|Content Type||Conference Paper|
|Title||Corrosion Behavior of Carbon Steel Influenced by Sulfate Reducing Bacteria in Soil Environments|
|Authors||Seon Yeob Li, Young Geun Kim, and Young Tai Kho, Korea Gas Corp.; Tak Kang, Seoul National University|
|Source||CORROSION 2003, March, 2003 , San Diego Ca|
|Copyright||2003. NACE International|
|Keywords||microbiologically influenced corrosion (MIC), sulfate-reducing bacteria (SRB), anaerobic soil, thin-film electrical resistance (TFER) sensor, cathodic protection (CP), disbonded coating, current distribution|
This work is a comprehensive study of microbiologically influenced corrosion (MIC) of carbon steel in soil environments, induced by sulfate-reducing bacteria (SRB). Four experimental phases were involved in this research: (1) field study, (2) anaerobic corrosion study, (3)localized corrosion study, (4) mathematical modeling of cathodic protection (CP) current/potential distribution. Based on the field survey and statistical approach, the predicting equation for the maximum corrosion depth of steel in soil was presented and it was proved that the predicted values were well matched with the field data. The contribution of microbial factors on soil corrosion was also discussed. It was also concluded that the presence of SRB and the resultant biogenic iron sulfide film on steel surface greatly changed the metal/electrolyte interface properties, and therefore the corrosion behavior of steel from anaerobic study. Using the thin-film electrical resistance (TFER) sensors, it was possible to detect and monitor the localized corrosion behavior induced by SRB. The mathematical model using the boundary element method (BEM) is capable of predicting (1) local potential; and (2) local cathodic current density inside crevice for a given applied holiday potential and soil resistivity. The model predicted a very short depth of current penetration depending on the crevice geometry. From the results of modeling and the following regression analysis, the optimization plots for cathodic protection inside crevice under the disbonded coating was proposed, which makes it possible to predict the current penetration depth.
It is widely recognized that microorganisms attach to, and influence the corrosion of metals and alloys exposed to the environments. Microorganisms are omnipresent in nature and their ability to grow and reproduce at rapid rates accounts for their presence in soil environments. Buried steel pipeline is designed to have a lifetime of about 30 to 50 years. In general, the protective organic coating and cathodic protection (CP) are applied together. Despite these protection measures, however, failure cases of underground pipelines due to corrosion have been continuously reported ~3. Microbiologically influenced corrosion (MIC) has been identified as one of the major causes of corrosion failures of underground pipelines.
The first MIC case was identified in 19345, where sulfate-reducing bacteria (SRB) resulted in failure of underground cast iron pipes. Since this observation, numerous works have been made concerning the effect of bacteria, especially SRB, on MIC in soil environments. According to recent survey program performed by authors, almost all corrosion of underground gas pipeline has occurred under the disbonded coating 6'7. The corrosion mechanism has been revealed mainly as the action of SRB. SRB are known as one of the key microbes in the MIC process. SRB are obligately anaerobic bacteria utilizing sulfate as a terminal electron acceptor and organic substances as carbon sources. During the metabolic process, sulfate is reduced to sulfide. These biogenic sulfides react with hydrogen ions produced by metabolic activities or by cathodic reaction of corrosion process to form corrosive hydrogen sulfide and iron sulfide precipitate on metal surface. The detailed mechanism of corrosion by SRB is reported elsewhere 8'9.
The risk of MIC must be exactly evaluated and the proper control methods prepared to mitigate MIC problems. The relationship between the activities of MIC-causing microbes, e.g., SRB, and various soil environmental factors should be investigated and assessed. In view of economic and engineering points, quantitative assessment of MIC is also important. The
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