Ceramic Oxide Coatings for the Corrosion Protection of Silicon Carbide

Abstract

Silicon carbide is currently used as a structural material for heat exchanger tubes and related applications because of its excellent thermal properties and oxidation resistance. Silicon carbide suffers corrosion degradation, however, in the aggressive furnace environments of industrial processes for aluminum remelting, advanced glass melting, and waste incineration. Adherent ceramic oxide coatings developed at Solar Turbines Incorporated, with the support of the Gas Research Institute, have been shown to afford corrosion protection to silicon carbide in a simulated aluminum remelt furnace environment as well as in laboratory-type corrosion testing. The coatings are also protective to silicon carbide-based ceramic matrix composites.

Functionaly Graded Alumina/Mullite Coatings for Protection of Silicon Carbide Ceramic Components From Corrosion

“…In contrast to SiO 2 , this liquid layer is not protective because the diffusion coefficient of oxygen in it is much higher than that in SiO 2 and because in the high temperature environment it is carried away from the surface through vaporization. The situation is exacerbated in the presence of moisture since more reactions that lead to formation of Na 2 O become thermodynamically more favorable (Van Roode et al, 1993). This corrosion process is not much different from the hot corrosion of turbine alloys that is observed under Na 2 SO 4 generating conditions and the corrosion that occurs in SiC heat exchanger tubes when alkali halide fluxes are used in the aluminum remelt industry.…”

“…Good oxidation resistance more or less requires that the coating be an oxide, but going through a database of thermal expansion coefficients of oxide ceramics, one soon comes to the realization that there is no oxide that both has thermal expansion coefficient matching that of SiC over the whole temperature range and provides acceptable protection against oxidation and corrosion. There is relatively good agreement between the thermal expansion coefficient of mullite and SiC, but, even though mullite does not contain free silica, there is some evidence in the literature that it tends to form sodium aluminosilicates and silicates in an alkali and sodium environment (Dietrichs and Krönert, 1982;Van Roode et al, 1993). As we mentioned in the previous section, much better corrosion resistance is displayed by alumina, but its thermal expansion coefficient is almost a factor of 2 greater than that of SiC.…”

“…This corrosion process is not much different from the hot corrosion of turbine alloys that is observed under Na 2 SO 4 generating conditions and the corrosion that occurs in SiC heat exchanger tubes when alkali halide fluxes are used in the aluminum remelt industry. Surface recession rates of almost 1 cm/yr may be observed under these circumstances (Goldfarb, 1988;Van Roode et al, 1993).…”

“…To reduce the mismatch between alumina and silicon carbide substrates, Federer et al (1989) and Van Roode et al (1993) produced graded coatings with composition varied in 25% steps between that of mullite (inner layer) and alumina (outer coating) using a plasma spraying method. Their corrosion tests showed that the mullite-alumina graded structures did very well during thermal cycling, showing no visible damage and developing only a few cracks.…”

Functionally Graded Alumina/Mullite Coatings for Protection of Silicon Carbide Ceramic Components From Corrosion

“…This corrosion process is not much different from the hot corrosion of turbine alloys that is observed under Na 2 SO 4 generating conditions and the corrosion that occurs in SiC heat exchanger tubes when alkali halide fluxes are used in the aluminum remelt industry. Surface recession rates of almost 1 cm/yr may be observed under these circumstances (Goldfarb, 1988;Van Roode et al, 1993).Given the exceptional properties of SiC and of other silicon-based ceramics but their problematic performance in alkali and sulfur containing environments, a protective coating must be used on surfaces exposed to the combustion environment to protect them from corrosion. For proper performance, such a coating must have good oxidation resistance and chemical stability (up to at least 1300 o C), good adherence with the base material, and good tolerance to thermal cycling.…”