Eight-Year Evaluation of the sti-P3 System for Protection of Buried Steel Tanks

A system termed P3 was developed by U.S. Steel Research to economically reduce the corrosion of carbon-steel tanks used in underground applications. This system combines three established methods for reducing corrosion: (1) the use of sacrificial anodes for cathodic protection; (2) the use of a protective coating; and (3) the use of insulating bushings, which are installed on the tank during manufacture. A subsequent cooperative program with a leading tank manufacturer resulted in the development of fabrication specifications for the design and the initial in-service installation in the latter part of 1969. Shortly thereafter, the Steel Tank Institute adopted the system and since that time approximately 5000 sti-P3 tanks have been installed. This paper describes an eight-year evaluation of the system on two 18.9-m3 (5000-gal) steel tanks buried at a gasoline service station in Chicago, Illinois. Both previous experience with steel tanks used for the underground storage of gasoline at this station, and the results of various soil tests conducted on samples obtained from the tank excavation show that the soil is corrosive to steel. Readings obtained with electrical-resistance probes indicate that after eight years of service, essentially no corrosion is occurring on the sti-P3-protected tanks. Also, the results of electrochemical measurements show that the tanks are being cathodically protected by the sacrificial anodes and that the anodes should have a long life in this soil.