Concentration cell corrosion occurs when two or more areas of the same metal surface are in contact with electrolytic solutions of different concentrations. One type of concentration cell corrosion is metal ion concentration cell corrosion, which typically occurs in underground pipes and tanks.
In metal ion concentration cells, the electromotive force (EMF) is the difference in the concentration of metal ions in the water in contact with the metal's surface. In the presence of water, a higher concentration of metal ions usually occurs under the surface of a buried pipe's joints. Relatively lower concentrations of metal ions exist near the surfaces of crevices in the metal structure. A small electric potential develops due to the difference in metal ion concentration. The area of the metal surface in contact with the lower concentration of metal ions will become a cathode and remain protected. The area of metal surface in contact with the higher concentration of metal ions will become anodic and eventually corrode.
The same metal has different electrical properties in the presence of different concentrations of the same electrolyte – in this case a water solution of the ions of the same metal. The difference in potential forces the metal to develop anodic and cathodic regions within it. The metal surface and the electrolyte solution complete the electrical circuit (a path for movement of electrons), which enables the current to flow and for electrochemical corrosion to occur.
Studying this type of corrosion is important because soil is a combination of metal ions at different concentrations. This becomes most severe when pipelines pass through swampy areas that are adjacent to dry areas or when a tank is located in dry soil but the water table makes some parts of tank's surface wet.
Sealing the joints and other exposed metallic surfaces to exclude moisture usually eliminates such corrosion. Applying an inorganic zinc primer protective coating also tends to protect metal structures.