Whonchee Lee

Role of Sulfate-reducing Bacteria in Corrosion of Mild Steel: a Review

The influence of sulfate‐reducing bacteria on corrosion of mild steel is reviewed, with special emphasis on the effects of biofilm structure and function, medium composition (dissolved oxygen and ferrous ion concentrations) and the physical and chemical properties of iron sulfides. A summary of different corrosion mechanisms is critically discussed, based on electrochemical and rate process analyses. A mechanism is proposed which explains the high corrosion rates observed in the field.

Corrosion of mild steel under anaerobic biofilm

Abstract 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 closed channel flow reactor was continuously fed with low concentration substrate at different dilution rates that influenced biofilm accumulation. No direct correlation was observed between corrosion and SRB activity in the absence of ferrous iron. Furthermore, corrosion of mild steel in the SRB environment was determined by the nature of the metal and environmental conditions such as dissolved iron concentration. When formation of an 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 × 109/cm2). However, when the 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 ...

Dissolved Oxygen and pH Microelectrode Measurements at Water-Immersed Metal Surfaces

Abstract Dissolved oxygen (DO) and pH were measured at metal/artificial seawater interfaces using microelectrodes in biotic and abiotic systems. Measurements in a closed system proved that presence of electrochemical and/or biological reaction products substantially influence the conditions at the metal surface. For long-term studies, only open (e.g., continuous flow) reactors should be used. An open channel flow reactor suitable both for microbiological and electrochemical measurements has been constructed and successfully tested.

Corrosion of mild steel underneath aerobic biofilms containing sulfate‐reducing bacteria part II: At high dissolved oxygen concentration

Microbial biofilms containing sulfate‐reducing bacteria (SRB) and general anaerobic bacteria (GAB) were grown in a closed flow channel reactor in air‐saturated bulk liquid. The SRB proliferated within anaerobic microniches even when dissolved oxygen penetrated the entire biofilm at some locations. Corrosion of mild steel during aerobic/anaerobic biofilm accumulation was classified as aerobic corrosion and SRB‐enhanced corrosion. Aerobic corrosion dominated during the early stages of biofilm accumulation. The corrosion rate decreased as the biofilm became more uniform over the surface. SRB‐enhanced corrosion occurred after the SRB community was established within the deposits and significant amounts of iron sulfides contacted the bare steel surface. The initiation and propagation of SRB‐enhanced corrosion in an aerobic/anaerobic biofilm system was explained through the establishment of an FeS/Fe galvanic cell.

Corrosion of mild steel in an alternating oxic and anoxic biofilm system

The effect of alternating oxic and anoxic conditions (12 h oxic‐12 h anoxie) on sulfate reducing activity, iron‐sulfur chemistry and the corrosion of mild steel, has been studied in biofilm reactors. During the experiment (35 d) an increasing activity of sulfate reducing bacteria was observed. A part of the produced sulfide and iron sulfide (FeS) was oxidized during oxic periods and resulted in a mixture of acid volatile sulfides (mainly mackinawite, FeS), chromium reducible sulfur (mainly pyrite, FeS2) and elemental sulfur (S°). At the end of the experiment an amount of total S corresponding to 157 umol cm−3 was found within the deposit. Corrosion rates were measured electrochemically during the experiment and were found in the range of 3–5 mpy after 7 d to 120–160 mpy after 34 d. An extended aeration of the biofilm system for 1 month without addition of any organics showed that the pools of Fe‐S compounds in the deposit and the corrosion rate remained high. Microsensor studies of dissolved oxygen penetr...

Impact of biofouling on the electrochemical behaviour of 304 stainless steel in natural seawater

Biofilm formation on 304 stainless steel (S30400) does not necessarily result in an ennoblement of the corrosion potential. Instead, biofilms composed of aerobic and anaerobic bacteria from Gulf of Mexico water formed an anaerobic biofilm/metal interface and caused the corrosion potential to move in the negative direction. Biofilms from the same source containing photosynthetic diatoms in the presence of light produced aerobic biofilm/metal interfaces and a positive shift (ennoblement of the corrosion potential). Corrosion potentials of stainless steels exposed in natural seawater cannot be predicted without an understanding of the composition of the biofilm and its impact on interfacial chemistry. In this paper, measurements of corrosion potential, interfacial pH and dissolved oxygen have been correlated with SEM/EDAX surface analyses to evaluate the electrochemical behaviour of stainless steels exposed to Gulf of Mexico water. The interfacial chemistries that influence the corrosion potential are also d...

Oxygen and ph microprofiles above corroding mild steel covered with a biofilm

O2 and pH microprofiles were measured above corroding mild steel covered with a biofilm. The pH in the anodic areas (tubercles) ranged from 5 to 7 and was always 9.45 at the surface of the cathodic areas. After 1 month of biofilm development, O2 was depleted at the anodic area but could reach the cathodic surface where it was reduced. Consequently, differential O2 concentration cells were the driving force for corrosion. The O2 microprofiles indicated that O2 was consumed in the tubercles, probably by microbial activity, while O2 was reduced electrochemically in the cathodic areas. It was concluded that O2 transfer to the cathodic surface was the rate limiting step for the corrosion process.

Review Article on the Influence of Dissolved Oxygen on Sulfate-Reducing Bacteria Related Corrosion

Corrosion by sulfate-reducing bacteria (SRB) has been intensively studied during the last 40 years, but until now the importance of oxygen in SRB-related corrosion has rarely been emphasized (Hardy and Bown, 1984; Starkey, 1985; Hamilton, 1990; Hamilton, 1991). The impact of oxygen on SRB-related corrosion is attributed to a direct effect on the sulfur-related corrosion products rather than to any stimulation of SRB activity (Hamilton, 1990; Hamilton, 1991). Pitting corrosion is the characteristic mode of attack and deep pit is usually found underneath a porous corrosion products. However, the role that oxygen plays in the aerobic/anaerobic environments in relation to corrosion has not been clearly defined. The system is complex and dynamic. The role of SRB must be viewed in the context of biological consortia (biofilms) and/or mixed ecosystems. In addition to the biological factors, the chemical environments which influence corrosion are also complicated by the introduction of oxygen. The following statement is quoted from Starkey. “ Factors that have been suggested or may be concerned with anaerobic corrosion relate particularly to the effect of ferrous sulfide, sulfur, ferrous hydrate and all other products of the corrosion process, differential aeration cells, and various combinations of all of those factors”. In this review, we intended to focus on aspects of experimental systems that more accurately reflect those environmental conditions generally associated with corrosion in the field. Describing the role of dissolved oxygen on SRB-related corrosion, we will summarize the current published papers which describe the corrosion of mild steel underneath aerobic biofilms containing SRB (Lee and Characklis, 1990; Lee et al. 1992). Finally, we will discuss various experimental approaches in an attemp to elucidate the true mechanism of SRB-related corrosion in aerobic environments.