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” into 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 corrosion-inhibiting coatings for aluminum. Most aircraft are constructed of aluminum base alloys, and, obviously, avoiding corrosion is highly desirable. Unfortunately, hexavalent chromium is carcinogenic to humans, and, in 2009, DOD 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 base 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. SERDP’s mission is to “meet DOD’s environment challenges,” through programs it sponsors in partnership with EPA and DOE.
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 protective conversion coatings. Praseodymium-based inhibitors are dispersed in the primer coating, where they migrate to the surface to inhibit corrosion.
The group is studying the coatings on substrates made of two aluminum base alloys commonly used in aerospace applications, 2024-T3 and 7075-T6. Both are susceptible to localized galvanic corrosion.
The quality of the Ce-base conversion coating is strongly dependent 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 Ce 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, such as focused ion beam/scanning electron microscopy, 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, they 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-base 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 it forms a compound over the intermetallic regions. Fahrenholtz says the compound that forms is a praseodymium hydroxycarbonate, however, the exact phase and composition are not know. “It is a really difficult compound to isolate,” he says.
The Pr-epoxy primer approach was recognized in 2007 as a R&D 100 winner. Deft, Inc. (Irvine, Calif.) is an industrial partner on the project and incorporates the Pr inhibitors in several of its primer products.
Sounding a little like a proud father, 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 Ce 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 like aluminum and magnesium, which are more reactive and need to be protected from the environment.