Existence of a critical temperature for corrosion of alloy 617 in He-CO-CO 2 environment
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1043-T01-03
Existence of a critical temperature for corrosion of alloy 617 in He-CO-CO2 environment
© 2008
Deepak Kumar and Gary S. Was Materials University ofResearch Michigan
Society
MRS Fall Meeting Boston, MA November 26 - 30, 2007 Support provided by US Dept. of Energy, NERI program
Introduction ¾ Helium gas is used as coolant in the Very High Temperature Gas-Cooled Reactors (VHTR) – Main impurities are CO, CO2, H2, H2O and CH4 (in ppm levels) • low PO2 • low a C
© 2008 Materials Research Society – Target outlet temperature range: 850 - 1000oC
¾ Depending on the impurity concentrations, temperature and alloy composition, oxidation/ carburization/ decarburization of a metallic surface can occur
Background Cr23C6 C arbon potential
Log(a C )
¾ Corrosion behavior in impure helium is determined by: gas (a – carbon potential C ) , gas – oxygen potential( PO2 ) in helium, – alloy composition
Cr2O3
Rapid carburization
oxidation + slow carburization
a∗C
∗
PCO
Cr
© 2008 Materials Rapid Research Society decarburization ∗
PO2
∗ ¾ PO is given by: 2 Cr2O3 ↔ 4Cr + 3 O2 2 a∗C is given by: Cr23C6 ↔ 23Cr + 6 C ∗ PCO is given by: 2 Cr2O3 + Cr23C6 ↔ 27Cr + 6 CO
oxygen potential
Log(PO ) 2
Schematic of modified Ellingham diagram for alloy 617, assuming Cr is the major element that forms carbides and oxide in the alloy. [Quadakkers et al,Werkstoffe and Korrosion 36,141(1985)]
Previous works on alloy 617 ¾ T ≤ 900oC, oxidation/ carburization T ≥ 950oC, rapid decarburization ¾Quadakkers et al. [1] : He +500ppm H2+1.5ppm H2O+15ppm CO+20 ppm CH4 - carburization at 900oC and rapid decarburization at 950oC, total exposure duration of 2000 hours.
© 2008 Materials Research Society
¾Christ et al. [2] : He +10 ppm H2O+60ppm CO+ 6 ppm CO2 - oxidation at 850 - 900oC and rapid decarburization at 1000oC, total exposure duration of 150 hours. ¾Brenner et al. [3] : He +500ppm H2+7.5ppm H2O+15ppm CO+20 ppm CH4 - rapid decarburization at 1000oC, total exposure duration of 1400 hours.
Previous works on alloy 617 contd.. ¾Decarburization at T ≥ 950oC occurs by following reaction: 2Cr2O3(solid) + Cr23C6(solid) → 27Cr + 6CO↑ ¾Decarburization mechanism by Brenner et al.: microclimate reaction, catalyzed by H2 and H2O, i.e. 2Cr2O3 + 6 H2 → 6H2O + 4Cr 6 H2O + Cr23C6 →23 Cr + 6 CO + 6H2
© 2008 Materials Research Society gas Cr2O3
Cr23C6
¾Christ et al. showed that decarburization occurs in an He-H2O-CO-CO2 environment.
Objective ¾ Determine whether a critical temperature exists that separates carburization from decarburization of alloy 617 in He-COCO2 without H2 and water vapor.
¾ In this simplified chemistry, oxidation potential ( PO2 ) is by: ©established 2008 Materials Research Society 2 ⎞ ⎛ CO 2 CO2 → 2 CO + O2 ; PO ∝ ⎜⎜ 2 ⎟⎟ 2 ⎜⎜ CO ⎟⎟ ⎠ ⎝ and carburization potential (ac) by: 2 ( CO ) aC ∝ 2 CO → CO2 + C ; CO 2
Material ¾ Composition of alloy 617: Ni wt%
Cr
Co
Mo Al
C
other
54.6 21.8 11.5 9.7 1.1 0.09 1.2
¾ Average grain size = 65μm ¾ M23C6 carbide present both along the g.b. and interiors of grains. ¾ Volum
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