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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51 MATERIALS PERFORMANCE: VOL. 57, NO. 3 MARCH 2018 Continued on page 52 state but "splash" outward in a way that looks like liquid. According to Schuh, it was an eye opening observation. For a metal to bond to metal, the splash needs to be solid—a liquid splash doesn't bond. at phenomenon was found in a variety of these metal processing methods, he says. With the new ability to observe the process, the researchers note that the conditions needed to induce that bond can now be identified. e findings could be relevant for pro cesses used to coat components. In addi tion to coatings, the new information could also help in the design of some metal based additive manufacturing systems, known as 3 D printing. ere, as with coatings, it is critical to ensure that one layer of the print ing material adheres solidly to the previous layer. By determining the optimal conditions to ensure a solid bond, Schuh says, this work provides an accurate and mathemati cal approach rather than an empirical approach for improving the bond of sprayed metal coatings. e work was supported by the U.S. Army through MIT's Institute for Soldier Nanotechnologies, the U.S. Army Research Office, and the U.S. Office of Naval Research. Source: Massachusetts Institute of Technology News Office, Ceramic Coating Performance in Aggressive Environments Although protective coatings are widely used to mitigate corrosion of structural steel bridges in aggressive humid environ ments, their service life is often diminished. Repairs can be costly due to materials, la bor, and environmental controls. As part of an investigation to assess novel coatings that are commercially available for steel bridge application, a chemically bonded phosphate ceramic (CBPC) coating was ex amined by researchers with the Florida In ternational University (Miami, Florida, USA) and the Florida Department of Trans portation (Gainesville, Florida, USA). e study considered various exposure environ ments such as inland, beach, and salt fog exposure. e CBPC coating consists of an acid phosphate and a water based slurry that contains base minerals and metal oxides (e.g. magnesium oxide/magnesium hydrox ide [MgO/Mg(OH) 2 ]). ese two compo nents are mixed together and sprayed on a metal surface with a dual component spray gun. According to the commercial litera ture, the acid phosphate and oxides in the slurry interact with the metal substrate to form an insoluble passivation layer of stable oxides (2 to 20 µ m thick) that contains ~60% iron with phosphate, potassium, mag nesium, silicon, hydrogen, and oxygen. Additionally, the commercial literature says the coating can be used in environments with temperatures ranging from 35 to 200 °F (1.7 to 93 °C) and 0 to 99% humidity. e researchers assessed the perfor mance of the CBPC coating in outdoor exposures by placing steel coupons with a CBPC coating at a beach test site in the Florida Keys as well as an inland test site in South Florida ~10 mi (16 km) from the coast. CBPC coated coupons for testing were provided by the manufacturer. e coated samples were exposed at 45 degrees facing south. Exposure times of four, eight, and 24 months were proposed to identify coating degradation and corrosion develop ment over time. Temperature and relative humidity at the two outdoor test sites were comparable, but the precipitation for inland site was higher than for the beach site. Samples also were exposed to salt fog conditions with 5% sodium chloride (NaCl) solution for 2,200, 5,800, or 14,600 h (according to ASTM B117, "Standard Prac tice for Operating Salt Spray (Fog) Appara tus") to evaluate the effects of the aggressive exposure conditions on the integrity of the coating and corrosion durability of the steel coupons. e salt fog chamber temperature was ~32 °C. e samples were placed at a ~40 degree inclination with support along the bottom edge of the coupon. To assess the corrosion damage incurred, the coating samples were evalu ated by visual inspection, coating thickness, adhesion measurement, and x ray

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