Microbial influenced corrosion on different grades of carbon steel coupons used in pipelines were investigated in this study. Microbes were isolated and cultivated from oil well produced water samples, and immersion tests were conducted with these waters, microbes and coupons. Bacteria were identified with 16S rRNA gene sequencing. Sequencing analyses revealed the existence of different kinds of bacteria including iron, manganese, and sulfate-reducing bacteria. Corrosion products were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) to characterize biofilm formation and pitting corrosion with resultant pit depth and morphology evaluated. XRD identified the presence of FeS, FeO(OH), and Fe3O4 in the corrosion products. In this investigation, the API X70 pipeline steel coupons experienced more severe MIC than API X52. The vulnerability of X70 to MIC suggests a causation role of metal microstructures. The microstructure analyses show that the smaller grain size steel has a higher susceptibility to MIC. This characterization of MIC in an oil-water environment offers specific information on the types of microbes that cause MIC, and this information may be used for future comprehensive analytical tests for establishing MIC mechanisms.
Microbiologically influenced corrosion is defined as corrosion that can be initiated or accelerated by the presence and activity of microorganisms1. Microbes may adhere to metal surfaces forming biofilms, complex microbial ecosystems, which can alter the chemistry at the metal surface and affect the kinetics of the corrosion processes of metals2. Microbial activity in any environment occurs in the presence of water, a carbon source, an electron donor and an electron acceptor3, all of which can be present in oil pipelines. Localized corrosion, such as the pitting of iron alloys4, is often the result of MIC. Microorganisms in oil fields can cause numerous problems, including MIC, for the oil industry5. The majority of internal corrosion of oil pipelines and storage tanks is associated with MIC, and the primary internal corrosion mechanism in many oil pipelines is pitting along the bottom surface of a pipe6. The oil and gas industries are demanding high strength steel (yield strength of more than 70 Ksi), which can have larger diameters and smaller wall thickness, as these steels are thought to be highly resistant to corrosion. In the oil industry, microbes are usually identified by culture-based methods as described in NACE TM0194 and ASTM D44127,8. However, over the past two decades genetic techniques, specifically the sequencing of identifiable DNA genes have been used to identify microorganisms in natural environments9. These techniques often include the amplification of 16S rRNA gene sequences by Polymerase Chain Reaction (PCR) 10. DNA segments produced by PCR can be evaluated by community fingerprinting techniques (denaturing gradient gel electrophoresis) or preferably by DNA sequencing. These DNA sequences are compared to databases (e.g., the GenBank database) to identify the microbes from which the DNA has been amplified10. DNA can be isolated from cultured microbes, or directly from environmental samples for 16S rRNA gene sequencing11 to identify all organisms present in the source environment.
