High Temperature Salt Deposit-Assisted Corrosion of Silicon Carbide-Based Ceramic Matrix Composites: Effects of Composite Chemistry and Architecture

Hot corrosion behavior of SiCfiber/BNinterphase/SiCmatrix Ceramic Matrix Composites (CMCs) was investigated to better understand molten Na2SO4 deposit-induced degradation of this class of composites proposed for use in turbine engines operating in marine environments. Model materials were studied to isolate the CMC chemistry from the composite architecture. Model materials included additive-containing sintered-α SiC (Hexoloy), Chemical Vapor Deposited (CVD) SiC, and Si coupons. SiC coupons were studied in the uncoated condition, or with a C- or BN-coating to simulate the fiber/matrix interphase material. Coupons were loaded with 2.5 mg/cm2 Na2SO4 and exposed in a tube furnace at temperatures between 900 and 1100°C, for times between 0.75 and 96 hours, in gas atmospheres containing either 2.5 or 1000 ppm SO2/O2. Additionally, CMC materials were tested in the tube furnace by the same procedure as model materials and in a Mach 0.3 burner rig with salt injection to determine their hot corrosion resistance. The solubility of SiO2 and B2O3 in molten Na2SO4 was explored at 900°C as a function of Na2O activity using an electrochemical cell.
Specimens were characterized after hot corrosion using mass change measurements, scanning electron microscopy, energy dispersive x-ray spectroscopy, profilometry, and x-ray diffraction analysis. The elemental composition of the corrosion products was analyzed using inductively coupled plasma optical emission spectrometry after removal from specimens by stepwise digestion in water, HCl, and HF.
The presence of boron was found to have a negative impact on the hot corrosion resistance of the model materials and CMC coupons. CMC coupons exposed in the tube furnace and the burner rig showed that a dense SiC matrix (from CVD and chemical vapor infiltration) is more resistant to hot corrosion while any chemical inhomogeneities or interfaces (from matrix processing by polymer infiltration and pyrolysis or melt infiltration) result in increased hot corrosion attack. The solubility of SiO2 and B2O3 in molten Na2SO4 was determined over a range of SO3 partial pressures. As the SO3 partial pressure decreases, the solubility of B2O3 and SiO2 is predicted to increase, indicating that low sulfur contents in fuels will lead to increased dissolution of silica or borosilicate scales grown on SiC.