The growth of sulfate-reducing bacteria (SRB) affects several important parameters at the metal/solution interface of carbon steel in liquid media such as pH and redox potential values, as well as modii3cations of the composition and structure of corrosion product layers.
Electrochemical techniques for corrosion assessment and surface analyses by energy dispersion X-ray analysis (EDAX), X-ray photoelectron spectra (XPS), X-ray distraction (XRD) and electron microprobe analysis (EPMA) complemented with scanning electron microscopy (SEM) and atomic force microscopy (MM) observations, were used to study the structure and composition of protective films on carbon steel in abiotic and biotic media containing different sulfur anions.
The results revealed that in biotic and abiotic sulfide films the outer layers were formed by both FeS and FeSp, although the relative content of these compounds varied in each case. Usually, the corrosion product films biotically formed were more adherent to the metal surface than those developed abiotically. The latter were flaky and loosely adherent, thus differing their function during the corrosion process.
SRB metabolism brings to the metal/ solution interface several sulfur compounds of corrosive characteristics, either as final metabolic products (sulfides, bisulfides or hydrogen sulfide) or intermediate metabolic compounds (thiosulfates, polythionates). These compounds are corrosive to carbon steel mainly through the transformation to sulfide anions that stimulate corrosion by a mechanism of anodic depolarization]. Thus, biocorrosion of carbon steel is strongly influenced by the nature and structure of sulfide films produced during the corrosion process. The environmental characteristics of the metal bio film solution interface and its surroundings (pH ionic composition, oxygen levels) influence the chemical and physical nature of sulfide films and may change their effects from corrosive to protective. Whereas thin adherent films of iron sulfide are protective, bulky and loosely adherent precipitates enhance corrosion rates.
Alternating cycles of oxic and anoxic conditions are able to produce high corrosion rates related to high corrosion currents and fairly stable anodic currents. SRB and biogenic sulfide and sulfur corrosion products are able to enhance cathodic reaction by acting as current carriers between the metal and the oxide interface within the biofilm. This assumption would be supported by previous work by Newman3 suggesting that the main action of biogenic hydrogen sulfide would be to stimulate localized attack at anaerobic sites, while the cathodic current would be provided by oxygen or sulfur reduction.
In a series ofpapers4?5,Crolet aimed to clarify several aspects of SRB-induced corrosion of carbon steel. According to this author, SRB are able to regulate local pH to a value corresponding to the lack of acidity. This pH value can be altered by secondary reactions involving minimal amounts of oxygen or ferrous ions. Thus, SRB would be able to stabilize pitting corrosion of carbon steel in saline media where other bacteria (e.g. Bacillus sp.) induce the opposite effect i.e. preventing corrosion.
