Current densities for cathodic protection of steel in tropical sea water

Abstract

Mass losses have been measured for sacrificial anodes used for the cathodic protection of steel structures in tropical sea water off the coast of Singapore. By converting these mass losses to average current density values over the exposure time, information has been provided which may be used to predict the mass of sacrificial anodes needed to ensure adequate cathodic protection during the design life of steel structures in this environment. The average current density for cathodic protection of steel during the first day of exposure in Singapore sea water was found to lie in the range 711–2134 mA m⁻², depending on the sea water current, anode type (zinc or aluminium), steel type (carbon content), and depth of exposure (2 or 12 m). The average current density fell with increasing time of exposure to 114·0, 58·0, and 19·1 mA m⁻² for 2 months, 9 months, and 13 years exposure respectively. Equations expressing the average current density as a function of time for different depths in Singapore sea water can be used to calculate the initial, final, and mean current densities for a given cathodic protection design life. For a 10 year cathodic protection design life, the initial, mean, and final current densities are found to be 92·2, 21·1, and 12·2 mA m⁻² for 2 m depth and 81·2, 20·2, and 12·3 mA m⁻² for 12 m depth. For estimating anode weight a higher value of mean current density should be used because additional protection current is required after the removal of marine growths or the breakdown of cathodic protection caused by storms, underwater inspection, mechanical damage, etc. Similarly, a higher final current density should be used for cathodic protection design because at the end of the design life there is a higher possibility of breakdown of protection, coating deterioration and exposure of more bare steel, and more disconnection of anodes from the structure owing to aging.