COMPARISON STUDY ON OXIDATION BEHAVIOR OF ADVANCED...

17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors August 9-13, 2015, Ottawa, Ontario, Canada

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COMPARISON STUDY ON OXIDATION BEHAVIOR OF ADVANCED NICKEL-BASED SUPERALLOYS IN SUPERCRITICAL WATER (SCW)

Mohsen Sanayei1, Majid Nezakat1, Hamed Akhiani2, Sami Panttilä3, Jerzy Szpunar1. 1Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon,

SK, S7N 5A9, Canada 2Mitsubishi Hitachi Power Systems Canada, LTD., 826 58th Street East, Saskatoon, SK, S7K 5Z4,

Canada 3VTT Technical Research Center of Finland, Materials for Power Engineering, P.O. Box 1000, FI-

02044 VTT, Finland

ABSTRACT

In this study, we evaluate the oxidation behavior of two nickel based Superalloys including Haynes 230 (Ni-Cr-W-Mo alloy) and Hastelloy N (Ni-Mo-Cr alloy) with a view to its feasible application in the Gen IV supercritical water-cooled reactors. The samples were oxidized in supercritical water with 150 ppb inlet-dissolved O2 at 600 °C and 25 MPa. Results show that degradation behavior of these two alloys were different. It was found that Haynes 230 possesses better oxidation resistance in a supercritical environment in comparison with Hastelloy N. The maximum geometric area normalized weight changes for Hastelloy N and Haynes 230 were, respectively, 0.629 and 0.088 mg/cm2 after 1000 h SCW exposure time. Raman spectra and XRD characterization illustrate the formation of spinel and chromium oxides as the major available phases in the outmost oxide layer in both Superalloys. Surface morphology characterization tests also reveal the effect of exposure time on the surface roughness of oxidized samples. Morphological differences of Hastelloy N and Haynes 230 in different exposure times are also discussed.

Keywords: Supercritical Water (SCW), Oxidation, Nickel-based Superalloys

1. INTRODUCTION

Over the past fifteen years, ongoing research and development of Generation IV nuclear reactor concepts has been conducted all the world. The Gen-IV international forum has accepted six different concepts to improve the efficiency and safety of future nuclear power stations [1]. Among them, supercritical water-cooled reactor (SCWR) has gained much interest due to its simplified design, smaller volume, and higher thermal efficiency compared to light water reactors [2]. Water transforms to supercritical state upon passing its thermodynamic critical point (374.2 °C and 22.1 MPa). Supercritical water (SCW) exhibits unique thermo-physical properties in terms of specific heat and thermal conductivity [3]. Therefore, SCW is considered to serve as cooling and heat transfer medium for future generation of nuclear reactors [4-7]. In the Canadian design of a SCWR, water enters the reactor core at ~350 °C and exits at 625 °C. It is worth noting that most commercial water-cooled reactors operating today have a coolant temperature barely exceeding 320 0C. Hence, the service environments expected for SCWRs pose significant challenges regarding materials selection [8-15]. One of the major challenges for implementing the SCWR is to find the reactor structural materials such as fuel cladding. SCW environment is highly corrosive so that it could dramatically degrade the exposed materials [3]. The structural components will experience higher operating temperatures, higher neutron doses, and an extremely corrosive environment. Therefore, a systematic study on corrosion of candidate materials is required before they can be used in SCWRs [1].


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As a result, an international quest, coordinated by the Generation IV International Forum (GIF), was initiated with the task to find appropriate materials which can tolerate the exposure to SCW that the reactor core components will experience in service [16].To date, austenitic stainless steels [17-26], Ni-base alloys [22-25,27-35], Ferritic–martensitic steels [11,25,36-41], and oxide dispersion strengthened steels [42-45] are the main candidate materials for SCWRs. Among these materials, the nickel alloys have better high temperature properties. Nickel and iron base super-alloys, due to their superior high temperature properties (e.g. oxidation, creep) would be good choices [3]. Among nickel alloys, Haynes 230 as Ni-Cr-W-Mo receives more attention due to is combined excellent high temperature strength and oxidation resistance with superior long term stability and good fabricability. In addition, Hastelloy N as Ni-Mo-Cr alloy with high oxidation and corrosion resistance properties can be one of the candidate materials for next generation nuclear reactors. To date, there is quite a few substantial literature investigating the supercritical water oxidation of these two alloys for long term exposure. It is worthy to mention that most supercritical water corrosion studies on other alloys have been done using a static autoclave which often leads to lower oxidation and less spallation than dynamic/recirculating autoclaves [3].

In the present study, we assessed the SCW oxidation resistance of two nickel-based Superalloys. Moreover, we focused on relation of between oxide surface morphology and SCW exposure time.

2. EXPERIMENTAL PROCEDURE

2.1 Materials and specimens

Hot rolled sheet of Haynes 230 and Hastelloy N with an average grain size of 21µm and 34.6 µm respectively, were used as a starting material to be exposed in a SCW environment. Table 1 shows the chemical composition of starting alloys. Prior to SCW exposure, specimens with a length of 24 ± 0.01 mm, width of 14 ± 0.01 mm, and thickness of 1.6 ± 0.01 mm were cut from the hot rolled sheets. Mechanical grinding of the specimen surfaces was performed using abrasive papers up to grit number 2000 (10 µm SiC particles).

2.2 Apparatus

Tests were conducted in an autoclave connected to a recirculation water loop, as illustrated in Figure 1. Table 2 shows values for temperature, pressure, inlet and outlet water conductivity, oxygen content and flow rate, which were monitored and controlled. To avoid leakage in the piping junctions, the water flow was kept low as the operating temperature was high (600 °C). The specimens were attached to a special sample holder where they were electrically insulated from the rack and from the autoclave body.

2.3 Methodology

The specimens were oxidized in supercritical water at 600 °C and 25 MPa for 100, 300 and 1000 h and the weight of each specimen was measured before and after 100, 300 and 1000 h exposure and the weight changes were calculated per unit area. To have a better statistical results, 3 specimens were exposed for each exposure time and the average amounts were reported. Consequently, a Hitachi SU6600 field emission gun scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy (EDS) was used to characterize surface morphology and phase identification of oxidized samples. The secondary electron images were recorded with the SEM operated at 5 kV at a working distance of typically 3-4 mm. The near-surface composition of the oxide layers was analyzed using EDS in the SEM with an accelerating voltage of 15 kV and a working distance of about 10 mm.

Corrosion products analysis was performed using a Bruker D8 Discover diffractometer with Cr Kα radiation and a Raman system. Raman spectra were obtained using a Renishaw model 2000 spectroscope equipped with 514 nm Ar ion laser with a spot size of approximately 2 μm and the maximum incident laser power of 1.5 W. The integration time used was 20 seconds. After exposure to SCW, the surface


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roughness of oxidized samples was measured using a ZYGO NewViewTM 8000 Series 3D optical surface profiler.

3. RESULTS & DISCUSSION

3.1 Weight change

Figure 2 illustrates the weight changes normalized to sample surface area of all the tested samples at 100, 300 and 1000 h SCW exposure periods with respect to the original weight of specimens before exposure. The maximum geometric area normalized weight changes for Hastelloy N and Haynes 230 were respectively 0.629 ± 0.018 and 0.088 ± 0.016 mg/cm2. The sample weight change after the SCW exposure was first used as a measure of corrosion resistance. Therefore, the small weight changes for Haynes 230 compared to that of Hastelloy N, shows better general oxidation resistance of Haynes 230. However, the figure also depicts a general trend of weight gain for Hastelloy N and weight loss for Haynes 230, during 1000 hours SCW exposure. The different trend of the weight change may be caused by the competing processes of weight gain due to oxidation/precipitation and weight loss due to oxide dissolution, pitting, or exfoliation [46, 47, and 48]. Therefore, weight changes trend in different exposure time should be investigated to have a better comparative study of oxidation resistance [49].

Haynes 230 shows a small weight gain (around 0.003 mg/cm2) first and reaches a peak after 100 h exposure which indicates its lower oxidation rate. After 100 h exposure time, the dissolution rate of the oxide film increased with further increase in exposure times (up to 1000 h). As a result, the weight loss illustrates its lower oxide retention ability [49]. On the other hand, Hastelloy N shows a quick increasing trend of oxidation rate until 100 hours exposure which shows its poor oxidation resistance. It seems the competing process of oxidation and dissolution keep balance from 100 to 300 h exposure time [50]. Therefore, the weight change in this period is negligible. After 300 h exposure time, the weight gain trends continue with increasing exposure times. It is worthy to note that oxidation rate for Hastelloy N and also dissolution rate for Haynes 230 in primary exposure times (100 hours) was higher than that of 300 to 1000 hours which shows oxidation rate is decreasing in long term exposures.

3.2 Raman spectra

Figure 3 shows the Raman spectra of the oxide films on Haynes 230 (Figure 3a) and Hastelloy N (Figure 3b) after exposure tests in SCW at 600 °C for different times. As Figure 3a illustrates the peaks on the spectra show no obvious change with increasing exposure time. Cr2O3 and spinel structures are responsible for peaks 551 cm-1 and 693 cm-1, respectively for Haynes 230 [51, 52]. With increasing exposure time, the intensity of peak 693 cm-1 is increasing which shows higher amount of spinel in the oxide film. On the other hand, Figure 3b show several peaks on the spectra suggesting that different oxide compounds such as Fe2O3, NiO, Cr2O3 and also spinel structures such as NiCr2O4, NiFe2O4, FeCr2O4 and CrMoO6 are responsible for peaks at 209 cm-1, 339 cm-1, 486 cm-1, 693 cm-1 and 1652 cm-1, in the oxide films of Hastelloy N after different exposure times.

The present results indicate that the outermost layer of the oxide film for Haynes 230 and Hastelloy N is spinel. Due to the limited detection depth of Raman spectroscopy, only the outmost surface of the oxide films can be detected. To complement the information obtained from Raman spectroscopy, XRD analysis was studied.

3.3 XRD analysis

Figure 4 illustrates XRD pattern of Haynes 230 (Figure 4a) and Hastelloy N (Figure 4b) after exposure to supercritical water at 600 0C and 25 MPa for 100, 300, and 1000 h. As Figure 4a shows, chromium oxide Cr2O3 is the major oxide that formed on the surface of Haynes 230. The major phase in the oxide film of Haynes 230 is Cr2O3 [49]. It is worthy to note that the intensity of the matrix peaks decrease with increasing exposure time while the intensity of peaks related to oxides are increasing.


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As it is shown in Figure 4b, all samples exhibit multiple peaks corresponding to different oxides such as Cr2O3, NiO and also spinels such as NiCr2O4, CrMoO4, CrMoO6, and NiMoO4 in different exposure times [1]. However, NiCr2O4 and Cr2O3 are the two major oxides formed at the surface of Hastelloy N. The intensity of peaks related to the content of phases in the oxide films, indicating that the spinel NiCr2O4 in the oxide film increases with increasing exposure time, while the intensity of matrix peaks are decreasing. Furthermore, after 1000 h exposure time, the intensity of peaks regarding Fe2O3, FeO and MoO2 are increasing. Since XRD patterns for spinels concerning different alloying constituents (Ni, Cr, Fe, Mo) are very similar, the exact spinel type cannot be clarified now [50]. A further analysis is needed to identify the composition of the oxide film.

3.4 Surface morphology

Figure 5 shows the SEM images for the oxidized surface of Haynes 230 after 100 h of SCW exposure. Multiple distinctly different types of oxides can be seen in the figure: Sparsely distributed discrete particles in the oxide matrix, large spherical particles, islands containing uniform polyhedral oxide particles, and large oxide spots are among different morphologies shown in this Figure [49].These distinct morphological regions can be identified as shown in the magnified images in the insets. The region highlighted in blue box is oxide spots which are covered by small and fine oxide particles. The region highlighted in red box shows the spherical particles which are adjacent to islands and consequently the region highlighted in green shows the morphology of uniform polyhedral oxide particles that form the oxide islands. The morphological difference suggests that the structures and chemical compositions of these oxides may be different [49].

With increasing the exposure time, some morphological changes on the surface of Haynes 230 exposed to SCW is identified as shown in Figure 6. As Figure 6a and Figure 6b illustrate, after 300 h exposure, some cracks and exfoliations are formed on the surface of the spots and the matrix, respectively. The rate of observed cracks on the spots was even increased for sample exposed for 1000 h (Figure 6c) which is in agreement with weight loss results (Figure 2) for Haynes 230. Furthermore, after 1000 h SCW exposure, the amount of fine triangular particles in the matrix was increased (Figure 6d) and also the morphology of particles adjacent to islands and spots was altered to plate/needle shaped oxides (Figure 6e) [50]. Also it is worthy to note that after long term exposure time, spots seems to be buried by the matrix oxide layer which is getting thicker (Figure 6f).

In contrast to Haynes 230, morphological differences were less observed in Hastelloy N (Figure 7) that indicates a high rate of uniform oxidation phenomenon which is consistent with weight gain results (Figure 2). The major morphology of oxide film for 100h exposure time is defined by triangular and polyhedral particles (Figure 7a). Also, some spherical particles have been formed on the surface of SCW exposed specimens (Figure 7b). By increasing the exposure time to 300 h, amount of spherical particles has been increased and they have been formed mostly adjacent to each other (Figure 7c). Furthermore, some observed exfoliation area (Figure 7d, e and f) competing with oxidation process leads to a balance between these two phenomena, as illustrated in weight changes results (Figure 2). However, after 1000 h exposure time, depth of exfoliation are seems to be less which means oxidation process is the predominant phenomena that leads to weight gaining trend (Figure 2).

3.5 Surface Roughness

As Figure 8 shows, surface roughness of oxidized Haynes 230 and Hastelloy N samples are completely different. Due to higher rate of oxidation for Hastelloy N, its surface roughness is much higher than that of Haynes 230. For both alloys, by increasing the exposure time, surface roughness has been increased due to higher oxidation process. As table 3 illustrates the rate of roughness change for Haynes 230 is lower than that of Hastelloy N. It is also worthy to note that the difference in roughness amount of Hastelloy N for 100 and 300 h exposure is negligible in comparison with that of 300 and 1000 h exposed specimens which is in consistent with weight changes results (Figure 2).


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4. CONCLUSIONS

The weight gain, phase compositions, surface morphology and surface roughness of the oxide films formed on Haynes 230 and Hastelloy N exposed to 600 °C and 25 MPa supercritical water has been investigated. The following conclusions can be drawn:

(1) General oxidation, pitting corrosion and exfoliation are observed on Haynes 230 and Hastelloy N exposed to SCW. However, Haynes 230 showed higher rate of exfoliation while Hastelloy N exhibited higher rate of oxidation. Thus, with increasing exposure time, weight gain for Hastelloy N and weight loss for Haynes 230 were observed up to 1000 h exposure time. The maximum geometric area normalized weight changes for Hastelloy N and Haynes 230 were respectively 0.629 ± 0.018 and 0.088 ± 0.016 mg/cm2 after 1000 h SCW exposure time.

(2) Due to involvement of several processes such as oxide dissolution, precipitation and exfoliation in SCW corrosion, further investigations such as cross-section observation should be considered to have substantial comparison study.

(3) The surface roughness as a function of exposure time increases for both alloys. A greater increase is observed Hastelloy N than Haynes 230.


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Table 1. Chemical composition (wt %) of starting alloys.

Alloys Ni Fe Cr Mo W Cu Mn Si Co Al C B 230 57 3* 22 2 14 0 0.5 0.4 5* 0.3 0.1 0.015*N 71 5* 7 16 0.5* 0.35* 0.8* 1* 0.2* 0 0.08* 0

*Maximum

Table 2. Targeted and realized values for the supercritical water test environment.

Quantity Target value/range Notes/realized values (mean ± SD) Temperature 600 °C Recorded mean 599 ± 1.0 °C

Pressure 250 bar Recorded mean 250 ± 0.9 bar Inlet conductivity 0.1-0.5 µS/cm Recorded mean 0.053 ± 0.001 µS/cm

Outlet conductivity 1.0-3.0 µS/cm Recorded mean 0.29 ± 0.11 µS/cm Inlet dissolved O2 150 ppb Recorded mean 150 ± 0.5 ppb

pH of the inlet water 7.0 (Pure water) Flow rate ~5 ml/min Full renewal about every 2 h

Table 3. Surface roughness of Haynes 230 and Hastelloy N exposed to SCW for various exposure times.

Sample Exposure time (h) Roughness (nm) Haynes 230 100 452.035 ± 45 Haynes 230 300 567.823 ± 58 Haynes 230 1000 654.254 ± 71 Hastelloy N 100 1541.488 ± 117 Hastelloy N 300 1595.255 ± 106 Hastelloy N 1000 2780.045 ± 156


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Figure 1. The supercritical autoclave system at VTT [1].

Figure 2. Weight changes of the specimens after exposure to SCW for various exposure times.


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Figure 3. Raman spectroscopy of the oxide films formed on Haynes 230 (a) and Hastelloy N (b) exposed

to SCW for various exposure times.


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Figure 4. XRD patterns of the oxide films formed on Haynes 230 (a) and Hastelloy N (b) exposed to

SCW at 600 °C and 25 MPa for various exposure times.


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Figure 5. Typical morphology of oxides that form on Haynes 230 after 100 h exposure to SCW at 600 °C

and 25 MPa.

Figure 6. Set of SEM images for Haynes 230 and morphological changes with increasing exposure times.

(a and b) for 300 h, (c, d, e and f) for 1000 h exposure time.


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Fig. 7. Set of SEM images for SCW exposed Hastelloy N and morphological changes with increasing

exposure times. (a and b) for 100 h, (c, d and e) for 300 h and (f) for 1000 h exposure time.


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Fig. 8. Surface roughness images of Haynes 230 (a, b, and c) and Hastelloy N (d, e, and f) exposed to SCW for various times. (a and d) for 100 h, (b and e) for 300 h and (c and f) for 1000 h exposure time.

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