Studies of the products from the reactions of co-acids containing concentrated HF and dilute HNO3 with Zircaloy-4

Studies of the products from the reactions of co-acids containing concentrated HF and dilute HNO3 with Zircaloy-4

Journal of Fluorine Chemistry 166 (2014) 28–33 Contents lists available at ScienceDirect Journal of Fluorine Chemistry journal homepage: www.elsevie...

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Journal of Fluorine Chemistry 166 (2014) 28–33

Contents lists available at ScienceDirect

Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor

Studies of the products from the reactions of co-acids containing concentrated HF and dilute HNO3 with Zircaloy-4 H.F. Gu a,b, L.F. Zhang a,b, M.Y. Li a,b, L. Wang c, X.N. Li c, S.Q. Wu a,b, S. Peng c, B. Gao c, G.P. Li a,* a

Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, People’s Republic of China c State Nuclear BaoTi Zirconium Industry Company, Baoji 721003, People’s Republic of China b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 6 May 2014 Received in revised form 5 July 2014 Accepted 7 July 2014 Available online 15 July 2014

The pickling of Zircaloy with HF–HNO3 solution as an important surface treatment has been widely used in the nuclear materials industry. However, a thorough knowledge of the products from the reactions of HF–HNO3 system with Zircaloy still needs to be well established. In the present study, the acid solution containing HF (40 wt.%) and HNO3 (65 wt.%) with four volume ratios, 100:0, 94:6, 89:11 and 80:20, was employed and it reacted with Zircaloy-4 violently with gas releasing at an air pressure of 1 bar, a relative humidity of 30  5% and a temperature of 25 8C. Meanwhile, white products formed within minutes. These products were identified by using X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The usage of the concentrated HF solution leads to the formation of crystalline ZrF4(H2O), while the usage of co-acids containing concentrated HF and dilute HNO3 results in the formation of crystalline ZrF4(H2O) and NH4ZrF5. In addition, with decreasing the volume fraction of HF and increasing that of HNO3 in the co-acid used, the weight fraction of crystalline ZrF4(H2O) decreases while that of crystalline NH4ZrF5 increases in the products obtained. ß 2014 Elsevier B.V. All rights reserved.

Keywords: Zircaloy-4 Hydrofluoric acid and nitric acid solution Aquatetrafluorozirconium Ammonium pentafluorozirconate

1. Introduction The pickling of Zircaloy with the mixed acid, HNO3 (45 vol.%)– HF (5 vol.%)–H2O (50 vol.%), which is composed of dilute HF and concentrated HNO3, is an important and economical surface treatment in the Zircaloy manufacturing process [1]. In the pickling bath, the mixed acid can react with Zircaloy violently to remove surface contaminants, rust and scale. After pickling, rinsing the surface of Zircaloy is performed immediately, in order to remove the remaining acid solution. Whereas after pickling, rinsing and air-drying, fluoride compounds that are invisible to the naked eyes may still form on the surface of Zircaloy, which contributes to the negative influence on further reactor application of Zircaloy [2]. Therefore, subsequent treatments such as abrasive blasting and girt blasting [3] are employed to minimize these unfavorable surface fluoride compounds. However, the studies of the Zircaloy pickle products have been rarely reported since the amount of residual fluoride compounds on the surface of Zircaloy is too slight to be collected and considerable Zircaloy pickle products aimed to be identified may only be found in the

* Corresponding author. Tel.: +86 024 83978619; fax: +86 024 83978619. E-mail address: [email protected] (G.P. Li). http://dx.doi.org/10.1016/j.jfluchem.2014.07.003 0022-1139/ß 2014 Elsevier B.V. All rights reserved.

bottom of a spent HF–HNO3 solution which has been used for a long time [2]. Hence, the pickle products remain laboratory curiosity. In the present study, considerable products are obtained by dropping the acid solutions on the surfaces of Zircaloy-4 samples without rinsing and subsequent air-drying treatment. That is a magnified simulation of the situation, especially the transition in the air from the pickling bath to the rinsing bath [1], when the possibly remaining acid film still reacts with Zircaloy on its surface. However, the acid solution used in this study consists of concentrated HF and dilute HNO3, which is different from the one used in the nuclear materials industry containing dilute HF and concentrated HNO3. Such a study is a significant complement to a thorough knowledge of the products from the reactions of HF– HNO3 system with Zircaloy, which contributes to the understanding of products (amorphous films that are still being studied) obtained from the same procedure by using co-acids containing dilute HF and concentrated HNO3. The present study indicates that the co-acids containing concentrated HF and dilute HNO3 can react with Zircaloy-4 violently producing crystalline ZrF4(H2O) and NH4ZrF5, which are identified by using XRD, FTIR and XPS. This way that the crystalline NH4ZrF5 produces has never been reported. It is noticed that zirconium tetrafluoride (ZrF4) can be obtained from NH4ZrF5

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[4] by the following reaction (Eq. (1)): NH4 ZrF5 ! ZrF4 þ NH3 " þ HF "

(1)

Zirconium tetrafluoride together with zirconium oxide and partially fluorinated zirconium oxide are being used as a catalyst for the fluorination of chloro-hydrocarbon to chloro-fluoro hydrocarbons [4,5]. Zirconium tetrafluoride is also applied to the synthesis of ZrF4-based glasses, which possess somewhat unusual properties, namely a relatively high anionic electrical conductivity [6,7] and a high optical transparency [8] from the near ultraviolet (230 nm) to middle IR (8000 nm). These glasses are used as good candidates for laser windows and optical waveguide materials [9,10]. In the present study, due to the convenience of the production of a relatively simple mixture of crystalline ZrF4(H2O) and NH4ZrF5, it provides a possibility to produce useful ZrF4 by purifying NH4ZrF5 from this kind of mixture and subsequently obtaining ZrF4 from NH4ZrF5. In addition, the most useful test to evaluate the quality of Zircaloy surface is XPS [3] since the amount of residual surface fluoride compounds is so less that it is beyond the abilities of other tests such XRD and FTIR. However, the XPS data of zirconium fluoride still needs to be well established. This study supplies the XPS data of crystalline ZrF4(H2O) and NH4ZrF5, which may be useful for Zircaloy surface evaluation. 2. Experimental A rod of Zircaloy-4 with the composition of Zr–1.5Sn–0.2Fe– 0.1Cr was supplied by State Nuclear BaoTi Zirconium Industry Company. Intermediate annealing at 600 8C for 5 h was performed for this rod. Four cylindrical samples with a thickness of 4 mm and a diameter of 10 mm were cut from this rod by electrical discharge machining. These samples were ground to 2000 grit SiC paper and

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polished. The hydrofluoric acid (HF) and the nitric acid (HNO3) used have the concentration of 40 wt.% and 65 wt.% respectively. By mixing HF and HNO3, four different kinds of acid solutions were prepared with four volume ratios, 100:0, 94:6, 89:11 and 80:20. The treatment of subjecting Zircaloy-4 samples to the acid solutions was carried out in a constant temperature and humidity chamber at an air pressure of 1 bar, a relative humidity of 30  5% and a temperature of 25 8C (1 bar, 30%  5% RH and 25 8C). When four kinds of acid solutions with the same volume of 0.06  0.005 ml were dropped on the surfaces of Zircaloy-4 samples respectively, all the reactions were boiling-like violent with massive gas releasing. Meanwhile, considerable solid products formed within about 5 min on the surfaces of Zircaloy samples. These products were subsequently kept for 24 h for air-drying in the constant temperature and humidity chamber at 1 bar, 30  5% RH and 25 8C. After enough drying, four groups of products were peeled off from the surfaces of Zircaloy-4 samples, ground and mixed into powders, in order to ensure that different kinds of compounds in every group, if they exist, distribute homogeneously for the following tests. X-ray diffraction (XRD) spectra were obtained with a Rigaku D/ max 2500PC X-ray diffractometer (XRD) using the Cu Ka radiation at 40 kV and 300 mA. All the XRD spectra were taken from 58 to 908 for 2u. The chemical structure of the products obtained was identified using BRUKER TENSOR 27 Fourier transformation infrared spectroscopy (FTIR) instrument. All the FTIR spectra were taken at a resolution of 4 cm1 from 4000 to 400 cm1. The total number of scans done for every FTIR test sample was 16, and all the FTIR spectra were smoothed once by OPUS software. An ESCALAB 250 (Thermo VG) X-ray photoelectron spectroscopy (XPS) spectrometer with Al-Ka X-rays (1486.6 eV, width 0.1 eV), operating at 15 kV and 10 mA, was used to obtain XPS spectra. XPSPEAK4.1 software was used for XPS component peak fitting and optimizing. The zirconium 3d (Zr 3d), fluorine 1s (F 1s) and nitrogen 1s (N 1s)

Fig. 1. Photographs of the products obtained by dropping the acid solutions on the surfaces of Zircaloy-4 samples. The volume ratios of HF (40 wt.%) and HNO3 (65 wt.%) in the acid solutions used are as follows: (a) 100:0; (b) 94:6; (c) 89:11; (d) 80:20.

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spectra were fitted to Lorentzian–Gaussian component peaks of equal full-width at half maximum (FWHM). The position and intensity of XPS component peaks were optimized to give the best fit to the experimental spectrum. The binding energies are quoted relative to hydrocarbon C 1s at 284.6 eV. 3. Results and discussion When the concentrated HF solution reacted with the Zircaloy-4 violently, the gas released was colorless. Whereas during the reactions of the co-acids containing concentrated HF and dilute HNO3 with the Zircaloy-4, the gas released was reddish-brown. Fig. 1 shows photographs of the products obtained by dropping four kinds of acid solutions on the surfaces of Zircaloy-4 samples respectively and subsequent air-drying treatment. These products covered the surfaces of Zircaloy-4 samples thoroughly and formed hemisphere thin shells due to the release of gas. The hemisphere shell might break in the center (shown in Fig. 1(d)). These shells look like predominantly snow-like white with some ice-like transparent spots. Fig. 2 shows the XRD spectra of the products obtained. When the acid solution used only contains HF, there are many sharp peaks (marked by solid squares in Fig. 2(a)) in the XRD spectrum with four strongest peaks centered at 2u = 13.78, 22.38, 26.98 and

48.68 respectively. The whole pattern belongs to crystalline ZrF4(H2O) based on XRD analysis. Even with addition of HNO3 in the acid solution used, crystalline ZrF4(H2O) remains to be detected readily (shown in Fig. 2(b)–(d)) because of concentrated HF. However, due to the addition of HNO3 in the co-acid used, several additional new sharp peaks (marked by solid triangles in Fig. 2(b)–(d)) appear with two strongest ones centered at 2u = 13.08 and 26.28 respectively. Their intensity increases with increasing the volume fraction of HNO3 (shown in Fig. 2(b)–(d)). The presence of these new peaks suggests the existence of a new crystalline phase (prior to further analysis, the new peaks are assumed to belong to one kind of crystalline phase, Phase a). In addition, it must be relevant with nitrogen (N) because it is attributed to the addition of HNO3 in the co-acid used. Based on XRD analysis, these new peaks all belong to crystalline NH4ZrF5 with high possibility. Whereas it is insufficient to identify Phase a as crystalline NH4ZrF5 definitely because only a few of its peaks could be detected due to its relatively low weight fraction in the products obtained. Hence, other kinds of tests to identify the products, such as XPS and FTIR, are necessary. It is also noticed that the relative intensity of the strong peaks centered at 2u = 13.08 and 13.78, which are associated with Phase a and crystalline ZrF4(H2O) respectively, is 0%, 8%, 26% and 35% in Fig. 2(a), (b), (c) and (d) respectively, which indicates the increasing trend. Therefore, with

Fig. 2. XRD spectra of the products obtained by dropping the acid solutions on the surfaces of Zircaloy-4 samples. The volume ratios of HF (40 wt.%) and HNO3 (65 wt.%) in the acid solutions used are as follows: (a) 100:0; (b) 94:6; (c) 89:11; (d) 80:20.

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Fig. 3. FTIR spectra of the products obtained by dropping the acid solutions on the surfaces of Zircaloy-4 samples. The volume ratios of HF (40 wt.%) and HNO3 (65 wt.%) in the acid solutions used are as follows: (a) 100:0; (b) 94:6; (c) 89:11; (d) 80:20.

decreasing the volume fraction of HF and increasing that of HNO3 in the co-acid used, the weight fraction of crystalline ZrF4(H2O) decreases while that of Phase a increases in the products obtained. Fig. 3 shows the FTIR spectra of the products obtained. Crystalline ZrF4(H2O) identified by XRD mentioned above has the FTIR spectrum (shown in Fig. 3(a)) with an apparent absorption peak centered at 1400 cm1 (Absorption Peak 1 shown in the deconvoluted FTIR of Fig. 3(a)) in the frequency region of 1500– 1350 cm1. Comparing Fig. 3(b), (c) and (d) with Fig. 3(a), the most

obvious difference is the presence of the additional new absorption peak centered at 1430 cm1 (Absorption Peak 2 shown in the deconvoluted FTIR spectra of Fig. 3(b), (c) and (d)), which is attributed to the presence of Phase a. It is also noticed that the intensity of Absorption Peak 2 becomes stronger with increasing the addition of HNO3 in the co-acid used. Furthermore, the frequency of 1430 cm1 falls into the strong frequency region, 1480–1390 cm1 (strong) [11], of NH4+ ions. In addition, with the volume fraction of HNO3 in the co-acid used up to 20 vol.%, two

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peaks centered at 3330 cm1 and 3230 cm1 (shown in Fig. 3(d)) become apparent to be distinguished readily, which also fall into the strong frequency region of NH4+ ions, 3340–3030 cm1 (strong) [11]. Therefore, the absorption peaks centered 3330, 3230 and 1430 cm1 mentioned above, which fall into the strong frequency region of NH4+ ions, synthetically suggest the existence of NH4+ ions in the products obtained, that is, in Phase a. Fig. 4 shows the XPS spectra of the products obtained. Every N 1s spectrum can only be deconvoluted into one component, namely, 401.7  0.1 eV (shown in Fig. 4(b), (c) and (d)), which is in good agreement with the binding energy, 401.8 eV, of N 1s of NH4+ in NH4NO3 [12] but different from the binding energy, 407.3 eV, of N 1s of NO3 in NH4NO3 [13]. Hence, the presence of this binding energy of N 1s indicates that NH4+ ions exist in the products obtained, that is, in Phase a. The F 1s spectrum of crystalline ZrF4(H2O) can only be deconvoluted into one component, namely, 685.1 eV (shown in Fig. 4(a)). The F 1s spectrum of the mixture of crystalline ZrF4(H2O)

and Phase a can also be deconvoluted into one component, namely, 685.4, 685.2 and 685.1 eV shown in Fig. 4(b), (c) and (d) respectively. These binding energies were in agreement with the binding energy, 685.1 eV, of F 1s in ZrF4(H2O) with very little difference due to the slight inference of F ions in Phase a. The Zr 3d spectrum of crystalline ZrF4(H2O) can be deconvoluted into two components, namely, 185.1 and 187.4 eV (shown in Fig. 4(a)). With HNO3 added in the co-acid used, the Zr 3d spectrum of the products obtained can be deconvoluted into four components, namely, 185.0  0.1 eV, 186.5  0.1 eV, 187.3  0.1 eV and 188.7  0.1 eV, which suggests that there are only two kinds of crystalline phases containing zirconium (Zr). It is in agreement with the assumption that the unknown additional peaks belong to one kind of crystalline phase, Phase a, in the XRD analysis. Hence, the presence of peaks centered at 185.0  0.1 eV and 187.3  0.1 eV is attributed to Zr4+ ions in crystalline ZrF4(H2O), while the presence of peaks centered at 186.5  0.1 eV and 188.7  0.1 eV is due to zirconium ions in Phase

Fig. 4. N 1s, F 1s and Zr 3d XPS spectra of the products obtained by dropping the acid solutions on the surfaces of Zircaloy-4 samples. The volume ratios of HF (40 wt.%) and HNO3 (65 wt.%) in the acid solutions used are as follows: (a) 100:0; (b) 94:6; (c) 89:11; (d) 80:20.

H.F. Gu et al. / Journal of Fluorine Chemistry 166 (2014) 28–33 Table 1 XPS components of N 1s, F 1s and Zr 3d spectra of the products obtained by dropping the acid solutions containing concentrated HF and dilute HNO3 on the surfaces of Zircaloy-4 samples. Components ZrF4(H2O) F 1s Zr 3d3/2 Zr 3d5/2 Phase a (NH4ZrF5a) N 1s F 1s Zr 3d3/2 Zr 3d5/2 a

4. Conclusions The following conclusions were obtained in this work.

Binding energy (eV) 685.1 eV 187.4 eV 185.1 eV 401.7  0.1 eV 685.2  0.2 eV 188.7  0.1 eV 186.5  0.1 eV

With high possibility.

a. The individual components and their corresponding binding energies are summarized in Table 1. FTIR and XPS results synthetically suggest that NH4+, F and zirconium ions exist in Phase a. When combining with the XRD result that the sharp peaks marked by solid triangles in Fig. 2 belong to Phase a with high possibility, it can be concluded that Phase a is crystalline NH4ZrF5 definitely. In addition, with decreasing the volume fraction of HF and increasing that of HNO3 in the co-acid used, the weight fraction of crystalline ZrF4(H2O) decreases while that of crystalline NH4ZrF5 increases in the products obtained. It is also noticed that there are only two kinds of crystalline phases observed and no third phase detected in XRD spectra (shown in Fig. 2), which means that the amount of the compounds containing the alloyed elements like Sn, Cr and Fe in Zircaloy-4, if they exist in the products obtained, is too small to be detected by XRD. Hence, the alloyed elements make little contribution to the products obtained and can be not considered in this work. During the reaction of the concentrated HF solution with Zircaloy-4, colorless gas was released and crystalline ZrF4(H2O) was produced. Due to charge conservation in this reduction– oxidation process, the colorless gas should be H2, which can be further identified. Therefore, the main reaction of HF solution with Zircaloy-4 can be written as Eq. (2): Zr þ 4HF þ H2 O ! ZrF4 ðH2 OÞ þ 2H2 "

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(2)

During the reactions of the co-acids containing concentrated HF and dilute HNO3 with Zircaloy-4, reddish-brown gas was released and crystalline ZrF4(H2O) and NH4ZrF5 were produced. Hence, HNO3 in the concentrated HF surrounding can react with Zircaloy4, producing NH4+ ions. In addition, the reddish gas resulting from the addition of HNO3 in the co-acid used may be NO2, which entails further identification. Therefore, the main reaction of the co-acids containing concentrated HF and dilute HNO3 with Zircaloy-4 may follow Eq. (3): 3Zr þ 5HNO3 þ13HF ! 2ZrF4 ðH2 OÞ þ NH4 ZrF5 þ 4NO2 " þ 5H2 O (3)

A. Predominantly snow-like white products were obtained by dropping the acid solutions, which consist of HF (40 wt.%) and HNO3 (65 wt.%) with four volume ratios, 100:0, 94:6, 89:11 and 80:20, on the surfaces of Zircaloy-4 samples respectively at 1 bar, 30  5% RH and 25 8C. B. During the reaction of the concentrated HF solution with Zircaloy-4, colorless gas was released and crystalline ZrF4(H2O) was produced. C. During the reactions of the co-acids, which consist of HF (40 wt.%) and HNO3 (65 wt.%) with three volume ratios, 94:6, 89:11 and 80:20, with Zircaloy-4, reddish-brown gas was released and crystalline ZrF4(H2O) and NH4ZrF5 were produced. D. With decreasing the volume fraction of HF and increasing that of HNO3 in the co-acid used, the weight fraction of crystalline ZrF4(H2O) decreases while that of crystalline NH4ZrF5 increases in the products obtained. E. The alloyed elements like Sn, Cr and Fe with finite amount in Zircaloy-4 make little contribution to the products obtained from the reactions of the acid solutions, which consist of HF (40 wt.%) and HNO3 (65 wt.%) with four volume ratios, 100:0, 94:6, 89:11 and 80:20, with Zircaloy-4.

Acknowledgement The authors thank Dr. X.P. Song, Dr. Y.L. Hao and Dr. Q. Kang for XRD, FTIR and XPS measurements respectively in the present study. References [1] H.G. Zimmermann, J. Nucl. Mater. 11 (2) (1964) 247–248. [2] J. Rynasiewicz, J. Nucl. Mater. 12 (2) (1964) 153–158. [3] Sang Jin Han, Ju Pil Yoon, In Kyu Kim, Jong Yeol Kahng, Hung Soon Chang, The Second Asian Zirconium Workshop, Baoji, People’s Republic of China, October 15–19, (2013), p. 11. [4] M.M. Makhofane, J.T. Nel, J.L. Havenga, A.S. Afolabi, International Conference on Chemical, Mining and Metallurgical Engineering, Johannesburg, South Africa, November 27–28, (2013), pp. 213–216. [5] T. Tanuma, H. Okamoto, K. Ohnishi, S. Morikawa, T. Suzuki, Appl. Catal. A 359 (1–2) (2009) 158–164. [6] D. Leroy, J. Lucas, M. Poulain, D. Ravaine, Mater. Res. Bull. 13 (11) (1978) 1125–1133. [7] D. Leroy, D. Ravaine, Compt. Rend. Acad. Sci. Paris, Serie C 286 (1978) 413–416. [8] M. Poulain, M. Chanthanasinh, J. Lucas, Mater. Res. Bull. 12 (2) (1977) 151–156. [9] M. Poulain, J. Non-Cryst. Solids 56 (1–3) (1983) 1–14. [10] R.H. Nielsen, J.H. Schlewits, H. Nielsen, Kirk–Othemer Encyclopedia of Chemical Technology (5th Ed.), Vol. 26, Wiley, New Jersey, 2004, pp. 638. [11] C.N.R. Rao, Chemical Applications of Infrared Spectroscopy, Academic Press Inc., New York/London, 1963, pp. 337–340. [12] W.E. Swartz Jr., R.A. Alfonso, J. Electron Spectrosc. Relat. Phenom. 4 (4) (1974) 351–354. [13] K. Burger, F. Tschismarov, H. Ebel, J. Electron Spectrosc. Relat. Phenom. 10 (4) (1977) 461–465.