corrosion rate by electron flow from the metal (4Fe0 → 4Fe2+ + 8e−) through semiconductive sulfides to the crust-colonizing cells reducing sulfate (8e− + SO42− + 9H+ → HS− + 4H2O). Hence, anaerobic microbial iron corrosion obviously bypasses H2 rather than depends on it. SRB with such corrosive potential were revealed at naturally high numbers at a coastal marine sediment site. Iron coupons buried there were corroded and covered by the characteristic mineral crust. It is speculated that anaerobic biocorrosion is due to the promiscuous use of an ecophysiologically relevant catabolic trait for uptake of external electrons from abiotic or biotic sources in sediments.

The dual role of microbes in corrosion

“…Recently, two different mechanisms of iron corrosion were described, that is, chemical MIC (CMIC) and electrical MIC (EMIC) (Enning et al, 2012;Enning and Garrelfs, 2013;Venzlaff et al, 2013). These two mechanisms differ in the source of electrons used by the SRB, which was shown to be species specific.…”

“…This explains why microorganisms and the crust can continue to grow, which is not the case in CMIC, where corrosion and crust formation stop upon loss of direct metal contact (see Figure 1). Direct electron uptake is also shown for methanogens, but the produced corrosion crust is not conductive and so corrosion will probably stop once direct metal contact is lost, that is, the methanogenic Archaea are no longer in direct contact with the metal surface (Enning et al, 2012).…”

“…Also, there is an increase in contradictory reports on both accelerating and inhibiting actions on the corrosion process of the same functional group of microorganisms, such as sulfate reducers (Enning et al, 2012;Venzlaff et al, 2013), iron reducers (AlAbbas et al, 2013) and methanogens (Uchiyama et al, 2010). These studies show a strong species specificity for either MIC or MICI, which indicates that there is an urgent need to focus on the mechanisms rather than on the presence of certain functional groups, which is common practice at the moment.…”

Enhanced microbial electrosynthesis by using defined co-cultures

“…Besides methanogenic archaea and homoacetogenic bacteria, Fe(0)-corroding microorganisms have been intensively investigated for their outstanding electron transfer capabilities (Dinh et al, 2004;Uchiyama et al, 2010;Enning et al, 2012;Venzlaff et al, 2013;Enning and Garrelfs, 2014;Kato et al, 2015;Beese-Vasbender et al, 2015b). Although direct electron uptake has been proposed in these microorganisms, no mechanism has been identified to date Venzlaff et al, 2013;Enning and Garrelfs, 2014;BeeseVasbender et al, 2015b).…”

Biocorrosion on Surface of ASTM A283 Carbon Steel, Exposed in Diesel S10 and Tap Water

“…Therefore, this protective film may have caused failures due to the disruption of microbial action, bulky growth and oxidation, thus contributing to localized corrosion. According to Enning et al 31 , some microorganisms converted the lactate to acetate through pyruvate to produce FeCO 3 . According to Sun et al 32 , the corrosion product is formed by the ions resulting from the anodic and cathodic reactions, as shown in Eqs.…”