This paper offers an overview of sulfate-reducing bacteria (SRB) induced corrosion and protection of carbon steel in marine media.
The complexity of the local environment at the steel/seawater interface is enhanced in the presence of microorganisms and their extracellular polymeric substances (EPS). As a consequence of the biofilm heterogeneity, areas with different ion concentrations are formed and the development of corrosion product layers of dissimilar protective characteristics occurs.
Electrochemical aspects, microbial interactions within biofilms, the significance of medium composition, the role of iron sulphides, and hydrogen effects are discussed. A brief description of recent experiments for SRB induced corrosion assessment, involving the use of electrochemical techniques, surface studies employing energy dispersive X-ray analysis (EDAX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and electron microprobe complemented with electron microscopy observations and atomic force microscopy (AFM), is provided.
Sulphate Reducing Bacteria (SRB) are frequently found in coastal waterlogged clay soils, in polluted marine environments, such as those found in the vicinity of harbours and fish processing industry1, and in off-shore oil extraction equipment.2 The activity of SRB can be markedly enhanced within microbial consortia (biofilms) thriving at the seawater/steel interface.3 Such biofilms frequently develop anaerobic regions, even in aerobic bulk water4, providing SRB with a favorable environment for growth. The result of biofilm development i s the formation of a wide variety of sites on the steel surface that are markedly chemically and physically different from neighboring sites, thus facilitating the initiation of localized corrosion processes.
Pitting attack is a predominant feature of SRB-influenced corrosion of carbbol steel5, with the pits being open and filled with soft black corrosion products, generally consisting of different types of iron sulfides. After the removal of these products, the metal underneath appears bright but rapidly rusts on exposure to air.
In corroded pipelines and other industrial structures, SRB activity involves a number of chemical entities, especially metastable sulfur compounds such as thio-sulfate and polythionate.6 The sulfur anions derived from the dissimilatory reduction of sulfate anions by SRB (sulfides, bisulfides and hydrogen sulfide) are recognised for their deleterious effects on ferrous metals.7
When exposed to sulfur species, iron and steel first develop a film of mackinawite of poor protective characteristics, that later changes, through several chemical and electrochemical paths, to more stable iron sulfides.8 In all cases iron sulfides are characterized by their strong cathodic effects on the hydrogen reduction reaction, causing an indirect increase in the corrosion rate. This is one of the most important features of SRBinfluenced corrosion of carbon steel, where the metal surface is seldom free of deposits of diverse nature (e.g. iron sulfides, iron oxides, iron hydroxides and even biofilms). Thus, the biocorrosion process is related to the breakdown of steel passivity by corrosive metabolic products generated by SRB. In marine environments, the initiation of localized corrosion is further facilitated by the presence of chloride anions that develop a synergistic action with sulfur compounds.9
