Hydrothermal corrosion behavior of CVD SiC in high temperature water

Abstract The hydrothermal corrosion of polished and as-cut high purity chemical vapor deposited (CVD) SiC was studied in a constantly refreshing water loop. Light water reactor (LWR) conditions were simulated at 288, 320, and 350 °C with dissolved gas concentrations between 0.15 and 3 ppm H2 or between 1 and 4 ppm O2. In hydrogenated water, the rate of material loss was low, calculated to be ∼1.3 μm of recession after 5 years of service in 320 °C water. Moreover, there was no observed localized attack at any temperature. In oxygenated conditions, the corrosion rate was higher, with a calculated material loss >10 μm after 5 years of service in 1 ppm O2, 320 °C water. Mass loss significantly increased when grain fallout became significant (as early as 200h with 4 ppm O2 at 350 °C or after 1000–2000h with 2 ppm O2 at 288 °C). Grain fallout more than doubled the corrosion rate and a steady state corrosion rate in the grain fallout regime was not observed but expected to eventually occur once large grains begin to be removed. Polished specimens had lower mass loss than unpolished coupons. A kinetic analysis of the data in this work suggests that the corrosion rates are controlled by a single activation step in both oxygenated and deoxygenated conditions, with the reaction order with respect to oxygen being 1. A resulting reaction rate equation to predict corrosion of SiC (in mg/cm2s) in high purity water from 288 to 350 °C and up to 4 ppm O2 was constructed: Rate = 0.1458 1 + SA  T ( 1.09 ( 1 − 10 − 3 T ) [ O 2 ] e − 1.275 x 10 4 T + 7.91 x 10 − 6 e − 7.39 x 10 3 T ) .