Abstract
Recent field reports have suggested that microbiologically influenced corrosion (MIC) is occurring at biologically favorable sites in equipment handling ethanol fuels. Recent laboratory corrosion testing has demonstrated the susceptibility of engineering alloys to corrosion in ethanol solutions containing microbes cultivated from an ethanol fuel terminal. This suggests handling and transportation of larger volumes of ethanol and ethanol fuel blends may increase the likelihood of MIC related failures. Mitigating this threat will likely increase the cost of integrity management of fuel transportation equipment comprising pipeline systems, terminals, and fueling stations. Microbial communities in ethanol spillage and runoff tanks at an ethanol fuel terminal were identified using Sanger sequencing of ribosomal rRNA genes. Microbes were cultivated from the samples for a laboratory MIC study. MIC of API-5L X52 and X70 steels was evaluated using electrochemical impedance spectroscopy (EIS) and electron microscopy. Acid producing bacteria (APB) are associated with a high tendency for pitting. Based on experimental observations, it is believed that the pitting occurs by two mechanisms: (1) acetic acid-enhanced corrosion driven by galvanic coupling between the ferrite matrix and second phase particles and (2) local colonization by APB cells. Sulfate reducing prokaryotes (SRP) enhanced general corrosion and also caused pitting. Corrosion behavior is believed to be controlled by the evolution of the iron sulfide corrosion product layer. The integrity of this film is observed to be dependent on the presence of acetic acid as well as microbial colonization of the film. This laboratory investigation, along with recent field reports, demonstrates and describes a threat of MIC at sites in ethanol transportation systems where conditions are biologically suitable.
