On Monday, June 11, 2012, Dr. James L. Smialek delivered a brief overvie, “High Temperature Oxidation and Corrosion,” on concepts regarding high temperature oxidation and corrosion to a mixed audience of National Aeronautics and Space Administration (NASA) Academy members, other interns and faculty fellows. Topics focused on a “NASA perspective” of issues, specializing in environments beyond 30,000 feet above ground level (AGL).
The presentation commenced with a brief overview of corrosion topics. In general, any environment rich in oxygen, or an oxygen-containing compound such as carbon dioxide or water, is conducive to oxidation. Exposure to oxides, sulfides, nitrides, and carbines may result in oxidation, however, prevention focuses mainly on resisting oxides. Various topics of interest included alloys, oxide stability, oxygen versus metal diffusion, scale microstructure, and cyclic oxidation. All of the above have relevance in the study of oxidation.
Not surprisingly, the material selected for a component weighs heavily on its susceptibility towards oxidation. Materials are ranked in a general scale ranging from “protective” to “alkaline”, referring to their reactivity. Aluminum, chrome, and silicon have desirable, stable traits, while titanium tends to react quickly, and magnesium will explosively react. The challenge in creating a corrosion-resistant, strong alloy comes from combining materials which give the best oxidation resistance while using the least amount of alloying metal. In other words, use the strongest metal with the least amount of weak alloying material possible to give it corrosion resistance.
Currently, high performance materials include nickel-based and “single crystal” superalloys, offering very high thermal and corrosive resistance with respectable strength. As research continues, the trends for these alloys show increasing amounts of aluminum and chromium, with decreased dependence on titanium for strength. The materials of the future may belong to the platinum aluminide group, created by a complex process. Interestingly, science today remains unsure why platinum aluminides are so beneficial; the material seems to have value well beyond the sum total of its parts. Unfortunately, the platinum aluminides face problems too. Relying on a thermal barrier coating only 200 nanometers thick, mere water droplets can cause rapid failures, potentially due to hydrogen embrittlement.
Testing of materials for these applications must simulate hot corrosion, and its tendencies of speedy failure progressions with dramatic results. The process is known as cyclic furnace testing. Various coupons of the materials in question are attached to a rotary plate, and spun at very high rates behind a jet of heated gas. These cycles vary greatly in time.
The presentation lasted around an hour and terminated with a review. Titanium and aluminum alloys remain problematic in terms of corrosive issues, while Fe(Ni) and Cr(Al) materials show promise with impressive amounts of success. The presentation was certainly informative, and the topic quite relevant from an engineering perspective.