Corrosions indirectly caused by metabolic byproducts of microbes is called "Microbiological Corrosions". The microbes do not consume the metals as nutrition sources causing the corrosions, but the metabolic byproducts and/or the germs themselves form corrosion cells causing anodic/cathodic reactions. The corrosion causing microbes are mainly bacterium, and sometimes mold fungus. Examples are iron-oxidizing bacteria that cause corrosions on steel, and steel/copper corrosions caused by sulfate-reducing bacteria. Additionally, aluminum alloy corrosions caused by mold fungus and bacteria in aircraft fuel are known. Classifications are shown below.

(1) Corrosion by Aerobic Bacteria

Aerobic bacteria require oxygen for growth and propagation, with the iron-oxidizing bacteria and sulfur bacteria being well known. The iron-oxidizing bacteria grows by absorbing the energy generated when oxidizing divalent iron ions (Fe2+) into trivalent iron ions (Fe3+) using water dissolved oxygen. This causes formation of rust bamps(tubercle) inside steel water pipes resulting in pipe blockages. The sulfur-oxidizing bacteria grows by utilizing the energy generated when oxidizing the remainder of incompletely oxidized sulfur, and eventually forms sulfuric acid. It can survive in strongly acidic environments of pH2~4.

(2) Corrosion by Anaerobic Bacteria

Anaerobic Bacteria does not require oxygen for growth, rather cannot survive in environments with oxygen. It was originally discovered in soil with little oxygen. Well known of anaerobic bacteria are sulfate-reducing bacteria, nitrate-reducing bacteria, and Methan producing bacteria. Sulfate-reducing bacteria reduces sulfates down to H₂S. The H₂S then oxidizes Fe and creates FeS. Contaminated metropolitan sewage water is low in dissolved oxygen due to decomposed organic compounds as well as contains large amounts of sulfate, providing ideal habitats for sulfate-reducing bacteria. The old contaminated Sumida river in Tokyo used to emit a stench during the summer, and caused blackening of copper alloys. The bacterial corrosions do not occur with AL, Zn, Pb and their alloys, and limited with Cu, but steel alloys are severely affected. The nitrate-reducing bacteria reduces nitrates and generates ammonia. The Methan producing bacteria create Fe(OH) 2 and CH4 (methane gas) from Fe and CO2.

(3) Microbiological Corrosion of Aluminum Alloys

Microbes living in water mixed in jet fuel and dissolved inorganic salts are know to cause problems such as Jet aircraft wing corrosions.

As countermeasures to prevent microbiological corrosions, it is essential to make the environment inhabitable to the microbes. In applications for aerobic bacteria the air is eliminated, and for anaerobic bacteria air circulation is provided.