The Influence Of Environment Condition On The Design Of Cathodic Protection Systems For Marine Structure

Abstract

ABSTRACT Environmental conditions should and definitely do influence the design of cathodic protection systems for corrosion mitigation on marine structures in all areas of the world. Selection of components of the corrosion mitigation systems are influenced by environmental conditions from both an electrochemical and mechanical standpoint. Corrosion rates on metals are influenced by their environments, and there is quite often a definite relationship between the corrosion rate and the protective current density required to prevent corrosion. 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 area. A cathodic protection system designed for corrosion prevention on a structure in straight sea water would not perform satisfactorily for the same structure in brackish or polluted waters. Environmental conditions should and definitely do have an influence on the corrosion rates of submerged steel and the design of cathodic protection systems for corrosion mitigation on marine structures. In orderfor a cathodic protection system to perform properly and thus prevent corrosion, the environment in which the system must operate must be taken into account as a primary consideration. 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 selection of the type of cathodic protection system depends largely on the environment and its overall effect on the structure to be protected and on the protective system. The effects of environment will be handled in two broad classifications, electrochemical and mechanical, and they will both be related in this discussion to the natural sea water offshore in the Gulf of Mexico as an average or "norm" for environmental conditions. It is logical to select the Gulf as an average of environments since it contains more offshore platforms and other related marine facilities than any other body of water in the world. It follows that experience withcorrosion and cathodic protection on steel structures in the Gulf is very extensive. For steel structures in natural sea water there is an approximate equality between the corrosion rate, expressed in mils per year (mpy) and the amount of cathodic protection current (direct current), expressed in milli per sq. ft. of submerged steel area, 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 (.005 inches year) and the cathodic protection current density for attainment of a minimum protective to-water potential indicative of complete corrosion control is 5 milliamperes anodic per sq. ft. of submerged steel area.