ABSTRACT
API-RP38 culture medium was used to culture sulfate SRB isolated from an oilfield wastewater sample. SRB test bottle method was used to verify bacterial activity, and SEM was employed for identifying the microbial morphology. Corrosion behaviors were studied via weight loss methods and electrochemical tests including polarization curve and EIS measurements. The results of corrosion weight loss show that the corrosion rate of carbon steel was accelerated significantly after the inoculation of SRB, characterized by pitting corrosion. The real oilfield conditions with high content of H2S and high salinity were simulated for tests and the results indicate that severe pitting corrosion also took place under the conditions of SRB which survived in the unfavorable environments. The results of polarization curves show that, in H2S environments with the existence of SRB, the corrosion potential of carbon steel negatively shifted and the corrosion current density increased as the passage of testing time. However, without SRB, the corrosion potential shifted positively and the corrosion current density decreased due to the protection of uniform corrosion product film. EIS results show that, with the existence of SRB, both the corrosion scale impedance and charge transfer impedance decreased as the passage of testing time. As the time lengthened, this tendency turned more apparent. While under the sterilized conditions, both the corrosion scale impedance and charge transfer impedance increased as the testing time elapsed.
INTRODUCTION
About 80% of the underground metal damages are caused by microbiologically-induced corrosion (MIC). Sulfate-reducing bacteria (SRB) are found the main microbes that cause MIC.1,2 As anaerobic species, SRB feed on organic matters and can metabolically reduce sulfate to sulfide. Widely existing in the soil, sea, river, underground pipelines, oil and gas wells, etc. SRB induce the corrosion of metallic structures, resulting in overwhelming economic losses.3-5 It has been found that, under anaerobic conditions, SRB thrive and produce mucus, which contributes to the formation of scale, blocking water injection pipelines. In turn, the sediment scales in the pipeline are favorable for the reproduction of SRB, often resulting in severe localized corrosion on the pipe surfaces and further pitting perforation of the pipe walls.6-8 In US, statistics indicate that more than 77% of oil well corrosion failures are related to SRB, dominantly characterized by pitting.9 Different mechanisms have been proposed to explain the reason for corrosion induced or accelerated by SRB, including, for instance, cathode depolarization theory and localized corrosion cell theory.10,11 However, SRB corrosion has not yet been studied in depth in certain special environments. Some researchers have shown that SRB cannot survive in environments with high concentration of H2S or high salinity.12-14 however, it is not rare that severe corrosion phenomena caused by SRB have been observed under such conditions in oil and gas gathering pipelines and well tubings. Under such a background, this study focuses on the effects of SRB on pitting corrosion behavior of carbon steel in H2S environment.
