An overview of mechanisms by which sulphate-reducing bacteria influence corrosion of steel in marine environments

This communication provides an overview of the literature on the biocorrosion of steel in marine media, influenced by the presence of sulphate-reducing bacteria (SRB). Electrochemical aspects, microbial interactions within biofilms, the significance of medium composition and the role of iron sulphides, and hydrogen effects are discussed. A brief description of recent experiments involving the use of electrochemical techniques for corrosion assessment, 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, as well as the application of novel techniques, such as micro sensors and atomic force microscopy, is given. The growth of SRB in marine environments causes significant modifications of many physicochemical parameters at the steel/seawater interface, including local changes in pH and redox potential values, variations in anion and cation concentrations and alteration of the composition and structure of corrosion products. Complex chemical and biological reactions and equilibria are also markedly altered during bacterial proliferation. These effects, which are absent in abiotic media, often lead to significant changes in the corrosion behaviour of steel. The complicated nature 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 biofilm heterogeneity, areas with different ion concentrations are formed and the development of corrosion product layers of dissimilar protective characteristics occurs.