A Practical Mechanistic Model for MIC Based on a Biocatalytic Cathodic Sulfate Reduction Theory

Microbiologically induced corrosion (MIC) due to sulfate-reducing bacteria (SRB) is a major problem facing the oil and gas industry as well as other industries such as water utility. Current risk-factor probability models are useful in predicting the likelihood of MIC. However the reliable prediction of the progression of MIC pitting must depend on mechanistic modeling. This paper presents a mechanistic model for the prediction of MIC pitting progression based on a biocatalytic cathodic sulfate reduction (BCSR) theory. The hydrogenase system in the sessile SRB cells at the interface of biofilm and metal surface is treated as a bio-electrocatalyst for sulfate reduction. The model considers both charge transfer resistance and mass transfer resistance. It can be calibrated using an experimentally measured electrochemical parameter recast as “biofilm aggressiveness” for a particular SRB biofilm. Other charge transfer and mass transfer parameters are used as available in the literature or from existing experimental correlations. Computer simulation indicates that charge transfer resistance is important initially when the biofilm thickness is small. However, mass transfer resistance becomes dominant after pit grows to a sizable depth. In fact, the growth of any deep pits will always be mass transfer controlled regardless of how aggressive the biofilm is.