Materials Performance

MAR 2018

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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15 MATERIALS PERFORMANCE: VOL. 57, NO. 3 MARCH 2018 Corrosion at the SS collar and poor sealing of the flash joint with the roof. Photo courtesy of Vinod S. Agarwala. Continued on page 16 Information on corrosion control and prevention the primer to the metal surface through crevice or general corrosion." As a result, Agar- wala and other mem- bers of the technical team recommended a new solution based on superior protective coatings and practices more appropriate for the environment. Given the issue with coating porosity, Agarwala says the solution is to provide corrosion inhibitors to the system, both on the surface of the metal as well as in the coatings. "The most recommended practice is to give a corrosion protective surface treatment to the steel surface or pipe stay, like conversion coatings, before the primer is applied," he says. To accomplish this, anodizing, galva- nizing, and zinc-phosphate corrosion coatings are well-known practices capa- ble of protecting the metal surface from corrosion while also providing a good bonding base for the primer and topcoat. The surface should be sandblasted to a bare metal surface f inish prior to apply- ing any such surface treatment, he adds. For the coating itself, Agarwala says a system based on epoxy compounds is suitable, since epoxies offer excellent water resistance, good exterior durability, strong adhesion, and chemical resistance. However, epoxies have limited pot life and require an application temperature of 50 °F (10 °C) for proper curing, he cautions. With regards to crevice or pitting cor- rosion of the pipe stays, bracing plates, f langes, and f lashing, Agarwala says that if the general metal loss is more than 20%, some strengthening of the select areas may be required. However, no SS base f langes or collars should be used, given the aforementioned conf lict with the CS pipe stays. The recommendations from Agar - wala's team for selective repairs are to either weld with metallic pipe or use a carbon-f iber reinforced f iberglass plastic (CFR P) reinforcement system known as V-Wrap, a technolog y developed by Struc- tural Technologies, Inc. (Columbia, Mary- land, USA). Agarwala says repairs with this system are often performed on air- craft structures, and the wrap is then coated with the same paint as used on the stays. "CFR P wrap is a laminate of carbon f iber and f iberglass fabrics using a resin as binder and has high dielectric constant and toughness," he explains. "It serves as an insulator. There should be no electri- cal contact with the substrate metal stays." According to Agarwala, the graphite carbon wrap system can potentially get compromised when wet, leading to ionic conductivity and ultimately delamination in the resin system. To address this, he Galvanic corrosion where the SS collar was placed, showing heavy metal loss and deep pitting. Photo courtesy of Vinod S. Agarwala.

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