CORROSION PROTECTION AND PREVENTION UNDERGROUND

Corrosion of buried structures is often accelerated by stray electrical currents and differential environmental cells. Due to the difficulty in inspecting buried structures many are never inspected until they are removed from service or fail.

Cathodic protection, usually combined with protective coatings, is the most widely used means for controlling underground corrosion. Cathodic protection not only can effectively control corrosion on buried structures, periodic confirmation of the proper operation of the cathodic protection system through electrical inspections can be used to infer the condition of the buried structure.

Electrolyte.

Soils contain mineral matter, organic matter, water, and air. The electrical conductivity of the soil can be affected by many factors but the amount of water and the soluble salts in the mineral and organic matter have the greatest effect. The resistivity of soils is conveniently measured using the four pin method shown.

In general, a soil with low resistivity will be more corrosive than a soil with high resistivity, but substantial corrosion can still occur in high resistivity soils under certain conditions. High soil resistivity should not be used as the sole criteria for determining the necessity to control corrosion on buried structures.

Commonly Used Materials.

Steel is the most commonly used material for buried structures. Cast iron is also used in buried pipes and valves. Aluminum is sometimes used underground for fuel lines where internal corrosion from steel lines would cause unacceptable fuel contamination. Lead is often used as an outer sheath on buried communication lines.