Biofilms and differential aeration cells.

By definition, any biofilm attached to a metallic surface is a differential aeration cell.

Biofilm, being a multi-species society, containing many types of bacteria, aerobic and anaerobic, living together and has a layered structure.

At the outside layers there are distributed the aerobic bacteria. As the water and the dissolved oxygen diffuse the extracellular matrix, the dissolved oxygen is consumed from the aerobic species for its respiration.

So, at the inner layers the oxygen concentration drops, and finally at the metal surface the conditions becomes anaerobic, even if the bulk water is highly aerated.