Abstract
So far, laboratory experimental pitting tests and published literature on microbiologically influenced corrosion (MIC) have overwhelmly focused on sulfate reducing bacteria (SRB) that usually respire on sulfate (terminal electron acceptor) because SRB are often found at pitting sites. Many laboratory pure-culture SRB pitting 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 respire on nitrate or nitrite. 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. Some failures in pipelines carrying crude oil and produced water, purportedly due to MIC, have been reported in the literature indicating very high pitting rates (as high as 10 mm/year) that are much higher than the short-term laboratory MIC pitting 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 rates due to organic acids (represented by acetic acid) and acidic pH corrosion through mechanistic modeling to show that APB are capable of very fast MIC pitting and 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 APB instead of focusing too much on SRB.
