The Role of Oxygen In Corrosion and Cathodic Protection^★

The oxidation of iron is accompanied by the simultaneous reduction of some other material. In the case of buried steel, above pH 4.5 the reduced substance is oxygen. The two electrons furnished by the corrosion of an iron atom reduce one-half molecule of oxygen to hydroxyl ion. Corrosion ceases if access of oxygen to all points of the surface is eliminated.

Complete cathodic protection is achieved when electrons are supplied at the steel surface at the same rate oxygen molecules diffuse to the surface. Current drainage of 1 ma./sq.ft. is equivalent to 6.24 × 10¹⁵ electrons/sq.ft./sec; this amount reduces 1.56 ×10¹⁵ molecules of oxygen, or the amount of oxygen in 318 monolayers of water. This establishes the role of diffusion of oxygen in cathodic protection; any factor which affects diffusion of oxygen to the steel surface will affect the current requirements for cathodic protection. The Baytown Refinery cathodic protection system drains 12,000 amperes, an amount just sufficient to reduce one gram of oxygen per second reaching the buried surface area of about 10,000-000 sq. ft. It is roughly estimated that the average thickness of the soil diffusion layer in the Baytown Refinery is equivalent to one inch of quiescent water.

The influence of the thickness of the diffusion layer on the current density requirements for complete cathodic protection can be developed from Fick's Law of Diffusion in the simple form: ma./sq. ft. where δ is in centimeters.