1. Introduction
A supercritical water reactor (SCWR) is considered a promising Generation IV nuclear reactor owing to its simple design and high thermal efficiency. An SCWR is a high-temperature, high-pressure, and water-cooled reactor, which operates above the critical point of water (374 °C, 22.1 MPa) [1-4]. The typical design of an SCWR is a once-through, direct-cycle system operating at a pressure of 25 MPa and temperature range from 280 °C to 620 °C [5-7]. One of the major problems faced by SCWRs is the lack of data related to the material behavior of supercritical water (SCW). The resistance to corrosion and stress corrosion cracking (SCC) under SCW conditions are important requirements [8, 9]. It is well known that SCW is highly corrosive to metallic materials, especially in an oxidizing environment, and the fuel cladding in an SCWR is likely to be subjected to severe corrosion. Therefore, selecting appropriate candidate materials for fuel cladding is an important feature of SCWR design [10]. In addition, SCC is one of the major concerns in the selection of materials and is still an important technical issue [11-16].
Austenitic stainless steels have been widely used as structural materials in nuclear reactors owing to their excellent high-temperature corrosion resistance and good mechanical properties [17-19]. Typical 310S stainless steel is considered a promising material for fuel cladding of the Chinese CSR1000 [20] and the Japanese SCWR design [2]. Owing to the radiation effect, there is an elevated dissolved oxygen (DO) concentration. It is generally known that the SCC behavior of stainless steel is subject to water chemistry such as oxygen. However, many corrosion studies have been conducted in low DO or deaerated environments [21, 22]. There are few studies on the SCC susceptibility of 310S with different DO concentrations in SCW. Therefore, a thorough understanding of the SCC behavior of 310S in SCW with different DO concentrations is significant for the development of SCWRs.
Therefore, in this study, the effect of DO on the SCC susceptibility of 310S was studied by carrying out slow strain-rate tensile (SSRT) tests.
2. Experimental
2.1. Materials
The materials used in this work were extruded bars of 310S austenitic stainless steels (Pang Steel Corporation). The extruded bars were subjected to solution heat treatment at 1050 °C for 1 h, followed by quenching in water. The chemical composition of 310S is listed in Table 1. The specimens for the SCC test were sampled from the extruded bar parallel to the loading direction along the rolling direction, and processed into a round bar with a gauge of length 20 mm and diameter 4.0 mm according to the ASTM standard E8. The geometry of the round bar tensile specimen is shown in Fig. 1. The gauge section was burnished with 1000 grit emery paper, washed with ethanol in an ultrasonic cleaner, and cleaned with distilled water. The polishing direction was parallel to the loading direction.
| C | Si | Mn | S | P | Cr | Ni | Fe |
|---|---|---|---|---|---|---|---|
| 0.042 | 0.45 | 0.95 | 0.002 | 0.025 | 24.42 | 20.34 | Bal. |
2.2. Characterization
To investigate the SCC susceptibility of 310S in SCW, SSRT tests were performed on specimens in a supercritical environment corrosion testing facility, as shown in Fig. 2, which can be operated at a temperature up to 650 °C at the pressure of 30 MPa. The loading rate of the testing facility can be continuously adjusted from 0.0001 mm·s-1 to 1 mm·s-1. There are two loop systems in the testing facility, one each for establishing specific SCW environments and monitoring water chemistry at room temperature (25 °C). Deionized water with conductivity less than 0.1 μS/cm was pressurized to 25 MPa and heated to 620 °C to establish the SCW environment. The volume of the autoclave was 2.5 L and the flow rate of inlet water could reach up to 15 L/h, yielding an autoclave water refreshment rate of approximately 6 times per hour. A stable and reliable water chemistry environment was obtained during the testing. The DO concentration was measured directly on the equipment using an Orbisphere oxygen analyzer. The DO concentration in the SCW environment during the SCC tests ranged from zero to 8000 ppb with continuously bubbling argon-oxygen mixture gases. The strain rate of the specimen was maintained at 7.5×10-7 s-1 in this work, which has been shown to be appropriate to evaluate the SCC susceptibility of stainless steel at high temperatures [6].
After the test, the stress–strain curves, fracture morphology, and chemical composition of the oxide were analyzed to evaluate the SCC susceptibility and mechanical properties of 310S. The analysis of fractographic features of the specimens was conducted using a field-emission scanning electron microscope (FE-SEM JSM-7500, Japan), and an energy-dispersive X-ray spectroscopy (EDS) model OX-FORD IE-250 attached to the SEM was utilized to analyze the chemical composition of the oxide layer. The electron probe microanalyzer (EPMA-1720, Japan) analysis was also used to study the distribution of the major elements in cracks on the cross-section along the gauge section.
3. Results and discussion
3.1. Stress–strain curves
The stress–strain curves of 310S obtained from the SSRT tests in SCW at 620 °C are shown in Fig. 3 and the results are presented in Table 2. Elongation reduction can be used as a quantitative method to evaluate the SCC susceptibility. The elongation of 310S decreased with the increase of DO concentration, which indicates its dependence on DO concentration. The elongation under different DO conditions ranged from 36.7% to 45.6%. Compared with the data measured in the deaerated SCW, the elongation was decreased by 19.5% when DO reached 8000 ppb. This tendency follows the increase in the oxidation potential of SCW. According to Zhu [23], DO changes the oxidation potential of SCW, leading to an increase in the oxidation rate. Lu [24] investigated the effect of DO on the SCC of 316NG in simulated boiling water reactor environments using a different technique. His study indicated that the potential gradient between the crack mouth and crack tip increases with the increase in DO concentration in high-temperature water, and the crack growth rate increases with the increase in DO owing to the acceleration of the oxidation rate at the crack tip. A similar result was also obtained by Zhang [25]. The crack growth rate of 316L increases with the increase in DO concentration in the simulated pressurized water reactor primary water, as DO may affect the oxide film structure. DO has a negligible effect on both the ultimate tensile strength (UTS) and yield strength (YS). The insensitivity of UTS and YS to DO is consistent with the literature data on 316Ti stainless steel tested at the temperature of 650 °C [17].
| DO (ppb) | UTS (MPa) | YS (MPa) | Elongation (%) | Fracture mode |
|---|---|---|---|---|
| 0 | 229 | 186 | 45.6 | Brittle, IG |
| 500 | 225 | 170 | 44.7 | Brittle, IG |
| 1000 | 234 | 184 | 39.8 | Brittle, IG |
| 2000 | 229 | 174 | 38.9 | Brittle, IG |
| 8000 | 233 | 190 | 36.7 | Brittle, IG |
3.2. Fracture behavior
3.2.1. Fracture morphology analysis
The SEM analysis of broken specimens was used to characterize the fracture behavior. Fig. 4 (a) shows the SEM image of the fracture surface of 310S tensile strained in deaerated SCW. The failure surface of 310S exhibited brittle and intergranular morphology. Almost all the specimens were covered with grains. Intergranular facets appeared in both the center and edge regions of the fracture surface, which are characteristics of brittle fracture, as shown in Figs. 4 (b) and 4 (c). These results are very close to the data of HR3C, tested in SCW at temperatures of 600 °C and 650 °C [26]. The specimens tested in SCW with different DO concentrations showed similar characteristics, suggesting that DO has no significant effect on the fracture surface morphology.
3.2.2. Gauge surface morphology analysis
Micrographs of the gauge surfaces showed similar characteristics after SCW tests with different DO concentrations at 620 °C. Fig. 5 shows that the cracks on 310S were widely distributed at the gauge surface near the fracture surface, and most of the cracks were perpendicular to the loading direction. By using EDS to analyze the oxide film on the gauge surface, chemical changes were observed in the oxide film formed in the SCW with different DO concentrations, as presented in Table 3. The detected oxide film was located at the gauge surface close to the crack mouth (see label 1 in Fig. 5). The EDS results show that the oxide layer was mainly composed of Cr, O, and Fe, and the Cr concentration increased with the increase in DO. Numerous studies on austenitic stainless steel have shown that the oxides have a two-layered structure [7, 27, 28]. The inner layer is rich in chromium and the outer layer is mainly composed of magnetite. According to Was [7], for most high nickel–chromium stainless steels, the outer oxide layer loosely adheres to the inner layer and exhibits a tendency to spall. Similar results can be found in Ref [29]. Furthermore, the DO affects the structure and composition of the oxide layer formed on stainless steel. At a high DO concentration, the incorporation of chromium into the oxide layer is promoted, inhibiting the formation of magnetite at the top of the oxide layer [22]. When oxygen is present, the solvation and oxidation of SCW are enhanced. The solubility of Fe-rich outer oxides is likely to increase, leading to the decrease in the concentration of iron oxide with the increase in DO and the opposite trend for Cr concentration.
| DO (ppb) | Time to fracture (h) | Cr (wt%) | Fe (wt%) | O (wt%) |
|---|---|---|---|---|
| 0 | 169 | 7.55 | 62.22 | 30.22 |
| 500 | 165 | 12.82 | 56.84 | 30.34 |
| 1000 | 147 | 16.60 | 52.83 | 30.36 |
| 2000 | 144 | 40.28 | 28.77 | 30.95 |
| 8000 | 133 | 63.91 | 4.61 | 31.48 |
3.2.3. Cross-sectional analysis
EPMA analysis was performed on the cross-sections of fractured specimens to investigate the relationship between a crack and oxide layer. Figs. 6(a) and 6(b) show the results obtained from EPMA using a wavelength-dispersive spectrometer (EPMA–WDS) on the cross-section of the specimens. The Fe–Cr oxide was distributed along the crack, and the oxide film can be divided into two layers. The outer oxide layer was rich in Fe, whereas the inner oxide layer was rich in Cr. This observation of a double oxide layer is consistent with the studies on 316 austenitic stainless steel in SCW [26, 27]. It is speculated that diffusion plays a key role in the formation of the double oxide layer. Oxygen diffused inward, whereas metal elements such as Cr and Fe diffused outward. The iron was enriched in the outer layer but was depleted in the inner layer, whereas the concentration of chromium exhibited the opposite trend. These differences in the distribution of iron and chromium may be due to a lower diffusion rate of chromium in the oxide layer compared with that of iron. It is believed that an inner layer was formed on the original surface of the polished specimen, and the oxide was grown via selective oxidation and diffusion of chromium. The iron ions diffused outward to form an outer oxide layer, whereas fewer chromium ions are transferred to the outer oxide layer owing to the low diffusion rate. Ni enrichment was detected at the oxide/metal interface, which can be explained by the low diffusion and incorporation rate of nickel in the oxide layer [30]. This phenomenon has also been observed by Behnamian [31], who attributed the formation of Cr-rich oxide to selective oxidation. The oxide layer formed in SCW with DO of 8000 ppb has an approximate thickness of 10 μm, and the oxide layer formed in the deaerated SCW has an approximate thickness of 17 μm. The oxide layer formed in the SCW with DO of 8000 ppb is thicker than that in the deaerated water, indicating an accelerated corrosion effect caused by oxygen.
It is well known that chromium oxide is a passive film and can mitigate corrosion effectively. Therefore, the integrity of the Cr-rich inner oxide layer may play an important role in the SCC resistance of tensile specimens in deaerated SCW and SCW with DO of 8000 ppb. Figs. 6(c) and 6(d) show the cross-sectional images of the tensile specimens after SSRT in deaerated SCW and SCW with DO of 8000 ppb, obtained using EPMA with an EDS (EPMA-EDS). Notably, the distribution of Cr-rich oxide layer along the crack changes with the oxygen concentration. For the specimen exposed to deaerated SCW, the Cr-rich oxide layer appears to be compact and has no discontinuity, as shown in Fig. 6(c). In SCW with DO of 8000 ppb, the Cr-rich oxide layer in Fig. 6(d) is relatively discontinuous at some locations and the thickness of Cr-rich oxide layer is non-uniform. As shown in the morphology, the thickness of Cr-rich layer increases, particularly in the region of the cracks near the surface of the specimens. The thicker Cr-rich oxide layer can lead to higher internal stress in the oxide film, resulting in a higher risk of breaking of the oxide film in SCW with DO of 8000 ppb. It is expected that a thin and continuous Cr-rich oxide layer prevents further oxidation, whereas the discontinuous Cr-rich oxide layer may increase the SCC susceptibility. Notably, only the inner oxide layer is detected in the region of the cracks near the surface, which can be attributed to the dissolution of the outer oxide layer or spallation caused by the internal stress inside the oxide layer. It is consistent with the EDS results presented in Table 3.
4. Conclusion
SSRT tests of 310S were carried out in SCW at a temperature of 620 °C, pressure of 25 MPa, and strain rate of 7.5×10-7 s-1. The tests were performed with DO concentrations of 0, 500, 1000, 2000, and 8000 ppb. According to the above experiments, the main results are as follows:
(1) DO in the SSRT test had a negligible effect on YS and UTS, whereas the elongation was strongly dependent on the DO concentration.
(2) A brittle fracture mode was observed on the fracture surface, and dense cracks were observed on the gauge section. The entirely brittle character and a similar fracture surface indicate that the effect of DO on the fracture morphology is negligible.
(3) Oxides were observed inside the cracks with two-layered structures: an Fe-rich outer oxide layer and a Cr-rich inner oxide layer. The Cr-rich inner oxide layer inside the cracks was more continuous in deaerated SCW compared with that in oxygenated SCW.
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