Characterization of FeCrAlY Thin Film Deposited by Magnetron Sputtering and Its Corrosion Resistance under High-temperature Water Vapor Environment

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(1.School of Environment and Chemical Engineering, Shenyang Ligong University, Shenyang 110159, China;2.Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China)

ABSTRACT: The work aims to deposit FeCrAlY thin film on Zr alloy substrate and use it as a protection layer to alleviate the corrosion under high-temperature water vapor environment.The FeCrAlY thin film was deposited on Zr and glass substrate via magnetron sputtering method.The XRD and SEM were used to characterize the structure and morphology of FeCrAlY thin film.The XPS was employed to investigate the valence states change before and after corrosion under high-temperature water vapor environment.The corrosion property of Zr and FeCrAlY thin film coated on Zr was tested under high-temperature water vapor environment of 900 ℃.The results showed that FeCrAlY thin film coated on Zr displayed better anticorrosion property compared to Zr.The weight gain of FeCrAlY thin film coated on Zr was nearly 30% less than that of Zr.The FeCrAlY thin film deposited on Zr via magnetron sputtering method plays the role of protection layer under high-temperature water vapor environment.During the process of corrosion, Al2O3 passivation layer separates out with help of Cr because of “third element effect”.The separated Al2O3 layer plays as protection layer and inhibits corrosion.

KEY WORDS: magnetron sputtering; Zr alloy; FeCrAlY thin film; high temperature water vapor environment; Al2O3 passivation layer; corrosion property

1 Introduction

Zr alloy, because of its excellent overall property,such as: good mechanical properties, anti-corrosion property and low thermal-neutron capture cross-section,had been chosen to use as cladding material in the thermal neutron reactors.However, Zr would rapidly oxide under water vapor environment at high temperature when the coolant system was no longer efficient.At this time, the oxide layer of Zr would form, overall mechanical property would degrade, and Zr would react with hydrogen that formed ZrH2, which was brittle product that could cause the Zr cladding crack.The mixture of hydrogen and oxygen in the air was source of explosive accident.In 2011, Fukushima nuclear leakage accident happened by this way[1].Since then, how to avoid hydrogen explosion and development of accident tolerant fuel became significant research topics in the nuclear security field[2].

The long-term way for the cladding materials in the nuclear reactor was to find substituted materials.Now the candidate materials were FeCrAl alloy[3], ODS steel[4-6], Mo[7-8]and SiC/SiCfcomposite[9].Every material had its pros and cons.Before the above candidate materials came into the final application in the nuclear reactor, there were still many tests to be done in the future.The relevant nuclear security research, development and evaluation were also required.Under this background, surface coating on Zr was a short-term solution for cladding materials[10].FeCrAl and Cr were the metal candidates for the coating materials[11-13].FeCrAl, due to the good oxidation resistance exposed to the high temperature vapor, was considered the most promising coating material for the accident tolerant fuel(ATF) program[14].It had been attracted a lot of attention and widely investigated in the recent years.

In this manuscript, FeCrAlY thin film deposited on the Zr surface via magnetron sputtering approach along with different substrate temperature (room temperature,200 ℃, 400 ℃).The corrosion resistance of Zr deposited with FeCrAlY thin film was studied under water vapor environment at high temperature.For the FeCrAlY thin film, due to the “third element effect”, Cr would accelerate Al to diffuse to the surface of FeCrAlY thin film, and Al2O3passivation layer would form with help of Cr under water vapor environment at high temperature.The existence of Y would promote to form stable α-Al2O3.The Al2O3passivation layer was an effective layer to protect Zr alloy from corrosion[15-21].The high temperature water vapor corrosion test demonstrated that FeCrAlY thin film deposited on Zr substrate could efficiently slow the corrosion rate under water vapor environment at high temperature.

2 Experimental details

2.1 Deposition of FeCrAlY thin film

The Fe-18Cr-5Al-0.2Y target was prepared by vacuum melt, solidification.After material processing, the thickness of the Fe-18Cr-5Al-0.2Y target was 1 mm.The chemical composition was as follows: Fe 76.8%, Cr 18%, Al 5%, Y 0.2% (weight percent ratio).The Fe-18Cr-5Al-0.2Y thin film was deposited on Zr alloy substrate by magnetron sputtering method.The composition of Zr alloy was in the following: Sn 1.5%, Fe 0.2%,Cr 0.1%, O 0.013%, Zr 98.187%.The parameters for the Fe-18Cr-5Al-0.2Y thin film deposition were in the following.The voltage was 220 V, and the current was 1 A, and the sputtering time was 45 min.The thin film deposited on the Zr substrate at room temperature, 200 ℃and 400 ℃.

2.2 Characterization of FeCrAlY thin film

The FeCrAlY thin film sample was characterized by Scanning Electron Microscopy (SEM, FEI Inspect F50).The phase composition of the FeCrAlY thin film before and after corrosion was characterized by X-ray diffraction (XRD, RIGAKU D/max-2500PC) with CuKα radiation.The adhesive force between FeCrAlY thin film and Zr substrate was evaluated by measuring the adhesive strength.A Quad Group Sebastian Five tensile tester (Quad Group Inc, Spokane WA) was applied where a 2.7 mm diameter stud with a thin f ilm of epoxy was bonded on FeCrAlY thin film.The valence states and depth profiles of the FeCrAlY thin film sample were estimated by X-ray photoelectron spectroscopy(XPS) with MgKα (1253.6 eV) and AlKα (1486.6 eV)twin anode X-ray using an ESCALAB250 system.The Ar+-ion repetitive bombarded etching was applied to obtain the quantification and valence states of the elements along the film depth.The sputtering rate was 0.1 nm/s for the Ar+-ion gun of 3 kV and 2 μA.The FeCrAlY thin film deposited on the Zr substrate at room temperature was chosen to conduct the high temperature water vapor environment test.The detail water vapor corrosion test was performed at 900 ℃ for 8 hours.The pressure of the water vapor was 0.1 MPa.All the specimens for corrosion test were 15 mm×10 mm×1 mm.

3 Result and discussion

3.1 XRD analysis of FeCrAlY thin film deposited by magnetron sputtering method

Fig.1 displayed the structure of as-deposited FeCrAlY thin film on the Zr and glass substrate at room temperature, 200 ℃ and 400 ℃.The FeCrAlY thin film deposited on the Zr and glass substrate showed uniform cubic structure (Fig.1a, Fig.1b).Because the content of Y in the target was 0.2% (wt), it was too difficult to detect Y in thin film.Compared the as-sputtered FeCrAl and FeCrAlY thin film under the same sputtering condition, the main peaks of FeCrAlY shifted to the low angles (Fig.1c).It illustrated that the crystal cell of FeCrAlY was larger than the FeCrAl, which demonstrated that Y was in the thin film.The XPS result (Fig.1d)also proved above result.The 3d peaks at 156.5 eV and 157.7 eV showed that Y existed in the thin film.The adhesive force of the FeCrAlY thin film was tested,which were 76 MPa, 82 MPa, 84 MPa, respectively.It showed that the higher of the substrate temperature, the higher of the adhesive force for the thin film.

3.2 SEM analysis of FeCrAlY thin film

Fig.2 showed surface view and cross-section images of FeCrAlY thin film deposited on the glass and Zr substrate at room temperature, 200 ℃ and 400 ℃.When the substrate temperature was higher for glass and Zr substrate, the grain grew larger in situ during the deposition process.From the images of surface view and cross-section, small pinholes could be seen (Fig.2k).It illustrated that the higher deposited temperature was, the more pinholes were.The pinholes would become diffusion channel under high temperature and high pressure water vapor environment.The corrosion rate would accelerate.The role FeCrAlY thin film cannot efficiently protect Zr substrate from water vapor corrosion.When the temperature was high to deposit FeCrAlY thin film, the quality of the thin film was not good enough to protect Zr substrate to resist water vapor corrosion.

3.3 XRD analysis of FeCrAlY thin film after corrosion test at 900 ℃ water vapor environment

In the Fig.3, after corrosion test at 900 ℃ water vapor environment, metal oxide compounds, such as Al2O3, Cr2O3, Fe2O3, and FeCr2O4were formed.In the beneficial to alleviate the corrosion rate.According to previous work, the as-formed Al2O3, Cr2O3layer were the XRD pattern, the as-formed Al2O3layer was α-Al2O3.During the process of FeCrAlY thin film under water vapor environment at high temperature, metastable γ-Al2O3was formed because γ-Al2O3could exist at 550 ℃.When the temperature elevated, they transformed to stable α-Al2O3[22-23].As for Zr substrate with out FeCrAlY thin film, ceramic ZrO2was formed during the process of water vapor corrosion.However, when the FeCrAlY thin film was deposited on the Zr substrate,the as-formed metal oxide could retard corrosion rate,and the weight gain for the tested sample was smaller than the Zr sample in the Fig.6.

3.4 XPS analysis of FeCrAlY thin film after corrosion test at 900 ℃ water vapor environment

Fig.4 showed the element valence states of Fe, Cr,Al, respectively.It illustrated that after water vapor test at 900 ℃ for 8 hours, the Fe3+, Cr3+, and Al3+formed.The three metal cations corresponded to the compounds of Fe2O3, Cr2O3, Al2O3and FeCr2O4.Under water vapor environment at high temperature, Fe, Cr, Al would oxide by high temperature water vapor.The stable metal oxide could form.As for FeCrAlY thin film, the asformed metal oxide would protect Zr substrate from water vapor corrosion.Among the as-formed metal oxide,Al2O3played the key role because the Al2O3was a kind of robust metal oxide which possessed high corrosion resistant property.For the FeCrAlY alloy, the existence of Cr would accelerate Al to diffuse to the surface of thin film.This phenomenon was “the third element effect”.It had been demonstrated by the previous research[22].

3.5 Elemental change with depth of FeCrAlY thin film before and after corrosion test at 900 ℃ water vapor environment

Fig.5 displayed the Fe, Cr, Al element content changed with surface depth.Because the content of Y was too small to detect, there was no Y element content changed in the Fig.5.As can be seen from the Fig.5a,the content of Fe was in good accordance with the target; however, the Cr was less than the target, and the Al was higher than the target.When the content of Al was higher than the target, it was beneficial that Al would separate out during the water vapor corrosion and Al2O3was useful layer that protected the Zr substrate from the corrosion.In the Fig.5b, after the water vapor corrosion test, the Al content rapidly increased to 75.62% from 14.63% on the surface of the thin film.The phenomenon could be taken into account that Cr would accelerate Al to diffuse to the surface under water vapor environment at high temperature.

3.6 Corrosion data of FeCrAlY thin film deposited on the Zr substrate and Zr

Fig.6 illustrated the weight gain of Zr and FeCrAlY thin film deposited on the Zr substrate.As can be seen from the Fig.6, The weight gain of Zr was 34 mg/cm2,however the weight gain of FeCrAY was only 25 mg/cm2.The weight gain of FeCrAY was nearly 30% less than that of Zr.It showed that FeCrAlY could use as protection thin film that alleviated water vapor to corrode Zr substrate.

3.7 SEM analysis of FeCrAlY thin film deposited on the Zr substrate after corrosion test at 900 ℃ water vapor environment

Fig.7 showed surface and cross-section images of FeCrAlY thin film deposited on the Zr substrate under water vapor environment at high temperature.From the surface images of Fig.7a and b, metal oxide films were formed during the corrosion process.Some pinholes and cracks could observe.These pinholes and cracks could be the channel for water vapor to diffuse into the Zr substrate and corrode it.From the cross-section images Fig.7c and d, the Zr substrate did not corrode seriously.It demonstrated that FeCrAlY thin film could effectively retard the water vapor diffusion and alleviate the corrosion rate of Zr substrate.In the Fig.7g and h,Cr2O3and Al2O3thin film could observe.These two thin films played the vital roles to resist water vapor corrosion to Zr substrate.The existence of Cr would accelerate Al to separate out and the separated Al would oxide to form Al2O3protection film.Element Y could also promote Al to form stable α-Al2O3when Al content was above 5% in the thin film under water vapor environment at high temperature[21].Cr2O3and Al2O3were efficiently protection layers to retard corrosion rate in 900 ℃ water vapor environment.Therefore, the overall weight gain of FeCrAlY thin film deposited on the Zr substrate slowed down.

4 Conclusions

FeCrAlY thin film was deposited on glass and Zr alloy substrate via magnetron sputtering approach under different temperature.The grain of the as-deposited FeCrAlY thin film was larger increased with the temperature of substrate.FeCr2O4、Fe2O3、Al2O3and Cr2O3were formed in 900 ℃ high temperature water vapor environment after 8 h corrosion test.Passivation layers of Cr2O3and Al2O3were separated out to protect Zr substrate and slowed down the corrosion rate of the FeCrAlY thin film deposited on Zr substrate.FeCrAlY thin film deposited on Zr substrate paved the way to slow the corrosion rate under water vapor environment at high temperature.