Corrosion of mild steel under an anaerobic biofilm

Corrosion of mild steel under completely anaerobic conditions in the presence of a mixed population biofilm, including sulfate-reducing bacteria (SRB), has been studied in a continuous flow system. The open channel flow reactor was continuously fed with low concentration substrate at different dilution rates which influenced biofilm accumulation. No direct correlation was observed between corrosion and SRB activity in the absence of ferrous ion. Furthermore, corrosion of mild steel in the SRB environment was mainly determined by the nature of the metal and environmental conditions such as dissolved iron. When formation of iron sulfide film on mild steel was prevented before the biofilm accumulated, the metal surface retained its scratch lines after a 21-day experiment (SRB at 2.6 {dollar}times{dollar} 10{dollar}sp{lcub}9{rcub}{dollar}/cm{dollar}sp{lcub}2{rcub}{dollar}). However, when iron sulfide film was formed before the accumulation of biofilm, visible localized corrosion appeared after 14 days and increased up to 21 days. Intergranular and pitting attack were found in the localized corrosion area. Inclusions (Al, Mn, and Fe) were usually found in the localized corrosion area. At high iron concentration (about 60 mg/L in the bulk water), all biogenic sulfide was precipitated and corrosion rate significantly increased. Intergranular attack was found over the entire metal surface.; The anaerobic corrosion processes of mild steel in the presence and absence of anaerobic biofilm, including sulfate-reducing bacteria, was compared. In the biotic experiment, biofilm accumulated on a chemically-formed iron sulfide film for 21 days. The abiotic experiment was designed to simulate this hydrogen sulfide environment which had similar hydrogen sulfide concentration, redox potential, and pH. There was no difference in the metal matrix at three days in the two conditions. However, corrosion propagation was different for the two cases after 3 days. Localized corrosion was found in the presence of biofilm while uniform intergranular attack was observed in the absence of biofilm.