Coatings and Cathodic Protection for Underground Pipelines - What a Corrosion Engineer Should Know

Coatings control corrosion by providing a barrier against oxygen and water. Although coatings are extremely effective, all organic coatings are semi-permeable to O2 and water. The water permeability coefficient of fusion bonded epoxies, one of the best coating systems, is approximately 10 gms/mil/24 hours, but minimum concentration of oxygen and water that can start the corrosion process is 10 M. In this paper, Nernst equation is used to calculate these concentrations. Cathodic Protection, CP, on the other hand controls corrosion by providing excess electrons to the pipe surface. This system is based on the electrical circuit, where the negative terminal of electrical source is connected to the pipe and the positive terminal to the anode that is placed in the soil. Both systems have limitations in providing the absolute protection single handedly. A combination of CP and coating is the best solution for this complex problem. In the CP-Coating System, coating undertakes the major role to provide the needed protection and CP assumes the secondary role. However, in all cases where the coating fails to provide protection, CP becomes critically important and should provide the necessary protection. Therefore, the limitations of each system should be evaluated and their properties must be adjusted so that they will work in union. In this paper, the properties of coatings that are critical in making the coating a part of the CP-Coating Combination System are discussed. Adhesive Strength and Cohesive Strength are two important and two distinctively different properties. Adhesive Strength is a total sum of several components. Mechanical adhesion, polar-polar adhesion and chemical adhesion are three major components of Adhesive Strength of the coating. The role of these (adhesion and cohesion) properties in providing long-term protection are explained. Along with the chemistry of the coating, the application parameters have a major role in the final Adhesive Strength of the coating. In this paper, the important parameters that control the adhesive strength and how they depend on some of the application parameters are also explained.