Native iron in the continental lower crust: petrological and geophysical implications

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

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.

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

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.

Redox history of the Earth's interior since approximately 3900 Ma: implications for prebiotic molecules

The history of the oxidation state in the Earth's mantle has been constrained using (a) the whole-rock abundances of Cr and V in ancient volcanics, and (b) the composition of Cr-rich spinels in ancient volcanics. Results indicate that the Earth's mantle has been at-or-near its current oxidation state (+/- 0.5 log-unit fO2) since at least 3600 Ma, and probably since at least 3960 Ma. Volatiles released into the Earth's atmosphere by high-temperature (T > or = 1300 K) volcanism during this time have been dominated by H2O, CO2, and SO2. This blend of volatiles is known to provide smaller yields of prebiotic, organic molecules by atmospheric and surface processes than gas mixtures containing higher concentrations of reduced species such as H2, CO, and H2S (e.g., Miller, 1998; Zolotov and Shock, 2000). The results discussed in this article independently support the conclusion of Canil (1997, 1999). If the atmosphere was reducing (e.g., CH4, H2, H2S, NH3, CO) at any time during the last approximately 3900 Ma, high-temperature volcanic outgassing was not the cause of it.