Corrosion of Silicon-Based Ceramics in Combustion Environments Nathan S. Jacobson* National Aeronautics and Space Administration, Lewis Research Center, Cleveland, Ohio 44 135 Silicon-based ceramics and composites are prime candi- valves and piston heads,4 as shown in Fig. I(b). The use of dates for heat engine and heat exchanger structural compo- ceramic tubes as heat exchangers has been proposed for indus- nents. In such applications these materials are exposed to trial furnaces, such as glass remelt furnaces, steel soaking pits, combustion gases and deposit-forming corrodents. In this and aluminum reclamation furnaces.' An example is illustrated paper combustion environments are defined for various in Fig. I(c). In addition, ceramics are under consideration for applications. These environments lead to five main types of heat exchangers in coal-fired combustors.' A more developed corrosive degradation: passive oxidation, deposit-induced application is the use of ceramic tubes for indirect heating, corrosion, active oxidation, scale/substrate interactions, where hot combustion gases pass through the tube, heating a and scale volatility. Each of these is discussed in detail. The process on the out~ide.~The structural components of the hot key issues in oxidation mechanisms of high-purity silicon sections are subject to a range of chemical attack processes, carbide (Sic) and silicon nitride (Si,N4) in pure oxygen are depending on the temperature, pressure, and chemical discussed. The complicating factors due to the actual com- environment. bustion environment and commercial materials are dis- The focus of this paper is on the silicon-based ceramics sili- cussed. These discussions include secondary elements in the con carbide (Sic) and silicon nitride (SilN,,). These materials ceramics; additional oxidants, such as water and carbon are inherently unstable in air and form a thin layer of silicon dioxide (COJ; combustion environment impurities; long- dioxide (SO,) in an oxidizing environment. SiO?has the lowest term oxidation effects; and thermal cycling. Active oxida- permeability to oxygen of any of the common oxides and forms tion is expected in a limited number of combustion situa- an effective reaction barrier.' Therefore, silicon-based ceramics tions, and the active-to-passive transition is discussed. At have the potential of substantially better high-temperature oxi- high temperatures the limiting factors are scale melting, dation behavior than metals. The protective oxide scales on sili- scale volatility, and scale/substrate interactions. Deposit- con, Sic, and Si,N, are shown schematically in Fig. 2. Note induced corrosion is discussed, primarily for sodium sulfate that Si,N, forms a silicon oxynitride (Si2N20)layer below the (Na,SO,), but also for vanadate and oxide-slag deposits as SiOz layer.'SiC may also form an oxycarbide layer, but there is well. In applying ceramics in combustion environments it is only limited evidence for this." The analogous protective layers essential to be aware of these corrosion routes and how they on superalloys for high-temperature applications are alumina affect the performance of a component. (A120J and chromia (Cr203)." Extensive work has been done on the performance of AI,O, and Cr,O, scales in combustion I. Introduction environments. In general, our knowledge of the behavior of SiO, scales lags behind that of A120, and Cr20,. ANY potential uses of silicon-based ceramics and com- The purpose of this paper is to review our current state of Mposites involve exposure to combustion gases. These knowledge of the interaction of SiO, with combustion environ- applications range from hot-section structural components of ments. Because the focus is on the interaction of SiO? with the gas turbines and piston engines to heat exchanger tubes for environment, many of the conclusions apply to all Si0,-pro- industrial furnaces. Gas turbines are used in aircraft and electric tected materials. This includes composites of Sic and Si,N,, power generation and have been tested for automobiles. Poten- such as Sic-fiber-reinforced Sic matrices and SiC-fiber-rein- tial ceramic components of these engines include combustor forced Si3N, matrices, and SO,-forming alloys, such as molyb- liners and, perhaps some day, turbine The location of denum disilicide (MoSi,). In this paper the major chemical these components in a gas turbine engine is shown in Fig. I(a). degradation routes are discussed. Particular emphasis is on the In piston engines the potential ceramic components include mechanisms of corrosion and the key questions involving them. 11. Environments S. M. Wiedcrhorn~ontrihutingeditor Combustion environments vary widely, depending on fuel, temperature, pressure, and oxidizer. Fuels are complex mix- tures of hydrocarbons, and they are classified by boiling points. Manuscript No. 1955915. Received June 15. 1992; approved Septembcr 28, 19?2. Lower-boiling-point fuels include automotive gasoline, and Member, American Ccramic Society. higher-boiling-point fuels include aviation fuel and fuel oils.12 4 Journul ofthe American Ceramic Society-Jacobson Vol. 76, No. 1 ,- Fan blades r Turbine I blades I Stator vanes \ I Sample holder --,Flue gas (b) (4 Fig. 1. Schematics of proposed applications lor ceramics where ceramics will be subjected to combustion environments: (a) gas turbine engine, (b) piston engine, and (c) aluininum reclamation furnace (see Ref. 5). Table I lists some common fuels and their approximate impurity equivalence ratio of 1 is stoichiometric, an equivalence ratio of contents. "-lh Note that all fuels have some amount of sulfur, less than 1 denotes a fuel-lean region, and an equivalence ratio which may lead to corrosion. The last category of fuel oils con- greater than I denotes a fuel-rich region. Most gas turbines tains more sodium, potassium, and vanadium than the other operate in the fuel-lean or stoichiometric regions. These fuels. These lower-purity fuels are likely to be utilized more in regions contain large amounts of oxygen with the CO, and H,O. the future as petroleum resources decrease. Impurities have a Most corrosion studies have been performed in the fuel-lean major influence on corrosion. Table I does not include coal- region. However, some novel combustor designs may involve derived fuels, which generally have even higher impurity the fuel-rich region, which produces larger amounts of carbon levels. monoxide (CO) and hydrogen gas (HJ. However, this region Combustion is the oxidation of these fuels to the stable prod- also contains the combustion products C02and H,O. Note that ucts carbon dioxide (C02)and water (H20). Equilibrium com- these are equilibrium calculations; if equilibrium is not bustion products can be calculated by mixing the fuel and the attained, other species, such as elemental carbon, may form. oxidant together in a free-energy-minimization computer The goal of a corrosion study is to understand the chemical code. Figure 3(a) shows the equilibrium combustion products reactions that occur between these combustion products and the for a standard aviation fuel (Jet A-CH, 91xs) as a function of proposed hot-gas-path structural materials. equivalence ratio. Equivalence ratio is defined as the fuel-to-air Figure 3(b) shows the adiabatic flame temperature as a func- ratio at a particular point divided by the stoichiometric fuel-to- tion of equivalence ratio. This temperature is calculated by air ratio for complete combustion to CO, and H20. Thus, an using a free-energy-minimization computer code and setting January 1993 Corrosion of Silicon-Based Ceramics in Combustion Environments 5 LSi02 0 .5 1.o 1.5 2.0 Equivalence ratio 2500 r SiOp L 2000 1500 1000 500 (b) I I I ~~~ I 0 .5 1 .o 1.5 2.0 Equivalence ratio Fig. 3. Calculated gas composition and Rarne temperature as a func- tion of equivalence ratio: (a) equilibrium gas composition and (b) adia- batic flame temperature. H20,but now the impurities play a key role. An aluminum rec- lamation furnace and a glass remelt furnace may involve alkali- metal salts that can deposit on the heat exchanger.' In a coal- tired furnace the atmosphere may be more reducing." In addi- tion, a mixture of oxides may form a slag deposit, which can be quite corrosive, on the tubes. Fig. 2. Schematic of protective oxide scales: (a) on Si, (b) on Sic, Table I1 summarizes the composition of these combustion and (c) on Si,N,. environments. This list is by no means exhaustive. As noted, specific applications generate specific corrodents. For example, the net heat loss equal to zero." This is an idealized case that combustion of municipal wastes may generate hydrogen chlo- assumes no heat loss through the walls of the combustion cham- ride (HCI).'" In summary, combustion environments are com- ber. It does give the maximum temperature of the flame, how- plex, involving not only the combustion products COz and ever. In general, the wall materials and the components H,O, but a variety of other gases and possible deposits as well. downstream from the flame are at lower temperatures. Temperatures and pressures are again quite dependent on the Corrosion occurs not only by gaseous coinbustion products, particular system. The ranges are listed in Table II. An but also by deposits. Perhaps the most common deposit is important issue to consider is thermal cycling. Some applica- sodium sulfate (Na,SO,), which forms when sodium reacts with tions, such as a utility turbine or industrial furnace, involve sulfur fuel impurities.'X The sodium may originate from a essentially isothermal exposures for long periods. Other appli- marine environment, from a salted roadway, or as a fuel impu- cations, such as an aircraft gas turbine, involve thermal rity. Corrosion by Na,SO, is termed "hot corrosion" and is dis- cycling. cussed in detail herein. Other types of deposit-induced corrosion originate from vanadium fuel impurities and from oxide slags. Ill. Qpes of Corrosive Attack and In some ways the piston chamber of an internal combustion Experimental Techniques engine is a more complex environment than the hot section of a gas turbine.