Theoretical Modeling of the Possibility of Acid Producing Bacteria Causing Fast Pitting Biocorrosion

Biocorrosion, also known as microbiologically influenced corrosion (MIC), is caused by various corrosive biofilms. So far, laboratory experimental MIC pitting tests in the published literature have overwhelmingly focused on sulfate reducing bacteria (SRB) that use sulfate as the terminal electron acceptor because SRB and sulfate are often found at anaerobic pitting sites. Many laboratory pure-culture SRB pitting corrosion data have been reported and they are often less than or not much greater than 1 mm/year. There are also some limited data available for nitrate reducing bacteria (NRB) that use nitrate or nitrite as the terminal electron acceptor. Dedicated laboratory studies are lacking on anaerobic corrosion by acid producing bacteria (APB) that undergo anaerobic fermentation instead of anaerobic respiration in the absence of an external terminal electron acceptor such as sulfate and nitrate. Failures in pipelines carrying crude oil and produced water purportedly due to MIC have been reported in the literature. Some point to very high pitting corrosion rates (as high as 10 mm/year) that are much higher than the short-term laboratory MIC pitting corrosion rates for SRB. The pipeline failure cases discussed in this work occurred in relatively low sulfate conditions. This work explored the possibility of very high MIC pitting corrosion rates due to free organic acids (represented by acetic acid) and acidic pH corrosion through mechanistic modeling to show that APB biofilms are capable of very fast MIC pitting while mass transfer limitation on sulfate diffusion from the bulk-fluid phase to the biofilm cannot support very fast pitting caused by sulfate reduction in a low sulfate concentration environment. More efforts should be devoted to MIC by APB instead of focusing too much on SRB. Theoretical Modeling of the Possibility of Acid Producing Bacteria Causing Fast Pitting Biocorrosion