Materials Performance

MAY 2015

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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49 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 COATINGS & LININGS ESSENTIALS Continued f rom page 46 Continued on page 51 of Elements on Steels for Corrosion Protection at High Temperatures," by X. Montero, M. Schütze, and M. Galetz. Study Compares High-Build, Direct-to-Metal Atmospheric Coatings High-build, direct-to-metal (DTM) coatings are popular as atmospheric industrial maintenance coatings because of their ease of application, increased productivity, reduced labor costs, and reduced return-to-service times. Because of their advantages over multi-coat systems, these coatings are used in mildly corrosive environments. High-build coatings are characterized by their ability to provide corrosion protection from one application of a relatively thick flm (e.g., 125 to 230 µm dry flm thickness). As the coating thickness increases, the permeability of water and ionic species (salts) is reduced, which lowers the corrosion rate. While the barrier mechanism is efective in protecting intact flms, coating damage can expose areas of metal and enable rapid corrosion; so protection is augmented by anticorrosive pigments (e.g., zinc phos- phate) that leach out of the damaged coating and passivate the exposed steel substrate. Another important performance variable is binder selection, and a number of diferent resin chemistries allow for high-flm build, including epoxies, polyure- thanes, polyaspartics, and polysiloxanes. To compare application characteristics and physical properties of these coatings when applied to steel panels, a study was conducted with each coating's performance benchmarked against a two-coat epoxy/ polyurethane system. ASTM tests—salt fog, abrasion resistance, fexibility, impact resistance, dry-time, pull-of adhesion, and accelerated weathering—were used to characterize each technology. Epoxy coatings performed well in physical testing with the exception of weathering; polysilox- anes performed well in most tests with the exception of fexibility; both polysiloxane and epoxy coatings had longer dry times; polyaspartic coatings performed well overall, but demonstrated room for improvement in wet adhesion; and polyurethane coatings performed well in all testing, but sufered from application disadvantages. More detailed information on the study is presented in CORROSION 2015 paper no. 6072, "High-Build, Direct-to-Metal Atmo- spheric Coatings for Moderately Corrosive Environments," by C.T. Williams and A. Olson.

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