What Does Local Cell Mean?
A local cell is a galvanic cell that is formed due to physical or chemical differences between areas on a metal surface in an electrolyte.
Because it is a galvanic cell, it causes galvanic corrosion due to electrical contact with a more noble metal or a non-metallic conductor in a conductive environment.
It is a form of local corrosion, and is therefore limited to the contact zone or between metal surfaces.
Corrosionpedia Explains Local Cell
Local cells can be produced by differences among adjacent areas on a metal surface. They may result from differences in the metal or the environment, or from impressed currents. Metal variations may be the result of composition differences. It may have a different corrosion potential compared to that of an adjacent solid solution. The difference may be in the thickness of a surface film at adjacent sites, which in turn may reflect metal differences in the substrate. The resulting electrochemical potential then develops an electric current that electrolytically dissolves the less noble material.
When a surface film does not fully cover the entire surface, part of the metal surface is passivated and acts as the cathode, forming a local galvanic cell, increasing the corrosion rate of the non-passivated part of the surface, and possibly causing pitting corrosion. For example, aluminum is normally passivated in neutral aqueous solutions, but the extent of passivity is relatively low in solutions containing species such as chloride ions and may break down under certain conditions. When aluminum is coupled with steel, it acts as an anode in chloride solutions causing corrosion, whereas it acts as a cathode in tap water and distilled water.