IMPROVING THE CORROSION RESISTANCE OF SILICON CARBIDE … · IMPROVING THE CORROSION RESISTANCE OF...

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2017 Water Reactor Fuel Performance MeetingSeptember 10 (Sun) ~ 14 (Thu), 2017

Ramada Plaza Jeju • Jeju Island, Korea

IMPROVING THE CORROSION RESISTANCE OF SILICON CARBIDE FOR FUEL IN BWR ENVIRONMENTS

BY USING A METAL COATING

Ryo Ishibashi1, Shigetada Tanabe1, Takao Kondo1, Shinichiro Yamashita2, and Fumihisa Nagase2

1 Hitachi-GE Nuclear Energy, Ltd.: 3-1-1, Saiwai-cho, Hitach-shi, Ibaraki-ken 317-0073, Japan, [email protected]

2 Japan Atomic Energy Agency: 2-4, Shirakata, Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan, [email protected]

ABSTRACT: For improving the corrosion resistance of silicon carbide (SiC) in boiling-water-reactor environments, corrosion-resistant coatings on SiC were evaluated. Due to its hydrogen-generation rate and reaction heat being lower than those of conventional Zircaloy, SiC is expected to be an appropriate material for accident-tolerant fuels. However, there are

still many critical issues with the practical application of SiC fuel cladding and fuel channel boxes, one of which is hydrothermal corrosion. Silicon carbide is chemically stable, but silicon oxide formed by oxidation of SiC dissolves in high-

temperature water. Although the rate of SiC dissolution is very small, the dissolution must be suppressed to comply with regulations for dissolved silica concentration in reactor coolant. In this study, the corrosion behavior of candidate coatings for SiC substrates were evaluated before and after exposure to unirradiated high-purity-water environments with dissolved oxygen concentrations (DOs) of 0.3 and 8.0 mg/l at 561 K. The coatings were formed on chemical-vapor-deposition (CVD)-

SiC-overcoated surfaces of SiC composite base specimens by physical vapor deposition and subsequent heat treatment. During the hydrothermal corrosion test for 3.6 Ms (1 Ms = 106 s), both chromium (Cr) and titanium (Ti) coatings protected the CVD-SiC substrate, while the zirconium coating showed significant disbondment. The Ti coating had the lowest weight

loss, and oxidation was limited to the thinner parts of the coating, while the weight loss of the Cr coating was similar to that of the CVD-SiC substrate, and oxidation penetrated deeper into the coating at the higher DO. Solubility of corrosion

products is estimated to produce the difference between the corrosion behaviors of Ti and Cr coatings. KEYWORDS: Accident tolerant fuel, Fuel cladding, Hydrothermal corrosion, Silicon carbide, Coating.

I. INTRODUCTION In light of the lessons learned in the aftermath of the Fukushima Daiichi nuclear accident, we have been developing

various technologies for light water reactors (LWRs) with an emphasis on safety. To create the technical basis for the practical use of accident-tolerant fuels (ATFs) and other accident-tolerant components in LWRs, a program sponsored and organized by the Ministry of Economy, Trade and Industry (METI) of Japan has been initiated.1 Various alternative materials for fuel cladding and core structures of LWRs have been considered, and SiC-based materials—particularly SiC-fiber-reinforced SiC-matrix ceramic composites (SiC/SiC composites)— are thought to provide outstanding passive safety features in “beyond-design-basis severe-accident scenarios”.2,3 Replacement of conventional Zircaloy fuel cladding with silicon carbide (SiC) fuel cladding (which has a lower hydrogen-generation rate and lower heat of reaction) is expected to significantly decrease the amount of hydrogen generation from fuels during severe accidents.3 Thus, manufacturing and integration technology, such as end-plug technology, is under development to resolve certain issues.4,5,6

However, there are many critical issues with the practical application of SiC fuel cladding, one of which is hydrothermal

corrosion. Hydrothermal corrosion can cause immediate critical issues as well as swelling-induced cracking and fuel-temperature issues.4 In hydrothermal light water reactor (LWR) coolant environments, the silicon in SiC undergoes oxidation

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and produces silica that readily dissolves in water. In fuel-cladding applications, depending on the in-pile recession kinetics of SiC, loss of cladding thickness can potentially expose the fiber-matrix interface and allow the silica concentration in the coolant to build up to the point of saturation, beyond which silica can deposit in the cold regions of the power loop.7 The amount of dissolved silica from SiC in the coolant of a boiling water reactor (BWR) with higher dissolved oxygen activity is likely to be larger than that in the coolant of a pressurized water reactor (PWR) because the dissolved oxygen activity in water can greatly increase SiC recession.7 The dissolved oxygen concentration (DO) in the inlet water of actual BWRs is generally controlled at a low level, e.g., less than 200 g/l for a BWR-normal water chemistry (NWC) environment. However, some oxidants, such as oxygen and hydrogen peroxide, are generated by radiolysis and promote the oxidation of SiC in actual reactors. Silica concentration in the coolant of BWRs should be kept low because silica deposited on turbine vanes might decrease efficiency. Therefore, silica dissolution from SiC fuel cladding in BWRs needs to be suppressed.

Corrosion-resistant coatings and technology for decreasing the recession rate of SiC are under develpoment.8–12 Sintering

additives and the crystalline orientation of SiC affect corrosion behavior in high-temperature water.8,9 Silicon carbide formed by chemical vapor deposition (CVD-SiC) is expected to show a recession rate lower than that of SiC formed in other ways because of its high purity due to the absence of sintering additives. Corrosion-resistant coatings made using industrially available technologies, such as electrolytic deposition, physical vapor deposition (PVD), and vacuum plasma spraying have been examined.10–12 Metals, such as Cr,11,12 and Ti,12 alloys, such as Zircaloy-2,11,12 and metal nitrides, such as CrN11 and TiN,11 have also been coated onto SiC as a surface layer.

The objective of this study was to clarify the feasibility of corrosion-resistant coatings on SiC substrates in BWR coolant

environments by evaluating the corrosion resistance of Cr, Ti, and Zr coatings in oxygenated pure water at 561 K under unirradiated conditions.

II. EXPRIMENTAL PROCEDURES

II.A. Materials and Coating Process

Specimen characteristics are listed in TABLE I. Base specimens were prepared by overcoating SiC composite plates,

SA-TyrannohexTM (40.0×9.8×1.0(mm)), with high-purity CVD-SiC. The CVD-SiC coating was processed to be 130–180 m thick using thermal CVD by ADMAP Inc. Then coatings consisting of Cr, Ti, or Zr were formed on the overcoated surfaces of the base specimens by PVD processing and subsequent heat treatment. Base specimens were rotated in an argon gas atmosphere while the metal coatings were deposited using the unbalanced magnetron sputtering method with a bias voltage of 150 V. The coatings were layered up to 10–20 m thick by deposition from sputtering targets of pure Cr, pure Ti, or Zircaloy-2 after a Cr bonding layer had been applied to the CVD-SiC substrate. The Cr-coated specimens were heat-treated at 1273 K for 1.8 ks (1 ks = 103 s), while the Ti-coated and Zr-coated specimens were heat-treated at 1223 K for 1.8 ks in a vacuum. The conditions of the heat treatment were based on the results of hydrothermal corrosion testing in high-purity water with a DO of 8.0 mg/l at 561 K (Ref. 12). Coating thicknesses were calculated from the specimen-thickness differences measured using a micrometer.

TABLE I. Characteristics of Specimens

Specimen Surface Average Coating Thickness

[m] Average Coating Weight

[mg/cm2] Base CVD-SiC - -

Cr-coated PVD-Cr 20±2 14.2±0.5 Ti-coated PVD-Ti 12±4 7.0±0.3 Zr-coated PVD-Zr 14±2 9.8±0.3

II.B. Hydrothermal Corrosion Test

The conditions of the hydrothermal corrosion tests are listed in TABLE II. The test apparatus consisted of an autoclave

and circulation loop for high-purity water at high temperature. Water was controlled at an electric conductivity less than 0.010 mS/m and predefined DO in a water-chemistry-regulation tank. The water was let into a test chamber after being

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pressurized at 7.5±0.5 MPa and heated to 561±5 K. Two DOs, 0.3 and 8.0 mg/l, were used. The DO of 0.3 mg/l is intended to simulate the BWR-NWC environment in the Halden reactor, and the DO of 8.0 mg/l is intended to simulate a high oxidant concentration produced by water radiolysis. Three specimens for each coating were tested under the same conditions. The specimens were supported at both ends by a jig made of polytetrafluoroethylene. Water was designed to flow in the across-the-width direction of the specimens. The specimens were balanced and observed after time lapses of 0.72, 1.8 and 3.6 Ms (1 Ms = 106 s).

TABLE II. Conditions of Hydrothermal Corrosion Tests

Temperature [K] Pressure [MPa] Flow Rate in Test

Chamber [m/s] Inlet Electric Conductivity at Room Temperature [mS/m]

Inlet Dissolved Oxygen Concentration (DO) [mg/l]

561±5 7.5±0.5 Approximately

2.0×10−4 <0.010

0.3 8.0

II.C. Analysis of Coating

The coatings before and after hydrothermal corrosion testing were evaluated by weighing then by X-ray diffraction

(XRD) analysis to identify their constituent phases, external observation and scanning electron microscopy (SEM). The XRD profiles were measured using the 2/ method using copper K (:1.5418 Ǻ) and analyzed using standard data of International Centre for Diffraction Data (ICDD). Top views and cross-sectional images of the coatings were observed through SEM. The specimens for cross-sectional observation were finished by argon ion milling. Oxidation of the coatings was evaluated by cross-sectional line analysis using SEM energy-dispersive X-ray spectroscopy (EDX).

III. RESULTS

III.A. Hydrothermal Corrosion

The weight differences determined by weighing the metal-coated specimens of CVD-SiC-coated SiC composite before

and after hydrothermal corrosion testing are shown in Fig. 1. These differences are shown in Fig. 2 as percentages of the coating weights (listed in TABLE I). Hydrothermal corrosion tests at a DO of 8.0 mg/l resulted in larger weight differences than did tests at a DO of 0.3 mg/l. The base specimens (CVD-SiC surface) and the Cr-coated specimens lost weight at a DO of 8.0 mg/l, but lost little or no weight at a DO of 0.3 mg/l. The Ti-coated specimens slightly gained weight at both DOs, although they lost weight after 3.6 Ms at a DO of 8.0 mg/l. The Zr-coated specimens lost weight at both DOs. The percentage weight differences of the Zr-coated specimens reached approximately 90% after 3.6 Ms at a DO of 8.0 mg/l (Fig. 2).

The appearances of specimens before and after hydrothermal corrosion testing are shown in Fig. 3. Surface discoloration

and disbondment of coating were observed, and the discoloration of each surface depended on the DO. At a DO of 8.0 mg/l, disbondment occurred at the edges of some specimens and expanded to the main surfaces of the Zr-coated specimens. The disbondment corresponds to the weight losses plotted in Fig. 1. The large weight loss of the Zr-coated specimens at a DO of 8.0 mg/l was apparently caused by disbondment of coating from the main surface of the specimens. An edge of a specimen is the portion at which disbondment of coatings easily occurs because of the stress singularity around the vertex of the joint between coating and substrate. Thus, disbondment behavior of coating on fuel cladding without the edge portion is impossible to directly evaluate from the results in these tests, although adherent strength can be compared among the coatings.

As mentioned above, both the Cr-coated and Ti-coated specimens lost less weight than the CVD-SiC substrate, while the

Zr-coated specimen lost more weight than the CVD-SiC substrate, and this greater weight loss was mainly caused by disbondment of the coating. The adherent strengths of both the Cr-coated and Ti-coated specimens seem, from the disbondment behavior of the coatings, to be greater than that of the Zr-coated specimen. The difference between the Cr and Ti coatings is estimated to be related to the corrosive properties of the materials themselves.

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‐10

‐8

‐6

‐4

‐2

0

2

0 1 2 3 4

Weight difference, w/m

gcm

−2

Immersion time, t/Ms

‐10

‐8

‐6

‐4

‐2

0

2

0 1 2 3 4

Weightdifference, w/m

gcm

−2


Base specimens

Cr‐coated specimens

Ti‐coated specimens

Zr‐coated specimens

(a) DO: 0.3 mg/l (b) DO: 8.0 mg/l

Fig. 1. Weight differences of base and coated specimens after hydrothermal corrosion tests.

‐100

‐80

‐60

‐40

‐20

0

20

0 1 2 3 4

Weight difference as fraction of

coationg weight, w/ｗ

0(%

)


‐100

‐80

‐60

‐40

‐20

0

20

0 1 2 3 4

Weight difference as fraction of

coationg weight, w/ｗ

0(%

)


Cr coating

Ti coating

Zr coating

(a) DO: 0.3 mg/l (b) DO: 8.0 mg/l

Fig. 2. Percentage weight differences of coatings on coated specimens after hydrothermal corrosion tests.

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Dissolved Oxygen Concentration

0.3 mg/l 8.0 mg/l

Immersion Period

0.0 Ms 0.72 Ms 1.8 Ms 3.6 Ms 0.0 Ms 0.72 Ms 1.8 Ms 3.6 Ms

Base specimen (CVD-SiC

surface)

Cr-coated specimen

Ti-coated specimen

Zr-coated specimen

Fig. 3. Appearances of one specimen for each coating before and after hydrothermal corrosion test. 1 mm

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III.B. Analyses of Coatings The results of XRD analyses of specimen surfaces before and after hydrothermal corrosion testing are listed in TABLE

III. The substance with the maximum peak intensity among identified (certain) substances is categorized as major substance, while other substances among those substances are categorized as minor substances. Possible substances, which were not identified with certainty due to week peak intensities, are categorized as uncertain. The XRD analyses of specimens after hydrothermal corrosion tests reveal that no oxide was detected on the CVD-SiC surface (base specimens), and the main corrosion products were CrOOH and Cr2O3 in the Cr-coated specimens, TiO2 in the Ti-coated specimens, and ZrO2 in the Zr-coated specimens. Carbides and intermetallic compounds including Cr were formed by heat-treatment of the Ti-coated and Zr-coated specimens.

Secondary electron images of the coatings before and after hydrothermal corrosion testing at 561 K for 3.6 Ms are shown

in Fig. 4. Regions of corrosion products in cross-section images were identified as higher-oxygen-concentration regions by line analyses using SEM-EDX. For the base specimens (CVD-SiC surface), no significant change was observed after hydrothermal corrosion testing at a DO of 0.3 mg/l, while dissolution of SiC was observed after testing at a DO of 8.0 mg/l. For the Cr-coated specimens, more corrosion products were observed after hydrothermal corrosion testing at a DO of 8.0 mg/l than after testing at a DO of 0.3 mg/l, and oxidation penetrated the coating at a DO of 8.0 mg/l. For the Ti-coated specimens after hydrothermal corrosion testing, oxidation was limited to the coating surface and granular corrosion products were observed. No significant change in oxidation behavior was observed between DOs of 0.3 and 8.0 mg/l. For the Zr-coated specimens after hydrothermal corrosion testing, small cracks at the surface were observed and oxidation penetrated the coating. The penetration of oxidation seems to progress deeper into the coating at a DO of 8.0 mg/l than it does at a DO of 0.3 mg/l. Small cracking of corrosion products is possibly related to deep penetration of oxidation into the coating and the disbondment of the coating.

As mentioned above, although dissolution of CVD-SiC substrate was apparent after testing at a DO of 8.0 mg/l, the Cr

and Ti coatings protected the CVD-SiC substrate. Small cracking of corrosion products was observed in the Zr coating, which is possibly related to the coating’s adhesion strength. No oxide was detected on the CVD-SiC surface after hydrothermal corrosion tests, and the main corrosion products were CrOOH and Cr2O3 in the Cr coating, TiO2 in the Ti coating, and ZrO2 in the Zr coating.

TABLE III. Results of XRD Analyses of Specimen Surfaces Before and After Hydrothermal Corrosion Test

Specimens Hydrothermal Corrosion Test Conditions

Substances Identified

Coating Thickness of Coating

Certain Uncertain

Major Minor

None – Before test 3C-SiC – – – DO of 0.3 mg/l at 561 K for 3.6 Ms 3C-SiC – – – DO of 8.0 mg/l at 561 K for 3.6 Ms 3C-SiC – –

Cr 23 m Before test Cr – – 18 m DO of 0.3 mg/l at 561 K for 3.6 Ms Cr – CrOOH, Cr2O3 21 m DO of 8.0 mg/l at 561 K for 3.6 Ms CrOOH Cr2O3, Cr –

Ti

9 m Before test Ti Cr0.2Ti0.8C, TiCr2 Cr, CrTi4

13 m DO of 0.3 mg/l at 561 K for 3.6 Ms TiCr2 Ti, Cr0.2Ti0.8C, TiCr, TiO2(Anatase)

Cr, TiO2(Rutile)

9 m DO of 8.0 mg/l at 561 K for 3.6 Ms TiCr2 Ti, Cr0.2Ti0.8C, TiCr, TiO2(Anatase)

Cr, TiO2(Rutile)

Zr 13 m Before test Zr Cr2Zr, Cr, ZrC – 15 m DO of 0.3 mg/l at 561 K for 3.6 Ms ZrO2 – Cr 15 m DO of 8.0 mg/l at 561 K for 3.6 Ms ZrO2 – Cr, CrO(OH)

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Coating View Before test DO of 0.3 mg/l DO of 8.0 mg/l

None

Top view

Cross section

Cr

Top view

Cross section

Ti

Top view

Cross section

Fig. 4. Secondary electron images of coatings before and after hydrothermal corrosion test at 561 K for 3.6 Ms.

Resin Resin Resin

Resin Resin Resin

Resin Resin Resin

CVD-SiC CVD-SiC CVD-SiC



Cr coating Cr coating Cr coating

Ti coating Ti coating Ti coating

Corrosion product

Corrosion product Corrosion product

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Coating View Before test DO of 0.3 mg/l DO of 8.0 mg/l

Zr

Top view

Cross section

Fig. 4. Secondary electron images of coatings before and after hydrothermal corrosion test at 561 K for 3.6 Ms (continued). IV. DISCUSSION

The dissolution behavior of various coatings is considered by comparing the solubility of their corrosion products in pure water at 561 K. Dissolution behavior needs to be understood in terms of thermodynamic driving force as well as kinetics, and the solubility of the corrosion products is an important property related to their dissolution’s thermodynamic driving force. According to the results obtained when analyzing coatings after hydrothermal corrosion tests, the related corrosion products are SiO2, Cr2O3, CrOOH, ZrO2, and TiO2. Dissolution reactions of the related corrosion products are shown in the following equations.

SiO2(crystal) + 2H2O = SiO2･2H2O(aqueous) (1)

Cr2O3 + 5H2O = 2HCrO4- + 8H+ + 6e (2)

CrOOH + 2H2O = HCrO4

- + 4H+ + 3e (3) ZrO2 + H2O = HZrO3

- + H+ (4) TiO2 + H2O = HTiO3

- + H+ (5)

As shown in Eq. (1), silica is dissolved by hydration. The solubility of SiO2 in high-purity water at 561 K was estimated by interpolation of experimental data13 to be 9.23±0.67 × 10−3 mol/kg. The solubility of Cr2O3 or CrOOH in high-purity water at 561 K was determined using the corresponding dissolution reaction (Eqs. (2) or (3)), the H2O/O2 reaction is

2H2O = O2＋4H+ + 4e (6)

and Nernst equation is

, (7)

where E is the potential, F is the Faraday constant (96487 C/mol), and G0 is the standard Gibbs free energy difference. Since Henry’s law constant for oxygen gas in water at 561 K is estimated to be 1.55×10−1 Pa/mole-fraction by interpolation

Resin Resin Resin

CVD-SiCCVD-SiC CVD-SiC

Coating

Coating Coating

Corrosion product Corrosion product

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of experimental data,14 the equilibrium oxygen partial pressure is calculated to be 3.58×10−8 Pa at a DO of 0.3 mg/l and 9.53×10−7 Pa at a DO of 8.0 mg/l. Then the E for equilibrium oxygen partial pressure at pH 5.6 is calculated to be 0.398 V (SHE) at a DO of 0.3 mg/l and 0.438 V (SHE) at a DO of 8.0 mg/l. The G0 is 591 kJ/mol for Eq. (2) and 307 kJ/mol for Eq. (3).15 The solubilities of ZrO2 and TiO2 were calculated using the G0 of 138 kJ/mol (Ref. 15) and 188 kJ/mol (Ref. 16) respectively.

The solubility of the related corrosion products in pure water at 561 K at pH 5.6 is shown in Fig. 5, where CS is solubility. Titanium oxide is the least soluble of the related corrosion products. This is one of reasons that the Ti-coated specimens had the lowest weight lost in the hydrothermal corrosion tests. The solubility of Cr2O3 and CrOOH, on the other hand, depends on the potential. These corrosion products are more soluble than SiO2 at high potentials corresponding to DOs of 0.3 and 8.0 mg/l, although at V = 0 (corresponding to a very low DO), they have a solubility between that of SiO2 and that of ZrO2. The high solubility is a driving force of dissolution. Thus, this is one of reasons that the Cr-coating loses weight during hydrothermal corrosion tests. Despite the solubility of Cr corrosion products being much higher than that of SiO2, their dissolution rates are estimated to be low from the Cr-coated specimens’ weight loss during hydrothermal corrosion testing, which is similar to that of the CVD-SiC substrate.

1.0E‐14

1.0E‐12

1.0E‐10

1.0E‐08

1.0E‐06

1.0E‐04

1.0E‐02

1.0E+00

1.0E+02

1.0E+04

1.0E+06

1.0E+08

SiO₂ Cr₂O₃ CrOOH ZrO₂ TiO₂

Solubility in pure water at 561 K, CS/mol kg‐1

Fig. 5. Solubility of related corrosion products in pure water at 561 K and pH 5.6.

V. CONCLUSIONS

To clarify the feasibility of corrosion-resistant coatings on SiC substrates in a BWR coolant environment, the corrosion

resistance of candidate metal coatings (Cr, Ti, and Zr) was evaluated in oxygenated pure water at 561 K under unirradiated conditions. The coatings were formed on CVD-SiC-overcoated surfaces of SiC composite base specimens by PVD processing and subsequent heat treatment.

During the hydrothermal corrosion testing for 3.6 Ms, both Cr and Ti coatings protected the CVD-SiC substrate, which

at 561 K apparently dissolved in high-purity water with a DO of 8.0 mg/l, while the Zr coating showed significant disbondment.

The Ti-coated specimens had the lowest weight loss, and oxidation was limited to the thinner parts of the coating, while

the Cr-coated specimens had a weight loss similar to that of the CVD-SiC substrate, and oxidation penetrated deeper into the

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coating at the higher DO. Solubility of corrosion products is estimated to produce the corrosion-behavior difference between Ti and Cr coatings.

ACKNOWLEDGMENTS

This study is the result of ‘Development of Technical Basis for Introducing Advanced Fuels Contributing to Safety Improvement of Current Light Water Reactors’ carried out under the Project on Development of Technical Basis for Safety Improvement at Nuclear Power Plants by Ministry of Economy, Trade and Industry (METI) of Japan. We would like to express our gratitude to Dr. K. Ishida of Hitachi, Ltd. for discussing corrosion resistance of candidate coatings.

REFERENCES

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