The shielding effect of wild type iron reducing bacterial flora on the corrosion of linepipe steel

Microbiologically influenced corrosion (MIC) by microbes capable of iron reduction (iron reducing bacteria (IRB)) on API 5L ×52 carbon steel coupons was investigated. A wild type of IRB was isolated and cultivated from a water sample collected from a sour oil well located in Louisiana, USA. 16S rRNA gene sequence analysis indicated that the mixed bacterial consortium contained two phylotypes close to members of the Proteobacteria (Shewanella oneidensis sp.) and Firmicutes (Brevibacillus sp.). The corrosion behavior of carbon steel coupons exposed to different media, with and without these microbes, was characterized by open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and polarization resistance (Rp), and a corrosion mechanism has been proposed. The biofilm and pit morphology that developed with time were characterized using field emission scanning electron microscopy (FESEM). Interestingly, surface morphology and electrochemical evaluations confirmed that IRB metabolic activities and resulting biofilms inhibit the corrosion process. The maximum corrosion rate in the biotic system was 4 mpy, while it was 20 mpy in the abiotic solution. Minor isolated pits were revealed in the biotic system, whereas extensive general pitting was found in the abiotic system. Elemental analysis and corrosion product structures were characterized by energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). XRD confirmed the formation of a significant amount of iron oxide compounds that include iron, Hematite (Fe2O3), Magnetite (Fe3O4) and iron (II) hydroxide Fe(OH)2 on the steel surface exposed to a biotic system.