THE ELECTROCHEMISTRY OF SRB CORROSION AND RELATED INORGANIC PHENOMENA

Sulfate-reducing bacteria can cause or influence corrosion in both carbon (or low-alloy) steels and stainless steels. The electrochemical reactions and rate-controlling processes are quite different in these two cases, the common feature being the catalysis of anodic dissolution by adsorption of hydrogen sulfide. In this paper the inorganic background to SRB corrosion is reviewed, and recent experimental results are described. For carbon steel, it is shown that corrosion occurs at some 100-mu-A/cm2 in uniformly anaerobic conditions with hydrogen evolution on a large surface area of iron sulfide as the cathodic reaction. Electrochemical impedance spectroscopy and related techniques are not always useful in monitoring or understanding this corrosion, even when the attack is uniform, owing to effects introduced by the very large capacitance of the porous FeS film. Chloride ions and precorrosion help to create uniform rather than pitting-type attack. For stainles steel, a special chromium-depleted alloy (Fe-Cr-15-Ni-10) has been used to study and enhance the pitting induced by SRB in Cl- -containing media. Catalysis of pitting dissolution by H2S requires a remote O2 cathode to be effective; reduction of sulfur species such as polysulfide (HS(x)-) is only barely able to sustain pitting under the most ideal conditions. Chloride must be the predominant anion, otherwise pitting is inhibited. A micro-cell arrangement is demonstrated that enables the stability of SRB-induced corrosion in an anaerobic micro-environment to be studied under conditions of bulk aeration.