Critical review: Microbially influenced corrosion of buried carbon steel pipes

• Factors affecting corrosion include temperature, rain, soil type and biofilm type.
• Biofilms create galvanic cells, aeration cells, release enzymes and alter acidity.
• Some microorganisms “consume” electrons directly extracted from steel.
• Growth of Fe0 oxidisers with nitrate reducers may increase corrosion rates.
• Metabolic states are in flux so thermodynamic equilibrium cannot be reached.

External corrosion of buried carbon steel pipes is a problem of global proportions, affecting a wide range of industries and services. Many factors affect corrosion rates. Biofilms may secrete enzymes and compounds that attack metal, alter local acidity and create differential aeration and galvanic cells. An important consideration is that biofilm metabolisms and enzymatic reactions are constantly in flux, altering the impact of microorganisms on corrosion rates, and thermodynamic equilibrium is not reached. Recent research demonstrates that some anaerobic microorganisms catalyse the oxidation of metallic iron and directly consume the electrons, with serious consequences for corrosion. This review examines relationships between soil characteristics, microbiology and corrosion processes, focussing on the impacts of microorganisms on external corrosion of buried carbon steel pipes. Techniques for improving the understanding of microbially influenced corrosion are considered and critiqued, with the aim of assisting those who work in the area of corrosion mitigation.