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Soil and pitting corrosion of hot-dip galvanized steel : experimental and mathematical modeling Nakhaie, Davood


Hot-dip galvanized steel is widely used in the electric power utility industry for power transmission and distribution structures because of its suitable service life in many atmospheric and underground conditions. The Zn coating protects the carbon steel substrate via the formation of a passive layer and by providing a sacrificial anode. Transmission towers are designed to last decades and the slow corrosion rate of the Zn coating plays an important role in assuring this service. However, under certain conditions, the corrosion rate of Zn coatings is considerable, and it may threaten the integrity of the galvanized steel structure, thus limiting its service life. Therefore, evaluating the below-grade corrosion of galvanized steel transmission towers is necessary. In this work, different experimental and analytical techniques coupled with thermodynamic and mathematical modeling were employed to evaluate the soil corrosion of hot-dip galvanized steel transmission towers. Results showed that corrosion of the Zn coated steel takes place in three successive stages, mostly in the form of localized corrosion, likely arising from the complex nature of the soil environment. Mechanistic studies of the pitting corrosion of Zn revealed that it has very different behavior than other metals, such as Fe and stainless steel. While stable pit growth of other metals strongly depends on the presence of a metal salt film, stable Zn pits can grow at potentials well below the salt film precipitation potential, most likely due to Zn’s high tendency for complexation. Once a corrosion pit is formed in the Zn coating, which can penetrate the coating and expose the steel substrate to the environment, its growth is independent of the presence of the salt film. This distinct dissolution behavior of Zn, along with the localized nature of soil corrosion of hot-dip galvanized steel, were used to develop a mathematical model to predict the soil corrosion of galvanized steel. This new model, which is based on fundamental electrochemical equations, considers all three stages of the corrosion process. The model, which dynamically considers environmental variables to calculate the corrosion rate, was validated using existing models from the literature as well as in-situ corrosion testing.

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