Corrosion Behaviour of Bare and NiCrAlY Coated Alloy 214 in Supercritical Water at 700 °C

In response to increasing energy demand, Generation (Gen) IV International Forum was formed to jointly develop advanced reactors. The objective of this international effort is to design Gen IV reactors with improved sustainability, safety and reliability, economics and proliferation resistance, and physical protection [1]. The supercritical water-cooled reactor (SCWR) is one of several Gen IV reactor technologies being developed. Single-phase supercritical water (SCW) is utilized as the coolant in the Gen IV SCWR enabling a substantial increase in the thermodynamic efficiency, from 33% of the currently used light water-cooled reactors to 45% [2]. In the initial designs of the Canadian SCWR, the coolant is subjected to an operating pressure of 25 MPa, a core inlet temperature of 350 °C, and an outlet temperature of 625 °C [3]. Due to harsh environment of high temperature, high pressure, and radiation, there are challenges in material choice for in-core and out-of-core components.

Austenitic stainless steels (SS) are a class of materials considered for use in SCWRs because of their high resistance against creep and radiation. However, despite their superiority in corrosion resistance, studies have still found they are susceptible to stress corrosion cracking [4]. Also, there is an additional concern in using austenitic SS as they have been shown to be vulnerable to spallation which may eventually lead to component damage [5,6], covering/modifying critical heat transfer surfaces and contaminating different parts of the system. To address these concerns, a study by Otoguro et al. [6] demonstrated that with the addition of nickel, a reduced rate of spallation could be achieved. Also, in a separate study, modifying 310 SS with Zr demonstrated good general corrosion resistance at temperatures up to 600 °C [7].

Ni-based alloys have also been of particular interest for use in the Gen IV SCWRs due to their ability to maintain high strength and toughness at elevated temperatures. Compared to austenitic SS, Ni-based alloys have superior oxidation and corrosion resistance in SCW and exhibit good creep resistance at high temperatures [8]. Zhang et al. [9] studied various Ni-based alloys and reported only slight weight gains after exposure to SCW at 550 °C and 25 MPa for 1000 h. Alloy 214, containing 16 wt % Cr and 4.5 wt % of Al, is one of the Ni-based corrosion resistant alloys developed for high temperature services. In fact, Behnamian et al. [10] showed that alloy 214 is capable of forming a protective oxide layer following the exposure to SCW at 800 °C (25 MPa) for 12 h. After descaling, alloy 214 was found to have the least material loss as compared to other Fe-based (347 H, 316 L, 310 S, 800 H) and Ni-based (Alloy 625, C2000) alloys. It also exhibited great pitting resistance. Similar protective oxide formation was observed in another study by Wang et al. [11].

To further improve the corrosion resistance without jeopardizing mechanical or irradiation properties, corrosion resistant coating may be applied to the surface of the material [12]. NiCrAlY coating, in particular, has been shown to improve corrosion resistance when deposited on Alloy 625, with no signs of intergranular attack or pitting after 500 h exposure to low and high density SCW [13]. Similar results were found by Huang [14].

The current work aims to investigate the corrosion behavior of alloy 214 and NiCrAlY coated 214 by exposing the alloys to SCW at 700 °C/25 MPa for 1000 h. The selection of this temperature was based on the Canadian SCWR design where the coolant temperature is estimated to be 650 °C and the cladding temperature will experience a peak temperature greater than the coolant temperature. The results of SCW testing of bare and coated 214 are reported in this study.

Alloy 214 is a Ni–Cr–Al–Fe alloy (Table 1) designed to provide high temperature oxidation resistance. Its application temperature is rated to 950 °C. In a high temperature oxidizing environment, it has the ability to form an adherent Al₂O₃ protective scale, in addition to Cr₂O₃ [15]. Its creep resistance is more superior to that of 800 H and 310 SS. However, the ductility is much lower, a disadvantage for manufacturing cladding tubes. 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. 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 [16]. However, the Cr content in alloy 214 is lower than in several other candidate austenitic SS (310 and 800 H). As such, coated samples with higher Cr and Al surface content were also included in this study.

Six samples of 10 mm × 15 mm were sheared from a sheet with a 3 mm thickness. Three samples were coated with NiCrAlY (composition shown in Table 1) on all six sides. To hang samples in the test rack, a drill press was used to drill 3.175 mm diameter holes through the samples. Samples (including the coated ones) were grinded using 240, 320, 400, and 600 grit SiC abrasive papers. After grinding, the samples were cleaned in an ultrasonic bath (Branson 2510) using soap and water for 15 min followed by 15 min in acetone. To remove moisture, the samples were then placed in a furnace (Cole-Parmer Stable Temp) at 200 °C for 2 h. Prior to testing, sample dimensions were measured, and samples were weighed both before and after testing in SCW to calculate the weight change per unit surface area.

Exposure test was carried out in SCW autoclave at 700 °C/25 MPa with 150 parts per billion dissolved oxygen in the inlet flow and a pH of 7 (measured at room temperature in the low pressure part of the recirculation loop). The test duration was 1000 h. The autoclave is connected to a recirculation water loop (Fig. 1). The high pressure loop consisted of a high pressure pump, heat exchanger, preheater, and cooler. The flow rate during the exposure was around 7 ml/min resulting in refresh time of the autoclave roughly every 2 h. Specimens were hung on the test rack using electrically insulating ZrO₂ rings.

To deposit the NiCrAlY coating on all six surfaces of the flat samples, the samples were sand blasted to attain a clean and rough surface using no. 46 grit alumina abrasive (ALODUR) for about 3 min. A Mettech Axial III plasma spray system, manufactured by Northwest Mettech Corporation (Vancouver, BC, Canada), was used to coat the samples in this study. The coating powder composition and plasma spray parameters are given in Tables 2 and 3, respectively. This set of coating parameters was developed in another study and yielded a coating structure with less than 5% porosity. After coating application, the coated surfaces were polished up to 600 grit, and the final coating thickness was measured to be 15 μm.

The samples were weighed before and after each test period, using Mettler AT261 scale with a measuring uncertainty of ±0.002%. The mass change per unit area was calculated using coupon dimensions measured before testing. After SCW exposure testing, the as-exposed surface and cross section of the samples were examined using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) to determine the oxide layer thickness and composition. Phase composition of the surface oxides formed was analyzed using X-ray 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).

The weight change per unit surface area for each sample after 1000 h of exposure is shown in Fig. 2. Rather than using an average weight change, all measured weight changes are presented to show individual values. Weight gain was observed on all samples, indicating oxide formation on the surface. This is consistent with SEM observations where an oxide layer was observed. The results also revealed that greater weight gain for NiCrAlY coated 214 (214-4 −5, −6) was consistently attained as compared to that of bare alloy 214 (214-7, −8, −9). This is consistent with our previous studies of NiCrAlY coated alloys [17]; NiCrAlY alloy compositions have been specifically designed to enable protective oxide formation upon exposure to oxidative environment. In Fig. 3, the images of oxidized specimens 214-6 (coated) and 214-9 (uncoated) are presented after 1000 h of exposure to SCW at 700 °C. Both alloys had gray scale covering, but no scale spallation was observed. Although all the coated samples were produced using the same coating process parameters and coating material, the weight gain variations in the coated samples were greater than the bulk uncoated samples. This has been seen in many of our previous tests due to the manual coating sample surface polishing (to 600 grit) where the extent of polishing was determined by visual observation of the surface, leading to differences in actual exposed surface area. The standard procedure of polishing all SCW test samples to 600 grit was based on our previous comprehensive study of surface preparations [18]. In this study, samples made from five different alloys prepared with 600 grit abrasive paper consistently showed the least weight change.

In this section, we focus in on two particular specimens, bare alloy 214-7 and NiCrAlY coated 214-4. After 1000 h exposure to SCW, the entire surface of bare alloy 214-7 was observed to consist of a dark gray surface with light gray patches (Fig. 4). The qualitative EDS results revealed that the dark gray regions contain slightly elevated Al than found in the lighter gray region; all regions analyzed contained moderate amount of O, suggesting oxidation. However, the low oxygen content indicates limited extent of oxidation as most of the signals were from the underlying substrate alloy. There are also signs of surface cracking (in the oxide layer) observed (Figs. 4(a) and 4(b)), similar to those observed on Alloy 625 in previous studies by Selvig et al. [13]. A cross-sectional analysis of the sample revealed an uneven oxide layer, with occasional oxide penetrated to a depth of 4 μm (Fig. 5). The lack of oxide layer in some areas suggests possible scale spallation. Based on the qualitative EDS of the cross section, the presence of Ni, Cr, Al, and O content was observed (Fig. 5). XRD analysis suggests that a mixture of (Cr)₂O₃ and spinel is observed on bare alloy 214 (Fig. 6(a)). Based on a previous study on alloy 214 by Behnamian et al., the spinel is likely Ni(Cr, Al)₂O₄ [10].

The exposed NiCrAlY coated alloy 214-4 sample showed a surface with a mixture of dark and gray regions, indicating different compositions (Fig. 7). There were some areas with small pores, a typical microstructure of plasma sprayed coatings. EDS revealed that the darker regions had higher O content, suggesting greater oxidation formation. Based on both EDS analysis (Fig. 7) and XRD (Fig. 6(b)), the oxides include spinel Ni(AlCr)₂O₄ and Cr-oxide (Cr₂O₃) regions. The gray region, with limited oxygen content (Fig. 7), indicates limited oxidation. There was no surface topography observed to suggest scale spallation in the gray region.

Scanning electron microscopy cross-sectional analysis of NiCrAlY coated alloy 214-4 showed a uniform, thin oxide layer of about 1 μm thick. The EDS results of the surface oxide layer suggest the presence of Ni, Cr, and Al-containing oxides (Fig. 8), consistent with XRD analysis.

The harsh environment of elevated temperatures and pressures in addition to the radiation imposes challenges in finding a material for in-core and out-of-core components in SCWR. In the case of Canadian SCWR, the estimated peak cladding temperature of 850 °C is foreseen [19]. To reduce corrosion and oxidation in the reactor core, the formation of a stable and adherent oxide layer is crucial. This makes materials with oxide formers such as chromium, aluminum, silicon, niobium, and tantalum [19] ideal candidates for in-core components. Alloy 214 is a Ni-based alloy with additions of oxide formers such as chromium, aluminum, and silicon. However, as with other Ni-based alloys such as Alloy 625, it suffers from cracking of the exposed surface (Fig. 4). Our previous study showed that NiCrAlY coating could prevent surface cracking on Alloy 625 [13] and this is further demonstrated in this study.

In terms of oxide formation on the surface after SCW exposure at 700 °C/25 MPa, spinel Ni(Cr, Al)₂O₄ and Ni(Al, Cr)₂O₄ are assumed to have formed based on both coated and uncoated samples, however, more so on the coated samples. Some research reported duplex oxide structure on nickel-based alloys [20,21] after SCW exposure, with Cr-rich oxide, such as Cr₂O₃ near base metal and Ni-rich oxide on the outside. While most SS and solid solution Ni-based alloys owe their corrosion resistance to the formation of Cr₂O₃, alloy 214 containing 4.5 wt  % Al also enables the formation of Al₂O₃ or Al-containing spinel potentially improving oxidation/corrosion resistance further [10]. A previous study of alloy 214 exposed to SCW at 800 °C (25 MPa) for 12 h showed that the oxide layer comprised primarily of Ni, Cr, O, and Al; it was suggested that NiO and Ni(Cr, Al)₂O₄ were present on the outer layer and Cr₂O₃ and Al₂O₃ near the substrate [10]. The XRD results from the same study showed major peaks from the surface which were NiCr₂O₄ and Cr₂O₃, with weak Al₂O₃ peaks. Another study reported similar results for alloy 214 tested between temperatures of 850–1000 °C [22]. However, these studies did not report surface cracking following SCW exposure. In this study, the instability of scale formation on the surface of alloy 214 was observed (Fig. 4). Some areas were covered with nearly no oxide while others had thick oxide formation, up to 4 μm. This may be attributed to localized oxide exfoliation and subsequently a shortage of cation transmission from substrate to surface. Oxide exfoliation can be attributed to stresses developed during SCW exposure, due to oxide growth, and cooling from SCW. When the localized stress exceeds the bonding strength between the oxide scale and substrate, exfoliation occurs. Similarly, if the tensile stress present in the oxide layer is greater than the strength of the oxide, cracking ensues.

With the application of NiCrAlY coating, a uniform oxide layer was able to form and the exposed surface was free of cracking. Similar results were found for NiCrAlY coated Alloy 625 and 310 SS reported by Selvig et al. [13] and Huang [14].

This study investigated the corrosion resistance of bare alloy 214 and NiCrAlY coated 214 under SCW at a temperature of 700 °C and pressure of 25 MPa for 1000 h. SEM/EDS results indicate that oxide scale formed on the surface of all samples. This was consistent with the increase in weight following SCW exposure, attributed to oxide formation. SEM analysis revealed that bare alloy 214-7 sample exposed to SCW formed a uniform oxidized surface but an irregular oxidation depth along the cross section (up to∼4 μm). For the NiCrAlY coated 214-4 sample, different patches were formed on the surface, signaling different oxide formation. Based on the surface EDS, the bare alloy 214-7 had high levels of Ni, Cr, Al, and O while NiCrAlY coated 214-4 had high levels of Ni, Cr, and Al with some O. The oxides formed on bare and coated 214 are likely (Cr, Al)₂O₃ and spinel Ni(Cr, Al)₂O₄. While Ni-based alloys may not be a suitable bulk material for a nuclear reactor core considering their large neutron absorption cross section and vulnerability to irradiation damage, a thin coating layer of nickel-based material on the fluid side may offer fuel cladding materials exceptional corrosion resistance.

Funding to the Canada Gen-IV National Program was provided by Natural Resources Canada through the Office of Energy Research and Development, Atomic Energy of Canada Limited, and Natural Sciences and Engineering Research Council of Canada as well as Academy of Finland project IDEA (Interactive Modelling of Fuel Cladding Degradation Mechanisms) is gratefully acknowledged.

Bal. =

balance

EDS =

energy dispersive spectroscopy

Gen =

generation

SCW =

supercritical water

SCWR =

supercritical water-cooled reactor

SD =

standard deviation

SEM =

scanning electron microscopy

SS =

stainless steel

WD =

working distance, mm

XRD =

X-ray diffraction

θ =

the angle between the incident X-ray beam and the horizontal surface of the sample