The Impact of Impurities on Alloy Behavior in Supercritical CO2 at 700 °C

Abstract

As part of round robin testing, specimens of commercial alloys 316, 120, 625 and 740 were exposed to 20 MPa research grade CO2 at 700°C for up to 1500 h. The first set of specimens had higher mass gain likely due to impurities not flushed from the autoclave at startup. After this issue was corrected, an identical set of specimens exhibited lower mass gains for both the Fe-and Ni-based alloys. The differences in reaction products were characterized to understand the effect of impurities under these conditions. As suggested by the mass change, thicker oxides were formed in each case, primarily for the Fe-based alloys. For the Ni-based alloys, the difference was primarily in the transient stage of oxidation with no change in rate constant. Alloy 120 exhibited increased internal attack and differences in the scale phases formed were characterized to better understand the effect of impurities. Alloy 740 primarily exhibited a greater depth of internal attack in the first run.

A Tracer Study on sCO2 Corrosion with Multiple Oxygen-Bearing Impurities

“…The mass gains and oxide thicknesses correlate quite well with one another and the calculated rate constants are in the same order of magnitude as those of commercial alloys studied under similar conditions [32,38,70]. Interestingly, the oxidation rate constant for 282 alloy (k p = 2.9 × 10 -8 mg 2 cm −4 s −1 ) in sCO 2 was close to that of a Ni-based alloy with similar Cr, Ti, and Al concentrations (k p = 1.7 × 10 -8 mg 2 cm −4 s −1 ), when studied in air at 750 °C for 2000 h [63].…”

“…Only the external oxide has been taken into account and any internal degradation has been excluded from the thickness measurements. Since similar alloys have been reported to follow parabolic oxidation kinetics [32,33], Eq. 3 has been used to calculate parabolic rate constants presented in Table 2.…”

Review on the Corrosion Behaviour of Nickel-Based Alloys in Supercritical Carbon Dioxide under High Temperature and Pressure

“…The explanation is that with the addition of O2, the rate of Cr diffusion will be accelerated and lead to formation of the oxide layer with more defects. The results of another study [79] indicated that the existence of O2 led to a thicker oxide layer and higher mass gain.…”

Development of an Open-source Alloy Selection and Lifetime Assessment Tool for Structural Components in CSP

“…Essential corrosion data for model development and validation should include measured concentration profiles after exposures at the temperatures of interest (700-800°C) for different times. For exposures in sCO 2 , depletion of the alloying element forming the protective oxide can be correlated to the temporal evolution of specific mass change measurements for the three alloys (740H, 282 and 625), since these alloys have been reported to form compact and protective Cr-rich Cr 2 O 3 scales up to 800°C [2,4,[18][19][20][21][22][23][24][25]. However, for exposures in molten chloride salts, a combination of the extremely low fraction of species reacting in the boundary regions (a few microns) between phases or molecular regions and nonequilibrium reaction kinetics provide challenges in identifying corrosion products.…”