Long-term corrosion behavior of CVD SiC in 360 °C water and 400 °C steam

Abstract

Long-term corrosion tests were performed using chemical vapor deposition (CVD) SiC specimens in 360°C water and 400°C steam to evaluate the corrosion behavior of SiC cladding under normal operation conditions. The CVD SiC specimens exhibited weight loss caused by dissolution in both 360°C water and 400°C steam. For the corrosion test in 360°C water the dissolution rate was much higher in the static autoclave, where the dissolved oxygen content was higher, than in the loops, where the dissolved oxygen content was maintained at approximately 5ppb. The grain boundaries of the SiC dissolved preferentially during the early stage of corrosion, and the grains became thinner and detached from the surface, thereby leading to an acceleration of the weight loss. The pre-oxide was not able to protect the SiC from dissolving in water, even though it was effective, to a certain extent, at mitigating the dissolution of the SiC. In the corrosion test in 400°C steam, the weight loss was significant from the early stage of corrosion. No acceleration in the corrosion rate was observed in 400°C steam for up to 135days, and the weight loss at 90days was similar to that obtained from the 360°C water autoclave test.