While microbial-induced corrosion (MIC) does not produce a unique type of corrosion, understanding the dynamics of microbes is a key to avoiding a problem. Microbes thrive under the same conditions as present in many aqueous cleaning systems. They need heat and a food source to provide energy, moisture, and sometimes oxygen (although some more troublesome bacteria associated with MIC are anaerobic, i.e., neither requiring nor tolerating oxygen). Conditions that may be considered to be safer for workers, such as moderate temperatures and near neutral pH, can, at the same time, be more favorable towards microbial growth, and, therefore, increase the possibilities for MIC.

For the microbes (usually bacteria) that are involved in MIC, their food can be organic compounds, including oils, or in certain cases, the host metal itself. Bacteria known as “iron bacteria” use iron as an energy source and can attack stainless steel. Sulfur-reducing bacteria can attack sulfur-containing fluids, including lubricants, releasing sulfuric acid that attacks metal surfaces.

Some microbes create biofilms that produce a cocoon-like condition. Regions beneath these films are shielded from normally aerated water and can become oxygen deprived. Anaerobic bacteria may feed on the underlying metal or other food. Even if anaerobic bacteria are not present, these oxygen-deprived areas become anodic compared with the surrounding metal, allowing corrosive processes to proceed.

From: “Must It Rust? Part II: Microbial-induced Corrosion”