What Does Localized Differential Cells Mean?
When differences occur in a metal, the environment or from impressed currents, a localized differential cell is formed among small nearby areas on the metal surface, causing damage such as pitting and crevice corrosion. A concentration or differential cell develops between the electrolyte inside and outside the crevice. Accelerated corrosion occurs inside the crevice while the outside is protected.
Since biofilm tends to create non-uniform surface conditions, localized attack might start at some points on the surface leading to localized corrosion, usually in the form of pitting. Many of the byproducts of the microbial metabolism are corrosive. These materials can concentrate in the biofilm, causing accelerated metal attack through formation of localized differential cells.
Corrosionpedia Explains Localized Differential Cells
Deposits can cause accelerated localized corrosion by creating differential aeration cells. This same phenomenon occurs with biofilm. The non-uniform nature of biofilm formation creates an inherent differential, which is enhanced by the consumption of oxygen by organisms in the biofilm.
Microorganisms can influence corrosion in a variety of ways, like formation of localized differential cells and sulfate reduction. The formation of localized cells is the primary mechanism of corrosion caused by iron oxidizing bacteria. Corrosion may also develop when localized cells are formed, due simply to biofilms developing on metal surfaces (when no galvanic corrosion had ever existed). The oxidation of iron or manganese is not always a requirement for the development of a localized corrosion cell.
Microorganisms tend to attach themselves to solid surfaces, colonize, proliferate and form biofilms which may in turn produce an environment at the biofilm/metal interface radically different from the bulk environment in terms of pH, dissolved oxygen, organic and inorganic species. In crevice corrosion, localized differential cells result from a concentration difference between the electrolyte within the crevice and the electrolyte outside the crevice due to stagnation of electrolyte.
Localized differential cells in which a metal part is partly immersed in a liquid electrolyte above which the gaseous phase consists of a gas or gases other than air are also common. This type of cell corrosion occurs on buried metals as a result of their being in contact with soils that have different chemical compositions, water contents or degrees of aeration.