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Corrosion investigations on zircaloy-4 and titanium dissolver materials for MOX fuel dissolution in concentrated nitric acid containing fluoride ions
Journal of Nuclear Materials 473 (2016) 157e166
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Corrosion investigations on zircaloy-4 and titanium dissolver materials for MOX fuel dissolution in concentrated nitric acid containing fluoride ions J. Jayaraj, P. Krishnaveni 1, D. Nanda Gopala Krishna, C. Mallika, U. Kamachi Mudali* Corrosion Science and Technology Group, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam 603102, India
h i g h l i g h t s Zr-4 and CP-Ti exhibited high corrosion rate in boiling fluorinated nitric acid. Corrosion rate decreased in fluorinated nitric acid containing ZrO(NO3)2 and Al(NO3)3. High inhibiting efficiency is exhibited by 0.15 M ZrO(NO3)2 when compared to Al(NO3)3. Corrosion rates of CP-Ti were negligible in complexed fluorinated nitric acid. XPS analysis on CP-Ti confirmed the presence of TiO2 and absence of fluoride.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2015 Received in revised form 8 February 2016 Accepted 9 February 2016 Available online 26 February 2016
Aqueous reprocessing of plutonium-rich mixed oxide fuels require fluoride as a dissolution catalyst in boiling nitric acid for an effective dissolution of the spent fuel. High corrosion rates were obtained for the candidate dissolver materials zircaloy-4 (Zr-4) and commercial pure titanium (CP-Ti grade 2) in boiling 11.5 M HNO3 þ 0.05 M NaF. Complexing the fluoride ions either with Al(NO3)3 or ZrO(NO3)2 aided in decreasing the corrosion rates of Zr-4 and CP-Ti. From the obtained corrosion rates it is concluded that CP-Ti is a better dissolver material than Zr-4 for extended service life in boiling 11.5 M HNO3 þ 0.05 M NaF, when complexed with 0.15 M ZrO(NO3)2. XPS analysis confirmed the presence of TiO2 and absence of fluoride on the surface of CP-Ti samples, indicating that effective complexation had occurred in solution leading to passivation of the metal and imparting high corrosion resistance. © 2016 Elsevier B.V. All rights reserved.
Keywords: Zircaloy Titanium Nitric acid Passivation Fluoride Dissolution Complexing ions
1. Introduction Reprocessing of spent mixed carbide (PuC 70%:UC 30%) nuclear fuel discharged from the Fast Breeder Test Reactor (FBTR) at IGCAR, Kalpakkam, India has been successfully accomplished in the CORAL (COmpact Reprocessing of Advanced fuels in Lead shielded cell) facility at IGCAR, employing the Plutonium Uranium Recovery by EXtraction (PUREX) based aqueous process [1]. The PUREX process utilizes boiling 11.5 M nitric acid as the processing medium for the
* Corresponding author. E-mail address: [email protected] (U.K. Mudali). 1 Research scholar, Department of Chemistry, PSGR Krishnammal College for Women, Coimbatore-641004, India. http://dx.doi.org/10.1016/j.jnucmat.2016.02.018 0022-3115/© 2016 Elsevier B.V. All rights reserved.
dissolution of spent fuels. Owing to their high corrosion resistance in boiling nitric acid, zirconium and titanium are being considered for dissolver applications for the spent fuels discharged from fast breeder reactors (FBRs) [2,3]. The spent mixed carbide fuel from FBTR could be dissolved in a dissolver vessel made of commercial pure titanium (CP-Ti grade 2) without any significant corrosion problem of the titanium dissolver. On the other hand, the La Hague reprocessing plant at France uses equipment such as evaporator and dissolver made of zirconium [4]. A mock-up (Zr-4) dissolver vessel has been fabricated at IGCAR in collaboration with Nuclear Fuel Complex (NFC), Hyderabad, India, with the view to reprocessing spent fuels from future FBRs [5,6]. Continuous exposure of this Zr-4 mock-up dissolver vessel for 2500 h in boiling 11.5 M nitric acid containing simulated fission and corrosion products (i.e.,
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simulated dissolver solution without uranium and plutonium), did not show any corrosion attack on its outer surface or along the weld surface of the vessel [7]. It is worth mentioning that dissolvers made up of nitric acid grade and non-sensitized 304 L stainless steels cannot be used in concentrated nitric acid under boiling conditions, as they undergo severe intergranular corrosion in such corrosive media [8,9]. The experience gained during the successful reprocessing of mixed carbide spent fuel of FBTR will be utilized to reprocess the PuO2-UO2 mixed oxide fuel that will be discharged from India's first commercial Prototype Fast Breeder Reactor (PFBR) [10]. For the effective dissolution of PuO2 and PuO2-UO2 mixed oxide, fluoride as a dissolution catalyst is required in nitric acid [11,12]. The required concentration ranges for HNO3 and HF are 10e14 M and 0.001e0.1 M, respectively and temperature up to the boiling point. The rate of dissolution of PuO2 is proportional to 4th power of HNO3 concentration and 1.4th power of HF concentration, indicating the effectiveness of HF in the dissolution of PuO2 [12]. For instance, 20% PuO2-80% UO2 pellet did not dissolve in 10 M HNO3, but it was completely dissolved in 10 M HNO3 containing 0.05 M HF [12]. Nevertheless, it should be realized that the presence of these aggressive fluoride ions might affect the corrosion behavior of dissolvers made of Zr-4 and CP-Ti. The processing medium envisaged for the dissolution of Pu-rich mixed oxide spent fuels from PFBR would be boiling 11.5 M HNO3 þ 0.05 M NaF. Though dissolvers made of Zr-4 and CP-Ti exhibited high corrosion resistance in boiling 11.5 M HNO3 [2,3], their corrosion behavior needs to be evaluated in fluoride containing nitric acid. Thus, the focus of this study is to evaluate the corrosion behavior of the candidate dissolver materials Zr-4 and CP-Ti, when exposed to boiling 11.5 M HNO3 þ 0.05 M NaF. All the tests have been carried out in boiling liquid, vapor, and condensate phases of 11.5 M HNO3 þ 0.05 M NaF. The influence of complex forming agents with the fluoride ions, namely Al(NO3)3 and ZrO(NO3)2, has also been investigated in order to control the corrosion of Zr-4 and CP-Ti in fluorinated nitric acid environment. It is worth mentioning that addition of these complex forming agents might affect the dissolution rate of the spent MOX fuel. However, the main objective of this study is to identify the suitable dissolver material for extended service life in the chosen aggressive medium.
11.5 M HNO3 þ 0.05 M NaF (fluorinated nitric acid). Test solutions containing Al(NO3)3 and ZrO(NO3)2 as complexing agents in 11.5 M HNO3 þ 0.05 M NaF were used to evaluate the influence of the complexing agents in fixing the activity of the fluoride ions. The concentration of these complexing agents were varied as 0.05, 0.10 and 0.15 M. All the chemicals used to prepare the solutions were analytical grade reagents. Double distilled water was used to prepare all the solutions.
2. Experimental
2.4. Surface characterization
2.1. Materials
To understand the corrosion behavior of Zr-4 and CP-Ti samples, scanning electron microscopic (SEM) and X-ray photoelectron spectroscopic (XPS) analyses were carried out. A SNE3000M, desktop model SEM was used to observe the surface morphology of exposed samples in secondary electron mode at an accelerating voltage of 30 kV. XPS measurements were carried out using a SPECS Surface Nano Analysis GmbH, Germany spectrometer. For XPS, Al Ka was used as X-ray source at 1486.7 eV and the anode was operated at voltage of 13 kV with source power level at 300 W. The beam spot size of the X-ray was 1 mm diameter. The peak positions were compared with the standard values reported in literature and data bases for identification of different elements and their oxidation states. After correcting for background for all the peaks using “Shirley” approximation algorithm [15], the XPS data were
Zircaloy-4 (Zr-4) and commercial pure Ti plates (CP-Ti, grade 2) in annealed conditions were used and their compositions are given in Table 1. Coupons for corrosion test were prepared from the plates by cutting using diamond wheel in a flowing coolant. The coupons were subsequently wet ground using SiC paper from 80 to 1200 grit. The dimensions of Zr-4 and CP-Ti coupons were 15 10 5 mm with a surface area of 550 mm2. Prior to corrosion testing, these coupons were cleaned in distilled water and acetone. 2.2. Test environment The corrosion behavior of Zr-4 and CP-Ti were evaluated in
2.3. Corrosion testing The standard three phase corrosion setup described elsewhere [3,13] was used for the corrosion testing. A conical flask containing 300 ml of the test solution was heated to boiling condition (about 120 C). The vapors produced during boiling were condensed using a cold finger and the condensate (approximately 50 ml) was collected in the collecting cup. Through a siphon arrangement, the collected condensate was circulated back to the conical flask leading to achievement of equilibrium in the system. The process of collecting and re-circulating the condensate takes place continuously and the rate of renewal of condensate was fast [13]. The coupons were introduced to the liquid, vapor and condensate phases of these test solutions using a teflon thread. As recommended in ASTM A262 Practice C [14], the corrosion coupons were exposed for a period of 48 h, and experiments were performed for five such periods (i.e., total exposure period of 240 h) using fresh solutions in each period. After each testing period the coupons were removed and cleaned in distilled water. Further, the corrosion tested samples were ultrasonicated in acetone for 5 min. The corrosion coupons were air dried and the change in weight was measured in a micro balance with resolution of 0.0001 g. The corrosion rates from weight loss experiments were calculated using Eq. (1).
Corrosion rate ¼
8:76 104 W DAt
(1)
where, corrosion rate is expressed in mm/yr, W is weight loss in g, D is the density of the sample (Zr-4: 6.5 g/cm3, CP-Ti: 4.5 g/cm3), A is the exposed surface area in cm2 and t is exposed time in h.
Table 1 Elemental composition of Zr-4 and CP-Ti in weight percentage. Material/Identification
Zr
Ti
Sn
Fe
Cr
C
O
N
Zircaloy-4 (Zr-4) Commercial pure titanium (CP-Ti grade 2)
balance e
e balance
1.32 e
0.22 0.053
0.11 e
e 0.026
e 0.025
e 0.006
J. Jayaraj et al. / Journal of Nuclear Materials 473 (2016) 157e166
processed by SpecsLab2 software and analyzed by CasaXPS software. The individual high resolution spectra of all the elements were deconvoluted using a pseudo-voigt function.
3. Results 3.1. Corrosion behavior of Zr-4 and CP-Ti in boiling fluorinated nitric acid The corrosion rates of Zr-4 and CP-Ti coupons exposed to the liquid, vapor and condensate phases of boiling nitric acid containing fluoride ions (i.e. 11.5 M HNO3 þ 0.05 M NaF; also referred as fluorinated nitric acid) are presented in Table 2. After three testing periods of 48 h (i.e., 144 h) in boiling liquid of fluorinated nitric acid, the Zr-4 sample had completely dissolved. The average corrosion rate of Zr-4, calculated for 144 h, in the three different phases of 11.5 M HNO3 þ 0.05 M NaF follows the order: boiling liquid phase > condensate phase > vapor phase. On the other hand, after three testing periods of 48 h, the CP-Ti sample had completely dissolved in the condensate phase rather than in the boiling liquid phase of fluorinated nitric acid. The average corrosion rates of CP-Ti, calculated for 144 h, in the three different phases of 11.5 M HNO3 þ 0.05 M NaF follows the order: condensate phase > vapor phase > boiling liquid phase. Fig. 1a, b and c are the SEM images of the Zr-4 samples exposed to boiling liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF, respectively for the period of 144 h. The SEM images revealed uniform dissolution with the formation of micro pits in all the three phases of fluorinated nitric acid. Though the corrosion rates of Zr-4 were different in liquid, vapor and condensate phases of fluorinated nitric acid, the corrosion morphologies (Fig. 1a, b and c) confirmed that Zr-4 exhibited dissolution behavior in all the three phases. Fig. 2a, b and c are the SEM images of the CPTi samples that were exposed to boiling liquid, vapor and condensate phases for 144 h. The SEM image (Fig. 2a) of CP-Ti, exposed to boiling liquid phase of fluorinated nitric acid showed that the sample surface was smooth. The surface morphology (Fig. 2b) of the CP-Ti exposed to the vapor phase of fluorinated nitric acid exhibited roughened features with cracks along the surface and loosely adhered corrosion products all over the surface. When CP-Ti was exposed to the condensate phase of fluorinated nitric acid, an extremely roughened morphology with dimple like features and cracks was observed, as shown in Fig. 2c. This part of the study has confirmed that CP-Ti and Zr-4 exhibit high corrosion rates of the order of 2.9e78 mm/y, when exposed to boiling nitric acid containing fluoride ions. Such high corrosion rates are not allowed for dissolver application in the spent nuclear fuel reprocessing. However, to control the corrosion attack by fluoride ions, complex forming agents were added to the fluorinated nitric acid medium [16,17], so that the aggressive fluoride ions would not be available for corroding the dissolver vessels.
Table 2 Corrosion rates of Zr-4 and CP-Ti in boiling liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF. Material
Phase
Corrosion rate (mm/y) 48 h
96 h
144 h
Average
Zr-4
Liquid Vapor Condensate Liquid Vapor Condensate
80 6.1 20.1 3.1 4 72
77.5 6.0 18.3 2.9 3.9 70
77.3 5.8 18 2.8 3.8 73
78.3 6.0 18.8 2.9 3.9 71.7
CP-Ti
± ± ± ± ± ±
1.5 0.2 1.1 0.2 0.1 1.5
159
3.2. Effect of complexing agents on the corrosion of Zr-4 and CP-Ti in fluorinated nitric acid The effect of complexing agents Al(NO3)3 and ZrO(NO3)2, in fixing the activity of fluoride ions on the corrosion behavior of Zr-4 and CP-Ti were studied by adding different concentrations such as 0.05, 0.1 and 0.15 M of Al(NO3)3 and ZrO(NO3)2e11.5 M HNO3 þ 0.05 M NaF. With these complexing agents the corrosion rates were determined only in boiling liquid phase of fluorinated nitric acid for 48 h as shown in Table 3. It could be noted from Table 3 that the corrosion rates of Zr-4 and CP-Ti decreased as the concentration of the complexing agents increased. Lowest corrosion rate was obtained for the 0.15 M concentration of both the complexing agents. Subsequently, the corrosion behavior of Zr-4 and CP-Ti were evaluated in liquid, vapor and condensate phases of fluorinated nitric acid with 0.15 M Al(NO3)3 and 0.15 M ZrO(NO3)2 , respectively for five testing periods of 48 h duration and the results are given in Table 4. The corrosion resistance of Zr-4 improved marginally (as shown in Table 4) in boiling liquid phase of fluorinated nitric acid when complexed with 0.15 M Al(NO3)3. However, the corrosion rate of Zr4 in vapor and condensate phases has decreased significantly when complexed with 0.15 M Al(NO3)3. The SEM images presented in Fig. 3a, b and c revealed that the Zr-4 samples that were exposed to boiling liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3 for 240 h exhibited uniform dissolution. The corrosion rate of CP-Ti in the liquid, vapor and condensate phases of fluorinated nitric acid was lower when 0.15 M Al(NO3)3 was added to the test medium compared to those experiments where it is not added. Though a low corrosion rate was obtained for CP-Ti in the liquid phase of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3, the corrosion rates obtained in the vapor and condensate phases were still too high for practical dissolver application. SEM images of CP-Ti exposed to liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3 are shown in Fig. 5a, b and c respectively. Surface morphology of the CP-Ti coupon exposed to liquid phase of fluorinated nitric acid complexed with Al(NO3)3 was smooth (Fig. 5a) when compared to the surface morphologies (Fig. 5b and c) of the same samples exposed to vapor and condensate phases. In the vapor and condensate phases of fluorinated nitric acid with 0.15 M Al(NO3)3, a localized attack like feature (Fig. 5b) and roughened morphology with porous structure (Fig. 5c), respectively was observed on CP-Ti. When compared to 0.15 M Al(NO3)3 complexed solution, complexation with 0.15 M ZrO(NO3)2, decreased the corrosion rates for Zr-4 in liquid, vapor and condensate phases of fluorinated nitric acid (as shown in Table 4). However, SEM investigations carried out on the Zr-4 samples that were exposed to boiling liquid (Fig. 4a), vapor (Fig. 4b) and condensate phase (Fig. 4c) of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 for 240 h revealed uniform dissolution morphology under all conditions. Interestingly, lowest corrosion rates were obtained for CP-Ti in boiling liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. In the boiling liquid phase of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2, instead of weight loss a small weight gain was observed for CP-Ti and so mentioned as negligible corrosion rate in Table 4. SEM images of CP-Ti exposed to liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 are shown in Fig. 6a, b and c respectively. An un-attacked surface morphology (Fig. 6a) was observed over CP-Ti when exposed to liquid phase of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. Comparing with 0.15 M Al(NO3)3, a slightly attacked surface (Fig. 6b and c) was observed for the samples exposed to vapor and condensate phases of fluorinated nitric acid complexed with 0.15 M ZrO(NO3)2.
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Fig. 1. SEM images of Zr-4 samples exposed to 11.5 M HNO3 þ 0.05 M NaF for 144 h in (a) boiling liquid phase; (b) vapor phase; and (c) condensate phase.
Fig. 2. SEM images of CP-Ti samples exposed to 11.5 M HNO3 þ 0.05 M NaF for 144 h in (a) boiling liquid phase; (b) vapor phase; and (c) condensate phase.
Table 3 Corrosion rates of Zr-4 and CP-Ti in boiling liquid phase of 11.5 M HNO3 þ 0.05 M NaF þ x M Al(NO3)3 and 11.5 M HNO3 þ 0.05 M NaF þ x M ZrO(NO3)2, (where, x ¼ 0.05, 0.1 and 0.15 M of complexing agents). Complexing agent
Concentration of complexing agents (M)
Corrosion rate (mm/y) Zr-4
CP-Ti
Al(NO3)3
0.05 0.1 0.15 0.05 0.1 0.15
76.2 68.6 50.8 44.5 5.1 0.56
1 0.4 0.15 0.5 0.03 Negligible
ZrO(NO3)2
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161
Table 4 Corrosion rates of Zr-4 and CP-Ti in boiling liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3 and 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. Environment
11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3
Material
Zr-4
CP-Ti
11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2
Zr4
CP-Ti
a
Phase
Liquid Vapor Condensate Liquid Vapor Condensate Liquid Vapor Condensate Liquid Vapor Condensate
Corrosion rate (mm/y) 48 h
96 h
144 h
196 h
240 h
Average
50.8 1.3 1.7 0.15 0.89 2.57 0.56 0.15 0.15 a 0.03 0.1
51.1 1.4 1.8 0.18 0.97 2.69 0.58 0.18 0.2 a 0.03 0.1
50.6 1.1 1.5 0.13 0.81 2.44 0.56 0.2 0.18 a 0.03 0.1
51.1 1.4 1.8 0.18 0.97 2.69 0.53 0.15 0.2 a 0.03 0.1
50.6 1.1 1.5 0.13 0.81 2.44 0.56 0.18 0.15 a 0.03 0.1
50.8 ± 0.25 1.3 ± 0.15 1.66 ± 0.15 0.2 ± 0.03 0.89 ± 0.08 2.57 ± 0.13 0.56 ± 0.02 0.17 ± 0.02 0.18 ± 0.03 a 0.03 0.1
Negligible.
Fig. 3. SEM images of Zr-4 samples exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3 for 240 h in (a) boiling liquid phase; (b) vapor phase; and (c) condensate phase.
XPS investigations were carried out on the Zr-4 and CP-Ti samples exposed to the effective complex forming condition of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. The XPS high resolution spectra of Zr 3d recorded for the surface of Zr-4 samples exposed to the liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 are shown in Fig. 7a, b and c, respectively. The F 1s spectra (Fig. 7d) shows a peak at 686.8 eV was observed for the Zr-4 samples. In the spectra of Zr 3d (Fig. 7a, b and c), the peaks of Zr 3d5/2 and Zr 3d3/2 observed at the binding energies 183.4 and 185.8 eV, respectively corresponded to ZrO2 [18]. Apart from these peaks, a low intensity peak at 189.2 eV was observed only for the Zr-4 samples exposed to vapor and condensate conditions, as shown in Fig. 7b and c. From the de-convoluted spectra of Zr 3d, the presence of fluoride could be realized as zirconium oxy fluoride (ZrOF2) corresponding to the peaks at 184.6 and 187 eV for the Zr-4 sample exposed to liquid phase [19]. On the other hand, for the Zr-4 samples exposed to vapor and condensate phases, fluoride was realized as zirconium fluoride (ZrF4) corresponding to the peaks at 186.8 and 189.3 eV respectively [20]. The XPS spectra of Ti 2p observed on the CP-Ti sample's surface exposed
to liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 are shown in Fig. 8a, b and c, respectively. In the spectra of Ti 2p, the peaks of Ti 2p3/2 and Ti 2p1/2 observed at the binding energies 459.6 and 465.4 eV, respectively corresponded to TiO2 [21]. It is worth mentioning that peak corresponding to F 1s spectra was not observed for the CP-Ti sample's exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. 4. Discussion The high corrosion resistance of Zr-4 and CP-Ti samples in boiling 11.5 M HNO3 is due to the formation of respective passive films of ZrO2 and TiO2 [3,7]. However, it is evident from this study that addition of 0.05 M NaF to 11.5 M HNO3 changed the passive behavior of Zr-4 and CP-Ti to dissolution behavior. In general, metal ions hydrolyses in aqueous solution to form their corresponding metal hydroxides and oxides. In the present case, since the solution is more acidic in nature, both Zr-4 and CP-Ti samples would not form hydroxides [22]. The two major competing reactions that can occur in fluorinated nitric acid medium are formation of metal
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Fig. 4. SEM images of Zr-4 samples exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 for 240 h in (a) boiling liquid phase;(b) vapor phase; and (c) condensate phase.
Fig. 5. SEM images of CP-Ti samples exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3 for 240 h in (a) boiling liquid phase; (b) vapor phase; and (c) condensate phase.
fluorides and metal oxides. Zr-4 and CP-Ti may undergo dissolution in the presence of fluoride ions by forming their respective metal fluorides [23,24] as given in equations (2) and (3). Further, formation of stable metal oxides of Zr-4 and CP-Ti as given by equations (4) and (5) is also possible.
¼ 915
(3)
. Zr þ 2NaF þ 2HNO3 4ZrO2 þ 2NaNO2 þ 2HF ; DG0r kJ: mol1
Zr þ 6NaF þ 6HNO3 4ZrF4 þ 4NaNO3 þ 2NaNO2 þ 2HF . þ 2H2 O; DG0r kJ: mol1 ¼ 1162
Ti þ 6NaF þ 6HNO3 4TiF4 þ 4NaNO3 þ 2NaNO2 þ 2HF . þ 2H2 O; DG0r kJ: mol1
¼ 940 (2)
(4)
J. Jayaraj et al. / Journal of Nuclear Materials 473 (2016) 157e166
163
Fig. 6. SEM images of CP-Ti samples that were exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 for 240 h in (a) boiling liquid phase; (b) vapor phase; and (c) condensate phase.
Ti þ 2NaF þ 2HNO3 4TiO2 þ 2NaNO2 þ 2HF;
DG0r
. kJ: mol1
¼ 787 (5) As fluoride ion is highly electronegative in nature, it has a strong tendency even to attack the metal oxides. Hence, ZrO2 and TiO2 formed as per equations (4) and (5) can further convert to their corresponding metal fluorides instantaneously according to equations (6) and (7).
ZrO2 þ 4NaF þ 4HNO3 4ZrF4 þ 4NaNO3 . þ 2H2 O; DG0r kJ: mol1 ¼ 223
(6)
TiO2 þ 4NaF þ 4HNO3 4TiF4 þ 4NaNO3 . þ 2H2 O; DG0r kJ: mol1 ¼ 128
(7)
The values of Gibbs energy were calculated [25] at the boiling liquid phase temperature of 120 C. Values of Gibbs energy change in reactions 2, 3, 6 and 7 indicate that the formation of ZrF4 is more favored than the formation TiF4. This implies that severe corrosion of Zr-4 will occur in comparison to the CP-Ti sample (Table 2) in the boiling liquid phase of fluorinated nitric acid as the feasibility for the simultaneous conversion of ZrO2 to ZrF4 is more than that for TiO2 to TiF4. Owing to the formation of ZrF4, the Zr-4 sample exhibited dissolution behavior in all the three phases of fluorinated nitric acid as shown in Fig. 1a, b and c. The CP-Ti sample exposed to the boiling liquid phase of fluorinated nitric acid showed a smooth surface (Fig. 2a) due to the simultaneous process of removal of metal and formation of passive layer as given in equations (3) and (5). This process is similar to that of chemical polishing [26], wherein the metal undergoes uniform dissolution in the electrolyte
and a polished like smooth surface was obtained. The concentrations of nitric acid and fluoride ions in the vapor and condensate phases are expected to be lower than that of the liquid phase [13,16]. The concentration of condensate phase is reported to be about 7.2 M [13] for the liquid phase of 12 M nitric acid. As expected, lower corrosion rates were obtained for Zr-4 in the vapor and condensate phases of fluorinated nitric acid in comparison to its liquid phase. In contrast, higher corrosion rates were obtained for CP-Ti samples exposed to vapor and condensate phases of fluorinated nitric acid. Similar trend of higher corrosion rates in vapor and condensate phases than that of the liquid phase was reported for titanium when exposed to boiling 11.5 M nitric acid [2,3]. The corrosion resistance of titanium in the liquid and condensate phases of nitric acid was influenced by the concentration of its own corrosion product (Ti4þ) [13]. As the concentration of Ti4þ was high in the liquid phase, the stable TiO2 film formed on the surface of CP-Ti had resisted the corrosion attack and thus low corrosion rate was observed. On the other hand, Furuya et al. [13] reported that the formation of semi-protective Ti2O3 film was responsible for the high corrosion rate in the condensate phase of nitric acid. However, in this study, apart from the formation of semi-protective passive layer, the formation of TiF4 would have significantly contributed to the high corrosion rates in the vapor and condensate phases of fluorinated nitric acid. Ti2O3 can also get converted to TiF4 as per equation (8) and enhances the corrosion rate of CP-Ti in the condensate phase of fluorinated nitric acid.
Ti2 O3 þ 8NaF þ 8HNO3 42TiF4 þ 8NaNO3 þ 3H2 O . þ H2 [; DG0r kJ: mol1 ¼ 160
(8)
The corroded morphologies of CP-Ti exposed to the vapor (Fig. 2b) and condensate phases (Fig. 2c) of fluorinated nitric acid could be the result of the simultaneous formation and dissolution of an un-protective oxide layer. When Al(NO3)3 or ZrO(NO3)2 is added to fluorinated nitric acid, aluminum fluoride complexes or zirconium fluoride complexes will
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Fig. 7. XPS spectra of Zr 3d and F 1s for Zr-4 sample surface exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 for 240 h: (a) Zr 3d spectra, de-convoluted peaks of ZrO2 and ZrOF2 in boiling liquid phase; (b and c) Zr 3d spectra, de-convoluted peaks of ZrO2 and ZrF4 in vapor and condensate phases; and (d) F 1s spectra in boiling liquid, vapor and condensate phases.
be formed in the solution [16,17,27,28]. As the concentration of the complexing agents increased, the corrosion rates of Zr-4 and CP-Ti samples decreased, as shown in Table 3. The concentration of fluoride ions will decrease in the solution due to the complex formation, leading to a reduction in the corrosion rate of metallic samples [16,17]. It should be noted that for all the concentrations of the complexing agents, the corrosion rates (Table 3) were lower for ZrO(NO3)2 than that for Al(NO3)3. Several fluoro complexes of Al and Zr could be formed and they depend on the pH, fluoride content and ionic strength of the solution [27,28]. Assuming that only neutral fluoro compounds such as AlF3 and ZrF4 were formed in the respective solution, the former compound would require less fluoride ions than the later. The effectiveness of the complexing agents in reducing the corrosion rate depends on the stability constants of these fluoro complexes [16,17]. The stability constant of fluoro complex formed by Zr4þ is as high as about 1010, while that of Al3þ is 107 [17] thus, complexing with ZrO(NO3)2 is more effective than Al(NO3)3 for the corrosion control of Zr-4 and CP-Ti in fluorinated nitric acid, as evidenced in the present study. Table 4 reveals that Zr-4 sample had undergone less corrosion in both vapor and condensate phases of Al(NO3)3 complexed fluorinated nitric acid than that of its boiling liquid phase. Owing to the high corrosion of Zr-4 in the boiling liquid phase, the concentration of metal ions in the solution would gradually increase, resulting in the elevation of boiling point of the solution and reducing the
amount of vapor. Irrespective of the relatively lower corrosion rates for Zr-4 in fluorinated nitric acid with both complexing agents (Table 4) or severe corrosion rate of Zr-4 in fluorinated nitric acid alone (Table 2), the corroded morphologies obtained in all the three phases were similar, as shown in Figs. 1, 3 and 4. This suggests that the presence of even a very low concentration of fluoride ion left over after complexation could have resulted in the dissolution of Zr4. In the case of CP-Ti, the absence of visible surface attack and low corrosion rate indicated that 0.15 M Al(NO3)3 could effectively complex with fluoride to mitigate the corrosion in the boiling liquid phase of fluorinated nitric acid. Though localized corrosion and porous morphologies were obtained for the CP-Ti sample exposed to vapor and condensate phases of 0.15 M Al(NO3)3 complexed solution, the corrosion rates determined were less than that uncomplexed solution. In spite of lowering the corrosion rate of CPTi in fluorinated nitric acid, addition of excessive Al(NO3)3, does have an adverse effect during the vitrification process adopted for the management of high level liquid radioactive waste [16,29] and thus, it is not recommended. When complexed with 0.15 M ZrO(NO3)2, CP-Ti exhibited high corrosion resistance as evidenced from the corrosion morphologies and the corrosion rate in boiling liquid, vapor and condensate phases of fluorinated nitric acid. Hence, dissolvers made of CP-Ti shall be preferred for the dissolution of Pu-rich mixed oxide spent fuel in fluorinated nitric acid (11.5 M HNO3 þ 0.05 M NaF) complexed with 0.15 M ZrO(NO3)2.
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Fig. 8. XPS spectra of Ti 2p and de-convoluted peaks of TiO2 obtained on CP-Ti sample's surfaces that were exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 for 240 h in: (a) boiling liquid phase; (b) vapor phase; and (c) condensate phase.
The XPS analysis indicated the formation of FeZr bonding [19,20] for the Zr-4 sample exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 and the bonding would correspond to either ZrOF2 or ZrF4. In this highly oxidizing and boiling liquid phase, ZrOF2 would be converted to ZrF4. The area under the deconvoluted peaks in Fig. 7a, b and c gives the concentrations of the ZrO2 and ZreF state, after correcting for the sensitivity factor of the respective elements. The concentration of Zr4þ was found to be about 86, 91 and 94% and the concentration of ZreF was about 14, 9 and 6%, respectively at the sample's surface exposed to liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. This suggests the possibility that the fluoride ions were incorporated into the ZrO2 passive films and were responsible for the breakdown of the film leading to the corrosion of Zr-4. Even at the effective complex forming medium of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2, XPS analysis confirmed the presence of fluoride ion in the ZrO2 passive film, indicating that Zr-4 is highly sensitive to fluorinated nitric acid environment. The presence of TiO2 peak for the CP-Ti samples exposed to liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 reveals that the surface of CP-Ti sample was at passivated condition [7] and the absence of F 1s peak indicates that effective complexation of fluoride ions in solution would have occurred. Thus, the high corrosion resistance offered by CP-Ti in the boiling liquid, vapor and condensate phases of 11.5 M
HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 is attributed to the formation of protective TiO2 passive film. 5. Conclusion Extremely high corrosion rates were observed for Zr-4 and CP-Ti coupons in fluorinated nitric acid medium. The average corrosion rates of Zr-4 in the three different phases of fluorinated nitric acid solution follows the order: boiling liquid phase (78.3 ± 1.5 mm/ y) > condensate phase (18.8 ± 1.1 mm/y) > vapor phase (6.0 ± 0.2 mm/y). Similarly, the average corrosion rates of CP-Ti in the three different phases of fluorinated nitric acid follow the order: condensate phase (71.7 ± 1.5 mm/y) > vapor phase (3.9 ± 0.1 mm/y) > boiling liquid phase (2.9 ± 0.2 mm/y). The present study confirmed the complexing effect of Al(NO3)3 and ZrO(NO3)2 in reducing the corrosion rates of Zr-4 and CP-Ti in boiling fluorinated nitric acid. Based on the corrosion rates for Zr-4 and CP-Ti, it is ascertained that the maximum inhibiting efficiency is exhibited by 0.15 M ZrO(NO3)2 when compared to Al(NO3)3. SEM investigations on Zr-4 samples revealed dissolution morphologies even in the presence of fluoride complexing agents. XPS analysis confirmed the presence of fluoride on the Zr-4 sample's surface in all the three phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. This implies that the mechanism of corrosion attack on Zr-4 by fluoride ions is the formation of ZrF4, resulting in
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dissolution. The corrosion rates of CP-Ti in liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2, were negligible (<0.1 mm/y). XPS analysis of these samples confirmed the presence of TiO2 and absence of fluoride on the surface of CP-Ti samples indicating that effective complexation had occurred leading to passivation and imparting high corrosion resistance. From the results of this study, it is concluded that CP-Ti is a better dissolver material than Zr-4 for extended service life in the aggressive medium of boiling 11.5 M HNO3 þ 0.05 M NaF, when complexed with 0.15 M ZrO(NO3)2. Acknowledgments One of the authors, J. Jayaraj, thanks Dr. Ch. Jagadeeswara Rao, Scientific Officer, Corrosion Science and Technology Group, IGCAR, for his valuable comments. References [1] R. Natarajan, Baldev Raj, J. Nucl. Sci. Technol. 44 (2007) 393. [2] Baldev Raj, U.K. Mudali, Prog. Nucl. Energ 48 (2006) 283. [3] A. Ravi Shankar, V.R. Raju, M.N. Rao, U. Kamachi Mudali, H.S. Khatak, Baldev Raj, Corros. Sci. 49 (2007) 3527. [4] C. Bernard, J.P. Mourox, J. Decours, R. Demay, J. Simonnet, Zirconium made equipment for the New La Hague reprocessing plants, in: Proc. Int. Conf. On Fuel Reprocessing and Waste Management-record 91, 1991, pp. 570e575. Sendai, Japan. [5] U. Kamachi Mudali, A. Ravi Shankar, R. Natarajan, N. Saibaba, Baldev Raj, Nucl. Technol. 182 (2013) 349. [6] S. Tonpe, N. Saibaba, R.N. Jayaraj, A. Ravi Shankar, U. Kamachi Mudali, Baldev Raj, Energy Proced. 7 (2011) 459. [7] J. Jayaraj, K. Thyagarajan, C. Mallika, U. Kamachi Mudali, Nucl. Technol. 191 (2015) 58.
[8] U. Kamachi Mudali, R.K. Dayal, J.B. Gnanamoorthy, J. Nucl. Mater. 203 (1993) 73. [9] P. Fauvet, F. Balbaud, R. Robin, Q. Tran, A. Mugnier, D. Espinoux, J. Nucl. Mater. 375 (2008) 52. [10] S.C. Chetal, V. Balasubramaniyan, P. Chellapandi, P. Mohanakrishnan, P. Puthiyavinayagam, C.P. Pillai, S. Raghupathy, T.K. Shanmugham, C. Sivathanu Pillai, Nucl. Eng. Des. 236 (2006) 852. [11] G. Scott Barney, J. Inorg. Nucl. Chem. 39 (1977) 1665. [12] A.L. Uriarte, R.H. Rainey, Dissolution of High-density UO2, PuO2, and UO2-PuO2 Pellets in Inorganic Acids, Technical Report ORNL - 3695, The U.S. Atomic Energy Commission, 1965, pp. 1e81. [13] T. Furuya, J. Kawafuku, H. Satoh, K. Shimogori, A. Aoshima, S. Takeda, ISIJ Int. 31 (1991) 189. [14] ASTM, Standard A262-02a, Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels, ASTM International, USA, 2002, pp. 1e17. [15] M. Oku, S. Suzuki, N. Ohtsu, T. Shishido, K. Wagatsuma, Appl. Surf. Sci. 254 (2008) 5141. [16] S. Sathe, V. Kain, K. Chandra, Corrosion 68 (2012) 026002. [17] R.S. Ondrejcin, B.D. Mclaughlin, Corrosion of High Ni-Cr Alloys and Type 304 Stainless Steel in HNO3-HF, Report no. DP-1550, E.I. Dupont de Nemorse & Co, Savannah River Laboratory, Aiken, SC, 1980, pp. pp.1e43. [18] C. Morant, J.M. Sanz, L. Galan, L. Soriano, F. Rueda, Surf. Sci. 218 (1989) 331. [19] H.J.M. Bosman, A.P. Pijpers, A.W.M.A. Jaspers, J. Catal. 161 (1996) 551. [20] C. Sleigh, A.P. Pijpers, A. Jaspers, B. Coussens, R.J. Meier, J. Electron. Spectrosc. Relat. Phenom. 77 (1996) 41. [21] C. Fonesca, S. Boudin, M.C. Belo, J. Electroanal. Chem. 379 (1994) 173. [22] J. Kragten, Atlas of Metal Ligand Equilibria in Aqueous Solution, John Wiley and Sons Inc., USA, 1978. [23] J. Prono, T. Jaszay, A. Caprani, J.P. Frayret, J. Appl. Electrochem 25 (1995) 1031. [24] D.S. Kong, Y.Y. Feng, J. Electrochem. Soc. 156 (2009) C283eC291. [25] O. Knacke, O. Kubaschewski, K. Hesselmann, Thermochemical Properties of Inorganic Substances, second ed., Springer-Verlag, New York, 1991. [26] G.F. Vander Voort, Metallography: Principles and Practice, first ed., ASM International, OH, 1999. [27] R.M. Sawant, U.K. Thakur, K.L. Ramakumar, J. Solut. Chem. 34 (2005) 113. [28] M.S. Corbillon, M.A. Olazabal, J.M. Madariaga, J. Solut. Chem. 37 (2008) 567. [29] F. Sokolov, K. Fukuda, H.P. Nawada, Thorium Fuel Cycle-Potential Benefits and Challenges IAEA Technical Document 1450, 2005, pp. 1e105. Vienna.
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Corrosion investigations on zircaloy-4 and titanium dissolver materials for MOX fuel dissolution in concentrated nitric acid containing fluoride ions J. Jayaraj, P. Krishnaveni 1, D. Nanda Gopala Krishna, C. Mallika, U. Kamachi Mudali* Corrosion Science and Technology Group, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam 603102, India
h i g h l i g h t s Zr-4 and CP-Ti exhibited high corrosion rate in boiling fluorinated nitric acid. Corrosion rate decreased in fluorinated nitric acid containing ZrO(NO3)2 and Al(NO3)3. High inhibiting efficiency is exhibited by 0.15 M ZrO(NO3)2 when compared to Al(NO3)3. Corrosion rates of CP-Ti were negligible in complexed fluorinated nitric acid. XPS analysis on CP-Ti confirmed the presence of TiO2 and absence of fluoride.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 December 2015 Received in revised form 8 February 2016 Accepted 9 February 2016 Available online 26 February 2016
Aqueous reprocessing of plutonium-rich mixed oxide fuels require fluoride as a dissolution catalyst in boiling nitric acid for an effective dissolution of the spent fuel. High corrosion rates were obtained for the candidate dissolver materials zircaloy-4 (Zr-4) and commercial pure titanium (CP-Ti grade 2) in boiling 11.5 M HNO3 þ 0.05 M NaF. Complexing the fluoride ions either with Al(NO3)3 or ZrO(NO3)2 aided in decreasing the corrosion rates of Zr-4 and CP-Ti. From the obtained corrosion rates it is concluded that CP-Ti is a better dissolver material than Zr-4 for extended service life in boiling 11.5 M HNO3 þ 0.05 M NaF, when complexed with 0.15 M ZrO(NO3)2. XPS analysis confirmed the presence of TiO2 and absence of fluoride on the surface of CP-Ti samples, indicating that effective complexation had occurred in solution leading to passivation of the metal and imparting high corrosion resistance. © 2016 Elsevier B.V. All rights reserved.
Keywords: Zircaloy Titanium Nitric acid Passivation Fluoride Dissolution Complexing ions
1. Introduction Reprocessing of spent mixed carbide (PuC 70%:UC 30%) nuclear fuel discharged from the Fast Breeder Test Reactor (FBTR) at IGCAR, Kalpakkam, India has been successfully accomplished in the CORAL (COmpact Reprocessing of Advanced fuels in Lead shielded cell) facility at IGCAR, employing the Plutonium Uranium Recovery by EXtraction (PUREX) based aqueous process [1]. The PUREX process utilizes boiling 11.5 M nitric acid as the processing medium for the
* Corresponding author. E-mail address: [email protected] (U.K. Mudali). 1 Research scholar, Department of Chemistry, PSGR Krishnammal College for Women, Coimbatore-641004, India. http://dx.doi.org/10.1016/j.jnucmat.2016.02.018 0022-3115/© 2016 Elsevier B.V. All rights reserved.
dissolution of spent fuels. Owing to their high corrosion resistance in boiling nitric acid, zirconium and titanium are being considered for dissolver applications for the spent fuels discharged from fast breeder reactors (FBRs) [2,3]. The spent mixed carbide fuel from FBTR could be dissolved in a dissolver vessel made of commercial pure titanium (CP-Ti grade 2) without any significant corrosion problem of the titanium dissolver. On the other hand, the La Hague reprocessing plant at France uses equipment such as evaporator and dissolver made of zirconium [4]. A mock-up (Zr-4) dissolver vessel has been fabricated at IGCAR in collaboration with Nuclear Fuel Complex (NFC), Hyderabad, India, with the view to reprocessing spent fuels from future FBRs [5,6]. Continuous exposure of this Zr-4 mock-up dissolver vessel for 2500 h in boiling 11.5 M nitric acid containing simulated fission and corrosion products (i.e.,
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simulated dissolver solution without uranium and plutonium), did not show any corrosion attack on its outer surface or along the weld surface of the vessel [7]. It is worth mentioning that dissolvers made up of nitric acid grade and non-sensitized 304 L stainless steels cannot be used in concentrated nitric acid under boiling conditions, as they undergo severe intergranular corrosion in such corrosive media [8,9]. The experience gained during the successful reprocessing of mixed carbide spent fuel of FBTR will be utilized to reprocess the PuO2-UO2 mixed oxide fuel that will be discharged from India's first commercial Prototype Fast Breeder Reactor (PFBR) [10]. For the effective dissolution of PuO2 and PuO2-UO2 mixed oxide, fluoride as a dissolution catalyst is required in nitric acid [11,12]. The required concentration ranges for HNO3 and HF are 10e14 M and 0.001e0.1 M, respectively and temperature up to the boiling point. The rate of dissolution of PuO2 is proportional to 4th power of HNO3 concentration and 1.4th power of HF concentration, indicating the effectiveness of HF in the dissolution of PuO2 [12]. For instance, 20% PuO2-80% UO2 pellet did not dissolve in 10 M HNO3, but it was completely dissolved in 10 M HNO3 containing 0.05 M HF [12]. Nevertheless, it should be realized that the presence of these aggressive fluoride ions might affect the corrosion behavior of dissolvers made of Zr-4 and CP-Ti. The processing medium envisaged for the dissolution of Pu-rich mixed oxide spent fuels from PFBR would be boiling 11.5 M HNO3 þ 0.05 M NaF. Though dissolvers made of Zr-4 and CP-Ti exhibited high corrosion resistance in boiling 11.5 M HNO3 [2,3], their corrosion behavior needs to be evaluated in fluoride containing nitric acid. Thus, the focus of this study is to evaluate the corrosion behavior of the candidate dissolver materials Zr-4 and CP-Ti, when exposed to boiling 11.5 M HNO3 þ 0.05 M NaF. All the tests have been carried out in boiling liquid, vapor, and condensate phases of 11.5 M HNO3 þ 0.05 M NaF. The influence of complex forming agents with the fluoride ions, namely Al(NO3)3 and ZrO(NO3)2, has also been investigated in order to control the corrosion of Zr-4 and CP-Ti in fluorinated nitric acid environment. It is worth mentioning that addition of these complex forming agents might affect the dissolution rate of the spent MOX fuel. However, the main objective of this study is to identify the suitable dissolver material for extended service life in the chosen aggressive medium.
11.5 M HNO3 þ 0.05 M NaF (fluorinated nitric acid). Test solutions containing Al(NO3)3 and ZrO(NO3)2 as complexing agents in 11.5 M HNO3 þ 0.05 M NaF were used to evaluate the influence of the complexing agents in fixing the activity of the fluoride ions. The concentration of these complexing agents were varied as 0.05, 0.10 and 0.15 M. All the chemicals used to prepare the solutions were analytical grade reagents. Double distilled water was used to prepare all the solutions.
2. Experimental
2.4. Surface characterization
2.1. Materials
To understand the corrosion behavior of Zr-4 and CP-Ti samples, scanning electron microscopic (SEM) and X-ray photoelectron spectroscopic (XPS) analyses were carried out. A SNE3000M, desktop model SEM was used to observe the surface morphology of exposed samples in secondary electron mode at an accelerating voltage of 30 kV. XPS measurements were carried out using a SPECS Surface Nano Analysis GmbH, Germany spectrometer. For XPS, Al Ka was used as X-ray source at 1486.7 eV and the anode was operated at voltage of 13 kV with source power level at 300 W. The beam spot size of the X-ray was 1 mm diameter. The peak positions were compared with the standard values reported in literature and data bases for identification of different elements and their oxidation states. After correcting for background for all the peaks using “Shirley” approximation algorithm [15], the XPS data were
Zircaloy-4 (Zr-4) and commercial pure Ti plates (CP-Ti, grade 2) in annealed conditions were used and their compositions are given in Table 1. Coupons for corrosion test were prepared from the plates by cutting using diamond wheel in a flowing coolant. The coupons were subsequently wet ground using SiC paper from 80 to 1200 grit. The dimensions of Zr-4 and CP-Ti coupons were 15 10 5 mm with a surface area of 550 mm2. Prior to corrosion testing, these coupons were cleaned in distilled water and acetone. 2.2. Test environment The corrosion behavior of Zr-4 and CP-Ti were evaluated in
2.3. Corrosion testing The standard three phase corrosion setup described elsewhere [3,13] was used for the corrosion testing. A conical flask containing 300 ml of the test solution was heated to boiling condition (about 120 C). The vapors produced during boiling were condensed using a cold finger and the condensate (approximately 50 ml) was collected in the collecting cup. Through a siphon arrangement, the collected condensate was circulated back to the conical flask leading to achievement of equilibrium in the system. The process of collecting and re-circulating the condensate takes place continuously and the rate of renewal of condensate was fast [13]. The coupons were introduced to the liquid, vapor and condensate phases of these test solutions using a teflon thread. As recommended in ASTM A262 Practice C [14], the corrosion coupons were exposed for a period of 48 h, and experiments were performed for five such periods (i.e., total exposure period of 240 h) using fresh solutions in each period. After each testing period the coupons were removed and cleaned in distilled water. Further, the corrosion tested samples were ultrasonicated in acetone for 5 min. The corrosion coupons were air dried and the change in weight was measured in a micro balance with resolution of 0.0001 g. The corrosion rates from weight loss experiments were calculated using Eq. (1).
Corrosion rate ¼
8:76 104 W DAt
(1)
where, corrosion rate is expressed in mm/yr, W is weight loss in g, D is the density of the sample (Zr-4: 6.5 g/cm3, CP-Ti: 4.5 g/cm3), A is the exposed surface area in cm2 and t is exposed time in h.
Table 1 Elemental composition of Zr-4 and CP-Ti in weight percentage. Material/Identification
Zr
Ti
Sn
Fe
Cr
C
O
N
Zircaloy-4 (Zr-4) Commercial pure titanium (CP-Ti grade 2)
balance e
e balance
1.32 e
0.22 0.053
0.11 e
e 0.026
e 0.025
e 0.006
J. Jayaraj et al. / Journal of Nuclear Materials 473 (2016) 157e166
processed by SpecsLab2 software and analyzed by CasaXPS software. The individual high resolution spectra of all the elements were deconvoluted using a pseudo-voigt function.
3. Results 3.1. Corrosion behavior of Zr-4 and CP-Ti in boiling fluorinated nitric acid The corrosion rates of Zr-4 and CP-Ti coupons exposed to the liquid, vapor and condensate phases of boiling nitric acid containing fluoride ions (i.e. 11.5 M HNO3 þ 0.05 M NaF; also referred as fluorinated nitric acid) are presented in Table 2. After three testing periods of 48 h (i.e., 144 h) in boiling liquid of fluorinated nitric acid, the Zr-4 sample had completely dissolved. The average corrosion rate of Zr-4, calculated for 144 h, in the three different phases of 11.5 M HNO3 þ 0.05 M NaF follows the order: boiling liquid phase > condensate phase > vapor phase. On the other hand, after three testing periods of 48 h, the CP-Ti sample had completely dissolved in the condensate phase rather than in the boiling liquid phase of fluorinated nitric acid. The average corrosion rates of CP-Ti, calculated for 144 h, in the three different phases of 11.5 M HNO3 þ 0.05 M NaF follows the order: condensate phase > vapor phase > boiling liquid phase. Fig. 1a, b and c are the SEM images of the Zr-4 samples exposed to boiling liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF, respectively for the period of 144 h. The SEM images revealed uniform dissolution with the formation of micro pits in all the three phases of fluorinated nitric acid. Though the corrosion rates of Zr-4 were different in liquid, vapor and condensate phases of fluorinated nitric acid, the corrosion morphologies (Fig. 1a, b and c) confirmed that Zr-4 exhibited dissolution behavior in all the three phases. Fig. 2a, b and c are the SEM images of the CPTi samples that were exposed to boiling liquid, vapor and condensate phases for 144 h. The SEM image (Fig. 2a) of CP-Ti, exposed to boiling liquid phase of fluorinated nitric acid showed that the sample surface was smooth. The surface morphology (Fig. 2b) of the CP-Ti exposed to the vapor phase of fluorinated nitric acid exhibited roughened features with cracks along the surface and loosely adhered corrosion products all over the surface. When CP-Ti was exposed to the condensate phase of fluorinated nitric acid, an extremely roughened morphology with dimple like features and cracks was observed, as shown in Fig. 2c. This part of the study has confirmed that CP-Ti and Zr-4 exhibit high corrosion rates of the order of 2.9e78 mm/y, when exposed to boiling nitric acid containing fluoride ions. Such high corrosion rates are not allowed for dissolver application in the spent nuclear fuel reprocessing. However, to control the corrosion attack by fluoride ions, complex forming agents were added to the fluorinated nitric acid medium [16,17], so that the aggressive fluoride ions would not be available for corroding the dissolver vessels.
Table 2 Corrosion rates of Zr-4 and CP-Ti in boiling liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF. Material
Phase
Corrosion rate (mm/y) 48 h
96 h
144 h
Average
Zr-4
Liquid Vapor Condensate Liquid Vapor Condensate
80 6.1 20.1 3.1 4 72
77.5 6.0 18.3 2.9 3.9 70
77.3 5.8 18 2.8 3.8 73
78.3 6.0 18.8 2.9 3.9 71.7
CP-Ti
± ± ± ± ± ±
1.5 0.2 1.1 0.2 0.1 1.5
159
3.2. Effect of complexing agents on the corrosion of Zr-4 and CP-Ti in fluorinated nitric acid The effect of complexing agents Al(NO3)3 and ZrO(NO3)2, in fixing the activity of fluoride ions on the corrosion behavior of Zr-4 and CP-Ti were studied by adding different concentrations such as 0.05, 0.1 and 0.15 M of Al(NO3)3 and ZrO(NO3)2e11.5 M HNO3 þ 0.05 M NaF. With these complexing agents the corrosion rates were determined only in boiling liquid phase of fluorinated nitric acid for 48 h as shown in Table 3. It could be noted from Table 3 that the corrosion rates of Zr-4 and CP-Ti decreased as the concentration of the complexing agents increased. Lowest corrosion rate was obtained for the 0.15 M concentration of both the complexing agents. Subsequently, the corrosion behavior of Zr-4 and CP-Ti were evaluated in liquid, vapor and condensate phases of fluorinated nitric acid with 0.15 M Al(NO3)3 and 0.15 M ZrO(NO3)2 , respectively for five testing periods of 48 h duration and the results are given in Table 4. The corrosion resistance of Zr-4 improved marginally (as shown in Table 4) in boiling liquid phase of fluorinated nitric acid when complexed with 0.15 M Al(NO3)3. However, the corrosion rate of Zr4 in vapor and condensate phases has decreased significantly when complexed with 0.15 M Al(NO3)3. The SEM images presented in Fig. 3a, b and c revealed that the Zr-4 samples that were exposed to boiling liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3 for 240 h exhibited uniform dissolution. The corrosion rate of CP-Ti in the liquid, vapor and condensate phases of fluorinated nitric acid was lower when 0.15 M Al(NO3)3 was added to the test medium compared to those experiments where it is not added. Though a low corrosion rate was obtained for CP-Ti in the liquid phase of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3, the corrosion rates obtained in the vapor and condensate phases were still too high for practical dissolver application. SEM images of CP-Ti exposed to liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3 are shown in Fig. 5a, b and c respectively. Surface morphology of the CP-Ti coupon exposed to liquid phase of fluorinated nitric acid complexed with Al(NO3)3 was smooth (Fig. 5a) when compared to the surface morphologies (Fig. 5b and c) of the same samples exposed to vapor and condensate phases. In the vapor and condensate phases of fluorinated nitric acid with 0.15 M Al(NO3)3, a localized attack like feature (Fig. 5b) and roughened morphology with porous structure (Fig. 5c), respectively was observed on CP-Ti. When compared to 0.15 M Al(NO3)3 complexed solution, complexation with 0.15 M ZrO(NO3)2, decreased the corrosion rates for Zr-4 in liquid, vapor and condensate phases of fluorinated nitric acid (as shown in Table 4). However, SEM investigations carried out on the Zr-4 samples that were exposed to boiling liquid (Fig. 4a), vapor (Fig. 4b) and condensate phase (Fig. 4c) of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 for 240 h revealed uniform dissolution morphology under all conditions. Interestingly, lowest corrosion rates were obtained for CP-Ti in boiling liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. In the boiling liquid phase of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2, instead of weight loss a small weight gain was observed for CP-Ti and so mentioned as negligible corrosion rate in Table 4. SEM images of CP-Ti exposed to liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 are shown in Fig. 6a, b and c respectively. An un-attacked surface morphology (Fig. 6a) was observed over CP-Ti when exposed to liquid phase of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. Comparing with 0.15 M Al(NO3)3, a slightly attacked surface (Fig. 6b and c) was observed for the samples exposed to vapor and condensate phases of fluorinated nitric acid complexed with 0.15 M ZrO(NO3)2.
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Fig. 1. SEM images of Zr-4 samples exposed to 11.5 M HNO3 þ 0.05 M NaF for 144 h in (a) boiling liquid phase; (b) vapor phase; and (c) condensate phase.
Fig. 2. SEM images of CP-Ti samples exposed to 11.5 M HNO3 þ 0.05 M NaF for 144 h in (a) boiling liquid phase; (b) vapor phase; and (c) condensate phase.
Table 3 Corrosion rates of Zr-4 and CP-Ti in boiling liquid phase of 11.5 M HNO3 þ 0.05 M NaF þ x M Al(NO3)3 and 11.5 M HNO3 þ 0.05 M NaF þ x M ZrO(NO3)2, (where, x ¼ 0.05, 0.1 and 0.15 M of complexing agents). Complexing agent
Concentration of complexing agents (M)
Corrosion rate (mm/y) Zr-4
CP-Ti
Al(NO3)3
0.05 0.1 0.15 0.05 0.1 0.15
76.2 68.6 50.8 44.5 5.1 0.56
1 0.4 0.15 0.5 0.03 Negligible
ZrO(NO3)2
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Table 4 Corrosion rates of Zr-4 and CP-Ti in boiling liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3 and 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. Environment
11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3
Material
Zr-4
CP-Ti
11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2
Zr4
CP-Ti
a
Phase
Liquid Vapor Condensate Liquid Vapor Condensate Liquid Vapor Condensate Liquid Vapor Condensate
Corrosion rate (mm/y) 48 h
96 h
144 h
196 h
240 h
Average
50.8 1.3 1.7 0.15 0.89 2.57 0.56 0.15 0.15 a 0.03 0.1
51.1 1.4 1.8 0.18 0.97 2.69 0.58 0.18 0.2 a 0.03 0.1
50.6 1.1 1.5 0.13 0.81 2.44 0.56 0.2 0.18 a 0.03 0.1
51.1 1.4 1.8 0.18 0.97 2.69 0.53 0.15 0.2 a 0.03 0.1
50.6 1.1 1.5 0.13 0.81 2.44 0.56 0.18 0.15 a 0.03 0.1
50.8 ± 0.25 1.3 ± 0.15 1.66 ± 0.15 0.2 ± 0.03 0.89 ± 0.08 2.57 ± 0.13 0.56 ± 0.02 0.17 ± 0.02 0.18 ± 0.03 a 0.03 0.1
Negligible.
Fig. 3. SEM images of Zr-4 samples exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3 for 240 h in (a) boiling liquid phase; (b) vapor phase; and (c) condensate phase.
XPS investigations were carried out on the Zr-4 and CP-Ti samples exposed to the effective complex forming condition of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. The XPS high resolution spectra of Zr 3d recorded for the surface of Zr-4 samples exposed to the liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 are shown in Fig. 7a, b and c, respectively. The F 1s spectra (Fig. 7d) shows a peak at 686.8 eV was observed for the Zr-4 samples. In the spectra of Zr 3d (Fig. 7a, b and c), the peaks of Zr 3d5/2 and Zr 3d3/2 observed at the binding energies 183.4 and 185.8 eV, respectively corresponded to ZrO2 [18]. Apart from these peaks, a low intensity peak at 189.2 eV was observed only for the Zr-4 samples exposed to vapor and condensate conditions, as shown in Fig. 7b and c. From the de-convoluted spectra of Zr 3d, the presence of fluoride could be realized as zirconium oxy fluoride (ZrOF2) corresponding to the peaks at 184.6 and 187 eV for the Zr-4 sample exposed to liquid phase [19]. On the other hand, for the Zr-4 samples exposed to vapor and condensate phases, fluoride was realized as zirconium fluoride (ZrF4) corresponding to the peaks at 186.8 and 189.3 eV respectively [20]. The XPS spectra of Ti 2p observed on the CP-Ti sample's surface exposed
to liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 are shown in Fig. 8a, b and c, respectively. In the spectra of Ti 2p, the peaks of Ti 2p3/2 and Ti 2p1/2 observed at the binding energies 459.6 and 465.4 eV, respectively corresponded to TiO2 [21]. It is worth mentioning that peak corresponding to F 1s spectra was not observed for the CP-Ti sample's exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. 4. Discussion The high corrosion resistance of Zr-4 and CP-Ti samples in boiling 11.5 M HNO3 is due to the formation of respective passive films of ZrO2 and TiO2 [3,7]. However, it is evident from this study that addition of 0.05 M NaF to 11.5 M HNO3 changed the passive behavior of Zr-4 and CP-Ti to dissolution behavior. In general, metal ions hydrolyses in aqueous solution to form their corresponding metal hydroxides and oxides. In the present case, since the solution is more acidic in nature, both Zr-4 and CP-Ti samples would not form hydroxides [22]. The two major competing reactions that can occur in fluorinated nitric acid medium are formation of metal
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Fig. 4. SEM images of Zr-4 samples exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 for 240 h in (a) boiling liquid phase;(b) vapor phase; and (c) condensate phase.
Fig. 5. SEM images of CP-Ti samples exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M Al(NO3)3 for 240 h in (a) boiling liquid phase; (b) vapor phase; and (c) condensate phase.
fluorides and metal oxides. Zr-4 and CP-Ti may undergo dissolution in the presence of fluoride ions by forming their respective metal fluorides [23,24] as given in equations (2) and (3). Further, formation of stable metal oxides of Zr-4 and CP-Ti as given by equations (4) and (5) is also possible.
¼ 915
(3)
. Zr þ 2NaF þ 2HNO3 4ZrO2 þ 2NaNO2 þ 2HF ; DG0r kJ: mol1
Zr þ 6NaF þ 6HNO3 4ZrF4 þ 4NaNO3 þ 2NaNO2 þ 2HF . þ 2H2 O; DG0r kJ: mol1 ¼ 1162
Ti þ 6NaF þ 6HNO3 4TiF4 þ 4NaNO3 þ 2NaNO2 þ 2HF . þ 2H2 O; DG0r kJ: mol1
¼ 940 (2)
(4)
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163
Fig. 6. SEM images of CP-Ti samples that were exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 for 240 h in (a) boiling liquid phase; (b) vapor phase; and (c) condensate phase.
Ti þ 2NaF þ 2HNO3 4TiO2 þ 2NaNO2 þ 2HF;
DG0r
. kJ: mol1
¼ 787 (5) As fluoride ion is highly electronegative in nature, it has a strong tendency even to attack the metal oxides. Hence, ZrO2 and TiO2 formed as per equations (4) and (5) can further convert to their corresponding metal fluorides instantaneously according to equations (6) and (7).
ZrO2 þ 4NaF þ 4HNO3 4ZrF4 þ 4NaNO3 . þ 2H2 O; DG0r kJ: mol1 ¼ 223
(6)
TiO2 þ 4NaF þ 4HNO3 4TiF4 þ 4NaNO3 . þ 2H2 O; DG0r kJ: mol1 ¼ 128
(7)
The values of Gibbs energy were calculated [25] at the boiling liquid phase temperature of 120 C. Values of Gibbs energy change in reactions 2, 3, 6 and 7 indicate that the formation of ZrF4 is more favored than the formation TiF4. This implies that severe corrosion of Zr-4 will occur in comparison to the CP-Ti sample (Table 2) in the boiling liquid phase of fluorinated nitric acid as the feasibility for the simultaneous conversion of ZrO2 to ZrF4 is more than that for TiO2 to TiF4. Owing to the formation of ZrF4, the Zr-4 sample exhibited dissolution behavior in all the three phases of fluorinated nitric acid as shown in Fig. 1a, b and c. The CP-Ti sample exposed to the boiling liquid phase of fluorinated nitric acid showed a smooth surface (Fig. 2a) due to the simultaneous process of removal of metal and formation of passive layer as given in equations (3) and (5). This process is similar to that of chemical polishing [26], wherein the metal undergoes uniform dissolution in the electrolyte
and a polished like smooth surface was obtained. The concentrations of nitric acid and fluoride ions in the vapor and condensate phases are expected to be lower than that of the liquid phase [13,16]. The concentration of condensate phase is reported to be about 7.2 M [13] for the liquid phase of 12 M nitric acid. As expected, lower corrosion rates were obtained for Zr-4 in the vapor and condensate phases of fluorinated nitric acid in comparison to its liquid phase. In contrast, higher corrosion rates were obtained for CP-Ti samples exposed to vapor and condensate phases of fluorinated nitric acid. Similar trend of higher corrosion rates in vapor and condensate phases than that of the liquid phase was reported for titanium when exposed to boiling 11.5 M nitric acid [2,3]. The corrosion resistance of titanium in the liquid and condensate phases of nitric acid was influenced by the concentration of its own corrosion product (Ti4þ) [13]. As the concentration of Ti4þ was high in the liquid phase, the stable TiO2 film formed on the surface of CP-Ti had resisted the corrosion attack and thus low corrosion rate was observed. On the other hand, Furuya et al. [13] reported that the formation of semi-protective Ti2O3 film was responsible for the high corrosion rate in the condensate phase of nitric acid. However, in this study, apart from the formation of semi-protective passive layer, the formation of TiF4 would have significantly contributed to the high corrosion rates in the vapor and condensate phases of fluorinated nitric acid. Ti2O3 can also get converted to TiF4 as per equation (8) and enhances the corrosion rate of CP-Ti in the condensate phase of fluorinated nitric acid.
Ti2 O3 þ 8NaF þ 8HNO3 42TiF4 þ 8NaNO3 þ 3H2 O . þ H2 [; DG0r kJ: mol1 ¼ 160
(8)
The corroded morphologies of CP-Ti exposed to the vapor (Fig. 2b) and condensate phases (Fig. 2c) of fluorinated nitric acid could be the result of the simultaneous formation and dissolution of an un-protective oxide layer. When Al(NO3)3 or ZrO(NO3)2 is added to fluorinated nitric acid, aluminum fluoride complexes or zirconium fluoride complexes will
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Fig. 7. XPS spectra of Zr 3d and F 1s for Zr-4 sample surface exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 for 240 h: (a) Zr 3d spectra, de-convoluted peaks of ZrO2 and ZrOF2 in boiling liquid phase; (b and c) Zr 3d spectra, de-convoluted peaks of ZrO2 and ZrF4 in vapor and condensate phases; and (d) F 1s spectra in boiling liquid, vapor and condensate phases.
be formed in the solution [16,17,27,28]. As the concentration of the complexing agents increased, the corrosion rates of Zr-4 and CP-Ti samples decreased, as shown in Table 3. The concentration of fluoride ions will decrease in the solution due to the complex formation, leading to a reduction in the corrosion rate of metallic samples [16,17]. It should be noted that for all the concentrations of the complexing agents, the corrosion rates (Table 3) were lower for ZrO(NO3)2 than that for Al(NO3)3. Several fluoro complexes of Al and Zr could be formed and they depend on the pH, fluoride content and ionic strength of the solution [27,28]. Assuming that only neutral fluoro compounds such as AlF3 and ZrF4 were formed in the respective solution, the former compound would require less fluoride ions than the later. The effectiveness of the complexing agents in reducing the corrosion rate depends on the stability constants of these fluoro complexes [16,17]. The stability constant of fluoro complex formed by Zr4þ is as high as about 1010, while that of Al3þ is 107 [17] thus, complexing with ZrO(NO3)2 is more effective than Al(NO3)3 for the corrosion control of Zr-4 and CP-Ti in fluorinated nitric acid, as evidenced in the present study. Table 4 reveals that Zr-4 sample had undergone less corrosion in both vapor and condensate phases of Al(NO3)3 complexed fluorinated nitric acid than that of its boiling liquid phase. Owing to the high corrosion of Zr-4 in the boiling liquid phase, the concentration of metal ions in the solution would gradually increase, resulting in the elevation of boiling point of the solution and reducing the
amount of vapor. Irrespective of the relatively lower corrosion rates for Zr-4 in fluorinated nitric acid with both complexing agents (Table 4) or severe corrosion rate of Zr-4 in fluorinated nitric acid alone (Table 2), the corroded morphologies obtained in all the three phases were similar, as shown in Figs. 1, 3 and 4. This suggests that the presence of even a very low concentration of fluoride ion left over after complexation could have resulted in the dissolution of Zr4. In the case of CP-Ti, the absence of visible surface attack and low corrosion rate indicated that 0.15 M Al(NO3)3 could effectively complex with fluoride to mitigate the corrosion in the boiling liquid phase of fluorinated nitric acid. Though localized corrosion and porous morphologies were obtained for the CP-Ti sample exposed to vapor and condensate phases of 0.15 M Al(NO3)3 complexed solution, the corrosion rates determined were less than that uncomplexed solution. In spite of lowering the corrosion rate of CPTi in fluorinated nitric acid, addition of excessive Al(NO3)3, does have an adverse effect during the vitrification process adopted for the management of high level liquid radioactive waste [16,29] and thus, it is not recommended. When complexed with 0.15 M ZrO(NO3)2, CP-Ti exhibited high corrosion resistance as evidenced from the corrosion morphologies and the corrosion rate in boiling liquid, vapor and condensate phases of fluorinated nitric acid. Hence, dissolvers made of CP-Ti shall be preferred for the dissolution of Pu-rich mixed oxide spent fuel in fluorinated nitric acid (11.5 M HNO3 þ 0.05 M NaF) complexed with 0.15 M ZrO(NO3)2.
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Fig. 8. XPS spectra of Ti 2p and de-convoluted peaks of TiO2 obtained on CP-Ti sample's surfaces that were exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 for 240 h in: (a) boiling liquid phase; (b) vapor phase; and (c) condensate phase.
The XPS analysis indicated the formation of FeZr bonding [19,20] for the Zr-4 sample exposed to 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 and the bonding would correspond to either ZrOF2 or ZrF4. In this highly oxidizing and boiling liquid phase, ZrOF2 would be converted to ZrF4. The area under the deconvoluted peaks in Fig. 7a, b and c gives the concentrations of the ZrO2 and ZreF state, after correcting for the sensitivity factor of the respective elements. The concentration of Zr4þ was found to be about 86, 91 and 94% and the concentration of ZreF was about 14, 9 and 6%, respectively at the sample's surface exposed to liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. This suggests the possibility that the fluoride ions were incorporated into the ZrO2 passive films and were responsible for the breakdown of the film leading to the corrosion of Zr-4. Even at the effective complex forming medium of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2, XPS analysis confirmed the presence of fluoride ion in the ZrO2 passive film, indicating that Zr-4 is highly sensitive to fluorinated nitric acid environment. The presence of TiO2 peak for the CP-Ti samples exposed to liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 reveals that the surface of CP-Ti sample was at passivated condition [7] and the absence of F 1s peak indicates that effective complexation of fluoride ions in solution would have occurred. Thus, the high corrosion resistance offered by CP-Ti in the boiling liquid, vapor and condensate phases of 11.5 M
HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2 is attributed to the formation of protective TiO2 passive film. 5. Conclusion Extremely high corrosion rates were observed for Zr-4 and CP-Ti coupons in fluorinated nitric acid medium. The average corrosion rates of Zr-4 in the three different phases of fluorinated nitric acid solution follows the order: boiling liquid phase (78.3 ± 1.5 mm/ y) > condensate phase (18.8 ± 1.1 mm/y) > vapor phase (6.0 ± 0.2 mm/y). Similarly, the average corrosion rates of CP-Ti in the three different phases of fluorinated nitric acid follow the order: condensate phase (71.7 ± 1.5 mm/y) > vapor phase (3.9 ± 0.1 mm/y) > boiling liquid phase (2.9 ± 0.2 mm/y). The present study confirmed the complexing effect of Al(NO3)3 and ZrO(NO3)2 in reducing the corrosion rates of Zr-4 and CP-Ti in boiling fluorinated nitric acid. Based on the corrosion rates for Zr-4 and CP-Ti, it is ascertained that the maximum inhibiting efficiency is exhibited by 0.15 M ZrO(NO3)2 when compared to Al(NO3)3. SEM investigations on Zr-4 samples revealed dissolution morphologies even in the presence of fluoride complexing agents. XPS analysis confirmed the presence of fluoride on the Zr-4 sample's surface in all the three phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2. This implies that the mechanism of corrosion attack on Zr-4 by fluoride ions is the formation of ZrF4, resulting in
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dissolution. The corrosion rates of CP-Ti in liquid, vapor and condensate phases of 11.5 M HNO3 þ 0.05 M NaF þ 0.15 M ZrO(NO3)2, were negligible (<0.1 mm/y). XPS analysis of these samples confirmed the presence of TiO2 and absence of fluoride on the surface of CP-Ti samples indicating that effective complexation had occurred leading to passivation and imparting high corrosion resistance. From the results of this study, it is concluded that CP-Ti is a better dissolver material than Zr-4 for extended service life in the aggressive medium of boiling 11.5 M HNO3 þ 0.05 M NaF, when complexed with 0.15 M ZrO(NO3)2. Acknowledgments One of the authors, J. Jayaraj, thanks Dr. Ch. Jagadeeswara Rao, Scientific Officer, Corrosion Science and Technology Group, IGCAR, for his valuable comments. References [1] R. Natarajan, Baldev Raj, J. Nucl. Sci. Technol. 44 (2007) 393. [2] Baldev Raj, U.K. Mudali, Prog. Nucl. Energ 48 (2006) 283. [3] A. Ravi Shankar, V.R. Raju, M.N. Rao, U. Kamachi Mudali, H.S. Khatak, Baldev Raj, Corros. Sci. 49 (2007) 3527. [4] C. Bernard, J.P. Mourox, J. Decours, R. Demay, J. Simonnet, Zirconium made equipment for the New La Hague reprocessing plants, in: Proc. Int. Conf. On Fuel Reprocessing and Waste Management-record 91, 1991, pp. 570e575. Sendai, Japan. [5] U. Kamachi Mudali, A. Ravi Shankar, R. Natarajan, N. Saibaba, Baldev Raj, Nucl. Technol. 182 (2013) 349. [6] S. Tonpe, N. Saibaba, R.N. Jayaraj, A. Ravi Shankar, U. Kamachi Mudali, Baldev Raj, Energy Proced. 7 (2011) 459. [7] J. Jayaraj, K. Thyagarajan, C. Mallika, U. Kamachi Mudali, Nucl. Technol. 191 (2015) 58.
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