Assumptions of protective current requirements and bare metal areas. To obtain a starting point, certain general assumptions have been found helpful: (reference)
For bare metal in the ground, a current of 11 to 22 mA/m2 of bare metal surface has been found adequate, except under extreme or unusual conditions. This value must then be modified to suit the particular conditions.
For coated pipe, the current required is difficult to estimate without field tests. The primary reason is the unknown condition of the protective coat which can vary from nearly 0% to 98% coverage. For a fairly new protective coat properly applied,. assume 2 percent bare and 22 mA/m2 for use in tentative calculations. Field test may show that this figure should be modified.
Bare pipelines can usually be protected by 11 to 22 mA/m2. This is seldom justifiable economically for extensive or long lines, however, and the necessary protection is usually afforded by the application of cathodic protection to localized areas called "hot spots."
Bare steel tanks are treated the same as bare pipelines. Inside steel surfaces in contact with fresh water at zero or low velocities require from 22 to 65 mA/m2, depending on the nature of the water. The low value is used for water which is scale forming. That is, the water will form a calcareous coating on the surface of the metal.
Protecting steel surfaces in contact with water in motion presents another problem. Water in motion produces a scouring effect which prevents the formation of the above-mentioned coating and even the formation of a hydrogen film. Therefore, surfaces exposed to water in motion require a higher current density. The amount required is hard to predict. In this case, an experimental determination of the current requirement should be made.