Mechanism of destruction of the protective oxide layer on Alloy 230 in the impure helium atmosphere of Very High Temperature Reactors.

Abstract : Alloy 230 which contains 22wt.% chromium could be a promising candidate material for structures and heat exchangers (maximum operating temperature: 850°-950°C) in Very High Temperature Reactors (VHTR). The feasibility demonstration involves to valid its corrosion resistance in the reactor specific environment namely impure helium. The alloys surface reactivity was investigated at temperatures between 850 and 1000°C. Two main behaviours have been revealed: the formation of a protective Cr/Mn rich oxide layer at 900°C and its following destruction at higher temperatures. Actually, above a critical temperature called TA, oxide is reduced at the oxide/metal interface by carbon in solution in the alloy. To ascribe the scale destruction, a model is proposed based on thermodynamic interfacial data for the alloy (chromium and carbon activity), oxide layer morphology and carbon monoxide partial pressure in helium. The proposed mechanism is then validated regarding experimental results and observations on alloy 230 and model alloys.