The existence of anodic and cathodic [?]on the surface of a piece of metal implies that [?]in electrical potential are found on the surface. These potential differences have a number of causes. One important mechanism is oxygen [?]corrosion, in which the oxygen concentration in the [?]varies from place to place. An underground pipe that passes from clay to gravel will have a high oxygen concentration in the gravel region and almost no oxygen in the [?]clay. The part of the pipe in contact with the clay becomes [?]and suffers damage.