Evaluation of the corrosion kinetics of SiC with and without mitigation coatings in LWR chemistries^*

Since the Fukushima-Daiichi accident in 2011, accident-tolerant fuel (ATF) cladding concepts such as FeCrAl alloys and ceramic matrix composites (CMCs) of SiC have been extensively researched. SiC is desirable for excellent radiation tolerance, mechanical properties, and resistance to oxidation under accident scenarios. However, under standard operating conditions, it may corrode too quickly for use in LWRs. To mitigate these issues, coatings of Cr, CrN, Cr/CrN, and TiN were applied via physical vapor deposition (PVD) on SiC prepared by chemical vapor deposition (CVD). These were exposed, along with uncoated CVD SiC, to temperatures between 288 and 350°C, with 0.15-3 wppm dissolved H2 or 1-4 wppm O2. TiN and CrN coatings did not observe a permanently-protective oxide, potentially due to coating impurities. TiN was found to never be viable, slowly degrading in hydrogenated water, and spalling after ~0.5 µm of oxide growth. In HWC and PWR conditions, CrN did not perceptibly corrode but otherwise degraded, while the multilayer Cr/CrN immediately failed. Cr was a successful mitigation coating in the absence of oxygen. The corrosion kinetics of the SiC substrate was dependent on the surface preparation, with polished SiC leading to predictable linear kinetics, with one rate-limiting step for reactions with oxygen and water.