Species of corrosive microbes and corrosion mechanisms

Corrosion is a global problem affecting a wide variety of the mechanical structures of piping, buildings, transportation, sewage, and automotive parts. Corrosion is an abiotic electrochemical reaction of metal oxidation with oxygen and water. Under anoxic conditions, the only reactant available for iron oxidation is water-derived protons. The kinetics of this reaction is extremely slow. However, this behavior contrasts with extreme corrosion observed in anoxic environments, demonstrating that biological processes play an important role in iron and steel corrosion. Therefore, among the different corrosion mechanisms, microbiologically influenced corrosion (MIC) is the most common and the most closely related to the complex processes connected with microorganism activity. Biocorrosion is a well-established, highly destructive phenomenon, and MIC can accelerate the deterioration of metal, plastics, stone, concrete, and wood, leading to human and environmental risks as well as substantial economic losses, which make MIC an important research topic. It is estimated that 20% or more of corrosion losses can be attributed to MIC. The main types of bacteria associated with corrosion are SRB, SRA, NRB, APB, IOB, IRB, SOB, and bacteria-producing organic acids, exopolymers, or slime. MIC is always associated with biofilm. Although classical corrosion theories can explain some MIC phenomena, the limitations of these mechanisms are exposed when MIC becomes a serious concern in real industrial applications. With increasingly more research on corrosive bacteria, people have a more comprehensive and in-depth understanding of the mechanism of MIC. In this work, the species and characteristics of microorganisms easily leading to corrosion are analyzed, such as sulfate-reducing bacteria, nitrate-reducing bacteria, and iron-oxidizing bacteria. Different mechanisms of MIC are discussed using the concepts of bioenergetics, electron transfer theories, and respiration types. The latest research progress on the microbial corrosion mechanism, including extracellular electron transport, metabolite corrosion, and the concentration differential battery, was reviewed. The process of microbial corrosion often involves more than one mechanism. Different microorganisms grow in different environments, and their metabolic processes differ. Therefore, obtaining a unified corrosion mechanism is difficult, so we can only judge which mechanism plays the main role according to the specific situation. This review provides theoretical guidance for the diagnosis, prediction, and prevention of microbial corrosion under anaerobic and aerobic conditions in the industry.