This paper discusses a comparative evaluation of the performance of various types of protective coatings available for the corrosion protection of structural steel components located in marine environments. The study is based on accelerated testing conducted at the Corrosion Research Laboratory of the Florida Department of Transportation (FDOT) in the United States and at the Centro de Estudios de Corrosión (CEC) of the Universidad del Zulia in Venezuela. An inorganic zinc primer, thermally applied zinc (99%), aluminum (99.5%), 85/15 zinc aluminum alloy, and a new dual coat system comprised of one coat of thermally applied zinc with a top coat of thermally applied aluminum were evaluated at the FDOT. All the systems were also evaluated with an epoxy or wash primer top coat sealer. Thermally applied zinc (99%), aluminum (99.5%), and the new dual coating system comprised of one coat of zinc and a top coat of aluminum were evaluated at the CEC. Some of the thermally applied systems were evaluated with and without a sealcoat. Also evaluated were hot dipped galvanized specimens with and without the top coat sealer, and specimens treated with a thermal diffusion galvanizing process.
Discussion of the results from salt fog chamber (ASTM B 117) and salt water immersion testing at FDOT, and Prohesion chamber (ASTM G 85 A5) and natural exposure to an aggressive coastal-marine atmosphere (outdoor evaluation) testing at CEC are presented. Both evaluations showed excellent behavior of the dual thermally applied system.
Thermally applied coatings (metalizing) is not a new technology. It has been used to protect steel bridges in Europe since the 1930s and has been in use by the United States Navy to protect ship hulls from corrosion since the 1970s.1 Thermally applied coatings serve as protection barriers as well as anodes for cathodic protection. At this time, one of the most recognized coating methods for corrosion protection of steel consist of a three-coat paint system made up of a zinc primer, an intermediate coat and a top coat. When used to protect steel structures in very aggressive corrosion environments, these coating systems can offer protection from corrosion for up to 10 to 15 years before the system needs to be removed and reapplied. On the other hand, thermally sprayed applied coatings have been known to have a service life of 30-50 years but are not widely used. Thermally applied coatings require less stringent surface preparation procedures and no cure time, but the higher initial cost of 30-40% more than conventional coating systems poses a major disadvantage when the long-term benefits are not considered.