Ceramics in the Environment

mar13_2013

ceramics in the environment Rare-earth compounds defeat not-so-rare corrosion on aluminum fighter jets Corrosion nibbling away on a $30 million F-15 fighter jet is a bad thing, and the paint covering one is more than camouflage—it is a sophisticated multilayer coating system that also provides corrosion protection. A typical coating system comprises an inorganic conversion coating, a primer, and a topcoat. A conversion coating is not applied directly. Rather, the surface of the metal is “converted” to a coating layer by means of a chemical or electrochemical reaction. Anodizing is an example of a conversion coating. (Presumably, the native oxides on metallic surfaces could be classified as a type of conversion coating, too.) Chromate conversion coatings are among the most effective corrosioninhibiting coatings for aluminum. Most aircraft are constructed of aluminumbased alloys, and avoiding corrosion is a high priority. However, hexavalent is carcinogenic, and DOD is committed to eliminating chromate conversion coatings from its aircraft fleet. What to replace them with is the question that a group at the Missouri University of Science and Technology is addressing. ACerS Fellow and Society director, Bill Fahrenholtz, is working with Missouri S&T metallurgist Matt O’Keefe on rare-earth based corrosion-inhibiting coatings. The project was named one of only six “2012 Projects of the Year” by DOD’s Strategic Environmental Research and Development Program. Fahrenholtz and O’Keefe have been studying coatings incorporating rare-earth compounds of cerium and praseodymium and the mechanisms by which they inhibit corrosion. Their experiments show that rare-earth compounds are not inherently protective compounds, but, in the right circumstances, they are good alternatives to chromate coatings. Cerium-based compounds work well as corrosion-protection conversion coatings. Praseodymium-based inhibitors are dispersed in a primer coating, where they migrate to the surface to inhibit corrosion. The group is studying the coatings on substrates made of two aluminum-based alloys commonly used in aerospace applications, 2024-T3 and 7075-T6. Both are susceptible to localized galvanic corrosion. The quality of the ceriumbased conversion coating The DOD named a corrosion prevention project, led by Missouri S&T researchers Bill Fahrenholtz and Matt O’Keefe, a “2012 Project of the Year.” depends strongly on processing parameters, especially surface preparation. Aluminum is an electrochemically active material, which narrows the window where good coatings are achievable. In a phone interview, Fahrenholtz says, “We walk a fine line between getting a panel that is electrochemically active enough to make the coating, but not so active that it dissolves away.” Within that narrow window, he says, processing conditions that produce the best coatings also tend to favor formation of subsurface crevices. According to Fahrenholtz, the cerium coating covers 90 percent or more of the surface and prevents corrosion by forming a simple barrier layer. However, up to 10 percent of the surface may be exposed to crevices. Using element mapping tools, the team determined that oxides form within the crevices. During the salt spray exposure, corrosion products build up within the crevice, effectively closing it as it fills with oxide and providing a self-limit to the extent of corrosion. However, the team also found that the corrosion protection of the cerium conversion coatings is strongly dependent on the phase, structure, pH, and processing parameters. When processed properly, the conversion coating meets the military requirement to inhibit corrosion for two weeks in the ASTM B117 salt spray test. Praseodymium-based inhibitors are not used as coatings themselves. Rather, Pr2O3 or Pr6O11 powders are dissolved in the epoxy primer coating. The dissolved praseodymium ions inhibit corrosion of the substrate by migrating through the primer to the intermetallic, electrochemically active areas of the substrate, where they form a compound over the intermetallic regions. Fahrenholtz says the compound that forms is a praseodymium hydroxycarbonate. However, the exact phase and composition are not known. “It is a really difficult compound to isolate,” he says. The praseodynium-epoxy primer approach won recognition in 2007 as a R&D 100 winner. Deft Inc. (Irvine, Calif.) is an industrial partner on the project and incorporates the praseodynium inhibitors in several of its primer products. Fahrenholtz says, “Because this is now a commercial product, it’s pretty much a finished project, and our work on it is done.” The coatings are already in service on F-15 aircraft and Apache helicopters, and there are plans to apply them to other military aircraft systems. Work continues, however, on the cerium conversion coatings. Fahrenholtz says there are applications for this family of coatings in commercial aviation, military aviation, and automotives. He says automobile weight reduction, for example, drives the development of materials, such as aluminum and magnesium, which are more reactive and need to be protected from the environment. n 16 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 2 (Credit: Missouri S&T.)


mar13_2013
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