Hydrothermal Corrosion of SiC Coupons Suppressed by Magnetron Sputtered Cr Coatings

SiC-based components are sometimes susceptible to aqueous dissolution in LWR coolant environments. To address this issue, ~10 μm thick Cr coatings was deposited on reaction-bonded silicon carbide (RBSC) plates by magnetron sputtering. Corrosion behavior of Cr-coated SiC and -uncoated SiC coupons was studied by immersing in autoclave (345°C and 16.5 MPa). The weight loss of the Cr coated SiC coupons (3.02% after the 93-days) in the autoclave tests was effectively reduced due to their Cr-coated surfaces, in contrast to the uncoated ones (20.4% after the 78-days). Moreover, microstructural and compositional evolutions were examined by using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. It was revealed that a continuous and dense Cr2O3 layer formed on the surface after the hydrothermal corrosion, which can suppress the in-diffusion of corrosive medium.


Introduction
Since the nuclear accident in Japan in 2011, considerable research has been devoted to the development of accident tolerant fuel (ATF) cladding for light-water reactors (LWRs). Nowadays, SiC/SiC f ceramic matrix composites are considered as a potential material for nuclear fuel claddings in USA, Korea, France, and Japan [1][2][3][4][5][6][7][8][9][10][11]. The silicon carbide (SiC) exhibits excellent oxidation resistance under the accident conditions such as in a typical loss of coolant accident (LOCA) scenario [12][13][14][15][16], where a protective SiO 2 layer can form on the surfaces in the >1000°C steam, preventing breakaway oxidation [17]. However, SiC-based components are prone to hydrothermal corrosion under the supercritical water conditions [7,9,[18][19][20], as the SiO 2 layers on surfaces can dissolve rapidly in the >290°C water [9,19,20]. For example, a SiC-based fuel cladding proceeded a recession rate of 0.1 mg/cm 2 -month and thickness loss of 3.7 mm/year (assuming fully dense cladding), resulting in the dissolution of~55 kg of SiO 2 into the primary coolant annually [8]. The main problem we have to address is how to improve the hydrothermal corrosion resistance of SiC at the normal operating conditions.
Recently, it has been proposed that some aqueous corrosion-resistant coatings can be used to mitigate the dissolution of SiC [1,21,22]. For example, Oak Ridge National Laboratory successfully deposited adherent CrN, TiN, and Cr coatings onto CVD SiC and SiC/SiC composite coupons and rods via cathodic arc [5]. They have previously compared the hydrothermal corrosion resistance of CVD SiC samples with different coatings, all of which were exposed in an autoclave of 288°C water for 400 hours. The Cr coating exhibited the least amount of mass change [6]. Some initial tests show favorable corrosion resistance of the Cr-based coatings. In addition, The Cr coatings are already applied on the Zr-alloy claddings [4,23,24], as their hot water corrosion resistance and high temperature steam oxidation resistance are evaluated as level 5 and level 4 (where 1, worst; …; 5, best) [24], respectively.
This work confirmed that due to the protection of the Cr coatings, the corrosion of reaction-bonded silicon carbide (RBSC) coupons can be effectively suppressed in the supercritical water environment (345°C, 16.5 MPa). Both the Crcoated SiC coupons and uncoated ones were immersed in the autoclave, followed by the comparison on their evolutions of weight loss, composition, and microstructure. Finally, analysis and discussion were performed on the corrosion resistance mechanism of the Cr-coated SiC in the supercritical water.

Experimental Details
2.1. Coating Preparation. The Cr coatings were deposited on reaction-bonded silicon carbide (RBSC) coupons (density = 3:01 g/cm 3 , 20 mm in diameter, and 2 mm in height, Ningbo Yinzhou seal factory, China) by magnetron sputtering. In the chamber, the Cr target (99.9 at. % in purity) was driven by a radio frequency (RF) power supply (Comdel CV-1000) in parallel to another DC power supply. The deposition parameters are given in Table 1. The thickness of the Cr coatings was about 10 μm.

Coating
Characterization. The microstructure of the samples was analyzed by a Bruker D8 Advance X-ray diffraction diffractometer with Cu Kα radiation at a step of θ = 0:02°. The chemical species of the surfaces were probed by Raman spectroscopy on a Renish inVia-reflex system with an excitation laser of 532 nm. The surface and cross-sectional morphologies of the specimens were examined by a Gemini SEM 300 or a FEI Quanta 250™ FEG field emission scanning electron microscope (SEM). The elemental compositions of the specimens were determined by an energy dispersive X-ray spectroscopy (EDS), with an accelerating voltage of 20 keV. The coating-substrate adhesion was evaluated utilizing a scratch tester (CSM Instruments, Switzerland) with a Rockwell C diamond indenter (200 μm in radius) under a load range of 1 − 100 N and a scratch length of 5 mm with a loading rate of 5 N/s. The nanoindentation hardness of the as-deposited coating was tested with MTS Nano Indenter G200 tool with a Berkovich diamond tip (the diameter of 20 nm), and the indentation depth was 1.5 μm. 10 points are randomly tested for each sample, and the average value is finally selected. The corrosion tests were carried out in an autoclave with deionized water at 345°C and 16.5 MPa. Before and after each exposure segment, the samples were weighed on the electronic balance with an accuracy of 0.1 mg for 10 times.

Results and Discussion
3.1. The Microstructure and Mechanical Properties of the As-Deposited Coatings. Figure 1 shows the surface and cross-sectional SEM images of the as-deposited Cr coating on the SiC coupon. The coating presents a compact and columnar structure, being the result of competitive growth [25,26]. The adhesion of the Cr coating on its SiC substrate was evaluated by the scratch method. The morphology of the scratch track is presented in Figure 2. The coating was pressed under the load of 10 N, without any sign of crack or delamination, indicative of good adhesion between the coating and the SiC coupon. The H of the deposited coating obtained by nanoindentation test is~2:022 ± 0:271 GPa.

Corrosion
Behavior of the Uncoated SiC in the Supercritical Water. The uncoated SiC coupons and the Crcoated ones were simultaneously immersed in an autoclave of supercritical water environment. Figure 3 lists the weight loss percentages (W Loss %) of the two kinds of specimens as a function of the immersion times (t). A great weight loss occurred in the uncoated coupon during the immersion processes. For example, the W Loss % values of the uncoated coupon are 8.5%, 14.9%, and 20.4% for t = 30, 48, and 78 days, respectively. Figure 4 displays XRD patterns and Raman spectra of the uncoated SiC specimens after the immersion tests for various durations. Both SiC-6H (PDF#29-1131) and Si (PDF#27-1402) phases were mainly observed in Figure 4(a), meaning that some free Si also exists in the SiC coupon. Two peaks of Si at 28.60 cm −1 and 47.41 cm −1 , etc. are evident in the 0-day specimen. Intensities of the two peaks gradually decrease and finally disappear when the immersion time extends to 123 days. By comparison, there is no change for the diffraction peaks of the SiC before and after the immersion process. This observation can suggest that the SiC component did not change during the process, whereas the free Si was preferentially dissolved in the supercritical water. Moreover, the Raman characteristic peaks of SiC are~788 cm −1 ,~966 cm −1 , and~1500 cm −1 , and the characteristic peak of Si is~520 cm −1 [27] in Figure 4(b). The Raman peak of Si (~520 cm −1 ) decreases gradually with the extension of the immersion time, showing the same change trend as the XRD peaks, which can further confirm that the corrosion of the free Si occurs preferentially. Figure 5 presents surface morphologies and EDS composition maps of the uncoated SiC specimens before and after the immersion in the autoclave for 93 days. Before the  Figures 5(a)-5(c)). After the 93-days immersion, many corrosion pits and microcracks appear on the surface of the SiC coupon ( Figure 5(d)). Combining with Figures 5(e) and 5(f), it can be inferred that these severely corroded area mainly corresponds to the original Si enriched areas, that is, the free Si is preferentially dissolved in the corrosion process. Kim et al. [18] also found that the weight loss of the RBSC ceramics increases as the content of free Si increases. The oxidation reaction in the presence of water vapor is considered to progress as follows [28][29][30]: SiC + 2H 2 O g ð Þ = SiO 2 + CH 4 g ð Þ ΔG = − 327:687 kJ mol : Generally, the SiO 2 layer plays a protective role and decreases reaction rates under most conditions. However, the SiO 2 layer is believed to be unstable in the hightemperature and high-pressure water, and the dissolution of silica into water takes place by [28,[31][32][33]: Hirayama et al. [28] observed that a nonprotective Si (OH) 4 hydrosilica sol film, instead of an SiO 2 layer, formed in the high-temperature water. The hydrothermal reaction rate of the Si phase may be faster than that of the SiC phase, which may lead to preferential corrosion of the free Si. To evaluate reaction (1), Opila [29] took a piece of silicon wafer for oxidation at a temperature of 1200°C in a 90 vol% H 2 O/ O 2 mixture. The SiO 2 scale that generated on the surface remained amorphous and featureless, in contrast to the scale that formed on SiC under identical conditions. The results also proved that the reaction rates of Si and SiC with water are different under the same conditions. Figure 3 reveals that the W Loss % values of the Cr-coated SiC coupons are much lower than those of the uncoated one after the same tests. For example, the W Loss % of the Cr-coated coupon is only 3.02% for t = 93     Figure 6 displays XRD patterns and Raman spectra of the Cr-coated SiC specimens before and after the 93-day  5 Scanning immersion test. In comparison with the as-deposited specimen, the corroded specimen exhibits similar XRD pattern, except for the appearance of some weak peaks that could be assigned to hexagonal Cr 2 O 3 (PDF#38-1479) (Figure 6(a)). The Cr (PDF#06-0694) coating remained the original crystal structure during the 93-day immersion in supercritical water. Furthermore, the Raman spectrum in Figure 6(b) can accurately determine the products on the surface, showing the characteristic peaks of Cr 2 O 3 (~335 cm -1 and~558 cm -1 ) [34] and spinel (~490 cm -1 and 700 cm -1 ) [35,36]. It indicates that there is indeed a Cr 2 O 3 layer formed on the coating surface. As for the appearance of spinel (one kind iron-chromium mixed oxides), one potential cause is the incorporation of the impurity Ni or Fe ions into the oxide during its formation. The other is the precipitation of oxides from the coolant, without regard to the oxidation rate of the coatings, each arising from the impurity metals in the autoclave water [37].

Corrosion Resistance of the Cr-Coated SiC in the Supercritical Water.
The morphologies and elemental maps of the corroded specimen are displayed in Figure 7. Many whiskers form on the surface, and a 1.0-1.5 μm thick layer of Cr-O scale locates underneath the whiskers layer. The composition analysis shows that the atomic ratios of Cr and O account for~35.19% and~58.82% in the Cr-O scale, respectively. The Cr-O scale is covered with some "whiskers," being common features of Cr after oxidation under a moist atmosphere. Based on the XRD pattern and the Raman spectra, the Cr-O scale mainly composes of Cr 2 O 3 . Guillou et al. [38] also found the thin external Cr-O scale is denser without any pores. Hansel et al. [39] proved that local surface catalysis of H 2 O dissociation at low P O2 values causes local acceleration in the oxidation rate and the growth of protruding oxide whiskers. So the corrosion process in the supercritical water can be regarded as a slow oxidation process [40]: Additionally, energy spectrum analysis shows that Cr (~97.27 at %) is the main element below the oxide layer, and almost no O composition was detected, which confirms that the reaction (4) occurred on the surface of the Cr coating. The residual Cr coating still maintains the columnar crystal structure and close contact with the Cr 2 O 3 scale. Our previous work also revealed that very thin Cr 2 O 3 tissue formed on the outmost surface and along the columnar boundaries during the immersion process [41,42], which could effectively prevent the inter-diffusion of corrosive medium towards the underneath SiC substrate. Generally speaking, the columnar grain boundary will act as a fast diffusion channel of corrosion medium. Cr 2 O 3 is preferentially corroded at the columnar grain boundary, and the fast diffusion channel is filled, which may reduce the internal diffusion of oxidant and reduce the corrosion kinetics. At the same time, the layer of dense Cr 2 O 3 formed on the surface also protects the Cr coating below to prevent further corrosion of the Cr coating. Thus, the Cr coatings can substantially suppress the dissolve corrosion of the SiC coupons in the supercritical water, also presenting potential of application on SiC/SiC f ceramic matrix composite cladding.

Conclusions
It is confirmed that free Si is preferentially dissolved in hydrothermal corrosion of reaction-bonded silicon carbide. The Cr coatings were successful magnetron sputtered on the RBSC coupons, and Cr coating showed good adhesion to the SiC substrate. The corrosion resistance of the uncoated SiC coupons and the Cr-coated SiC coupons was simultaneously evaluated in the 345°C and 16.5 MPa autoclave. The weight loss percentage of the coated SiC coupons was only 3.02% after the 93-day immersion in the autoclave, in contrast to that the weight loss percentage of the uncoated SiC coupons reach up to 20.4% after the 78-day immersion. The results showed that the corrosion resistance of SiC can be greatly improved by plating Cr coating. Meanwhile, it demonstrates that the coatings can prevent the SiC coupons from the corrosion of supercritical water, due to a continuous and dense Cr 2 O 3 layer formed on the surface. The work would be helpful for developing new generation ATF coatings of SiC claddings.

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.