R&D Partnerships Accelerate Use of New Coating

When the U.S. Navy christened its new class of close-to-shore combat ships, known more formally as littoral combat ships, the pervasive threat of corrosion became clear during the vessels’ initial operations. The vessels’ low draft required substituting steel with a light-weight alloy of aluminum and magnesium. But this alloy proved susceptible to corrosion and cracking, escalating maintenance costs and raising concerns about structural integrity throughout the fleet.

The Navy turned to the University of Virginia School of Engineering’s Center for Electrochemical Science and Engineering to solve this engineering problem. To appreciate the challenge, an analogy might help: Monkey bread—biscuit dough balls baked together with cinnamon—is a good way to think about it. The dough balls are crystals within the aluminum-magnesium alloy’s structure; the cinnamon seam is the interface—or grain boundary—where two or more crystals touch. The cinnamon is also where the corrosion and cracking occurs.

In the language of materials science, this problem is called intergranular corrosion. When the material is loaded, or placed under stress such that it’s pulled apart at its ends, then intergranular stress corrosion cracking can occur. Intergranular corrosion and intergranular stress corrosion cracking result from sensitization—the formation of particles at the grain boundaries that are susceptible to dissolution in seawater.

With funding from the Office of Naval Research, the center’s research staff identified the root cause of the corrosion and cracking. Armed with this knowledge, the team received further funding to engage with industry partners to figure out how to design strategies to stop the cracking and develop an engineering approach to manage the damage across the fleet.

Supported by Small Business Innovation Research program funding, the Center partnered with Luna Innovations, based in Charlottesville, Va., to design a metal-rich primer that infuses metal pigments into a protective coating. When wetted, this metal galvanically couples with the aluminum-magnesium material, effectively stopping the oxidation reactions on the metal of the structure being protected.

The application of a coating is not always practical, however. Structural integrity managers needed a tool to help predict the onset and progression of such damage on naval vessels. As part of the Office of Naval Research’s Future Naval Capability program, the center partnered with VEXTEC Corporation to develop a prediction code for intergranular corrosion and intergranular stress corrosion cracking. VEXTEC is an engineering consulting company based in Brentwood, Tennessee, that offers software based on integrated computational materials engineering to predict product durability. This research effort involved interaction with Navy engineers to design and coordinate on-ship exposures to validate the models. 

Extensive undergraduate and graduate student efforts underpin all of these efforts. An exciting result is that several of the researchers have since joined the Navy and industry labs and continue to use the expertise they developed at UVA to orchestrate the integration of these research efforts and real-world impact.