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Inconel alloy 617 has been selected as the material of choice for the retort of the furnace for the Tritium Extraction Facility (TEF). A concern was raised regarding the efficacy of this material for use at the design temperature of 1100° C and in a potentially aggressive environment. A literature search and review was conducted to address this concern. Although the performance of this material in the exact environment in which the TEF will operate has not been investigated, results in similar environments with relatively low oxygen partial pressures and high temperatures have been reported in the literature. Based on the reported testing and publications, the retort is not expected to experience significant degradation during operation. Although, there are conditions under which the retort will operate that may adversely impact the strength and lifetime of the retort. For instance, under extremely low oxygen partial pressures (10^-14 atm, which are achievable in mixed hydrogen gasses) surface and whole cross-sectional decarburization may occur which will potentially weaken the retort and cause a reduction in the creep strength. In addition, at temperatures in excess of about 1000° C, it is possible that chromium will form a volatile oxide which can cause surface dealloying with either a reduction in the creep strength or an increase in the extent of internal carburization. The precise gas mixture, temperature, and partial pressures that cause the material degradation can only be obtained through materials testing. The proof of principle (POP) furnace should not experience substantial degradation with an Inconel 617 retort. To monitor retort performance and suitability for continued service, representative sample coupons should be suspended on the inside (process side) and outside (annulus side) of the retort for periodic metallographic analysis or hardness testing. Surface dealloying and hardness could be used to monitor the "health" of the retort in this surveillance program.

Production units should be fabricated with a surface coating that will promote aluminum oxide (alumina) formation. Alumina does not form a volatile oxide at 1100° C. Alternatively, the high temperature corrosion improvement could be realized by fabricating the retort from alumina forming alloys. The major drawbacks of alumina forming alloys are that they are generally more difficult to fabricate and weld than Inconel 617. Three commercially available materials are precipitation hardened Astroloy, solutioned Cabot 214, and mechanically alloyed Inconel MA 6000. These materials offer significant oxidation resistance improvements and mixed mechanical property improvement, relative to Inconel 617, at the expense of fabricability. Inconel 617 with an aluminum rich coating offers a reasonable compromise and will be more resistant to degradation than uncoated Inconel 617.

Inconel alloy 617 has been selected as the material of choice for the retort of the furnace for the Tritium Extraction Facility (TEF). A concern was raised regarding the efficacy of this material for use at the design temperature of 1100° C and in a potentially aggressive environment. A literature search and review was conducted to address this concern. Although the exact environment in which the TEF will operate has not been reported, similar environments with relatively low oxygen partial pressures and high temperatures have been tested. Based on the testing and publications, it is likely that the retort will not experience significant degradation during operation.

The Tritium Extraction Facility (TEF) at the Savannah River Site (SRS) will extract tritium from "Tritium Producing Burnable Absorption Rods" (TPBAR) that have been irradiated in a Commercial Light Water Reactor (CLWR). The composition and contents of the TPBAR are such that tritium is produced during irradiation. Extraction occurs by heating the TPBAR in the extraction furnace and driving the tritium out. The specific application poses a unique operating environment. The design temperature is 1100° C in an environment that contains hydrogen isotopes (protium, deuterium, tritium), helium isotopes (He³ and He⁴), nitrogen, water, carbon- monoxide and dioxide, low partial pressures of oxygen, and possibly other gaseous species. These gaseous species can influence the high temperature corrosion behavior of metallic alloys, depending on many variables such as the heating rate, gas composition, alloy composition, and surface treatment.

For example, it is possible to form (1) a stable and protective oxide, with few transient oxides, on the surface, (2) a stable surface oxide with internal oxides, (3) a non-protective surface oxide with internal oxides and an alloy depletion zone, (4) a carburized zone in addition to nitrides and carbonitrides, among many other complex gas-metal interactions. A difficulty in determining the exact species present during operation arises since the TEF furnace will not operate at a single temperature, rather it will be ramped from ambient to the extraction temperature (1100° C) likely with intermediate steps along the way to dehydrate the TPBAR.

Inconel alloy 617, chosen to be the material of construction for the hot-walled retort in the TEF extraction furnace, has been the subject of a significant amount of research and much of this is available in technical publications. Inconel alloy 617 was selected as a potential material for high temperature gas cooled reactors (HTR). There have been extensive studies of the environmental resistance of this alloy in helium environments with trace amounts of hydrogen, water, carbon dioxide, etc. Unfortunately, the studies do not replicate the expected thermal or chemical environment of the TEF. However, extrapolations can be made with respect to the oxygen potential, carbon potential, etc to make educated guesses regarding the effects of the environment on the retort material.

A schematic of the furnace/retort configuration is shown in Figure 1. This system will be loaded with a "basket" of TPBARs. It will operate with a low pressure of inert gas in the annulus and under high vacuum and a gaseous atmosphere in the retort. The retort is supported near the top by a flange and must be able to support its own weight without distortion. It also must remain leak tight. The interactions of the environment and stress suggest that the creep and oxidation properties will be important properties to examine for this application.

In addition to alloying for strength, high temperature alloys are designed to selectively oxidize either chromium or aluminum to form an adherent oxide layer. Trace elemental additions of reactive metals are often added to improve the oxide adherence and spall resistance. Inconel alloy 617 is a nickel-chromium-cobalt-molybdenum-aluminum alloy, Table 1, that is solid solution strengthened. The high chromium promotes the formation of chromium oxide as a protective oxide under adequately oxidizing conditions. Cr and Mo are carbide formers that act to improve the creep resistance. In addition, Mo and Co enhance the strength. Al acts synergistically to enhance the oxidation resistance and to improve the strength.

Table 1. Nominal composition of Inconel alloy 617 (29)

Thermodynamic considerations of the oxidation of Inconel alloy 617 indicate that aluminum will oxidize and the kinetics of the alloy suggest that this oxidation will occur internally. Titanium will also oxidize; the extent will depend significantly on the conditions. The primary surface protection for this alloy will be by the selective oxidation of chromium, which is stable, adherent, slow growing, and "replenishable". An Ellingham diagram, a plot of the free energy of formation as a function of temperature, is shown in Figure 1. The comments made above about the formation of an appropriate oxide film, not the rate at which it forms, can be validated at the design temperature. In addition, the relative partial pressures of oxygen required can also be estimated to ensure that the desired protective oxide film is formed. From evaluation of the thermodynamic data presented in Figure 1, it can be seen that at 1100° C, Al will oxidize in an environment that has an oxygen partial pressure of 10^-32 atm or greater, titanium will oxidize in an oxygen partial pressure of 10^-25 atm or higher, while chromium will oxidize in an oxygen partial pressure of 10^-19 atm or higher. In general, a vacuum environment will be oxidizing to these elements unless a reducing species such as hydrogen is present. If inadequate oxygen is present, a non-protective oxide film may be formed which could promote alloy depletion and loss of strength.

The TEF design basis average gas composition, assuming NQ₃ (ammonia) equal to CO₂, is listed in Table 2. This gas exhibits a sizable fraction of water and some carbon dioxide and carbon monoxide. Assuming these are equilibrium compositions, the partial pressure of oxygen can be estimated from either reaction shown below (32):

Based on these chemical reactions and the free energy calculation, the partial pressure of oxygen at 1100° C is 1.6x10^-14 atm based on the water reaction or 1.2x10^-12 atm for the carbon dioxide/monoxide reaction. The difference between the two suggests a non-equilibrium outlet gas composition. The actual gas composition at temperature will ultimately determine the corrosion conditions; similar conditions have been examined with the following results.

Table 2. TEF Furnace Outlet Gas Composition, mole percent (35).

P_CO/P_CO2 = 0.06/0.12 = 2 P_H2/P_H2O = 60.5/26.8 = 2.25: "Q" is a combination of H and T.

The oxidation behavior of Inconel alloy 617 is complex but has been investigated in several different environments. The alloy was being considered for use in a high temperature gas cooled reactor, Thus, the corrosion behavior of alloy 617 was tested in air, helium-, argon-, and hydrogen- based gas mixtures or vacuum atmospheres with low impurity additions (1,4,5,6,9,10,12,13,18,29). The reported testing covered a broad range of conditions in which alloy 617 had a variety of responses.

The oxidation of Inconel alloy 617 in air indicates that it is by a diffusion process with a time dependence proportional to the square root (5, 12). Internal oxidation along grain boundaries also occurs and follows a parabolic rate. The internal oxides are primarily aluminum. The cyclic oxidation behavior in air up to a temperature of 2150° F (1150° C) has been tested (29). A weight loss of approximately 40 mg/cm^2 was observed after 500 hour exposure for Inconel alloy 617 in cyclic oxidation in air, Figure 2. This corresponds to a thickness loss of 50 m m based on the oxidation to form only chromia and the loss of the metal based on a density of 8 g/cc.

Christ, et al., tested Inconel alloy 617 in air and a in low partial pressure of oxygen in an argon hydrogen water gas mixture at a temperature range of 850-1000° C with the results shown in Figure 3 (12). The low oxygen partial pressure tested was about 10^-19 atm, which was shown to be adequate for the oxidation of chromium from a thermodynamic standpoint. There was little difference in oxidation rate between the low partial pressure and air despite the 15 to 18 orders of magnitude difference in oxygen partial pressure.

There is a significant change in the oxidation/corrosion behavior when the oxygen partial pressure is further reduced. Graham (18) and Christ (9) have tested Inconel alloy 617 in mixed helium environments and defined regimes of oxidation, carburization, and decarburization based on the amount of impurity and impurity ratio and the temperature.

Graham (6, 18) tested alloy 617 in a variety of helium gas mixtures with minute additions of H₂, H₂O, CO, and CH₄. The results are striking since he was able to produce decarburization, carburization, and "protective" oxidation simply by modifying the partial pressures of the above species at pressures less than 75 millitorr (10 Pa); the total pressure was not given but expected to be about 760 torr (101,000 Pa or 1 atm). The conditions and outcome are listed in Table 3. A microclimate concept was introduced in this paper, and it suggests that there is a critical temperature for a given gas mixture that will promote formation of a protective oxide film.

mtorr is millitorr

Christ, et al. also tested alloy 617 in impure helium with similar test results (9). The conditions and outcome are listed in Table 4. These results also show that minor variations in gas content can cause differences in the high temperature corrosion behavior of nickel base alloys. A typical oxidation curve from the He-H₂-H₂O test gas is shown in Figure 4. The protective oxidation exhibits parabolic behavior while the decarburization/oxidation behavior is more complex.

*Complete decarburization was observed on a sample 2 mm thick after 150 hours.
**Decarburization and oxidation occur simultaneously.

Shikama, et al. conducted oxidation tests in a hydrogen based gas, 80% H₂ + 15% CO + 5% CO₂, between 650 and 1000° C (4). Their results are consistent with those reported for the helium atmospheres with little effect at 650° C, a mixed mode of carburization and oxidation at 900° C, and oxidation at 1000° C. Samples exposed to this gas at 1000° C had chromium depletion zone depths of about 0.004 inch after 200 hours.

Mazandarany and Lai tested a number of alloys including Inconel 617 at temperature between 649 and 871° C in impure He (1500 m atm H₂, 450 m atm CO, 50 m atm CH₄, and 50 m atm H₂O) [i.e.,1140 millitorr H₂, 340 millitorr CO, 38 millitorr CH₄, and 38 millitorr H₂O) ], and demonstrated only oxidation for Inconel 617 (10). A thin surface oxide was formed at 649° C while internal oxidation and surface oxidation was observed at 760 and 871° C. The internal oxides were primarily aluminum and titanium while the external oxide consisted of two layers with the outer layer being manganese, titanium and chromium and a slight enrichment of silicon and the inner layer being chromium and titanium. An alloy depletion zone, chromium loss, about 0.003 inch deep was noted after 5000 hour exposure at 871° C. The total depth of oxidation was modeled using the formula:

where d is the depth; t is time; n is the power law; and k is a constant. The data were fit with a least squares fit and the results shown in Table 5. The exponent n has a value of 0.5 for diffusion controlled oxidation, i.e., "parabolic" oxidation rate.

A sample of Inconel 617 that was initially carburized was subsequently decarburized in the following impure He gas mixture 500 m bar H₂, 20 m bar CH₄, 15m bar CO, 4 m bar N₂, and 1.5 m bar H₂O (13), (i.e., 375 millitorr H₂, 15 millitorr CH₄, 11 millitorr CO, 30 millitorr N₂, and 1.1 millitorr H₂O. This material response can be attributed to the time-dependent depletion of chromium and titanium (two strong carbide formers) which can increase the carbon activity in the alloy with respect to the environment.

The creep properties of Inconel 617 have been investigated under a number of conditions that are germane to TEF. These results also show the complexities of environmental conditions on not only corrosion resistance but also mechanical properties.

Hosoi and Abe tested Inconel 617 in a variety of grades of helium and vacuum (1). The oxygen content ranged from 0.05 ppm to air (21% O₂). The creep rupture life as a function of oxygen content in helium is shown in Figure 5. The creep life decreases with increasing oxygen to a minimum life that occurs near 500 ppm of O₂, and then increases to the same level as the low oxygen pressure. The carbon content follows the rupture life curve, which suggests that the creep resistance is primarily due to the presence of the carbides. Complete decarburization was observed in a 2mm thick sheet test specimen after about 100 hours.

Hosoi and Abe also tested in variable oxygen contents using a vacuum system (1). The rupture life and carbon content decrease with decreased pressure and exhibit a minimum at pressure of 10^-2 mm Hg (=10^-2 torr and 2.75 m atm [2.1 millitorr]O₂). With further reductions in pressure, the rupture life and carbon content are recovered as shown in Figure 6. The microstructures correspond well with the chemical and mechanical property data.

Ohnami and Imamura evaluated the effects of vacuum on the creep rupture properties of Inconel 617 and observed a reduction in the rupture time for samples tested in a vacuum of 0.3 torr compared to air; the reduction was between 1/6 to 1/3 (14). The creep life was dominated by crack initiation, which occupied 70 to 80% of the creep rupture life. The rupture life was also dependent on the stress concentration factor. The crack initiation time was decreased by 1/3 for stress concentration factors increasing from 2.0 to 6.4. It was surmised that a non-protective oxide film was formed in the vacuum environment that was tested.

Ennis, et al. tested the creep resistance of Inconel 617 under significantly different conditions and found that the creep strength of oxidized Inconel 617 which had a surface carbide free zone was greater than Inconel 617 which was aged at the same conditions but did not have a carbide free zone (16). The samples were made by machining then exposing and by exposing then machining, respectively. They attributed the increase in life to the surface dissolution of carbides due to selective oxidation of the chromium carbides and the release of the carbon. This carbon migrates towards the center of the sample and increases the volume of carbide. The loss of surface carbide is offset by the increase in the center carbide precipitation.

The post-creep test microstructure was evaluated by Mankins, et al. (28). The evaluated samples had been tested as indicated in Table 6. The samples examined had not reached tertiary creep conditions, accelerated creep rate conditions under which voids form. Characteristic microstructures can be seen in Figure 7. The influence of time, temperature and stress can be seen from these micrographs. Samples exhibit coarsening of the grain boundary and intergranular carbides after the 649° C exposure (Fig. 7b). A significant increase in intergranular carbide precipitation is observed after 716, 816, and 871° C (Figs. 7c, 7d, and 7e) exposure. A number of carbides apparently precipitated on twin boundaries after the 816° C exposure (Fig. 7d). Intergranular carbides increase in size and the number of carbides decrease during the 982 to 1093° C exposures (Figs. 7f and 7g)

Aging of the samples at temperatures to 871° C and then tensile testing at room temperature results in an increase in the room temperature yield and tensile strengths. The largest increase in strength was for samples that were aged for 1000 hours at 704° C. This aging condition promoted the formation of g ', an intermetallic compound having a superlattice structure with a nominal formula of Ni₃Al. It is a commonly used strengthening phase in nickel based alloys.

Table 6. Creep test Conditions for samples that were microscopically examined.

The tensile properties of Inconel 617 were determined after long time exposure in air and vacuum at 1093K (7). There was little effect on the yield strength for testing at cryogenic to high temperatures as shown in Figure 8. The vacuum exposed samples were not exposed to a system that readily removed any volatile oxides and weight loss measurements revealed a loss of only 0.3 mg/cm² after 7914 hours exposure while air exposure resulted in a weight gain of 1 mg/cm² after 10,000 hours and 1.8 mg/cm² after 22,500 hours (5). The average internal oxidation depth was 0.0016 inch.

Hydrogen permeation can be reduced by the presence of an adherent surface oxide. Many researchers have investigated the permeation reduction in nickel based and other alloys. A combination of diffusivity and solubility give the general equation for permeation. The governing equation is

where V*_M is the permeation rate; K_T is the permeation coeffecient; A is the permeation area; x_eff is the effective wall thickness; and p₁ and p₂ are hydrogen pressures on the upstream and downstream sides, respectively. Further,

where K_o is the preexponential factor; Q is the activation enegry; R is the gas constant; and T is the absolute temperature (33).

Bare Inconel 617 was tested and the activation energy and preexponential factors were determined to be 64.9 (kJ/mol) and 2.5 x 10^-2 (cm³ (STP) · s^-1 · cm^{-1 ·} Ö bar^-1), respectively. The permeation reduction factor V_M/V_ox depended on the test temperature but varied from 45 to 150 with an oxide present (33).

Le Claire conducted a critical examination of the literature and determined that the measured permeabilities all lie within a factor of 1.5 to either side of:

For which the activation energy is within about 3% of the previous data while the preexponential factor varies by about a factor of 2.3. The permeation equation is valid for a temperature range of 500 to 1000° C. The permeabilities of high nickel alloys are on average about 2-4 times larger than austenitic stainless steels.

The selection of Inconel 617 for the retort in the tritium extraction facility furnace poses some concern from a durability standpoint. As can be seen from the information presented in this document, the material response to the conditions that are encountered during extraction cannot be easily predicted. The exact conditions, gaseous species, temperature, pressure, time, etc. will determine how the retort will respond. The oxygen and carbon potentials will strongly influence whether a protective chromium oxide film is formed. If there is adequate water and carbon dioxide to form stable chromium oxide, Cr₂O₃, without excessive chromium depletion, then the situation will be one of protective/stable oxide growth. Under some conditions, chromium oxide becomes less stable at temperatures in excess of about 1000° C. The chromium oxide may form a volatile, non-protective oxide of CrO₃. The rate of formation and the significance of this reation for the TEF is uncertain. Previous experience with other chromia forming alloys (IN738 Ni-16Cr, 3.45 Al, 3.45 Ti, 8.5Co, 1.75 Mo, 2.6W, 1.75Ta) in a high velocity oxidation/erosion environment at 1038° C reveals metal loss corresponding to 2 mils surface recession after about 250 hours. The conditions, such as temperature, gas pressure, gas velocity, for the TEF furnace are quite different and this concern is significantly less, although an adverse reaction is still possible.

There were no articles that specifically discussed the effects of hydrogen on the mechanical properties of Inconel 617. Based on engineering judgement and that Inconel 617 is a face centered cubic austenitic type alloy hydrogen embrittlement would not have an appreciable adverse impact on this alloy. Data for Inconel 625, which is a nickel based alloy with a similar chemistry, exhibits a reduction in the tensile ductility in excess of 50% at room temperature and less than 10% reduction at 1250° F (36). These data are for samples that were tested in 5000 psi hydrogen, which is a much higher hydrogen pressure than what is expected in the retort. Despite the reduction in elongation, the samples still exhibited 23% and 53% elongation at room and 1250° F.

For the POP furnace, the selection of Inconel 617 was an acceptable first choice but clearly the conditions challenge this or any other chromia forming alloy to the limits of capability. These conditions may limit service life. For operating conditions in excess of 1000° C, an alloy or coating which forms alumina is preferred for extended life. In general most alloys that form alumina (4-6% Al, 15% Cr, 3-9% Ta, <2% Ti) are cast alloys since the ductility (and ability to be formed or wrought) is advesersly affected by the high aluminum content Two weldable wrought alloys that are likely to have adequate high temperature gas corrosion resistance are Astroloy and Haynes 214 alloy. Inconel alloy MA 6000 is a mechanically alloyed material that has good creep and rupture resistance but is not easily weldable. The best way to protect the inner and outer surfaces of the retort would be to use a coating that promotes the formation of alumina. The implementation of a coating presents some difficulties due to the retort size geometry. It will be difficult to ensure that all surfaces are coated since the retort is essentially a ten foot tall blind canister. Also because of its length (height), the thermal expansion mismatch could promote coating cracks for systems with a _{CTE coating} < a _{CTE substrate} and coating rumples for the reverse.

1. Monitor the condition of the retort using coupons in the annulus and process sections. These could be evaluated using metallography and hardness testing to determine the level of degradation that has occurred over the lifetime.
2. Consider coating the internal and annulus surfaces of the "production retorts" with an MCrAlY coating that has in excess of 6% Al and a coefficient of thermal expansion that closely matches the substrate Inconel 617.
3. Consider changing the alloy for the "production retorts" to Cabot alloy 214 or Astroloy provided the fabricability, weldability, and mechanical properties will meet or exceed the system requirements.
4. Conduct a hydrogen compatibility test program of tensile testing hydrogen charged notched and smooth tensile samples in low pressure hydrogen.

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