SOIL RESISTIVITY AS RELATED TO UNDERGROUND CORROSION AND CATHODIC PROTECTION

Abstract

CORROSION DATA ARE BASED ON MEASUREMENTS MADE ON ABOUT 4,500 SPECIMENS OF COMMONLY USED PLAIN FERROUS MATERIALS WHICH HAD BEEN BURIED IN BACK-FILLED TRENCHES AT 86 NATIONAL BUREAU OF STANDARDS SITES FOR PERIODS UP TO 17 YEARS. THE METALS EXPOSED CONSISTED OF OPEN HEARTH IRON, HAND-PUDDLED AND MECHANICALLY PUDDLED WROUGHT IRONS, AND OPEN HEARTH AND BESSIMER STEELS, ALL WITHOUT ALLOYING CONSTITUENTS. THE SOILS RANGED IN RESISTIVITY FROM 50 TO 54,000 OHM-CM AND IN PH FROM 2.6 TO 10.2 FOR PERIODS OF EXPOSURE UP TO 5 YEARS, THE MAXIMUM PIT DEPTHS ARE ON THE AVERAGE DEEPER IN SOILS WITH RESISTIVITIES UP TO 500 OHM-CM THAN IN SOILS WITH HIGHER RESISTIVITIES. IN THE HIGHER RESISTIVITY SOILS GREATER THAN 500 OHM-CM, THERE APPEARS TO BE NO REGULAR VARIATION BETWEEN MAXIMUM PIT DEPTH AND SOIL RESISTIVITY. HOWEVER, FOR PERIODS OF EXPOSURE LONGER THAN 5 YEARS, THE RATE OF MAXIMUM PENETRATION DECREASES AS THE SOIL RESISTIVITY INCREASES. THE CURRENT DENSITIES REQUIRED FOR CATHODIC PROTECTION ARE USUALLY GREATER THAT THE CURRENT DENSITIES ASSOCIATED WITH CORROSION. THE HIGHER CORROSION RATES EXISTING DURING THE FIRST FEW YEARS OF EXPOSURE INDICATE THAT IT IS PROBABLY ECONOMICALLY FEASIBLE TO WAIT FOR ABOUT 2 YEARS BEFORE MEASURING CURRENT DENSITIES NECESSARY FOR CATHODIC PROTECTION.