Six-Year Evaluation of Thermal-Sprayed Coating of Zn/Al in Tropical Marine Environments

The main objective of this research was to evaluate the performance of thermal-sprayed coating of Zn/Al (double layer) after six years of exposure, with and without the use of sealant (wash primer) in tropical marine environments of very high aggressiveness: La Voz Station (located at the Peninsula de Paraguaná/Falcón State) and Lake Maracaibo Crossing Station (located at Zulia State), in Venezuela. To that effect, carbon steel coupons (100？mm × 150？mm × 2？mm) were sprayed by flame process. The coupons were characterized by means of initial weight, thickness, metallographic, adherence, and roughness, being evaluated monthly by visual inspection during six years. After removal, the coupons were evaluated by microscopic analysis to determine the morphology of attack, microstructure, penetration of contaminants, composition, and morphology of corrosion products. The results showed that after six years, the double-layer system represents an excellent choice for corrosion protection of steel by combining the galvanic protection of zinc with the erosion resistance of aluminum. However, due to the erosion-corrosion effect, a sealant such as wash primer can be used in order to extend its service life. 1. Introduction Worldwide, many organizations have made numerous efforts in terms of studies and research looking for new coating systems to improve the durability of structures exposed to marine and coastal marine environments, providing protection to the steel during its service life and producing a minimal impact on the environment. In marine and coastal marine environments, where a number of factors are present such as high-chloride concentrations, strong winds, changing conditions of relative humidity and temperature (especially in tropical climates like Venezuela), organic coatings do not perform properly. In this regard, thermal-sprayed coatings appear as an alternative to protect steel structures in those environments. In the United States, Europe, and Japan [1, 2], these coatings are gaining importance for their promising low environmental impact, corrosion resistance, cost benefits, durability, and other advantages over other conventional paint systems. In 1974, the American Welding Society [3] performed a study that was considered a pioneer in thermal sprayed coating where it reported that these coatings with Zn and Al, with and without sealer, exposed in marine and industrial environments, provided protection against corrosion for 19 years. Moreover, in 1987, the Laque Center for Corrosion Technology, INC. [4] reported that the degree of protection of