Mechanisms of high-temperature corrosion in helium containing small amounts of impurities. I. Theoretical and experimental characterization of the gas phase

Christ, H.‐J.; Schwanke, D; Uihlein, Th.; Sockel, H. G.

doi:10.1007/bf00656642

Cited by 17 publications

(23 citation statements)

References 22 publications

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“…8,9,[11][12][13][14][28][29][30][31][32][33][34][35][36][37][38] Depending on the specific proposed application, different model chemistries have been developed, and the testing has focused on these. Several of the model impurity chemistries are shown in Table 6.…”

Section: Environmental Interactions Between Vhtr Helium and High Tempmentioning

confidence: 99%

“…Several of the model impurity chemistries are shown in Table 6. [28][29][30][31][32][33][34][35][36][37][38] Comparing the values in Table 6 with actual operating experience suggest that the model chemistries tend to have higher impurity levels of some species than those found in operating reactors. This is notable for H 2 in particular.…”

Section: Environmental Interactions Between Vhtr Helium and High Tempmentioning

confidence: 99%

“…Evaluation of this alloy for VHTRs began in the early 1980's with the most comprehensive work done by Quadakkers, Christ and Graham. [31][32][33][34][35][36][37][38] The newer alloy Haynes 230 is under consideration as an alternative to Inconel 617 because it has equivalent creep properties and may suffer from less internal oxidation.…”

Section: Environmental Interactions Between Vhtr Helium and High Tempmentioning

confidence: 99%

“…Quadakkers utilized a modified Ellingham diagram, Figure 2, to display results of the nickel-chromium alloy's stability calculations. [31][32][33][34][35][36]39 Five conditions are represented within the diagram: I-strongly reducing, II-highly oxidizing, III-stable external oxide with stable internal carbides, IV-strongly carburizing internally and externally, and, lastly, IVastrong external carburization with stable oxide layer. Zone III was determined to be the area of highest stability, an environment oxidizing yet slightly carburizing.…”

Section: Environmental Interactions Between Vhtr Helium and High Tempmentioning

confidence: 99%

“…43 Pinning down an exact value can be difficult, especially because of a high temperature phenomenon referred to as a 'microclimate,' or critical temperature event associated with irreversible destruction of chromium oxides and carbides. 33,34 A local chromium activity of 0.75 is associated with the following reaction: [33][34] 2 Cr 2 O 3 + Cr 23 C 6 = 6 CO + 27 Cr.…”

Section: Inl/ext-06-11494 Revision 0 June 2006mentioning

confidence: 99%

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Kinetics of Gas Reactions and Environmental Degradation in NGNP Helium

Wright¹

2006

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A number of very high temperature helium-cooled reactors have been built and operated for extended periods. The helium coolant in the primary circuit has been found to contain low levels of impurities after steady-state operation that can lead to an environmental degradation of the high temperature alloys used for internals and heat exchangers. Depending on the impurity concentration and the temperature, high temperature alloys can undergo oxidation, carburization, or decarburization. The concentration of H 2 O and CC are of particular interest because they essentially control the oxygen partial pressure and carbon activity, respectively. The optimum coolant chemistry for long-term stability of high temperature alloys is slightly oxidizing and results in formation of a tenacious and protective Cr 2 O 3 scale.The most critical metallic component of the Next Generation Nuclear Plant (NGNP) is the heat exchanger. Inconel 617 is the primary candidate alloy for this application because of its superior creep resistance. The mechanisms of environmental interaction between this alloy and prototype Very High Temperature Reactor (VHTR) helium chemistries have been extensively studied. A modified type of Ellingham diagram that maps the ranges of carbon activity and oxygen partial pressure that result in each of the degradation mechanisms has been developed. In addition, at temperatures above about 950°C, additional degradation mechanisms arise that effectively determine the maximum temperature for sustained service. While the mechanisms of degradation are relatively well-known, the kinetics of the reactions for reactor conditions of high helium pressure and velocity are not well characterized.

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Section: Environmental Interactions Between Vhtr Helium and High Tempmentioning

confidence: 99%

Section: Environmental Interactions Between Vhtr Helium and High Tempmentioning

confidence: 99%

Section: Environmental Interactions Between Vhtr Helium and High Tempmentioning

confidence: 99%

Section: Environmental Interactions Between Vhtr Helium and High Tempmentioning

confidence: 99%

Section: Inl/ext-06-11494 Revision 0 June 2006mentioning

confidence: 99%

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Kinetics of Gas Reactions and Environmental Degradation in NGNP Helium

Wright¹

2006

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Chromia-Assisted Decarburization of W-Rich Ni-Based Alloys in Impure Helium at 1273 K (1000 °C)

Adharapurapu

Kumar

Zhu

et al. 2010

Metall Mater Trans A

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The microstructure and surface stability of two experimental W-rich Ni-based alloys have been studied at 1273 K (1000°C) in an impure-He environment containing only CO and CO 2 as impurities. The alloy Ni-2.3Al-12Cr-12W contained 0.08 wt pct carbon in solution, whereas the second alloy Ni-2.3Al-3Mo-12Cr-12Co-12W contained M 6 C carbides at the same carbon level. Both alloys, which were preoxidized with~2.3 lm Cr 2 O 3 layer, were decarburized completely within 50 hours of exposure to the helium gas mixture at 1273 K (1000°C) via the following chromia-assisted decarburization reaction: Cr 2 O 3 (s) + 3C alloy (s) fi 2Cr (s) + 3CO (g). Microstructural observations, bulk carbon analysis, and microprobe measurements confirmed that the carbon in solid solution reacted with the surface chromium oxide resulting in the simultaneous loss of chromia and carbon. The Cr produced by the decomposition of the Cr 2 O 3 diffused back into the alloy, whereas CO gas was released and detected by a gas chromatograph. Once the alloy carbon content was reduced to negligible levels, subsequent exposure led to the uninterrupted growth of Cr 2 O 3 layer in both alloys. In the preoxidized alloys, chromia-assisted decarburization rates were slower for an alloy containing carbides compared with the alloy with carbon in solid solution only. The formation of Cr 2 O 3 is shown to be the rate-limiting step in the chromia-assisted decarburization reaction. Exposure of as-fabricated alloys to the impure-He environment led to the formation of a thin layer of Al 2 O 3 (<1 lm) between the substrate and surface Cr 2 O 3 oxide that inhibited this decarburization process by acting as a diffusion barrier.

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High-Temperature Oxidation of Alloy 617 in Helium Containing Part-Per-Million Levels of CO and CO2 as Impurities

Kumar

Adharapurapu

Pollock

et al. 2011

Metall Mater Trans A

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The objective of this study was to determine the mechanisms of carburization and decarburization of alloy 617 in impure helium. To avoid the coupling of multiple gas/metal reactions that occurs in impure helium, oxidation studies were conducted in binary He + CO + CO 2 gas mixtures with CO/CO 2 ratios of 9 and 1272 in the temperature range 1123 K to 1273 K (850°C to 1000°C). The mechanisms were corroborated through measurements of oxidation kinetics, gas-phase analysis, and surface/bulk microstructure examination. A critical temperature corresponding to the equilibrium of the reaction 27Cr + 6CO M 2Cr 2 O 3 + Cr 23 C 6 was identified to lie between 1173 K and 1223 K (900°C and 950°C) at CO/CO 2 ratio 9, above which decarburization of the alloy occurred via a kinetic competition between two simultaneous surface reactions: chromia formation and chromia reduction. The reduction rate exceeded the formation rate, preventing the growth of a stable chromia film until carbon in the sample was depleted. Surface and bulk carburization of the samples occurred for a CO/CO 2 ratio of 1272 at all temperatures. The surface carbide, Cr 7 C 3 , was metastable and nucleated due to preferential adsorption of carbon on the chromia surface. The Cr 7 C 3 precipitates grew at the gas/scale interface via outward diffusion of Cr cations through the chromia scale until the activity of Cr at the reaction site fell below a critical value. The decrease in activity of chromium triggered a reaction between chromia and carbide: Cr 2 O 3 + Cr 7 C 3 fi 9Cr+3CO, which resulted in a porous surface scale. The results show that the industrial application of the alloy 617 at T > 1173 K (900°C) in impure helium will be limited by oxidation.

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Mechanisms of high-temperature corrosion in helium containing small amounts of impurities. I. Theoretical and experimental characterization of the gas phase

Cited by 17 publications

References 22 publications

Kinetics of Gas Reactions and Environmental Degradation in NGNP Helium

Kinetics of Gas Reactions and Environmental Degradation in NGNP Helium

Chromia-Assisted Decarburization of W-Rich Ni-Based Alloys in Impure Helium at 1273 K (1000 °C)

High-Temperature Oxidation of Alloy 617 in Helium Containing Part-Per-Million Levels of CO and CO2 as Impurities

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