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24 DECEMBER 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 12 T FEATURE ARTICLE Assessing Galvanized Steel Power Transmission Poles and Towers for Corrosion Kathy Riggs Larsen, Editor The electric power utility industry commonly uses galvanized steel for power transmission poles, lattice towers, and other transmission and distribution as- sets—particularly high-voltage transmission line structures and substation structures—because it is known to be well-suited for service in most atmo- spheric and underground environments and has a long record of proven performance. According to the American Iron and Steel Institute, close to 1 million steel distribution poles have been installed in the United States since 1998 and are being used by more than 600 U.S. electric utilities. 1 Zinc galvanizing protects the carbon steel (CS) substrate by providing a barrier against corrosive compounds and also by acting as a sacrificial anode that protects the underlying CS surface if the coating is damaged. Adelana Gilpin-Jackson, P. Eng., a special- ist engineer with Canadian electric utility BC Hydro (Burnaby, British Columbia, Canada) comments that hot-dip galvanizing provides electric utility structures with a surface layer (Eta layer) of pure zinc for gal- vanic and barrier protection, as well as several inter- metallic zinc alloy layers (Zeta, Delta, and Gamma) that form as the zinc coating is applied under high temperatures. These layers are metallurgically bonded with the steel to form a tough and well- adhered coating that provides superior galvanic and barrier protection. Galvanized structures typically exhibit a low cor- rosion rate because a continuous passive film, known as a zinc patina, forms on the pure zinc top layer of the galvanized surface when it is exposed to the atmo- sphere. This passive surface film provides a protective barrier that prevents moisture and chlorides from corroding the underlying steel. As the patina starts to develop, a layer of zinc oxide (ZnO) quickly forms as the zinc reacts with oxygen in the air. The ZnO layer, when exposed to moisture, converts into a thin layer of zinc hydroxide [Zn(OH 2 )], which reacts with atmo- spheric carbon dioxide (CO 2 ) over time and becomes a dense, insoluble layer of zinc carbonate (ZnCO 3 ) that slows corrosion of the underlying zinc. Since zinc is anodic to steel, the hot-dip galvaniz- ing also acts as a sacrificial anode if the galvanized coating is physically damaged to some degree. If indi- vidual areas of underlying steel become exposed, the surrounding zinc will provide sacrificial cathodic pro- tection (CP) to the unprotected sites by corroding preferentially. The zinc is consumed as it sacrifices itself to protect the bare steel. Generally speaking, galvanized steel can last for many years in nonaggressive environments, and typi- cally does an excellent job of protecting steel when the structure is located in moderately corrosive environ- ments where oxidizing conditions prevail, says Mehrooz Zamanzadeh, FNACE, a NACE-certified Corrosion Specialist. He notes that during a recent field assignment in Texas, galvanized lattice towers dating back to the early 20th century exhibited an intact galvanized layer even after 90 years of service.