Corrosion occurs naturally in all cooling water systems. However, the presence of biofilms on system surfaces can accelerate the general corrosion process and shorten the lifetime of the system.

Biofilm contributes to corrosion in several ways. This picture illustrates the pitting caused by microbially influenced corrosion.  Biofilms shield metals from inhibitors designed to protect them, and alter the environment beneath the biofilm, making these areas more susceptible to the initiation of localized corrosion.  Differential oxygen cells, low local pH due to acidic metabolic by-products, the production of corrosive metal sulfides, and direct metabolic attack resulting in reduction of the metal are just some of the mechanisms by which biofilm communities promote pitting corrosion in cooling water systems.