corrosion behavior of zirconium, titanium, and their alloys in simulated ...

CORROSION BEHAVIOR OF ZIRCONIUM, TITANIUM, AND THEIR ALLOYS IN SIMULATED DISSOLVER SOLUTION OF FAST BREEDER REACTOR SPENT NUCLEAR FUEL USING ZIRCALOY-4 MOCK-UP DISSOLVER VESSEL

REPROCESSING

KEYWORDS: simulated dissolver solution, Zr-702 corrosion, commercially pure Ti corrosion

J. JAYARAJ, K. THYAGARAJAN, C. MALLIKA, and U. KAMACHI MUDALI* Indira Gandhi Centre for Atomic Research, Corrosion Science and Technology Group Kalpakkam 603102, India

Received August 18, 2014 Accepted for Publication November 21, 2014 http://dx.doi.org/10.13182/NT14-90

Long-term corrosion testing of a mock-up dissolver vessel to be employed in the aqueous reprocessing of spent nuclear fuels of fast breeder reactors has been initiated. In this work, a Zircaloy-4 (Zr-4) mock-up dissolver vessel was used as the testing facility to evaluate the corrosion rate of several candidate materials based on zirconium and titanium in the boiling and vapor phases of simulated dissolver solution (SDS) comprising fission and corrosion product ions in 11.5 M nitric acid. Several campaigns of 100, 250, 500, 1000, and 2500 h of operation were completed. The corrosion rates of the candidate materials are expressed both in micrometers per year (mm/yr) and mils per year (mils/yr). Zirconium702, Zr-4, autoclaved Zr-4, and commercial pure titanium (CP-Ti) exhibited low corrosion rates of 0.08 to 0.23 mm/ yr (0.003 to 0.009 mils/yr) in the as-received and welded conditions exposed to the boiling liquid phase of the dissolver solution for 2500 h. Whereas the CP-Ti and CPTi weld exhibited marginally higher corrosion rates of

1.0 mm/yr (0.04 mils/yr) and 1.9 mm/yr (0.075 mils/yr), respectively, in the vapor phase of the dissolver solution, the lowest corrosion rate of 0.08 mm/yr (0.003 mils/yr) was obtained for the autoclaved Zr-4 sample exposed to boiling SDS. Scanning electron microscope investigations did not reveal any corrosion attack for the titanium and zirconium samples. Laser Raman spectroscopic analysis confirmed that the origins of passivity of zirconium and titanium samples were due to the formation of ZrO2 and TiO2, respectively. However, the CP-Ti/AISI Type 304L stainless steel (SS 304L) and Zr-4/SS 304L dissimilar welds had undergone severe corrosion. Visual inspection of the Zr-4 dissolver vessel revealed no corrosion attack after operation for 2500 h. The results of this 2500-h campaign would serve as the baseline data for the analysis of future long-term campaigns.

I. INTRODUCTION

reprocessing of spent nuclear fuels discharged from both thermal reactors with low plutonium content and fast breeder reactors (FBRs) with high plutonium content.1 In the PUREX process, dissolution of spent nuclear fuel is carried out at different concentrations of nitric acid. Dissolution of irradiated UO2 fuel from the thermal

A Plutonium Uranium Recovery by EXtraction (PUREX)–based aqueous process is employed for the *E-mail: [email protected] 58

Note: Some figures in this paper may be in color only in the electronic version.

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CORROSION BEHAVIOR OF Zr, Ti, AND THEIR ALLOYS IN SIMULATED REPROCESSING SOLUTION

nuclear reactor is usually carried out in dissolver vessels made of AISI Type 304L stainless steel (SS 304L) due to its corrosion resistance in a boiling 8 M HNO3 medium.1,2 However, the reprocessing plants for Pu-rich spent fuels of FBRs are characterized by high concentrations of boiling nitric acid, a high radiation field, and highly oxidizing fission products.3,4 Dissolvers made up of nitric acid grade and nonsensitized SS 304L cannot be used in concentrated nitric acid under boiling conditions, as they undergo severe intergranular corrosion in such a corrosive medium.1,5,6 Titanium, zirconium, and their alloys are considered for the fabrication of dissolver vessels in FBR fuel reprocessing in order to meet the demand of the zero failure concept as they are inaccessible for direct maintenance in addition to their life expectancy of w40 yr (Refs. 1 and 7). The spent fuel of various burnups from 25 to 155 GWd/tonne discharged from the Fast Breeder Test Reactor (FBTR) at Kalpakkam is being reprocessed in the CORAL (COmpact Reprocessing of Advanced fuels in Lead shielded cell) reprocessing facility at Indira Gandhi Centre for Atomic Research (IGCAR). Dissolution of mixed carbide fuel comprising 30% UC and 70% PuC from the FBTR is accomplished in a titanium grade II dissolver vessel using 11.5 M boiling nitric acid without any significant corrosion problem. Although titanium exhibits excellent corrosion resistance in boiling nitric acid, it undergoes excessive corrosion in the vapor and condensate phases of nitric acid.1 Hence, for the future FBR reprocessing plants, zirconium and its alloys are being considered for dissolver and evaporator applications. Zirconium equipment and pipings have been used at the La Hague reprocessing plant, France, owing to its excellent corrosion resistance.8 To understand the corrosion behavior of zirconium, a high-temperature, high-concentration Zircaloy-4 (Zr-4) test facility was installed at IGCAR, and long-term corrosion testing of zirconium samples was conducted, as described elsewhere.7 The corrosion rates obtained for the zirconium samples were v1 mil per year (mil/yr) in 11.5 M boiling liquid, vapor, and condensate phases of nitric acid, and no significant attack was revealed.7,9 It should be mentioned that unlike titanium, the zirconium samples did not suffer by the vapor and condensate phases of nitric acid. The international as well as our experience on zirconium corrosion in high concentration of nitric acids under boiling conditions motivated us to fabricate a Zr-4 dissolver vessel for FBR fuel-reprocessing applications. To qualify and to assess the service life of the Zr-4 dissolver for the aqueous reprocessing of spent fuels of FBRs, long-term corrosion testing of the mock-up dissolver vessel using simulated dissolver solution (SDS) comprising fission and corrosion product ions in 11.5 M nitric acid at the boiling condition has been initiated. In this work, the Zr-4 mock-up dissolver vessel NUCLEAR TECHNOLOGY

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is being used as the testing facility to evaluate the corrosion behavior of several candidate materials based on zirconium and titanium in a dissolver solution of FBTR spent fuel, which is reprocessed at the CORAL pilot plant. Moreover, to understand the passivation behavior of the candidate materials, potentiodynamic polarization studies were carried out with the test samples in SDS, and the results were compared with 11.5 M HNO3, at room temperature.

II. EXPERIMENTAL PROCEDURE II.A. Fabrication Methodology of Zr-4 Dissolver Vessel

The fabrication methodology of the Zr-4 dissolver vessel is described elsewhere.10 In brief, the machined components and cold pilgered tubes of Zr-4 were fabricated using tungsten inert gas and electron beam welding. To ensure good quality, weld qualification was done by liquid penetration testing and radiography. The dissolver assembly was subjected to double autoclaving to provide dense and adherent ZrO2 film (black colored surface) as shown in Fig. 1. II.B. Materials and Composition

Apart from qualifying the Zr-4 dissolver vessel, several candidate materials were subjected to corrosion testing in SDS. The respective compositions and conditions of the candidate materials are given in Table I. Prior

Fig. 1. Zircaloy-4 dissolver assembly (black-colored surface due to ZrO2 film formed during double autoclaving). 59


— — 0.02 0.03

— — 0.12 —

to the corrosion studies, the coupons, except the autoclaved Zr-4, were ground up to 1200 grit SiC paper, and all the samples were washed and cleaned in acetone. The compositions of the fission products and corrosion products (given in Table II) are based on the theoretical calculation from the reprocessing group of IGCAR, for the mixed carbide fuel (PuC: 70%, UC: 30%) that would have undergone a burnup of 150 GWd/tonne in the FBTR at IGCAR and after a cooling period of 1 yr. To simulate the concentration of these fission and corrosion products, calculated quantities of metal oxides or nitrates were dissolved in 11.5 M nitric acid, and the

— — — 0.004 — — — 0.64

— — — 0.03

S Si

P

C

O

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— — — 1.64 — 0.005 — —

Composition of Fission and Corrosion Product Ions for the Mixed Carbide Fuel (PuC: 70%, UC: 30%) That Would Have Undergone a Burnup of 150 GWd/tonne in the FBTR at IGCAR and After a Cooling Period of 1 yr*

bal5balance.

a

— 2.4 — — bala bal — — Zr-4 Zr-702 Titanium (CP-Ti Grade 1) SS 304L

— — bal —

1.5 — — —

0.22 0.06 0.04 bal

0.11 0.05 — 18.33

0.005 0.005 — 10.12

Mn Cu Ni Cr Fe Sn Ti Hf Zr Material/Identification

Elemental Composition of the Dissolver Materials in Weight Percentage

TABLE I

TABLE II

60

SI Number

Element

Concentration (g/, )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Ag Ba Cd Ce Cr Cs Eu Fe Gd La Mn Mo Nd Ni Pd Pr Rb Rh Ru Sb Se Sm Sn Sr Tb Te Ti Y Zr Np Am Cm U Pu

0.0971 0.4846 0.0460 0.7694 0.1812 1.4054 0.0514 2.4401 0.0319 0.4176 0.0266 1.1151 1.1859 0.1383 1.0704 0.4098 0.0701 0.3497 1.0858 0.0171 0.0144 0.3419 0.0463 0.1460 0.0030 0.1600 0.0200 0.0844 0.9002 0.0174 0.0696 0.0002 23.6842 55.2631

*Neptunium, americium, curium, uranium, and plutonium were not included in SDS. NUCLEAR TECHNOLOGY

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resultant solution is called SDS. It should be mentioned that actinides such as neptunium, americium, curium, plutonium, and uranium were not included in the simulated solution. The presence of these actinides and the related radioactivity might influence the corrosion behavior of the dissolver materials. However, handling those radioactive materials in the corrosion testing facility described in Sec. II.D is very difficult. Also, further analyses of the corroded sample exposed to the radioactive environment possess difficulties, and thus, in this study the actinides are not simulated. II.C. Electrochemical Measurements in Nitric Acid and SDS Environments

A three-electrode cell with candidate materials of commercial pure titanium (CP-Ti), Zr-4, autoclaved Zr-4, or stainless steel (SS 304L) samples at an exposed surface area of 1 cm2 as the working electrode and Ag/AgCl (3 M KCl) and platinum as the reference and counter electrodes, respectively, was used to characterize the electrochemical behavior. Except the autoclaved Zr-4 sample, the surfaces of the other samples were ground up to 1200 grit SiC paper, and all the samples were cleaned with acetone, washed in distilled water, and air dried to

promote reproducible surfaces for the electrochemical studies. In all the electrochemical measurements, the potential was referred to with respect to the Ag/AgCl (3 M KCl) reference electrode. The open circuit potential (OCP) and potentiodynamic polarization behavior of the samples were evaluated in SDS and 11.5 M HNO3 in the temperature range 20uC to 22uC (hereinafter referred to as room temperature), in an aerated condition using a potentiostat. The OCP was monitored for 7200 s prior to the tests, and a stable OCP value was obtained for all the conditions. After measuring the OCP, potentiodynamic polarization tests were performed from 200 mV below the OCP to transpassive potential or 2.5 V, at the scanning rate of 0.166 mV/s. II.D. Corrosion Testing in Zr-4 Dissolver Vessel

The dissolver vessel along with the other equipment necessary to monitor the process condition is shown as a schematic diagram in Fig. 2. About 13.5 , of SDS was filled, and the solution level was up to the first horizontal limb of the dissolver vessel. A heating pad was wound over the right vertical limb of the dissolver for heating. The temperature of the dissolver vessel was maintained using a temperature controller at 110uC, which is the

Fig. 2. Schematic diagram of the mock-up Zr-4 dissolver vessel along with the other equipment necessary for corrosion testing. NUCLEAR TECHNOLOGY

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boiling temperature of SDS. To maintain a constant negative pressure of the 3-in. water column inside the dissolver, an air ejector was provided. For measuring the level of liquid, density, and pressure inside the dissolver, suitable probes connected to transmitters were fixed. The liquid level inside the dissolver was monitored using a purge method. The dissolver off-gas line was connected to a vent pot through a down draft condenser. A polytetrafluoroethylene (PTFE) sample holder assembly shown in Fig. 3a was designed and

fabricated to accommodate the corrosion coupons. This sample holder was placed inside the left vertical limb of the dissolver vessel in such a way that the samples would be exposed to both the vapor phase (85uC) and the boiling liquid phase (110uC) of SDS as shown in Fig. 3b. Several campaigns of 100, 250, 500, 1000, and 2500 h of operation were completed. After every testing period, the corrosion rates were calculated based on the weight loss in the coupons, and the average corrosion rate was reported.

Fig. 3. (a) PTFE sample holder to accommodate the corrosion coupons exposed to both the vapor phase (85uC) and the boiling liquid phase (110uC) of SDS. (b) Schematic of the sample holder assembly placed inside the left vertical limb of the Zr-4 dissolver vessel. 62

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II.E. Surface Morphology and LRS Analysis After Corrosion Testing

The surface of the candidate materials after exposure to the vapor phase (85uC) and the boiling liquid phase (110uC) of SDS for 2500 h was characterized by a scanning electron microscope (SEM) and by laser Raman spectroscopy (LRS). An HR800 (Jobin Yvon) Raman spectrometer equipped with 1800 groove/mm holographic grating was used. The samples were placed under an Olympus optical microscope mounted at the entrance of the Raman spectrograph. An argon ion laser of wavelength 488 nm was used as an excitation source. The laser spot size of 3-mm diameter was focused on the sample surface using a 50| (NA 5 0.75) objective. The laser power at the sample was *10 mW. The slit width of the monochromator was 400 mm. The backscattered Raman spectra were recorded using a supercooled charge-coupleddevice (CCD) detector, over the range 80 to 800 cm{1 with 5-s exposure time and 20 CCD accumulations. All the spectra were baseline corrected.

III. RESULTS AND DISCUSSION III.A. Potentiodynamic Polarization Behavior of Candidate Materials in SDS and Nitric Acid at Room Temperature

The polarization curves recorded for SS 304L and CP-Ti in 11.5 M HNO3 and SDS at room temperature are shown in Fig. 4a. Similarly, the polarization curves recorded for Zr-4 and autoclaved Zr-4 in 11.5 M HNO3 and SDS at room temperature are shown in Fig. 4b. Corrosion parameters such as the corrosion potential Ecorr and corrosion current density Icorr were calculated and are listed in Table III. The Ecorr and Icorr values were obtained by the well-known Tafel method.11 In comparison to nitric acid, the Ecorr values shifted to the noble direction in SDS for all the samples studied, indicating the oxidizing nature of the simulated fission and corrosion product ions. For SS 304L in SDS, the Ecorr value was 0.98 V, and it was closer to the transpassive potential Etp of 1.2 V, indicating that SS 304L was very actively corroding, as high current values were obtained when compared to its anodic polarization behavior in 11.5 M HNO3. It is known that the formation of a thin Cr2O3 film is responsible for the passive behavior of SS 304L at lower concentrations of nitric acid. Under transpassive conditions, the protective Cr2O3 film is oxidized to Cr6z, resulting in the formation of either chromic acid or dichromate. As Cr6z ions have high solubility, the corrosion resistance behavior of SS 304L deteriorates, resulting in severe intergranular corrosion of stainless steel even in nonsensitized conditions.2,5,6 The transpassive dissolution of SS 304L was observed at high concentrations of HNO3, at high temperature and in the NUCLEAR TECHNOLOGY

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Fig. 4. Polarization curves recorded for candidate materials in 11.5 M HNO3 and SDS at room temperature: (a) SS 304L and CP-Ti and (b) Zr-4 and autoclaved Zr-4 (AC Zr-4).

presence of highly oxidizing chemical species.2,5,6 In the present study, oxidizing ions present in SDS favored the formation of Cr6z rather than Cr3z, thus shifting Ecorr closer to the transpassive potential that led to severe corrosion of SS 304L. On the other hand, for CP-Ti in nitric acid at potentials wEcorr, the current values remained almost constant due to the formation of a protective passive film of TiO2 (Ref. 12). Figure 4a, reveals that the corrosion current values and passive current values for CP-Ti are 63

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TABLE III Polarization Parameters of SS 304L, CP-Ti, Zr-4, and Autoclaved Zr-4 in 11.5 M HNO3 (Nitric) and SDS*

Material/Electrolyte

Ecorr

Icorr

Epass

Ipass

Etp

V versus Ag/AgCl

| 10{6 A/cm2

V versus Ag/AgCl

| 10{6 A/cm2

V versus Ag/AgCl

SS 304L

Nitric SDS

0.77 0.98

4.88 23.4

0.85 —

15.2 —

1.2 1.2

CP-Ti

Nitric SDS

0.74 0.90

0.159 0.114

0.90 1.10

0.626 0.576

— —

Zr-4

Nitric SDS

0.84 0.92

0.103 0.061

0.93 1.0

0.296 0.22

1.5 1.8

Autoclaved Zr-4

Nitric SDS

0.92 0.93

0.00067 0.00016

1.5 1.5

0.205 0.049

— —

*Polarization parameters: corrosion potential Ecorr, passivation potential Epass, transpassive potential Etp, corrosion current density Icorr, and passive current density Ipass.

quite similar in both nitric acid and SDS at room temperature. In contrast to SS 304L, no transpassive behavior was observed for CP-Ti even until 2.5 V, indicating that the oxidizing ions present in the dissolver solution had no adverse effect on the passivation of CP-Ti (Refs. 13 and 14). In nitric acid and SDS, the Zr-4 sample also exhibited spontaneous passivation with a distinct passive region, until the Etp, as shown in Fig. 4b. Passivation of Zr-4 in nitric acid is attributed to the formation of ZrO2, and the Etp was *1.5 V in nitric acid.7 In SDS, the Etp shifted to 1.8 V. Although the Ecorr value and Etp shifted to the noble direction (as could be seen in Fig. 4b), it is evident that the Zr-4 sample exhibited a wider passive region with low current values in SDS when compared to nitric acid. This result implies that the oxidizing ions present in the dissolver solution have a beneficial effect on the passivation of Zr-4. To improve the corrosion behavior of Zr-4 further, autoclaving was done, and these samples were subjected to a potentiodynamic polarization test in both nitric acid and SDS. Autoclaving of Zr-4 resulted in the formation of a thick, black-colored oxide layer of ZrO2 (Ref. 15). Unlike other candidate materials studied, the ennoblement in the Ecorr value due to the presence of the oxidizing ions was negligible for the autoclaved Zr-4 sample, indicating that the presence of oxidizing ions had an insignificant effect. The lowest corrosion current values obtained for the autoclaved Zr-4 samples both in nitric acid and SDS suggested that the ZrO2 that formed during autoclaving acted as a barrier layer. Though current was fluctuating above Ecorr, in the passive region, the measured current values were lower than that for other candidate materials. Interestingly, the increase in current values due to the transpassive behavior observed for Zr-4 was not noticed for the autoclaved Zr-4 until 2.5 V. The potentiodynamic 64

polarization results (Figs. 4a and 4b) obtained in both nitric acid and SDS indicated that the highest corrosion resistance is offered by the autoclaved Zr-4 sample when compared to the other candidate materials used in this study. The candidate materials CP-Ti, Zr-4, and autoclaved Zr-4 have been demonstrated to possess high passivation ability from the electrochemical studies in 11.5 M HNO3 and SDS at room temperature. Thus, the corrosion resistance of these materials and their weldments were evaluated as discussed in Secs. III.B and III.C by weight loss measurements, by exposing the samples to the boiling liquid and vapor phases of SDS. III.B. Corrosion Behavior of CP-Ti and Its Weld in the Vapor and Boiling Phases of SDS

The average corrosion rates for CP-Ti and CP-Ti weld exposed for 2500 h in the boiling liquid and vapor phases of SDS are shown in Fig. 5. The average corrosion rates for the other materials such as zirconium-702 (Zr702), Zr-702 weld, Zr-4, Zr-4 weld, and autoclaved Zr-4 exposed for 2500 h in boiling liquid of SDS are also given in Fig. 5. The corrosion rates of the candidate materials are expressed in both micrometers per year (mm/ yr) and mils per year. The average corrosion rate of CP-Ti and CP-Ti weld in the boiling liquid phase of SDS was *0.1 mm/yr (0.004 mil/yr) and 0.23 mm/yr (0.009 mil/ yr), respectively. In comparison to the boiling liquid phase, higher corrosion rate values of 1.0 mm/yr (0.04 mil/yr) and 1.9 mm/yr (0.075 mil/yr) were obtained for CP-Ti and CP-Ti weld, exposed to the vapor phase of SDS. The lower corrosion rate values in boiling liquid clearly indicate that the simulated fission and corrosion product ions present in the dissolver solution did not have an adverse effect on passivation of the CP-Ti samples. It is NUCLEAR TECHNOLOGY

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Fig. 5. Average corrosion rates for all the candidate materials exposed for 2500 h in the boiling liquid and vapor phases of SDS (mpy 5 mil/yr).

reported that small amounts of dissolved species of Fe3z, Cr6z, Ti4z, Si4z, and various fission product ions can effectively inhibit the corrosion of titanium in hot boiling nitric acid solutions.13,14,16 An unaffected surface morphology observed (as shown in Figs. 6a and 6b) for the CP-Ti and CP-Ti weld samples immersed in the boiling liquid phase of dissolver solution for 2500 h indicated the high corrosion resistance of the titanium sample. The corrosion resistance of titanium in boiling nitric acid is due to the formation of a strong and stable TiO2 film that developed on its surface. Titanium dissolved as Ti4z ions

and the excessive Ti4z ions formed in the liquid facilitated the formation of such protective TiO2 film on the surface. Owing to the low concentration of Ti4z ions, it is reported that a semiprotective, loosely adherent, and hydrated TiO2 film is formed in the vapor and condensate phases of boiling 11.5 M HNO3 (Ref. 1). The SEM images (Figs. 6c and 6d) observed for the CP-Ti and CPTi weld samples exposed to the vapor phase of SDS for 2500 h indicated a corroded surface with rough morphology when compared to the surface of the titanium samples exposed to the liquid phase (Figs. 6a and 6b). It is also reported that the titanium samples failed at the weld and heat-affected zones in boiling nitric acid due to the preferential attack of interconnected needles of the Ti-Fe beta phase.1,16 The formation of the beta phase is due to the higher content of iron (w0.05 wt%). Preferential attack was not observed in the present study for the CP-Ti weld sample exposed for 2500 h in the vapor and boiling liquid phases of SDS, which indicates that the beta phase was not formed as the iron content was only 0.04 wt%. III.C. Corrosion Behavior of Zr-4, Zr-702, and Their Welds in the Boiling Phase of SDS

The average corrosion rate of Zr-4 and its weld was 0.15 mm/yr (0.006 mils/yr) and 0.23 mm/yr (0.009 mils/yr), respectively, when exposed to the boiling liquid phase of SDS for 2500 h. In the same corrosive environment, the average corrosion rate exhibited by Zr-702 in the as-received and weld conditions was 0.13 mm/yr (0.005 mils/yr), which is marginally lower

Fig. 6. SEM images of CP-Ti and CP-Ti weld samples exposed in the boiling liquid and vapor phases of SDS for 2500 h: (a) CP-Ti in liquid phase, (b) CP-Ti weld in liquid phase, (c) CP-Ti in vapor phase, and (d) CP-Ti weld in vapor phase. NUCLEAR TECHNOLOGY

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than that of the Zr-4. In the acidic medium, zirconium dissolves as zirconic ions, Zr4z, and zirconyl ions, ZrO2z, with the evolution of H2. These Zr4z ions are responsible for the formation of stable ZrO2 on the sample surface and ensuring the corrosion resistance of zirconium.9 Insignificant corrosion rates measured for all the zirconium samples in the boiling liquid phase of the simulated dissolver environment indicate that there is no adverse effect of the fission and corrosion product ions on the corrosion rate of the zirconium samples. This observation is corroborated by the literature reports that the presence of several oxidizing agents like oxygen, Fe3z, and heavy metal ions has little or no effect on the corrosion rate of zirconium.17–19 SEM images shown in Figs. 7a and 7b of Zr-4 and its weld exposed to boiling dissolver solution for 2500 h revealed no significant corrosion attack. A similar unaffected surface was observed for the samples of Zr-702 and Zr-702 weld, as shown in Figs. 7c and 7d, respectively. These results clearly indicate that the corrosion behavior of zirconium is

unaffected by the metallurgical state of the material, i.e., the welded conditions.8,9 In this study, when compared to other candidate materials, the lowest corrosion rate of 0.08 mm/ yr (0.003 mils/yr) was obtained for the autoclaved Zr-4 sample, exposed to boiling SDS for 2500 h. The surface morphology of the autoclaved sample exposed to boiling SDS revealed a smooth surface of ZrO2 grains, as shown in Fig. 7e. The low corrosion rate data obtained for Zr-4 and Zr-702 in boiling SDS up to 2500 h is motivating, and thus, a longer exposure period of *2 to 3 yr will be necessary to qualify these candidate materials for 40 yr of service life. III.D. Corrosion Behavior of Dissimilar Welds of Zr-4 and CP-Ti with SS 304L in the Vapor and Boiling Phases of SDS

In the reprocessing plant, the zirconium or titanium dissolver needs to be connected to the rest of the process vessels and pipings made up of SS 304L. Fusion welding

Fig. 7. SEM images of zirconium samples exposed to the boiling liquid phase of SDS for 2500 h: (a) Zr-4, (b) Zr-4 weld, (c) Zr-702, (d) Zr-702 weld, and (e) autoclaved Zr-4. 66

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of these dissimilar materials would produce brittle intermetallic precipitates at the interface that reduce the mechanical strength as well as the corrosion resistance of

Fig. 8. Corrosion rates for the CP-Ti/SS 304L and Zr-4/SS 304L dissimilar explosive welds exposed in the vapor and boiling liquid phases of SDS (mpy 5 mil/yr).

the joint. Thus, solid-state joining processes such as explosive and friction joining are employed to join Zr-4 and CP-Ti to SS 304L (Refs. 20 and 21). However, friction welding of these dissimilar welds was not considered, as the bend ductility is poor when compared to the values obtained for the explosive welding of dissimilar joints.20 Thus, in this work the corrosion rates of the explosively welded Zr-4/SS 304L and CP-Ti/SS 304L samples were evaluated in SDS. High corrosion rates were obtained for the dissimilar explosive welds, as shown in Fig. 8. The corrosion rate for the Zr-4/SS 304L dissimilar weld was as high as 6.9 mm/yr (273 mils/yr) in the boiling dissolver solution exposed for 1000 h. Figures 9a and 9b are the SEM images of Zr-4/SS 304L dissimilar welds exposed for 250 and 1000 h in boiling SDS. In Fig. 9a, dissolution was witnessed along the interface of the weld, and intergranular corrosion was observed at the SS 304L side. Moreover, the arrow mark indicated in Fig. 9a shows the preferential dissolution of Fe-rich intermetallics that formed during explosive welding. In a previous study, the higher electrochemical activity of the Fe-rich intermetallics, present along the interface of the Zr-4/SS 304L dissimilar weld, was characterized by scanning electrochemical microscopy in nitric acid.22 For a longer exposure period of 1000 h, Zr4/SS 304L dissimilar weld samples failed (i.e., completely detached) at the interface as shown in Fig. 9b. SEM images in Figs. 9c and 9d (marked area of Fig. 9b) confirmed that the Zr-4 side of the weld showed no sign

Fig. 9. SEM images of Zr-4/SS 304L dissimilar welds exposed to the boiling liquid phase of SDS: (a) exposed for 250 h, (b) exposed for 1000 h, (c) Zr-4 side of the dissimilar weld exposed for 1000 h, and (d) SS 304L side of the dissimilar weld exposed for 1000 h. NUCLEAR TECHNOLOGY

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(a)

(b)

Fig. 10. SEM images of CP-Ti/SS 304L dissimilar welds exposed to SDS for 500 h in (a) the boiling liquid phase and (b) the vapor phase.

of corrosion damage, whereas the SS 304L side had undergone severe grain dissolution. Similarly, high corrosion rates of 6.3 mm/yr (247 mils/yr) and 5.3 mm/yr (208 mils/yr), respectively, were obtained for the CP-Ti/SS 304L dissimilar weld, exposed to the vapor and boiling liquid phases of SDS for a period of 500 h. In this condition, the CP-Ti/SS 304L dissimilar weld failed at the interface, and the SS 304L side of the weld had undergone severe grain dissolution when exposed to the liquid and vapor phases as shown in Figs. 10a and 10b, respectively. Thus, for the safe operation of the plant, a code of practice is recommended that the dissolver solution should be transferred through the dissimilar weld joints, only at room temperature for the purpose of extending the service life of the dissimilar weld tubes. Also, the dissimilar welds should be placed far away from the dissolver vessel, so that the vapor phase attack can be minimized during the dissolution of the spent nuclear fuel. III.E. Origin of Passivation of CP-Ti and Zr-4 in SDS

Figure 11a shows the LRS spectrum of the CP-Ti sample exposed to the boiling liquid and vapor phases of SDS for 2500 h. The reported Raman frequencies for the rutile TiO2 phase are 144, 235, 320 to 360, 448, 612, and 827 cm{1, and for the anatase TiO2 phase, the frequencies are 147, 198, 320, 398, 448, 515, 640, and 796 cm{1 (Ref. 23). The high-intensity bands for the rutile and anatase phases are at 448 and 147 cm{1, respectively.23 It is worth mentioning that for the CP-Ti samples all the peaks did not appear. For the CP-Ti samples exposed to the vapor and liquid phases, the strongest band observed at 144 cm{1 with a minor shift of 3 cm{1 confirmed the presence of the anatase phase. The other band positions of anatase and rutile were also shifted, indicating the presence of the mixed phase of rutile and anatase in the passive films formed in the vapor and boiling liquid phases of SDS. Figure 11b shows the LRS spectra of Zr-702, Zr-4, and autoclaved Zr-4 samples exposed to the 68

boiling liquid phase of SDS for 2500 h. Hirata24 has assigned the frequencies of 170, 183, 213, 291, 329, 376, 468, 502, 529, 552, 602, 626, and 745 cm{1 for the monoclinic ZrO2 phase. For Zr-702, Zr-4, and autoclaved Zr-4 exposed to the boiling dissolver solution, a major peak around 175 to 180 cm{1 and minor peaks in the range 338 to 383 cm{1 correspond to the monoclinic ZrO2 phase, as shown in Fig. 11b. For the tetragonal ZrO2 phase, Raman peaks are reported to appear in the ranges 145 to 149, 259 to 269, 318 to 319, 461 to 473, 602 to 607, and 641 to 650 cm{1 (Refs. 25, 26, and 27). In Fig. 11b, the peak at 638 to 643 cm{1 and the minor peak at 272 cm{1 correspond to the tetragonal ZrO2 phase. The peak at 479 cm{1 can be assigned to both the monoclinic and the tetragonal phases of ZrO2. It should be mentioned that all the peaks corresponding to the monoclinic and tetragonal phases did not appear. The shift in the peak positions also indicated the formation of the monoclinic and tetragonal mixed phase of ZrO2 in the passive films formed in the boiling liquid phase of SDS. The formation of the mixed phases of anatase and rutile TiO2 on titanium samples as well as the monoclinic and tetragonal phases of ZrO2 on zirconium samples has been confirmed by LRS studies. These oxide films are responsible for the observed high corrosion resistance of Ti-based and Zr-based materials in SDS. III.F. Inspection of Mock-Up Zr-4 Dissolver Vessel

Visual inspection of the Zr-4 dissolver vessel was successfully carried out. The outer surface of the vessel and all the accessible welds were examined thoroughly. In general, no significant corrosion attack was observed in the Zr-4 dissolver vessel, operated for 2500 h. The significance of these inspection results is that they would serve as the baseline data for the values to be accrued during the subsequent inspection campaigns. Long-term corrosion data under a plant simulated condition will be obtained through 2 to 3 yr of NUCLEAR TECHNOLOGY

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(a)

(b)

Fig. 11. (a) LRS spectra of the CP-Ti sample exposed to the boiling liquid and vapor phases of SDS for 2500 h. (b) LRS spectra of Zr-702, Zr-4, and autoclaved Zr-4 samples exposed to the boiling liquid phase of SDS for 2500 h.

operation, and the result will be used to model and predict the life expectancy of the Zr-4 dissolver vessel and the other candidate materials.

IV. CONCLUSIONS

Potentiodynamic polarization studies of SS 304L in nitric acid and SDS revealed that the presence of fission and corrosion product ions in the dissolver solution promoted transpassive dissolution, leading to severe corrosion. However, in the case of CP-Ti, Zr-4, and autoclaved Zr-4, the fission and corrosion product ions in SDS have no adverse influence in their passivation ability. The corrosion current density and passivation current density confirmed that the highest corrosion resistance is offered by the autoclaved Zr-4 sample when compared to the other candidate materials investigated. Candidate materials such as Zr-702, Zr-4, autoclaved Zr-4, and CP-Ti exhibited low corrosion rates of 0.08 to 0.23 mm/yr (0.003 to 0.009 mils/yr) in the as-received and welded conditions exposed to the boiling liquid phase of the dissolver solution for 2500 h, whereas the CP-Ti and CPTi weld exhibited marginally higher corrosion rates of 1.0 mm/yr (0.04 mils/yr) and 1.9 mm/yr (0.075 mils/yr), respectively, in the vapor phase of the dissolver solution. SEM investigations revealed no sign of corrosion attack on the titanium and zirconium materials. The lowest corrosion rate value of 0.08 mm/yr (0.003 mils/yr) was obtained for the autoclaved Zr-4 sample exposed to boiling SDS for 2500 h, and its surface morphology revealed a smooth surface of ZrO2 grains. LRS analysis confirmed that the origins of the passivity of the NUCLEAR TECHNOLOGY

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zirconium and titanium samples were due to the formation of protective passive film composed of ZrO2 and TiO2, respectively. On the other hand, the CP-Ti/SS 304L and Zr-4/SS 304L dissimilar weld coupons failed at the interface when exposed to the boiling liquid and vapor phases of SDS. Thus, for the safe operation of the plant, we recommend a code of practice such that the dissolver solution is transferred through the dissimilar weld joints, only at room temperature, to extend the service life of the dissimilar weld tubes. Also, the dissimilar welds should be placed far away from the dissolver vessel so that the vapor phase attack can be minimized during the dissolution of the spent nuclear fuel. Visual inspection of the outer surface of the Zr-4 dissolver vessel and all the accessible welds revealed no corrosion attack on the Zr-4 dissolver vessel operated for 2500 h. Long-term corrosion data under a plant simulated condition will be obtained through 2 to 3 yr of operation, and the result will be used to model and predict the life expectancy of the Zr-4 dissolver vessel and the other candidate materials. ACKNOWLEDGMENTS The authors are thankful to U. Veeramani and S. Selvam of Reprocessing Group, IGCAR, for the operation of the Zr-4 dissolver facility. They acknowledge T. Nandakumar and S. Ramya of Corrosion Science and Technology Group, for recording Raman spectra and useful discussions.

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