Many microorganisms found in industrial water utilize hydrogen in their metabolic processes, which often results in the cathodic depolarization of the corrosion reaction . The release of hydrogen from the surface of a metal provides a means for continuation of the corrosion reaction. Oxygen released by algae as part of their metabolism also serves to cathodically depolarize the corrosion reaction.

Although the depolarization effects of microbes are important, their primary contribution to corrosion is related to under-deposit corrosion. A slime mass — or a combination slime/inorganic salt mass — produces a differential aeration cell. 

The area under the mass will, therefore, become actively anodic, causing severe localized attack. In this situation, the microorganisms not only constitute a deposit problem in themselves, but also entrap other materials which might normally have remained in the water phase; thus the deposit formation increases and, inevitably, more corrosion occurs.

Many microbiological species present special corrosion problems in addition to those inherent in the basic nature of their actions.

 Sulfate-reducing bacteria produce extremely dangerous hydrogen sulfide gas, which corrodes metals by low pH attack, and by the formation of ferrous sulfide, which is cathodic to ferrous construction materials in the system. 

Sulfide-oxidizing bacteria produce sulfuric acid, and as mentioned earlier, produce localized low pH areas in the system. Corrosion proceeds very rapidly in these low pH areas. 

Nitrifying bacteria, such as Nitrobacter and Nitrocystis, nullify the effectiveness of nitrite corrosion inhibitors by oxidizing nitrite to nitrate. This is most serious in closed recirculating systems which commonly use nitrite as a corrosion inhibitor.

The examples discussed above represent only a few of the special corrosion problem! caused by particular microbial species; many more could have been offered.

The corrosion problems caused by microbes place a strain on routine corrosion control programs, increasing the amount of inhibitors and dispersants needed to minimize corrosion and the formation of deposits. An effective control program to remove microbial growth deposits is the only method to prevent under-deposit corrosion before any damage is done.