Oxidation and electrochemical behaviors of Al2O3

Surface & Coatings Technology 334 (2018) 319–327

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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat

Oxidation and electrochemical behaviors of Al2O3 and ZrO2 coatings on Zircaloy-2 cladding by thermal spraying

T

⁎

Zhengang Duana, , Huilong Yangb, Sho Kanob, Kenta Murakamib,c, Yuhki Satohd, Yoichi Takedae, Hiroaki Abeb a

Department of Quantum Science and Energy Engineering, Graduate School of Engineering, Tohoku University, Sendai, Miyagi 980-8577, Japan Nuclear Professional School, School of Engineering, The University of Tokyo, Tokai, Ibaraki 319-1188, Japan c Institute of GIGAKU, Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan d Institute for Materials Research, Tohoku University, Sendai, Miyagi 980-8577, Japan e Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University, Sendai, Miyagi 980-8579, Japan b

A R T I C L E I N F O

A B S T R A C T

Keywords: Coatings Zirconium alloy Fuel cladding Corrosion resistance EIS

Alumina (Al2O3) and zirconia (ZrO2) coatings on Zircaloy-2 (Zry-2) with a thickness of 100 μm were prepared by thermal spraying. Corrosion behavior of the two coated and bare Zry-2 were evaluated by means of uniform corrosion at 360 °C and 19.2 MPa in distilled water, and by electrochemical measurements in 3.5 wt% NaCl solution. As-sprayed and corroded coatings were characterized by laser microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, nano-hardness and X-ray diffractometry. As-sprayed coated Zry-2 showed higher roughness than the as-received Zry-2. Alumina coating was observed with cracks on the surface and cross section, and it mainly consisted of γ-Al2O3, while some were transformed into γ-AlO(OH). The ZrO2 coating was observed with cracks on the surface and pores in its cross section. Corroded samples, on the other hand, were seriously degraded under corrosion test due to spallation of the ZrO2 coating. Regardless of coating material, the corrosion resistance of coated Zry-2 was mainly dependent on the substrate since the coating failed to block the diffusion of the electrolyte into the substrate.

1. Introduction

temperature steam in severe accidents would be irresoluble only by optimizing the Zr-alloy chemical composition and/or their fabrication processes. The development of substitutes for existing Zr-base alloys and coating technology have become more popular since the Fukushima accident. For alternative materials for Zr-base alloys, the advanced steels [7], Mo-based alloys [8] and silicon carbide (SiC) [9] are considered as the promising materials for fuel cladding in the future, however, the longer time and higher cost is needed to achieve a reasonable return for the alternative materials. On the contrary, the coating technology, as a mature technology in industrial materials to increase the corrosion and wear resistance [10], is much more economic than substitutes for Zr-base alloys without the necessity to modify the base materials, contributing to the possibility for commercial application in the near future [9]. Currently, metallic materials (such as Cr [11], and monolithic FeAlCr [12]) and carbide or nitride compounds (such as TiN [13], Cr3C2 [14], and MAX-phase materials [15]) have been proposed as the candidate materials for coatings and several methods have been applied to fabricate coatings on Zr-base alloys, such as magnetron sputtering,

Cladding tubes are a vital part of nuclear reactors because they not only provide an enclosure of the highly radioactive fuel but also remain in direct contact with the coolant during reactor operation which makes it vulnerable to corrosion [1]. Hence, the materials used for cladding tubes must have the important characteristics as follows: low thermal neutron capture cross-sections, high thermal conductivity, high strength, and high corrosion resistance. Zirconium (Zr) alloy has been employed as fuel cladding tubes because it has a much lower neutron absorption cross section as well as adequate mechanical properties than the other commercially available structural materials. The motivations to achieve acceptable safety margin at higher burnup are driving the evolution in the reliability of Zr-based cladding, thereby contributing to the birth of advanced Zr-based alloys like E635 [2], ZIRLO™ [3], M5® [4], HANA [5] and J-alloys [6], etc., which provide a major step forward in the improvement of corrosion resistance and mechanical performance. However, the well-known inherent demerits of Zr-based alloys such as rapid oxidation and hydrogen production under high-

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Corresponding author. E-mail address: [email protected] (Z. Duan).

https://doi.org/10.1016/j.surfcoat.2017.11.050 Received 5 July 2017; Received in revised form 27 October 2017; Accepted 17 November 2017 Available online 21 November 2017 0257-8972/ © 2017 Elsevier B.V. All rights reserved.


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physical vapor deposition (PVD), chemical vapor deposition (CVD), solgel process and high-velocity oxygen-fuel (HVOF) spraying [16]. Some encouraging results also have been achieved, e.g. higher corrosion resistance, lower hydrogen uptakes, and strong adhesion, although no firm conclusions could be made for the feasibility of their coatings for commercial use in nuclear power plants. Therefore, coating technology is a research hotspot in the field of development of advanced fuel cladding. In this paper, two common materials in the coating industry: alumina and zirconia were coated on Zircaloy-2 (Zry-2) by means of thermal spraying which is an accessible and economic method of coating preparation with the goal to determine the feasibility of single component and fabrication process and provide the reference for the future research. 2. Experimental 2.1. Specimens Commercial Zry-2 tubing was used as substrate and reference (noted as bare) in this study. Outer diameter, wall thickness and length of the tube were Φ11 mm, 0.7 mm and 80 mm, respectively. Thermal sprayed coatings with a desired thickness of 0.1 mm were applied using a Sulzer Metco F4HBS Gun/UNICOAT plasma spraying system. Prior to the deposition, the substrates were sandblasted to improve the adhesion of the coatings, and then preheated up to about 50 °C by spraying. The substrates were cooled in order to keep the temperature lower than 100 °C during thermal spraying process. Argon is the carrier gas, of which flow rate is 45 l/min. Arc current and arc voltage are 600 A and 70 V, respectively. Since the feedstock powders must go through a thin tube to the spray gun, good fluidity of the powders is necessary, meaning that the smaller powder is better. On the other hand, inertia of powders from the spray gun to the substrate is also critical to form a coating. Therefore, powders between 10 μm and 45 μm were selected by a sieve. As shown in Fig. 1, the feedstock powders, Al2O3 and ZrO2 powders, are in form of sphere and grain, respectively, and their mean diameter is 21.7 μm and 23.5 μm, respectively. The coated and bare (non-coating) Zry-2 tubes were cut into specimens with a length of about 15 mm by a high-speed cutter in the presence of water. Edges of specimens were ground with 2000 grit silicon carbide paper and polished with diamond paste (0.3 μm). The polished specimens were then cleaned ultrasonically in acetone.

Fig. 1. The morphologies of the starting powders. (a) Al2O3 and (b) ZrO2.

was used as a reference electrode. All electrochemical measurements were carried out using Salartron 1287/1260 system which was controlled by CorrWare and Zplot Software (Scribner Associates, Inc). Impedance spectra were analyzed using ZSimpwin 3.10 software. The EIS was measured after the stabilization of open circuit potential (OCP) in the frequency range from 10 kHz to 10 mHz at the AC amplitude of 10 mV vs. OCP. The potentiodynamic polarization curves were then recorded with a scan rate of 0.5 mV/s in a potential window from − 0.2 V to 1.2 V vs. OCP.

2.2. Autoclave corrosion Uniform corrosion experiments were performed in an autoclave equipped with a circulating water chemistry controlling system. The temperature in the autoclave was controlled at (360 ± 0.1) °C, and the operating pressure was kept by a high precision back pressure regulator at (19.2 ± 0.2) MPa. Feed water (distilled water) was continuously purified by a mixed-resin bed. High-purity argon (99.99%) was constantly purged into the water tank to achieve < 0.5 ppb dissolved oxygen (DO) in the feed water. Operation time was 21 days in total, with periodic shut down every 7 days for weighing of the samples.

2.4. Characterization The surface and cross-sectional images were obtained using a scanning electron microscopy (SEM) coupled with an energy dispersive X-ray spectroscopy (EDS) (HITACHI S-3400N). The surface topography of the samples was examined by a laser microscopy (VK-X100, Keyence Co.). The values of roughness parameters (Ra and Rq) were achieved according to the following equations.

2.3. Electrochemical measurement

Ra = Electrochemical impedance spectroscopic (EIS) and potentiodynamic polarization measurements were conducted to evaluate the corrosion performance in 3.5% NaCl solution at the temperature of 30 °C by a thermostat water bath. A conventional three-electrode system was used in a five-necked flask with two graphite electrodes (counter references) on the opposite sides of the working electrode to achieve uniform current distribution. The tube samples were used as working electrodes, and the edges and internal surface of the tubes were isolated from exposure to the solution with epoxy resin. A saturated calomel electrode (SCE) with a saturated KCl solution bridge and a Luggin probe

1 n

n

∑i =0

1 Rq = ⎛ ⎝n

n

yi

(1) 1/2

∑i =0 yi2 ⎞ ⎠

(2)

where, the subscript “a” and “q” stand for the arithmetic mean roughness and root mean square height, respectively. yi is a function describing the surface profile analyzed in terms of height (y) at the position (i) of the specimen over the evaluation length (n). X-ray diffractometry (XRD) measurements were performed using an Ultima IV instrument (Rigaku). A 3°/min scanning speed with Cu Kα 320


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Fig. 3. The back-scattered electron (BSE) images of the surface of Zry-2 coated with (a) Al2O3 and (b) ZrO2.

beneficial to enhance the adhesion strength of the coating to the substrate. As observed in Al2O3 coating, cracks propagated into the coating, but no penetration was detected. High density of pores was observed in ZrO2, but not in Al2O3. The molten Al2O3 particles could form a liquid film, and then solidified and shrunk, which produced circumferential stress concentration. The circumferential stress resulted in the formation and radial propagation of cracks. With regard to ZrO2 coating, the semi- and un-melted particles were surrounded by the melted particles, and then the pores could be formed when the melted particles solidified and shrunk. The distribution of the sprayed elements and Zr along the crosssectional direction in the coatings were measured with EDS as shown in Fig. 5. The abrupt interface was observed at the interface indicating that there is no elemental diffusion between the coating and substrate during the spraying process. Therefore, the bond strength to the substrate is ascribed to the mechanical force resulting from the rapid shrinkage and solidification of the molten and semi-molten particles when impacting the much colder surface of the substrate at high speed. Fig. 6 is the cross-sectional profiles of nano-hardness in as-sprayed coatings. It is obvious that all coatings were harder than the substrate. The hardness of Al2O3 coating was highest, although the Al2O3 coating was relatively brittle as cracks were formed and extended to the inner in Figs. 3(a) and 4(a). The hardness of the substrate is consistent with the Ref. [10]. In Al2O3 coating, the maximum hardness was about 16 GPa that appeared in the middle of the coating thickness. ZrO2 coating showed homogeneous and lowest hardness distribution because of uniform elemental profile although microspores also were found (Fig. 4(b)).

Fig. 2. 3D-scanning laser microscopy images of the surfaces of (a) bare Zry-2, and coated Zry-2 with (b) Al2O3 and (c) ZrO2.

radiation and the 2θ angle scan range was set from 20° to 80°. For the coated specimens, the microhardness along the cross-sectional direction in the coating through the substrate was measured using a microhardness tester (DUH-211, Shimadzu) with load and dwell time of 40 mN and 5 s, respectively. 3. Results and discussion 3.1. Specimen characterization The surface morphology of Zry-2 and as-sprayed coatings is shown in Fig. 2. The curvature of the image resulting from the tubular shape was processed to obtain plain images. According to the roughness parameter values, the roughness of the bare and two coatings are in the order: Al2O3 ≈ ZrO2 > bare. Fig. 3 shows the surface images of the as-sprayed specimens. All of as-sprayed coatings exhibited a rough surface. The molten traces of powders and micro-cracks were observed on the surface of as-sprayed coatings. Some semi- and un-melted particles formed bumps on the surface of ZrO2 coatings. As shown in Fig. 4, the thickness of the coatings was about 100 μm and quite uniform. The interface between coating and substrate was rough. This is due to the sandblast which is 321


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(a)

Al

100 80

Intensity

112µm Substrate

60

Coating

Resin

O

40 20

Zr 0

0

50

100 150 Distance (µm)

200

(b) 100

Intensity

80

Zr 108µm

60 Substrate

Coating

Resin

40 20 0 Fig. 4. The BSE images of the cross-section of Zry-2 coated with (a) Al2O3 and (b) ZrO2.

3.2. Corrosion in autoclave

Wt − w0 S

0

50

100 150 Distance (µm)

200

Fig. 5. Cross-sectional profiles of the elements of the as-sprayed coatings. (a) Al2O3 and (b) ZrO2.

ZrO2 coating peeled off from the interface of the coating/substrate, and separated into pieces during the first 7-days immersion. Fig. 7 shows the morphologies of outer (water side) and inner (substrate side) surface of ZrO2 coating and the surface of substrate. There were no obvious changes to the outer surface of ZrO2 coating after immersion. Some smooth areas surrounded by the fracture area were observed on the inner surface of ZrO2 coating. The fracture areas were also found on the surface of substrate, in addition, some grooves were left after the spalling of the coating. The molten powders can get together readily to form layers, and some layers connected with each other, but some did not. The semi- and un-melted particles were wrapped in the layers while some pores also formed among the semi-and un-melted particles and layers. The intersection of layers and the location of semi-and unmelted particles could be the weak point for spalling. After spalling, the layer could be the smooth area, while the fracture of layers could form the fracture area. Accordingly, the existence of semi-and un-melted particles presumably weakened the adhesion strength of the coating. The weight gain after immersion was calculated according to the equation as below:

∆W =

O

18

Nano-hardness (GPa)

16

Coating Substrate

Al2O3 ZrO2

14 12 10 8 6 4 2 0.00

0.05 0.10 0.15 Relative distance (mm)

0.20

Fig. 6. Cross-sectional profiles of nano-hardness of as-sprayed Al2O3 and ZrO2 coatings.

(3) surface and cross-sectional observations as shown in Table 2, on the surface of Al2O3 coating, the molten traces and cracks as observed in Fig. 3(a) became altered in the polygonal particles, meanwhile a loose layer was formed on the surface and propagated inward with immersion time. Accordingly, it is reasonable to deduce that the Al2O3 coating gradually dissolved into water or separated from the surface, resulting

where, ΔW is weigh gain (mg/dm2), Wt the weight of immersed sample. W0 the weight of initial sample, S the surface area of initial sample. As shown in Table 1, with an increase in immersion time, the weight gain of Zry-2 increased, implying a protective oxide film growth. The weight gain of Al2O3 was much higher than that of Zry-2 throughout the immersion but showed a declined tendency. With respect to the changes of 322


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hexagonal structure (α-Zr), and after the immersion, an oxide film with monoclinic structure (m-ZrO2) was formed on the surface. As shown in Fig. 8(b), the starting Al2O3 powders is composed of γ-, α-, κ-, and θ-phase according to the ICDD card n.10-0425, 43-1484, 461215, and 35-0121, respectively. After spraying, most of the other phases disappeared, but only γ-phase and trivial amounts of α- phase was detected in the as-sprayed coating. Consequently, during the coating fabrication, molten Al2O3 powders may transform into either α or γ phases. According to Refs. [17,18], the following mechanism is deduced on the formation of α and γ phase. A higher cooling rate during the solidification of liquid droplets favors the formation of γAl2O3 phase because of its lower interfacial energy. The γ-Al2O3 phase may transform into α-Al2O3 phase at the temperature of 800–1200 °C accompanied by volume shrinkage due to the increase in density from 3.6 to 3.99 g/cm3. Therefore, the differences in the cooling rates resulted in the different distribution of α- and γ-Al2O3 contents. Thus, the hardness variation as shown in Fig. 6 could be ascribed to the distribution of α- and γ-Al2O3 contents as well as the residual stress resulted from the γ-to-α-phase transformation and solidification of molten particles. Boehmite (γ-AlO(OH)) was found after immersion (ICDD card n.21-1307). The formation of γ-AlO(OH) in the range 150–400 °C is thermodynamically plausible, besides, the hydration process involves no drastic changes in the nearest neighbor environment of Al ion owing to the structural similarity between γ-Al2O3 and γ-AlO(OH) [19]. Hence, it is reasonable to assume that γ-AlO(OH) formed from the hydration of γ-Al2O3 following the reaction (4).

γ‐Al2O3 + H2 O → 2γ‐AlO(OH)

(4)

The weight change of Al2O3 coating can be concluded as the hydration reaction, the oxidation of substrate and dissolution or the separation of Al2O3 particles. The weight loss became dominant with immersion time. As shown in Fig. 8(c), the XRD pattern of starting powders coincides with the ICDD card n.13-0307 of m-ZrO2, suggesting that ZrO2 powder is in pure m-phase. In the as-sprayed ZrO2 coating, peaks agreeing with the ICDD card n.87-2105 of tetragonal ZrO2 (t-ZrO2) appeared, and then became weak after 7-day immersion. It was reported that m-to-tphase transformation is reversible in the thermodynamic sense, and the transformation temperature is 1174 ± 6 °C [20]. Accordingly, before impacting the substrate, m-ZrO2 firstly transformed into t-ZrO2, and melted subsequently. Most of t-ZrO2 transformed back to m-ZrO2 when cooled rapidly on the substrate. On the other hand, if exposed to moisture, t-ZrO2 can transform into m-ZrO2 even at low temperature (~ 100 °C) [21], although the phase transformation rate is strongly dependent on the nature of the chemically stabilizing element [22]. During the 7-day immersion, t-ZrO2 transformed into m-ZrO2 accompanied by a volume expansion. Thus, the spalling should be ascribed to the phase transformation as well as the weak adhesion. Fig. 9 presents the nano-hardness in Al2O3 coating after immersion at 360 °C and (19.2 ± 0.2) MPa for the period of 7, 14 and 21 days. Reduction in nano-hardness in the Al2O3 coating was observed with an increase of immersion time. In fact, the hydration and dissolution of Al2O3 loosened the whole coatings, but only the waterside part could be observed to become porous as observed in Table 2 by SEM observation.

Fig. 7. The morphologies of the outer and inner surface of ZrO2 coating and the surface of substrate after 7-day immersion.

Table 1 Weight gain (mg/dm2) of samples after corrosion test. Samples

7 days

14 days

21 days

3.3. Electrochemical behaviors

Bare Zry-2 Al2O3

6.6 34.7

7.2 33.0

7.8 17.4

3.3.1. EIS analysis The impedance data of the bare and the coated Zry-2 are shown in Fig. 10. As shown in Fig. 10(a), the real part of the impedance spectra of ZrO2-coated Zry-2 at the low frequency was much lower than the other samples, which is an indication of the lowest barrier property of ZrO2 coating since the real part of impedance at low frequency is related to the Faradaic process [23]. The lower corrosion resistance of ZrO2 coating may be attributed to the poor condensation of sprayed particles leaving a large area of surface and resulting in higher current. All

in reduction in weight gain at 21 days. The specimens after the immersion were subjected to XRD, and the results are summarized in Fig. 8. The XRD patterns marked as “N days” was achieved from the debris of samples after N-days immersion. As shown in Fig. 8(a), Zr in the as- received bare Zry-2 was identified as 323


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Table 2 Surface and cross-sectional observations in Al2O3 coating after corrosion test. Samples Al2O3

7 days

14 days

21 days

Surface

Cross section

samples exhibit high impedance (> 105 Ω) at the lower frequency while bare Zry-2 possesses the highest impedance shown in Fig. 10(b), which presumably indicate reasonably high density and dense packing in the oxide layer formed in atmosphere. Fitting analysis of the experimental data was performed using equivalent circuit models: “R(CR)” model for bare Zry-2 and “R(CR) (CR)” model for all coated Zry-2, where R and C denote the resistance and ideal capacitor, respectively. In order to account for the effects of the coarse electrode surface, all the capacitance elements were replaced with the constant phase elements (CPE), to reproduce the experiment results of which the formula is [24],

ZCPE =

1 Y0 (jω)n

Therefore, the real surface area of coated samples was higher than the measured one, thereby resulting in the increase of Rs. On the other hand, ceramic coatings greatly weakened the conductivity of working electrode (samples), which may also be a factor for variations of Rs for all samples. Rc is related to the resistance of the solution into the pores of the coatings. The lowest value of Rc for Al2O3 coating should be ascribed to the cracks shown in Fig. 4(a) since it is much easier for the electrolyte to diffuse into the coating along the cracks than pores. Similarly, a coating with higher Y0,c increased the diffusion of chloride ions through the coating. Accordingly, the lowest Rc and highest Y0,c for Al2O3 coating make a fastest diffusion reaction in the 3.5 wt% NaCl solution. nc for all coatings is about 0.5, which is coherent with coatings that do not protect the substrate clearly, and the electrolyte could go through the coating [25]. Additionally, the value of n in formula (4) is related to the fracture dimension (DF) of the coarse surface, and DF increases with increasing roughness. What's more, the relation of n and DF is shown as follows [24]:

(5)

where, ZCPE is the impedance of CPE, Y0 the admittance constant, n the dispersion parameter meaning the deviation of the model from pure R to C circuits (0 < n < 1), j the imaginary unit ( j = −1 ), ω the angular frequency. Thus, in the present case, the equivalent circuits are proposed as shown in Fig. 11 [25]. The Chi-squared was set within 10− 4 for the fitting process. For the proposed equivalent circuits, the corresponding impedance functions are achieved as follows:

Z = Rs +

R ct 1 + R ct ZCPEdl

(6)

Z = Rs +

Rc R ct + 1 + R c ZCPEc 1 + R ct ZCPEdl

(7)

n=

1 DF − 1

(8)

Consequently, the values of nc for Al2O3 and ZrO2 coating were almost identical due to nearly the same roughness (shown in Fig. 2). Rct represents the difficulty of charge transfer going through the interface between the solution and the electrode. A higher Rct stands for a higher resistance in the corrosion reaction. The double layer capacitance Y0,dl represents the wet area under the coating, which is the area in contact with the electrolyte and primarily affected by the surface roughness and surface oxidation [27]. As shown in Table 1, obviously, bare Zry-2 shows the highest Rct, indicating bare Zry-2 has a higher corrosion resistance. Rct is much higher than Rc, therefore, it is reasonable to deduce that the Rct has a direct and dominant influence on the corrosion behaviors of bare and coated Zry-2. In other words, the corrosion resistance of the coated and un-coated Zry-2 is mainly influenced by the Zry-2. Compared with bare Zry-2, a lower value of Y0,dl for Al2O3 coating indicates that less wet area of substrate was exposed to electrolyte. On the contrary, higher values of Y0,dl for ZrO2 coating suggest more wet area of the substrates, which is possibly the result of the pores formed in the interface between coating and substrate (shown in Fig. 4(b)) and poorer adhesion than Al2O3 coating. Similar with nc, ndl is also related with surface roughness of the substrate, which contributed to the decrease of ndl for coated samples since the rougher

Z is the impedance of the equivalent circuit, Rs is the solution resistance, Rct is the charge transfer resistance, CPEdl is the double layer capacitance, while the coating is modeled with the CPEc and resistance Rc. The impedance of CPEdl (ZCPEdl) and CPEc (ZCPEc) could be calculated according to the Eq. (5). Table 3 shows the calculated values of the coated Zry-2. The value of Rs for bare Zry-2 was much lower than these for all coated samples. Rs is related to the surface state (porosity, roughness) [24] and/or changing of the conductivity of the surface [26]. As shown in Figs. 2 and 3, the roughness of the as-sprayed samples was higher than that of the bare sample. In addition, the surface of coated samples was uneven and non-symmetrical, and the micro-cracks and micropores were observed on the surface and cross section of the coated samples. 324


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14

Al2O3 coating

7 days 14 days 21 days

Coating Substrate

Nano-ardness (GPa)

12 10 8 6 4 2 0.00

0.05 0.10 0.15 Relative distance (mm)

0.20

Fig. 9. Nano-hardness distribution of Al2O3 coating in the cross-sectional direction after exposure to high-temperature and pressurized water.

re

(a) 12 Ba

O3 Al 2

10

-Z" (106 ohm)

ZrO2

Ba

8

re

6

O3 Al 2 4

2

ZrO 2 0 0

1

2

3

4

5

Z' (106 ohm)

80

106

70

105

60 50

104 40 10

- F (deg)

|Z | (ohm.cm2)

(b) 107

3

102

Impedence Zry-4 Al2O3 ZrO2

10-2

10-1

30

Phase angle Zry-4 Al2O3

20

ZrO2

100 101 102 Frequency (Hz)

103

104

10

Fig. 10. EIS of bare and coated Zry-2 in 3.5% NaCl at 30 °C: (a) Nyquist plots and (b) Bode diagram.

substrate surface was applied to enhance the adhesion strength compared with that for the bare sample. In addition, for all coated samples, the roughness of the substrate increased the surface area, thus weakening the Rct. Besides, for ZrO2 coating, weaker adhesion, resulted in much lower Rct and higher Y0,dl than Al2O3 coatings.

Fig. 8. XRD patterns of (a) bare Zry-2 and coated Zry-2 with (b) Al2O3 and (c) ZrO2 before and after exposure to 360 °C pressurized water.

325


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3

A l 2 O

0.4

re

Al O 2

-0.2

3

re Ba

E (V/SCE)

0.2

0.0

Ba

2 ZrO

-0.4

ZrO2 -0.6

-0.8 10-11

Fig. 11. Equivalent circuits of (a) bare Zry-2, and (b) coated Zry-2.

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

2

I (A/cm ) 3.3.2. Potentiodynamic polarization test Fig. 12 displays the potentiodynamic polarization curves in the bare and coated Zry-2. The corrosion current density (Icorr), corrosion potential (Ecorr) and breakdown potential (Ebr) of the bare and the coated samples are listed in Table 4. The values of Icorr for all samples follow this order: ZrO2 ≫ Al2O3 > bare. Which is consistent with the order of impedance for all samples at low frequency as shown in Fig. 10(a). Icorr in ZrO2-coated sample was two orders of magnitude higher than those in the other samples, which is ascribed to the much lower impedance at low frequency. Since the Icorr is directly proportional to the uniform corrosion rate of the substrate, the uniform corrosion rate of the bare sample is lower than other samples, in agreement with the lowest weight gain of bare sample in the autoclave experiment. Additionally, it is well-known that a rough surface has high electrochemical activity. Thus, the value of Ecorr for a rough surface is expected to be lower than one in the same material with a smooth surface [28,29]. Accordingly, much lower Ecorr for ZrO2-coated sample is ascribed to the roughness of the substrate which was exposed to the electrolyte because of the weaker adhesion. In terms of Ebr for pitting attack, the bare sample with higher Ebr has a more excellent pitting corrosion resistance than the Al2O3-coated sample, meanwhile the a little higher Ebr for ZrO2-coated sample than bare sample would be ascribed to the formation of passive oxide layer during the larger passive potential region, where current density almost did not change with increasing scanning potential. What's more, a secondary passivation in the curves for Al2O3 -coated sample is the result of repassivation at the coating/substrate interface under the cracks or/and pores [30]. Comparing with the bare sample, two coatings are not competent to enhance the resistance to electrochemical attacks due to the presence of cracks and pores.

Fig. 12. Potentiodynamic polarization curves of the bare and coated samples.

Table 4 Corrosion current density (Icorr), corrosion potential (Ecorr) and breakdown potential (Ebr). Samples

Icorr (10− 10 A/cm2)

Ecorr (V)

Ebr (V)

Bare Al2O3 ZrO2

1.18 2.16 307

−0.246 −0.243 −0.415

0.072 0.003 0.095

Micro-cracks formed on the surface of as sprayed coatings. For the Al2O3 coating, the hoop stress resulting from the shrinkage and solidification of molten particles drove some cracks to propagate radially. A high density of pores was observed in the ZrO2 coating due to the presence of semi-and un-melted particles. Al2O3 coating is mainly composed of γ-Al2O3, some of which was hydrated into γ-AlO(OH) accompanied by a decrease in mean nano-hardness after the immersion. ZrO2 coating peeled off during the first 7-days immersion because of poor adhesion as well as t-to-m-phase transformation stress. The results of electrochemical experiments revealed that regardless of coating material, the corrosion resistance of coated Zry-2 was mainly dependent on the substrate since the coating failed to block the diffusion of the electrolyte into the substrate. The further optimization of spraying parameters, stress relaxation process as well as appropriate thickness are needed to protect the substrate, otherwise the thermal spray coating will be detrimental to the corrosion resistance of the substrate.

Acknowledgement 4. Conclusions

This study is the result of “The development of self-healing intelligence on nuclear fuel cladding” carried out under the Center of World Intelligence Project for Nuclear S&T and Human Resource Development by the Ministry of Education, Culture, Sports, Science and Technology of Japan.

Al2O3 and ZrO2 coating on Zry-2 were fabricated by thermal spay. Their characters in microstructure, hardness, as well as their performance on corrosion were investigated by SEM, XRD, microhardness tester, autoclave corrosion, EIS, and potentiodynamic polarization.

Table 3 Parameters of equivalent electrical circuit for bare and coated Zry-2 immersed in 3.5w%NaCl electrolyte. Samples

Bare Al2O3 ZrO2

Rs (Ω cm2)

45 651 889

Rc (Ω cm2)

– 6067 10,450

Rct (105 Ω cm2)

CPEc Y0,c (10− 6 Ω− 1 cm− 2 s− n)

nc

– 10.6 6.69

– 0.541 0.545

326

721 256 3.68

CPEdl Y0,dl (10− 7 Ω− 1 cm− 2 s− n)

ndl

11.3 9.72 101

0.901 0.793 0.754


Z. Duan et al.

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