The primary purpose of this document is to review and assess existing coating technologies in various industries in order to highlight their advantages and disadvantages for below-grade applications in transmission and distribution structures. Furthermore, practical information and standard guidelines on coating material selection, surface preparation, coating application, and coating assessment for buried sections of direct-embedded transmission poles are discussed.
Transmission and distribution (T&D) structures, including poles, are subject to aging and material degradation as a result of continuous exposure to environmental and mechanical stresses. It is well evidenced that below-grade corrosion is the main culprit responsible for the failure of T&D structures and the primary cause of in-service structural degradation.
Coating is potentially the most effective method (first defense) for below-grade corrosion mitigation at electric utility structures. From the perspective of corrosion control, the main function of coating systems is to protect the steel substrate against a corrosive environment by providing a barrier layer. For this purpose, the coating material itself must be chemically stable in corrosive soil environments and provide minimal permeability to oxygen, water (humidity), and corrosive ions (chemical salts). Furthermore, it should provide proper adhesion (wet) characteristics and exhibit adequate resistance against cracks, delamination, and cathodic disbondment in the presence of cathodic protection systems.
As a relatively new industry, the steel pole industry is required to be more aware of the problems associated with coating systems and the potential solutions. In fact, it has been a while since electric utility companies have realized the benefits of corrosion control technologies developed in the pipeline industry, and nowadays they show increasing interest in adapting successfully practiced techniques to minimize corrosion risk and improve the safety of their underground assets. Nonetheless, due to differences in design and application of transmission structures and pipelines, the electric utility industry is required to reiterate the lessons learned from the pipeline industry and develop specifically designed corrosion mitigation guidelines tailored to address the issues in transmission and distribution infrastructure. This is particularly true for galvanized steel and weathering steel, which have different surface characteristics compared with carbon steel.
Transmission poles; direct-embedded poles; underground corrosion; below-grade coating; pipeline coating; coating selection; surface preparation; coating application; coating assessment