The metal in the structure will be consumed by corrosion. If an insulating barrier were to be placed between the metal and the electrolyte, current could not flow. In theory this is sound, but not practical in the real world.

Such insulating barriers, or coatings, must be perfect when applied and must remain perfect throughout the life of the structure. It is not practical to apply a coating that will meet this stringent criteria.

In fact, a coated structure often will suffer leaks before a structure without coatings because current will discharge from the coated structure at pinholes in the coating, instead of discharging uniformly as it does from a bare structure. The current is concentrated at these pinholes at a much greater density.

The control of underground corrosion is usually cathodic protection with coating. The theory of cathodic protection is straightforward: If the anodic areas on structure are converted into cathodic areas, corrosion will stop.

Direct current is forced to flow into the earth through a ground connection outside the structure and then through the earth to the structure to be protected. The amount of current forced to flow is adjusted to a level that nullifies the current discharge in the anodic areas. Once the entire exposed metal surface of the structure is collecting current，the entire structure becomes a cathode, and corrosion ceases.