Corrosion of Silicon-Based Ceramics in Combustion Environments

Authors


  • S. M. Wiederhorn—contributing editor

  • Nathan S. Jacobson is a senior research scientist in the Materials Division at the NASA Lewis Research Center He received his B.A. degree in chemistry from the University of California at San Diego in 1975. He received his M.S and Ph.D. degrees in materials science from the University of California at Berkeley in 1978 and 1981, respectively. From 1981 to 1983, Dr. Jacobson was a post-doctoral fellow at the University of Pennsylvania. Since 1983, he has been with the Environmental Durability Group of the NASA Lewis Research Center, doing research on the oxidation and corrosion of ceramics and metals at high temperatures. His current research is focused on high-temperature interfacial reactions and vaporization processes, as limiting factors in applications of ceramics and composites. In 1992, he received a NASA Exceptional Scientific Achievement Medal for his contributions to the corrosion of ceramics. Dr. Jacobson has authored or coauthored more than 50 technical papers.

  • It is a pleasure to acknowledge the many helpful discussions I had in preparing this paper. In the combustion area, I would like to thank D. Blint of GM Research Laboratories and G. Seng of NASA Lewis. In the oxidation area, I would like to thank J. Smialek, L. Ogbuji, D. Fox, and E. Opila of NASA Lewis; K. Luthra of General Electric CR&D, Schenectady, NY; R. Tressler of The Pennsylvania State University, University Park, PA; and J. Porter II of MSNW, Inc., San Marcos, CA.

Abstract

Silicon-based ceramics and composites are prime candidates for heat engine and heat exchanger structural components. In such applications these materials are exposed to combustion gases and deposit-forming corrodents. In this paper combustion environments are defined for various applications. These environments lead to five main types of corrosive degradation: passive oxidation, deposit-induced corrosion, active oxidation, scale/substrate interactions, and scale volatility. Each of these is discussed in detail. The key issues in oxidation mechanisms of high-purity silicon carbide (SiC) and silicon nitride (Si3N4) in pure oxygen are discussed. The complicating factors due to the actual combustion environment and commercial materials are discussed. These discussions include secondary elements in the ceramics; additional oxidants, such as water and carbon dioxide (CO2); combustion environment impurities; long-term oxidation effects; and thermal cycling. Active oxidation is expected in a limited number of combustion situations, and the active-to-passive transition is discussed. At high temperatures the limiting factors are scale melting, scale volatility, and scale/substrate interactions. Deposit-induced corrosion is discussed, primarily for sodium sulfate (Na2SO4), but also for vanadate and oxide-slag deposits as well. In applying ceramics in combustion environments it is essential to be aware of these corrosion routes and how they affect the performance of a component.

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