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Robert A. RappDistinguished University Professor EmeritusPh.D., Carnegie Institute of Technology, 1960 Tel. (614) 292-6178 Office: 378 Watts Hall
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Compilation of measured solubilities for several oxides in fused pure Na2SO4 at 1200K. |
After four decades of research in the thermodynamic and kinetic studies of high temperature corrosion and protection, diffusion coatings, scaling mechanisms, point defects in compounds, solid-state electrochemistry, evaporation phenomena, chemistry and electrochemistry of fused salts, reactions in the solid state, and interfacial reactions, Professor Rapp's recent research has returned to process/extractive metallurgy.
In this research, from the scientific standpoint, electrochemical probes have been developed and demonstrated which provide a quantitative measure for the acid-base properties of cryolite-base (Na3AlF6) baths. A computer analysis has been developed to identify the three principal solutes for alumina in cryolite: Na2Al2OF6, Na2Al2OF4, and Na4Al2O2F6, and the equilibrium constants for their formation. The methodology was extended to explain the coupled solubilities of two or more oxides dissolved in cryolite. The existence of these oxyfluoride complexes acts as a buffer to limit the range of acid-base variation in cryolite-base melts. Measurements of the solubilities of oxides of Ni, Fe and Tn in such melts were made to test the theoretical predictions. An understanding of the cryolite chemistry is critical to the compatibility of materials, e.g. proposed inert anodes and cathodes, with the cryolite bath.
High-Temperature Materials Chemistry
High-temperature materials chemistry, encompassing thermodynamics, kinetics, electrochemistry, acid-base properties, heat and mass transfer and modeling, etc. provides important tools for understanding materials processing and degradation. Properties such as vapor pressures, solubilities, compound stabilities, electrode potentials, etc. are important for optimizing desirable high-temperature processes, e.g. CVD deposition, electrowinning, oxidation/reduction, evaporation/condensation, diffusion coatings, etc., and minimizing high-temperature corrosion processes by gases and fused salts. Today software with huge data banks are used to provide complex thermodynamic calculations and graphical representations, solutions to Fick's diffusion equations, and heat and mass transfer expressions. Identification of the rate-limiting- step and the interpretation of activation energy remain key issues in understanding and interpreting process kinetics. Such fundamental concepts continue to assist in the understanding, optimization or minimization of important materials interactions at high temperatures.
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Schematic representation of the "poisoned interface" interpretation of the Reactive Element Effect in high-temperature scaling. Large reactive element ions segregate to the metal/scale interface, thereby pinning the interfacial dislocations and preventing scale growth by cation diffusion. |
Dr. Rapp has published about 265 papers, chapters, and educational courses; he has authored 5 patents and submitted many other disclosures at OSU. He has served on the Review Board of several Journals: Corrosion, Oxidation of Metals, J. Electrochemical Soc., Metallurgical Transactions. He was named a Distinguished Engineering Alumnus by Purdue University, and has received the ASM Stoughten Young Teacher Award, and "Chavalier des Palmes academiques" from France. He has been awarded the ASM Howe Gold Medal, the British Inst. Corrosion's U. R. Evans Award, the NACE Whitney Award, and he presented the ASM Campbell Memorial Lecture in 1983. Two of his students have received BFGoodrich National Collegiate Inventor's Awards. He serves on the OSU President and Provost Advisory Committee.
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Sequence of photos from environmental hot-stage SEM showing growth of iron oxide (wustite) on iron at 1200C. (1400X) Screw dislocations intersecting the surface serve as sources for growth ledges. |
