In this study, an Al-containing alloy 214 was evaluated in superheated steam at 800 °C for a duration of 600 h. The purpose of using superheated steam was to simulate the supercritical water (SCW) condition at higher temperatures where no commercial SCW rig is currently capable of reaching (800 °C and beyond). After exposure to superheated steam, the weight change and surface oxidation were analyzed. Alloy 214 experienced greater weight gain than IN 625 and Ni20Cr5Al, due to its low Cr content. Formation of both Cr2O3 and Al2O3 was observed on the surface after 300 and 600 h of exposure. However, as exposure progressed, more Ni and Mn-containing spinel started to form, signaling Cr and Al depletion on the metal substrate surface.
Concerns with greenhouse gas emissions and uncertainty in the long-term supply of fossil fuels have resulted in renewed interest in nuclear energy as part of the energy mix for the future. In an effort to improve the current nuclear plant (often referred to as Gen-II and Gen-III reactors) efficiency, an international collaboration known as the Gen-IV International Forum  was established with the objective to develop generation-IV reactor technologies; higher efficiency, safety, reliability, and proliferation are some of the attributes the Gen-IV nuclear reactors would promise . Among these reactor designs is the Gen-IV supercritical water-cooled reactor (SCWR) technology. The SCWR design utilizes single-phase supercritical water (SCW) as the coolant, has a predicted thermodynamic efficiency of ∼45% (compared to 33% for current reactors), and is expected to provide for considerable plant simplification [2,3]. In the current Canadian SCWR design, 336 vertical channels are used in the core. A high-pressure inlet plenum feeds water into the core at a temperature of 350 °C. The channel outlets are connected to a drum, instead of feeder pipes, with water temperature of 625 °C. The outlet pipes from the drum connect to high-pressure turbine in a direct cycle, as shown in Fig. 1 . The estimated peak temperature at the cladding can be as high as 800 °C.
The identification of appropriate materials for in-core and out-of-core components to contain the SCW coolant is one of the major challenges in the design of the SCWR. Ferritic–martensitic and austenitic stainless steels and oxide dispersion strengthened alloys have been considered as materials for both fuel cladding and coolant-contacting components in the heat transport loop [5–8]. Precipitation-hardened and solid solution Ni-based alloys such as IN 718 and IN 625 have also been considered for applications where the radiation dose is lower. Unfortunately, most of these alloys have one or several disadvantages for their use in SCWR [9–13]. Nickel-based alloys in general exhibit better corrosion resistance in SCW than ferritic–martensitic and austenitic stainless steels and have enhanced creep resistance at higher operation temperature (Fig. 2); however, nickel-based alloys are susceptible to He embrittlement [14–16]. 310 austenitic stainless has attracted some attention as a potential fuel cladding material . However, the mechanical properties exhibited by 310 stainless steel (SS) make it less suitable for the fuel cladding of the Canadian SCWR concept as due to the projected peak temperature the material may be exposed.
Alloy 214 is a Ni–Cr–Al–Fe alloy designed to provide high-temperature oxidation resistance with composition provided in Table 1 [17,18]. Its application temperature is rated to 950 °C. At high-temperature oxidizing environment, it has the ability to form an adherent Al2O3 protective scale, in addition to Cr2O3. Its creep resistance is more superior than that of 800H and 310 SS. However, the ductility is much lower. Based on our previous research of Fe22Cr6AlY and Ni20Cr5AlY model alloys, the addition of Al played an important role in stabilizing the oxidation process and offered long-term protection to underlying metallic substrate [19–21]. It has been observed that the effect of water on oxidation performance of alloy 214 is minimal due to the formation of a protective external alumina scale .
This study was undertaken to investigate the oxidation/corrosion behaviors of alloy 214 under high-temperature steam at 800 °C. The use of high-temperature steam instead of SCW was based on our results of comparative studies of IN 625, 304 and 310 SS in both SCW and supercritical heat steam at 625 °C. Similar weight change and surface microstructure were observed after exposure to both conditions.
Alloy 214 samples were cut into small rectangular coupons measured 15 × 10 × 3.23 mm. They were then ground using 240, 320, 400, and 600 SiC abrasive papers on all six sides. After grinding, the samples were cleaned in an ultrasonic bath (Branson 2510) using soap and water for 45 min followed by 15 min in acetone. To remove moisture, the samples were placed in a furnace (Cole-Parmer Stable Temp) at 200 °C for 2 h. Sample weights were measured using Mettler Toledo AG285 scale, with ±0.1 mg precision. Weights measured before and after steam exposure were used to calculate the weight change per unit surface area.
A superheated steam rig was used to in lieu of SCWR. A water distiller supplies distilled water for the steam rig; a gear pump (Cole-Parmer, Vernon Hills, IL) was used to pump 1 kg/h water into steam generator manufactured by MHI. The steam generator discharges superheated steam into a tube furnace (Palmer 3 zone tube furnace) where the superheated steam reaches the final testing temperature of 800 °C. Exhaust steam from the loop is discharged into a water reservoir.
Following 600 h of testing, the surface microstructures were characterized using scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) to examine the progression of oxide formation on the surface. Phase composition of the surface oxides was analyzed using X-ray powder diffraction (XRD). A Co Kα radiation X-ray source was used with an applied voltage and current of 35 kV and 40 mA, respectively. All tests were carried out within a 2θ range of 25–80 deg (θ is the angle between the incident X-ray beam and the horizontal surface of the sample).
Results and Discussion
The average weight change during the 600 h exposure to high-temperature steam (Fig. 3) shows that the alloy 214 has its oxidation rate ranked between FeCrAlY and NiCrAlY coating materials tested within the same cycle. However, among the three Ni-based alloys (alloy 214, NiCrAlY, and IN 625), alloy 214 has the most weight gain (0.139 ± 0.012 mg/cm2 after 300 h and 0.258 ± 0.017 mg/cm2 after 600 h), likely due to the low Cr content. In our previous study, the addition of Al has been observed to increase initial weight gain during earlier stage of either SCW or steam exposure, with formation of Al2O3 observed.
SEM Surface Examination.
After 300 h in steam, the sample surface is already covered with oxide, as shown in Fig. 5. There seem to have three different oxide morphologies as indicated by A1, B1, and C1 in Fig. 6. EDS analysis (Table 2) reveals the presence of Cr, Al, Ni, and O on all three phases. XRD analysis indicates the presence of both Cr2O3 and Al2O3 on the surface (Fig. 7). Based on the surface morphology, SEM/EDS and XRD examination results, it is believed that NiO (or Ni-containing spinel) may have formed upon exposure to superheated steam, Cr2O3 and then Al2O3 formed below NiO. NiO spallation occurs leaving remnants of Ni-containing phase B1 on the surface. As Al2O3 usually forms just above the substrate and beneath Cr2O3 or other fasting growing oxide, the SEM/EDS is not able to reveal Al2O3; but XRD was able to detect subsurface, hence, the presence of Al2O3. Both phase A1 and phase C1 are essentially Cr2O3 with underlying Al2O3. The few needle-shaped phases on top of phase C1 may be Mn-containing spinel, resulting in the presence of Mn in EDS measurement from C1.
Figure 8 shows the surface of alloy 214 after 600 h of exposure to superheated steam. There are only two microstructurally different phases at this stage (A2 and C2 and their compositions are listed in Table 3). The blocky phase (B) found after 300 h (likely NiO) seemed to have spalled completely. The remaining phases are fuzzy featureless A2 and needle-shaped C2. The phase labeled with A2 has similar composition as A1, it is believed to be Cr2O3 on top of Al2O3 or a mixture of both. XRD analysis result shown in Fig. 9 confirms the presence of Cr2O3. The needle-shaped phase C2 seems to have increased as the duration of steam exposure increased. Usually as Cr and Al depletion occurs due to oxidation and Ni diffusion from substrate, spinel tends to form. On alloy 214, the spinel assumes a structure of (Ni, Mn)(Cr,Al)2O4 based on SEM analysis. Further examining the XRD spectrum, it is evident that Al2O3 indeed has formed. As SEM surface analysis did not reveal presence of Al2O3, it is believed the Al2O3 has formed under the Cr2O3, a known phenomenon of “gettering effect” provided by Cr .
The oxides formed on sample surfaces are similar to those report on alloy 214 under oxidation condition at 850–1000 °C  where Cr2O3, NiCr2O4, and Al2O3 were found. When being tested at a higher oxidation temperature between 1100 and 1200 °C, spinel surface and alumina subsurface oxides were found . Because of the formation of alumina, the cyclic oxidation resistance was also observed to be superior to other Cr-containing alloys in the temperature range of 982–1149 °C .
In this study, an Al-containing high-temperature oxidation resistant alloy 214 was tested under flowing steam heated to 800 °C. After 300 and 600 h, the samples were removed for weight measurements and examination of surface oxide formation by SEM/EDS and XRD. The results showed that NiO was formed initially on the sample exposed for 300 h; NiO was no longer present after 600 h. Both Cr2O3 and Al2O3 were found to form on samples, with Al2O3 being a subsurface oxide. Spinel was also observed on the surface but the amount seemed to have increased as test duration progressed, suggesting Cr depletion.
Funding to the Canada Gen-IV National Program was provided by the Natural Resources Canada through the Office of Energy Research and Development, the Atomic Energy of Canada Limited, and the Natural Sciences and Engineering Research Council of Canada. The authors also thank Dr. Yang at National Research Council for providing EDS and XRD services.
Natural Sciences and Engineering Research Council of Canada (Grant No. NNAPJ 424204).