Biocorrosion (also known as Microbiologically Influenced Corrosion or MIC) is caused by biofilms. Anaerobic MIC has several distinct types. Type I MIC involves utilization of extracellular electrons released by the oxidation of an energetic metal such as elemental iron (Fe0). These electrons are transported by sessile cells on or very close to the metal surface across their cell walls to the cytoplasm where the electrons are used for reduction of an exogenous electron acceptor (oxidant) such as sulfate and nitrate under biocatalysis. The biofilm benefits from the energy released by the respiration of the energetic elemental metal. For Type I MIC the culpable biofilm must contain bottom-layer electrogenic microbes that are capable of transporting electrons across cell walls. Type II MIC does not require this kind of electron transfer because the reduction of the oxidant such as proton and free acetic acid (secreted by a biofilm at a local high concentration) occurs extracellularly without biocatalysis. Based on experimental data and observations this work compares MIC mechanisms of Sulfate Reducing Bacteria (SRB) and Nitrate Reducing Bacteria (NRB) attacks on iron (energetic) and copper (not energetic) to illustrate the thermodynamics and kinetics of MIC.