High Temperature Behavior of Candidate Alloys in Helium Environments

C.J. Tsai, National Tsing Hua University, Taiwan T. K. Yeh, National Tsing Hua University, Taiwan

M.Y. Wang, National Tsing Hua University, Taiwan

Summary

The high-temperature gas-cooled reactor (HTGR) is deemed as the best candidate for next generation nuclear reactors. Helium is used as the coolant in the core of an HTGR that operates at temperatures of greater than 950 °C. Several iron- and nickel-based superalloys, including Alloy 800H, Hastelloy X, Alloy 617, and Alloy 625, with high corrosion resistance and superior mechanical strength are potential structural materials for the intermediate heat exchanger component in an HTGR. Impurities in the helium coolant, such as oxygen and water vapor, may lead to serious degradation in these alloys and increase their oxidation rates at high temperatures. In this work, the oxidation behavior of the selected alloys was investigated in high-temperature helium environments with various concentrations of O2 and H2O. Preliminary results showed that relatively thick and continuous chromium oxide layers were observed on the surfaces of the tested alloys. Based on the results of mass gain and SEM analyses, Hastelloy X alloy and Alloy 625 exhibited the best corrosion resistance after all designated corrosion tests.

1 Introduction A high-temperature gas-cooled reactor (HTGR) is categorized as a Generation IV nuclear reactor developed and designed in the 1970s-1980s and primarily dedicated to the cogeneration of electricity and hydrogen. Use of helium as coolant and ceramics as core structure material allows an operation temperature at core outlet as high as 1000 oC that makes hydrogen production easily achievable [1]. Accordingly, the material of the intermediate heat exchanger (IHX) in an HTGR may be exposed to a high temperature of up to 950 oC and should thus have high-performance and superior strength over a long lifetime. The helium coolant in an HTGR inevitably contains low levels of impurities during reactor operation, primarily consisting of small amounts of H2, H2O, CH4, CO, CO2, and N2. Impurities in the helium coolant may decrease the corrosion resistance of structural alloys and lead to increases in the corrosion rate of these alloys at high temperatures [2-7]. Furthermore, during an air ingress event, more severe oxidation of the IHX alloys could result in integrity breach of the IHX structure. Although the formation of stable oxides is preferred for preventing continuous degradation of alloy materials, unexpected spallation of thick oxides may harm the long-term integrity and hence the mechanical strength of the materials during prolonged operation of an HTGR. Several nickel- and iron-based alloys such as Alloy 617 and Alloy 800H are considered as promising candidates for IHX materials in an HTGR owing to their excellent creep resistance.

In this study, three nickel-based alloys (Alloy 617, Alloy 625, and Hastelloy X) and one iron-based alloy (Alloy 800H) were selected and underwent corrosion tests. Samples prepared from these alloys were exposed to helium gas environments with various amounts of impurities (i.e. oxygen and water vapor) at 950 oC for 48 to 144 hours. Tests results on the mass changes, surface morphologies, and composition of oxides of the samples are reported.

2 Experimental

2.1 Sample Preparation

Table 1 shows the chemical composition of the tested alloys of 800H, X, 617, and 625. These alloys were machined into samples in square disk shape with a dimen-sion of 20 mm×20 mm×1 mm. No thermal treatment was carried out before the corro-sion test. Both sides of the samples were mechanically polished with abrasive paper starting from 80 grit up to 1200 grit. Each sample was then ultrasonically cleaned in acetone (CH3COCH3), dried, and rinsed in distilled water for 10 minutes.

Table 1 Chemical Composition (wt%) of the Alloys Used in This Study

Alloy Ni Fe Cr Al Mo Mn Ti C Co W Cu Si

Alloy 800H 31.7 Bal 21.4 .4 - 1.5 .4 .05 - - .75 1.0

Hastelloy X Bal 22 18 - 9 1 - .1 1.5 .6 - 1

Alloy 617 Bal .9 21.6 1.2 9.5 .1 .3 .06 12.5 - .5 .015

Alloy 625 Bal 3.8 22.5 0.14 8.9 0.12 0.23 0.03 12.5 - - 0.21

2.2 Corrosion Test Conditions

A conceptual schematic of the experimental system is shown in Figure 1. In each corrosion test, eight samples, two from each alloy, were simultaneously placed on a ceramic holder inside a tube furnace and exposed to designated flowing gas mixtures at 950 oC. Four gaseous conditions were specifically engineered to simulate normal and various air-ingress environments in an HTGR, including pure dry helium, 90% helium with 10% oxygen, pure helium with 10% humidity, and pure helium with 50% humidity. Corrosion test durations varied from 48 to 144 hours. Gas input was controlled using a mass flow controller at a flow rate of 250 ml/min. The samples were heated up at a rate of 15 oC/min and cooled down at a rate of 7.5 oC/min in pure argon at a constant flow rate of 250 ml/min.

2.3 Sample Observation and Analyses

After the corrosion tests, the mass gain of each sample was measured using a high-precision balance. The oxide morphology and cross-sectional microstructure of a sample was directly observed using a scanning electron microscope (SEM), and the elemental composition of the oxide was analyzed using energy-dispersive X-ray spectroscopy (EDX). The surface microstructure of the oxide was characterized by X-ray diffraction (XRD) analysis.

Figure 1: Conceptual Schematic of the experimental system

3 Results and Discussion

3.1 Mass Gain Analyses

Figure 2 shows the mass gain of each alloy as a function of time (t1/2) after isothermal oxidation at 950°C up to 144 hours in various controlled helium environments. The oxidation rate of each alloy approximately follows the well-known parabolic rate law, as shown in Figure 2. In the four controlled helium environments, the parabolic rate constants of Alloy 625 and Hastelloy X alloy were almost the same and comparatively smaller among the four selected alloys. The parabolic rate constants of Alloy 800H and Alloy 617 were similar and comparatively greater in the two wet helium conditions. Furthermore, the mass gain of Alloy 800H was the highest, and that of Hastelloy X was the lowest among the four alloys under all test conditions.

The influence of environmental conditions on each alloy was also analyzed. It was observed that the mass gain of Alloy 800H was the lowest in pure helium, and no significant differences in mass gain of this alloy were observed in the other three helium environments. A similar behavior also occurred in Alloy 625. In addition, the mass gains of Hastelloy X were similar in the four test environments. For Alloy 617, the mass gain in the helium environment containing 10% oxygen was slightly higher than that in pure helium. In the meantime, the mass gains of Alloy 617 were similar under the two wet helium conditions.

(a) 100% He (b) 10% O2

(c) 10% RH (d) 50% RH Figure 2: Mass gains of the four alloys in various controlled helium environments.

3.2 Surface and Cross-sectional Oxide Morphologies

Figures 3 and 4 show the surface and cross-sectional oxide morphologies of the four alloys, respectively. On the surface of Alloy 800H, in addition to the uniform distribution of oxide particles, ridges of oxide formed along the grain boundaries in all four controlled helium environments. In pure helium, Ti existed on the grain boundary ridges as TiO2, based on the EDS and XRD results (not shown in this paper). A two-layer structure was observed on this alloy, with an outer oxide layer consisting of MnCr2O4 and MnFe2O4 and an inner layer of Cr2O3. Manganese concentrated on top of the spinel scale since it diffused faster than iron or nickel in the Cr2O3 layer via lat-tice diffusion [8,9]. The thickness of the oxide layer of Alloy 800H was the greatest among the four alloys and inceased with increasing test duration regardless of the environmental conditions, as seen in Figure 4. Furthermore, scattered internal oxides of Al2O3 were observed beneath the continous Cr2O3 oxide layer.

For Hastelloy X, corrosion along the grain boundary was less distinct when compared with that of Alloy 800H. The surface oxide of Hastelloy X consisted mainly of spinel oxides of MnCr2O4 and MnFe2O4, Fe2O3, and Cr2O3. The SEM cross-sectional micrographs of the samples exposed to various controlled helium environments appeared similar, consistent with the results of mass gain. Based on the EDS and GDS results (not shown here), two-layer oxides formed with a thin outer spinel layer of MnCr2O4 and MnFe2O4 and a thick inner layer of Cr2O3 right above the substrate. In the two helium environments containing water vapor, the surface oxide layer was additionally covered with Si-rich oxides, and the particle size and number increased with increasing test duration.

The surface oxides of Alloy 617 were mainly MnCr2O4, TiO2 and Cr2O3, and ridges of oxide also formed along the grain boundaries of this alloy. Based on the EDS mapping (not shown here), Ti existed on the grain boundary ridges. According to the XRD analysis results (not shown here), the Cr2O3 peaks were much greater than those of MnCr2O4 and TiO2. Local Al2O3 regions having formed due to internal oxidation scattered underneath and adjacent to the surface oxide layer. Because of its higher Al content, the formation of Al2O3 in Alloy 617 was more prominent than that in Alloy 800H.

The surface morphology of Alloy 625 after the corrosion test is also shown in Figure 3. In pure helium and 90% helium with 10% O2 environments, comparatively smallspinel oxide particles were observed. In the two helium environments with vapor, the

needle-shaped oxide particles were observed. The oxide layer was mainly composed of MnCr2O4, TiO2 and Cr2O3. Internal oxidation of Al led to the formation of Al2O3 underneath the oxide layer, but the amount of Al2O3 in Alloy 625 was much lower than those in Alloy 800H and Alloy 617 due to the less Al content in this alloy.

3.3 Oxidation Mechanism

The composition of elements plays an important role in deciding the properties and characteristics of an alloy. To maintain the long-term integrity of structural components, the presence of a protective oxide layer on the alloy surface is essential. When an alloy is exposed to high-temperature HTGR environments, the Gibbs free energies of formation of various metal oxides and the diffusivities of corresponding cations in the substrate and in the oxides must be considered simultaneously. In this study, the Cr contents in the selected alloys ranged from 18% to 20%, which exceed the critical chromium concentration for forming continuous oxide scales [10-13]. Therefore, a Cr2O3 layer formed and became the major structure on the surface of the tested alloy at the early stage of the corrosion process [14]. The Gibbs free energy of the formation of MnCr2O4 (ΔGf =-557.78 kJ/mol) is of the same order as Cr2O3 (ΔGf =-546.86 kJ/mol) and but somewhat lower, so that MnCr2O4 should be able to co-exist with Cr2O3. Mn in the tested alloy formed MnCr2O4 at the outermost layer because Mn generally diffuses two orders of magnitude faster than Cr, Fe and Ni in the Cr2O3 layer. In addition, the Gibbs free energy of formation of Al2O3 (ΔGf =-858.30 kJ/mol) is much lower than that of Cr2O3. Although the Al contents of the tested alloys in this study were all less than 2%, Al2O3 still preferentially formed underneath the surface oxide layer, leading to reduction in creep resistance of the alloy due to the tensile stress in the perpendicular direction [15]. In this study, Cr2O3 would be the main protective oxide, and a greater amount of internal oxidation of Al would result in a higher possibility of alloy failure.

(a) Alloy 800H

(b) Hastelloy X

(c) Alloy 617

(d) Alloy 625 Figure 3: Surface SEM micrographs of the four alloys at 950 oC up to 144 hours in various con-

trolled helium environments.

(a) Alloy 800H

(b) Hastelloy X

(c) Alloy 617

(d) Alloy 625 Figure 4: SEM cross-section micrographs for four test alloys at 950 oC up to 144 hours in vari-

ous controlled helium environments.

4 Conclusions Isothermal oxidation tests on Alloy 800H, Hastelloy X, Alloy 617 and Alloy 625 were conducted at 950°C up to 144 hours in various controlled helium environments. Major findings in this study are as follows.

1. In mass gain analyses, the parabolic rate constants of Alloy 625 and Hastelloy Xalloy were almost identical and exhibited the best corrosion resistance amongthe four alloys.

2. A continuous Cr2O3 layer was observed on the surface of every tested alloy andacted as the main protective oxide at 950 °C in the four selected environments.

3. The mass gain of Alloy 800H was the highest, and Hasetlloy X exhibited thelowest mass gain among the four alloys under every designated condition.

4. Grain boundary ridges were observed on the surface oxide of Alloy 800H in thefour controlled helium environments. Two-layer oxides formed with a thin outerspinel layer of MnCr2O4 and MnFe2O4 and a thick inner layer of Cr2O3 rightabove the Alloy 800H substrate.

5. The formation of an Al2O3 internal oxide underneath the Cr2O3 layer in Alloy 617was more prominent than that in Alloy 800H or in Alloy 625, and it could reducethe creep resistance of the alloy. The content of Al2O3 for Alloy 625 was muchlower than those for Alloy 800H and Alloy 617 due to its less Al content.

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