A lifetime model is being developed for supercritical CO 2 (sCO 2 ) compatibility for the 30 year duty life for concentrated solar power (CSP) applications at >700°C to achieve higher efficiencies than other power cycles. Alloys 740H, 282, 625 and Fe-base alloy 25 are being evaluated in 500-h cycles at 1 bar and 300 bar, and 10-h cycles in 1 bar industrial grade CO 2 at 700°–800°C. For comparison, companion experiments are being conducted in 1 bar air or O 2 . Using mass change, all of the alloys showed low mass gains with parabolic rate constants below the performance metric after 1000 h. However, alloy 25 showed a higher rate at 700°C in 300 bar sCO 2 and did not follow an Arrhenius relationship. After 1500 h in 1 bar CO 2 , a much faster rate was observed for alloy 25 due to the formation of Fe 2 O 3 , but a similar increase was not observed in 300 bar CO 2 . Oxide thickness measurements have been completed after 1000 h in each condition. Only minor differences were noted between the 1 and 300 bar exposures. Up to 4,000 h exposures in 10-h cycles has not resulted in evidence of scale spallation but very small mass losses for alloy 625 were consistently observed. As longer exposures times are completed, quantification of the reaction products as a function time will be used to better model the degradation rate and additional characterization techniques will be included to further develop the model.

A high-temperature, high-pressure, tube furnace has been used to evaluate the long term stability of different monolithic ceramic and ceramic matrix composite materials in a simulated combustor environment. All of the tests have been run at 150 psia, 1204°C, and 15% steam in incremental 500 h runs. The major advantage of this system is the high sample throughput; >20 samples can be exposed in each tube at the same time under similar exposure conditions. Microstructural evaluations of the samples were conducted after each 500 h exposure to characterize the extent of surface damage, to calculate surface recession rates, and to determine degradation mechanisms for the different materials. The validity of this exposure rig for simulating real combustor environments was established by comparing materials exposed in the test rig and combustor liner materials exposed for similar times in an actual gas turbine combustor under commercial operating conditions.