The Influence of Environment on Corrosion And Cathodic Protection

Abstract

A cathodic protection system designed for an offshore platform or drilling rig in the Gulf of Mexico may not operate with the desired results in the North Sea or Lake Maracaibo. This is because variations in environmental conditions affect the efficiency of the system, both electrochemically and mechanically. Such variations must, therefore, be taken into account in designing a cathodic protection system for preventing corrosion. preventing corrosion. Introduction Environmental conditions have a definite influence on the corrosion rate of submerged steel, and therefore must be a primary consideration in the design of cathodic protection systems for mitigating corrosion on marine structures. There are two general types of cathodic protection systems: galvanic anode systems, employing sacrificial aluminum, zinc, or magnesium anodes; and impressed-current systems, employing rectifiers and semi-inert anodes. The choice of system depends largely upon the environment and its effect on the structure to be prepared and upon the nature of the protective system itself. In our discussion here we shall handle the effects of environment in two broad classifications, electrochemical and mechanical, and then relate them to the natural sea water offshore in the Gulf of Mexico, considering conditions there to represent the average or "norm." The Gulf is a logical selection since it contains more offshore platforms and other related marine facilities than any other body of water in the world. Experience there with corrosion and cathodic protection of steel structures is very extensive. For steel structures in natural sea water there is an approximate equality between the corrosion rate and the amount of cathodic protection current (direct current) required to prevent corrosion. In Gulf of Mexico water the value is 5 i.e., the corrosion rate on a continuously submerged steel structure is approximately 5 mpy (0.005 in./ year) and the cathodic protection current density for attainment of a minimum protective structure-to-water potential indicative of complete corrosion control is potential indicative of complete corrosion control is 5 milliamp per square foot of submerged steel area. For the mud or soil zone of the structures i.e., that below the water zone the corrosion rate is approximately 1 mpy and the protective current density is 1 milliamp/sq ft. In practice we use design current densities of 6 milliamp in the water zone and 1 milliamp in the mud. This provides a reserve of protective current to cover some nonuniformity in applied current distribution and also provides a contingency for minor error of omission in the original calculations of structure areas to be protected. Such a reserve is also desirable in case additional submerged steel members are added to the structure later, and in case some cathodic protection current is lost to service boats, barges, pipelines, etc. The average corrosion rate of steel and iron specimens continuously immersed in natural sea waters all over the world is surprisingly uniform. For a rough estimate, in the absence of data applicable to a particular locality or condition of exposure, it is reasonable to use the 5 mpy that applies for the Gulf of Mexico. Any corrosion rates that are much higher than this may be assumed to result from some peculiar corrosion-accelerating factor, which must be environmental in nature. P. 323