Influences of Cr content and PWHT on microstructure and oxidation behavior of stainless steel weld overlay cladding materials in high temperature water

Journal of Nuclear Materials 467 (2015) 32e41

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Influences of Cr content and PWHT on microstructure and oxidation behavior of stainless steel weld overlay cladding materials in high temperature water X.Y. Cao a, X.F. Ding a, Y.H. Lu a, *, P. Zhu b, T. Shoji a, c a b c

National Center for Materials Service Safety, University of Science and Technology Beijing, 30 Xueyuan Road, 100083 Beijing, China Suzhou Nuclear Power Research Institute Co. Ltd., 1788 Xihuan Road, 215004 Suzhou, China Fracture and Reliability Research Institute, Tohoku University, 6-6-01 Aramaki Aoba, Aoba-ku, Sendai City 980-8579, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 June 2015 Received in revised form 1 September 2015 Accepted 8 September 2015 Available online 10 September 2015

Influences of Cr content and post weld heat treatment (PWHT) on microstructure and oxidation behavior of stainless steel cladding materials in high temperature water were investigated. The amounts of metal oxidized and dissolved were estimated to compare the oxidation behaviors of cladding materials with different Cr contents and PWHT. The results indicated that higher Cr content led to formation of more ferrite content, and carbides were found along d/g phase interface after PWHT. Higher Cr content enhanced the pitting resistance and compactness of the oxide film to reduce metal amount oxidized and dissolved, which mitigated the weight changes and the formation of Fe-rich oxides. PWHT promoted more and deeper pitting holes along the d/g phase interface due to formation of carbides, which resulted in an increase in metal amount oxidized and dissolved, and were also responsible for more Fe-rich oxides and higher weight changes. © 2015 Elsevier B.V. All rights reserved.

Keywords: Overlay cladding Cr content Post weld heat treatment (PWHT) Oxidation

1. Introduction Nowadays, weld overlay cladding materials (E308L and E309L stainless steels) are widely applied for reactor pressure vessels (RPVS) in nuclear power plants [1,2], where E309L is used as a transition layer and E308L is a welded overlay on the surface of E309L cladding. It is obvious that the corrosion resistance is an important issue for the cladding serviced in high temperature water, in which the oxide film inevitably forms on the surface of the cladding materials during service, which eventually determines the corrosion resistance of the cladding materials [3e6]. On the other hand, the post weld heat treatment (PWHT) can release the stress in the cladding, thus may influence corrosion behavior of the cladding materials [7]. Although neutron irradiation [8e10], SCC [11e13] and thermal aging of the weld overlay cladding [14e16] were reported widely, up to now the corrosion behavior of the cladding has been attracted little attention [17,18]. The study by Qiu et al. indicated that the cladding could exhibit different corrosion behaviors even though

* Corresponding author. Tel.: þ86 10 62332085; fax: þ86 10 62329915. E-mail address: [email protected] (Y.H. Lu). http://dx.doi.org/10.1016/j.jnucmat.2015.09.015 0022-3115/© 2015 Elsevier B.V. All rights reserved.

the properties of the oxide film formed on the cladding surface were almost the same [17]. Due to dissolution of Fe and Ni during the corrosion in high temperature water [19e22], the metal ions, such as nickel ions, were released through the oxide film to primary water and deposited on the fuel clad surface with boron, which might lead to the crud induced power shift (CIPS) or the axial offset anomaly (AOA) due to the neutron absorption nature of boron [19]. Accordingly, the ionic release and oxide film compactness were studied to illustrate the growth mechanism of the oxide film [4,19,20]. However, very limited reports are concentrated on the specific information concerning the metal ions oxidized and release with a mass resolution, especially in high temperature water, which results in an incomprehensive understanding of the corrosion procedure and dynamics. As we know, the oxide film formation is strongly dependent on Cr content of the stainless steels. Hamm et al. found an enrichment of Cr in a passive region, but a preferential dissolution of Cr occurred when the potential swept into a transpassive region in the oxide film [22]. Nevertheless, influences of the Cr content on metal ions as well as on the oxide film are still unclear in the cladding materials, even not exposed to PWTH. The objective of this work is to investigate the effects of Cr content and PWHT on microstructure and oxidation behavior of weld overlay cladding in high temperature water. The amounts of

X.Y. Cao et al. / Journal of Nuclear Materials 467 (2015) 32e41

metal oxidized and metal dissolved are estimated to compare the oxidation behavior in stainless steel cladding materials with different Cr contents and PWHT. 2. Materials and methods 2.1. Materials and welding Two types of E308L stainless steel alloys, named as A and B, were used as cladding materials on E36 base metal. The composition of E36 was similar to that of A36. Aiming to investigate the oxidation behavior of E308L weld metal, three-layer weld pads were deposited using SMAW (shielded metal arc welding) to obtain the undiluted weld metal. The chemical compositions of the base and weld metals are listed in Table 1. The Cr element content is 17.47% in B and 19.04% in A. The two cladding plates were separately cut into two parts after welding, respectively. One part is as-welded, named as A1 and B1, another part is subjected to a PWHT at 615  C for 16 h, named as A2 and B2, respectively. The present experimental welding samples under different conditions are listed in Table 2. 2.2. Microstructural examination The samples for microstructure examination, with a dimension of 15 mm  10 mm  2 mm, were cut from the top layer of the cladding plates. These samples were then mechanically ground to 2000# grit on SiC papers and then polished using a 2.5 mm diamond paste. The microstructures were examined by optical microscopy (OM) after etching by kalling's reagent (10 g CuCl2, 100 ml HCl and 100 ml ethanol). Each sample obtained more than 10 optical images, which were used to calculate the average amount of ferrite phase by image J software. Transmission electron microscope (TEM) specimens were first mechanically polished to approximately thickness of about 0.1 mm followed by polishing at 25 V in a solution of 10% perchloric acid in 90% methyl alcohol with a twinjet unit. The TEM observations were performed in a FEI-F20 transmission electron microscopy operated at 200 KV. 2.3. Oxidation testing All samples were gradually ground with silicon papers up to 800# grit and cleaned in ethanol and pure water, followed by dried in air immediately. The samples were measured for the weight first and subsequently immersed into the 2.6 L solution in 5 L static autoclave made of stainless steel. The solution in the autoclave contains 2.2 ppm Liþ as LiOH and 800 ppm B3þ as H3BO3. Just before test, the high pure argon was purged into the autoclave for half an hour to ensure low oxygen content in the solution. After that, the temperature and pressure of autoclave were gradually increased to 350  C and 21 MPa, respectively. After exposure for 750, 1500, 2250 and 3000 h, the samples were taken out to clean and dried first. The weight of each sample was subsequently measured to obtain the weight gain. In order to obtain the oxide weight, the oxide film of all the samples were removed by pickling method recommended by other's work [23] and the weight of samples after removing the

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oxide film was measured again. The X-ray photoelectron spectroscopy (XPS) analyses were performed by an AXIS ULTRADLD X-ray photoelectric spectroscopy. The photoelectron emission was excited by monochromatic Al Ka source operated at 150 W with initial photo energy of 1486.6 eV. The C1s peak from adventitious carbon at 248.8 eV was used as a reference to correct the wavelength shift. The depth profiling of XPS was performed by an ESCALAB 250 X-ray photoelectric spectroscopy, high-resolution spectra were obtained using a focusing X-ray monochromatic with a 0.65 mm spot size and monochromatic Al Ka source operated at 200 W was used as the sputtering gun. Quantification of the element in the oxide film was performed by XPS PEAK 4.1 peak-fitting software. 3. Results 3.1. Microstructure Fig. 1 displays the optical microstructure of the cladding materials with different Cr contents and PWHT. From Fig. 1, it is found that the typical microstructure of the stainless steel cladding includes austenite and ferrite phases, where the worm-like or islandlike ferrites (black phases in Fig. 1) are distributed uniformly in the austenitic matrix (gray phases in Fig. 1) in all the samples. There is no obvious difference in microstructure with different Cr contents and PWHT. The corresponding ferrite content of the cladding materials with different Cr contents and PWHT is displayed in Fig. 2. It is found that ferrite content of A (A1 and A2) is about 7.9%, which is more than that of B (B1 and B2) (6.8%). However, there is not much different in ferrite content between A1 and A2, as well as between B1 and B2. The results indicate that high Cr content results in an increase in ferrite content, while PWHT has no much influence on ferrite content in the cladding materials. Fig. 3(a) and (b) show bright-field TEM images of A1 and B2, for B2, the corresponding selected area electron diffraction (SAED) patterns are inserted. From Fig. 3(a), it is found that the relatively bright d-ferrite phase and dark g-phase exist in A1 without any precipitates are found and there is clear sharp interface between them. By comparison, The inserted SAED pattern in the d-ferrite phase in Fig. 3(b) indicates the ferrite exhibits a bcc structure, while that including precipitate and its surrounding austenite matrix, as shown in Fig. 3(b), indicates that the precipitates M23C6 exhibit a fcc structure, and show an orientation relationship with the austenite matrix of <001>M//<001>A. Fig. 3(c) shows the corresponding EDS result of the precipitate marked as “C” in Fig. 3(b), which reveals a high Cr concentration in the M23C6, as shown in Fig. 3(c). The corresponding chemical composition (wt. %) as follows: Cr (59.38), Mn (3.69), Ni (2.90) and Fe (bal.) 3.2. Oxidation behaviors Fig. 4(a) shows the weight gain of the samples after different exposure times. The corresponding surface morphologies of samples after 750 h and 3000 h test are inserted. As shown in Fig. 4(a), with increasing the test duration, weight gain of all the samples increases rapidly and then slowly to an almost stable value. The

Table 1 Chemical compositions of base and weld metals (wt. %). Alloy

C

Cr

Ni

Si

Mn

S

P

Mo

N

Fe

E36 A B

0.16 0.025 0.034

<0.20 19.04 17.47

/ 9.97 9.2

0.36 0.70 0.39

1.10 0.88 0.7

<0.015 0.0068 0.0075

<0.02 0.015 0.016

/ 0.083 0.09

/ 0.043 0.03

Bal Bal Bal

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Table 2 Present experimental welding samples under different conditions. Conditions

Samples

A

B

As welded

PWHT(615  C  16 h)

As welded

PWHT(615  C  16 h)

A1

A2

B1

B2

Fig. 1. Optical microstructure of (a) A1, (b) A2, (c) B1, and (d) B2. Austenite (g) is gray and ferrite (d) is dark.

Fig. 2. Ferrite contents of samples with different Cr contents and PWHT.

gain in weight of B (B1 and B2) is higher than that of A (A1 and A2). For the same cladding material, the samples after PWHT have higher weight gain than that of the as-welded ones for each test period. The corresponding surface morphologies of samples after

750 h and 3000 h exposure indicate that the oxide film is formed on all the samples with different Cr contents and PWHT after high temperature water exposure. As shown in Fig. 4(a), a few large oxide particles in size of several micrometers are formed after 750 h exposure. With increasing exposure time, more oxide particles form on the samples with different Cr contents and PWHT after 3000 h exposure. In addition, it is found that these oxide particles dispersed on the surface have a straight edge and planar face, and their maximum size are not more than 2 mm in diameter even after 3000 h exposure. Fig. 4(b) and (c) show the rate of weight gain and area percent of oxide particles of samples after different exposure times, respectively. As shown in Fig. 4(b), the rate of weight gain of the samples decreases rapidly and then slowly to an almost stable value with increasing exposure time. A similar result on the area percent of oxide particles is observed on all the samples, as shown in Fig. 4(c), which suggests that more oxide particles formation occur in all the samples with increasing exposure time. On the other hand, more oxide particles formation and more area percent of oxide particles in B than that in A with higher Cr content after 3000 h exposure, which suggests that the nucleation and growth of oxide particles are associated with Cr content. Besides, more oxide particles and area percent of oxide particles can also be observed in samples after PWHT (A2 and B2) than that in as welded samples (A1 and B1) in the same material, as shown in Fig. 4, which indicates that low Cr content and PWHT contribute to more oxidation of the cladding materials.

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Fig. 3. Bright-filed TEM morphologies of (a) A1, (b) B2 and (c) EDS result of the precipitate marked as “C” in Fig. 3(b). The diffraction patterns of the precipitate and its surrounding austenite matrix are inserted.

3.3. Structure and phase configuration of oxides Fig. 5 displays XRD patterns of oxide film formed on samples with different Cr contents and PWHT after 3000 h exposure. It is found that all the peaks coming from the oxide film are identified as Fe3O4, FeCr2O4 and Cr2O3, which indicates that Fe3O4, FeCr2O4 and Cr2O3 crystallites appear in the oxide films. Fig. 6(a) and (b) show bright-field TEM and high resolution images of the oxide film formed on A2 after 3000 h exposure. As displayed in Fig. 6(a), many fine oxide particles with a diameter size of 5 nme15 nm are distributed in the oxide film. The corresponding selected area electron diffraction (SAED) pattern of the oxide film indicates that the main phases of oxide film are FeCr2O4 and Cr2O3. From Fig. 6(b), it is found that the lattice fringe spacing of the fine oxide particle is 0.48 nm and 0.26 nm, respectively, which identifies that the oxides of the oxide film formed on A2 are FeCr2O4 and Cr2O3 respectively. The detailed XPS spectra of Fe 2p3/2, Cr 2p3/2 for the oxide film formed on the surface of the different samples after 3000 h exposure are shown in Fig. 7. The inserted is the complete Fe 2p and Cr 2p XPS spectra for the corresponding oxide film. The binding energies EB of XPS-peaks of the Fe, Cr and Ni after 3000 h exposure are listed in Table 3. As shown in Fig. 7(a) and (b), the Fe 2p3/2 peak is fitted into two possible peaks. One peak at about 710.5 eV is assigned to spinel (NiFe2O4 and FeCr2O4), as reported by Xu et al. [24], while another one at about 711.8 eV is assigned to FeOOH [25]. It can be found that the location of the XPS-peaks of the Fe 2p3/2 spectra is shifted from 710.5 eV to 711.1 eV (larger binding energy)

with a decrease in Cr content, which indicates that the high Cr content leads to formation of more Fe-hydroxides. Similarly, PWHT can result in formation of more Fe-hydroxides, as shown in Fig. 7. As displayed in Fig. 7(c) and (d), the main Cr 2p3/2 peak is fitted into two possible peaks, one peak at about 576.5 eV is assigned to Cr2O3 or FeCr2O4 [6,24,25], while another one at about 577.4 eV is assigned to Cr(OH)3 [6,24e26]. It is noted that the locations of the main Cr 2p peaks does not shift with increasing Cr content or after PWHT (Table 3), which indicate that the Cr content and PWHT have no much influence on the phase configuration of Cr-oxides. Fig. 8 shows XPS depth profiles of Fe, Cr and Ni of A1 and B2 after 3000 h exposure. It can be clearly seen from Fig. 8 that the oxide film of all the samples consists of two layers, an inner layer Cr rich and an outer layer Fe and Ni rich layers. As shown in Fig. 8(a), with increasing sputtering time, Fe content decreases firstly and then increases to the matrix level, while the variation tendency of Cr content is opposite to that of Fe content, as shown in Fig. 8(b), revealing a strong Cr enrichment in the oxide film by dissolution of Fe and Ni. As shown in Fig. 8(c), Ni content in the oxide film is very low and its change is not obvious. It should be noted that the element content of the oxide film between A1 and B2 is much different. As shown in Fig. 8(a), (b) and (c), higher concentration of Cr content and higher depletion of Fe, Ni is found in A1 than B2. In addition, the outer layer reveals larger composition disparity than the inner layer between A1 and B2, which indicates that the influence of Cr content and PWHT on the composition of outer layer is more significant than that of inner layer.

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X.Y. Cao et al. / Journal of Nuclear Materials 467 (2015) 32e41

Fig. 4. (a) Weight gain as a function of exposure time, (b) the rate of weight gain and (c) area percent of oxide particles formed on the surface of samples after different exposure times. The corresponding surface morphologies of samples after 750 h and 3000 h exposure are inserted.

Fig. 5. XRD patterns of oxide film formed on samples with different Cr contents and PWHT after 3000 h exposure.

Fig. 9(a) and (b) show the detailed fitted XPS spectra of Fe 2p3/2, Cr 2p3/2 and O 2p3/2 of the oxide film formed on the surface of A1 and B2 with sputtering time. For Fe, a main peak at about 710.5 ± 0.3 eV is found in Fe 2p3/2 spectrum, which is assigned to FeCr2O4 [5,24e26], the peaks of metallic Fe (706.7 ± 0.3 eV) and FeO (709 ± 0.3 eV) are found in Fe 2p3/2 beneath the surface of oxide film [5,24,25]. As shown in Fig. 9(a) and (b), the intensities of metallic Fe and FeO peak increase while that of FeCr2O4 peak decreases with increasing sputtering time. For Cr, a main peak of Cr in Cr 2p3/2 spectrum on surface of the oxide film is divided into two possible peaks, the peak at about 576.5 ± 0.3 eV is assigned to Cr2O3 or FeCr2O4 [5,24e26], the other peak at about 577.4 ± 0.3 eV is assigned to Cr(OH)3 [24e26]. The peak of Cr (574.1 ± 0.3 eV) is found in Cr 2p3/2 spectrum beneath the surface of oxide film. Form Fig. 9(a) and (b), it is found that the intensities of Cr2O3 (FeCr2O4) and metallic Cr peak increase while that of Cr(OH)3 peak decreases to zero with increasing sputtering time. For O, the peak of O in O 2p3/2 spectrum is divided into two possible peaks, the peak at about 529 ± 0.3 eV is assigned to O2

X.Y. Cao et al. / Journal of Nuclear Materials 467 (2015) 32e41

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Fig. 6. (a) Bright-field and (b) high resolution TEM images of the oxide film formed on A2 after 3000 h exposure. The corresponding selected electron diffraction pattern is inserted.

Fig. 7. (a) Fe 2p3/2 of A1 and B1, (b) Fe 2p3/2 of A1 and A2, (c) Cr 2p3/2 of A1 and B1, (d) Cr 2p3/2 of A1 and A2 XPS spectra of oxide film formed on the surface of samples after 3000 h exposure.

Table 3 Binding energies (EB) of XPS peaks of the samples after 3000 h exposure. XPS-peak

EB of XPS peaks (eV) A1

A2

B1

B2

Fe 2p3/2 Cr 2p3/2 Ni 2p3/2

710.65 577.00 855.85

711.20 577.15 855.85

711.05 577.10 855.80

711.10 577.05 855.85

oxide film of the cladding materials in high temperature water is composed of oxides primarily. However, as shown in Fig. 9(a) and (b), the high intensity of OH at 10 s indicates that more hydroxides form on the outmost surface of B2 than that of A1 since the area of fitting peak are used to estimate the relative proportion of component (%) [25]. 3.4. Metal amount dissolved and oxidized

[24e26]. The other peak at about 531 ± 0.3 eV is assigned to OH [24e26]. As shown in Fig. 9(a) and (b), the intensity of O2 increases beneath the surface with sputtering time, which indicates that the

Fig. 10 shows the variation of oxygen contents in the oxide film with the sputtering time in the A1 and B2. It is found that the oxygen contents both in A1 and B2 decrease with increasing

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X.Y. Cao et al. / Journal of Nuclear Materials 467 (2015) 32e41

Fig. 9. The fitted XPS spectra of Fe 2p3/2, Cr 2p3/2 and O 2p3/2 of the oxide film formed on the surface of (a) A1 and (b) B2 after 3000 h exposure with sputtering time.

coefficient and t is the exposure time. As shown in Equation (1), Cðx;tÞ varies linearly proportional to the change of the distance x, in case that D and t are constant. Taking the position of the half of surface oxygen concentration as the interface of the oxide film/ metal [28], the variation of oxygen content from the surface of oxide film to oxide film/metal interface is shown in Fig. 10. The oxygen concentration (O) and sputtering time (t) can be fitted by the following linear dependence:

O4DH ¼ 75:4130  0:036t

(2)

and

O4TR ¼ 71:2593  0:030t

Fig. 8. XPS depth profiles of (a) Fe, (b) Cr, (c) Ni of A1 and B2 after 3000 h exposure.

sputtering time, which can be explained by Fick's law. The form of Fick's second law in the one-dimensional direction can be simplified by Tayler's series as [27]



x Cðx;tÞ ¼ Cð0;tÞ 1  2 pffiffiffiffiffiffiffiffiffi 2 Dpt

(1)

where Cð0;tÞ and Cðx;tÞ are the oxygen concentration at the solution/ film surface and at the distance of x, respectively. D is the diffusion

(3)

It can be calculated the corresponding regression coefficients of the fits are R24DH ¼ 0.98 and R24TR ¼ 0.96, respectively. Fig. 11(a) shows the oxide weight of the samples after different exposure times, and the corresponding surface morphologies at 750 h and 3000 h are inserted. As shown in Fig. 11(a), with increasing test duration, oxide weight of all the samples increase rapidly and then slowly to an almost stable value. From Fig. 11(a), it is found that the oxide weight of B (B1 and B2) is higher than that of A (A1 and A2). For the same cladding material, the samples (A2 and B2) after PWHT have higher oxide weight than that of the aswelded ones (A1 and B1) for each test duration, which indicates that PWHT promotes the oxidation process. In the meanwhile, it is found that the surface morphologies of samples after removing the oxide film after 750 h and 3000 h reveal that pitting and preferential corrosion of ferrite occurred in the metals, as shown in Fig. 11(a), which indicates that the local corrosion takes place there.

X.Y. Cao et al. / Journal of Nuclear Materials 467 (2015) 32e41

Fig. 10. Oxygen contents in the oxide film of A1 and B2 with the sputtering time.

In addition, it is found that the number and depth of pitting decrease with increasing exposure time. Fig. 11(b) and (c) show the rate of oxide weight and the pitting depth with the exposure time. As shown in Fig. 11(b), the rate of oxide weight decreases rapidly

39

and then slowly to an almost stable value with increasing exposure time, which is consistent with the weight changes of the samples as shown in Fig. 11(a). As shown in Fig. 11(c), the pitting depth of the samples decreases with increasing exposure time, which indicates that uniform corrosion instead of the local corrosion gradually becomes predominant with increasing exposure time. The typical SEM morphology and 3-dimensional topography of pitting on the surface of A1 after removing the oxide film are also shown in Fig. 11(c). Form Fig. 11(c), it is found that the crystallographic pitting with maximum size of about 1 mm occur at or close to g/d phase interface, which indicates that g/d phase interface is the most susceptible site for the local corrosion. On the other hand, more and deeper pitting formation of samples after PWHT (A2 and B2) than samples as welded (A1 and B1) indicates that PWHT results in an increase in the pitting corrosion susceptibility, as shown in Fig. 11(a) and (c), which is in accordance with the previous work [7]. Besides, from Fig. 11(a) and (c), it is found that the pitting depth of B2 is higher than that of A2, which indicates that Cr content may has effect on the local corrosion of the cladding materials after PWHT in high temperature water. Metal amount dissolved and metal amount oxidized can be obtained by the Equations (4) and (5)

mweight

gain

¼ moxygen  mmetal

dissolved

(4)

Fig. 11. (a) Oxide weight as a function of exposure time, (b) the rate of oxide weight and (c) the pitting depth after different exposure times. The corresponding surface morphologies after 750 h and 3000 h exposure are inserted.

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moxide

X.Y. Cao et al. / Journal of Nuclear Materials 467 (2015) 32e41

weight

¼ moxygen þ mmetal

oxidized

(5)

where m oxygen is the mass of oxygen in the oxide film, m metal disis the mass of metal ions dissolve from materials into the solution and m metal oxidized is the mass of metal ions in the oxide film. Fig. 12(a) and (b) show the calculated metal amount oxidized and metal amount dissolved of samples exposed to 350  C water at different exposure time, respectively. As shown in Fig. 12(a), metal amount oxidized of all the samples rapidly increase at the oxidation initiation and then slowly increase. More metal amount oxidized is found in B than A, indicating that higher Cr content reduce the oxidation (Fig. 4), which is in agreement with the oxide weight of the samples (Fig. 11). A similar result can be observed between B2 (A2) and B1 (A1), which indicates that PWHT promotes oxidation of the cladding materials. As shown in Fig. 12(b), metal amount dissolved increases with increasing exposure time. On one hand, higher Cr content samples exhibit more metal amount dissolved, which indicates Cr content can affects metal ions diffusion during the oxidation process. On the other hand, PWHT seems to have little influence on dissolution of metal ions since metal amount dissolved is almost no difference between samples with the different PWTHs.

solved

4. Discussion As reported by many previous works [29e31], solidification mode of the austenite stainless steel welds contains a primary ferrite solidification mode (FA mode) and a primary austenite solidification mode (AF mode), which can be predicted by schaeffler equation (Creq/Nieq), where the Creq ¼ Cr% þ Mo% þ 1.5 Si% þ 0.5 Nb % and the Nieq ¼ Ni% þ 30C% þ 30% Nþ0. Mn% [29]. According to Table 1, the Creq/Nieq of A and B are 1.62 and 1.58 respectively, which indicate formation of FA mode in A and B during solidification. As a result, the worm-like or island-like metastable ferrite retain in the austenite stainless steel, as shown in Fig. 1, due to uncompleted d-g phase transformation during the quickly solidification of the weld pool, that amount has some influence on the mechanical properties and corrosion resistance of the weldments [32]. On the other hand, it is found that ferrite content of A (about 7.8%) is more than that of B (6.5%), which is contributed to higher Cr content in A because Cr is ferrite-stabilization element [30,32], resulting in a different oxidation performance for different Cr

contents (Fig. 4). It should be noticed that PWHT has little influence on ferrite content of cladding materials, as shown in Fig. 2. However, it greatly changes the microstructure, and produces M23C6 carbides along the d/g interface, which is consistent with other studies [7,18,33], resulting in an increase in the oxidation susceptibility of materials, as shown in Figs. 3 and 4. It is usually accepted that the metal ions should release from the metal/oxide film interface to solution and oxygen atoms transport in the opposite way when the film separates the reactants [34e36]. These ions or atoms movement result in the formation of the outer layer and the inner layer [5,6,24,26]. Thus, growth of the oxide film is dependant on not only diffusion of the oxygen atoms, but also the oxidization and dissolution of metal ions (Fe, Cr, Ni). The latter are closely related to the interfacial kinetics and solid-state transport and those cationic defects concentration dominate the protectiveness of the oxide film [37]. Therefore, in the present study, on one hand, Cr is easy to form the oxide at the interface of metal/oxide film with select dissolution of metal ions (Fe and Ni) [5], then the nucleation and growth of Cr2O3 occur [5,38], which are explained by the following reactions [5,24]. Cr3þ þ 3OH ¼ Cr(OH)3

(6)

Cr(OH)3 þ Cr þ 3OH ¼ Cr2O3 þ 3H2O þ 3e

(7)

The Cr-rich inner layer is completed in short time, as reported by previous work [24], which lowers the susceptible of the local corrosion. It is apparent that higher Cr content enhances such an effect, as shown in Fig. 11(c). On the other hand, high Cr content contributes to more compactness Cr oxides formation in the inner layer [3], which hinders the inward diffusion of oxygen ions and outward release of metal ions. In addition, the higher Cr content existed in the inner layer inhibits the diffusion of iron ions (Fe3þ and Fe2þ) mainly by occupying the octahedron sites [3,39]. The released Fe and Ni ions form oxides or hydroxides on the surface by dissolution-precipitation mechanism in the term of the following possible reactions [24]. Fe3þ þ 2H2O ¼ FeOOH þ 3Hþ

(8)

3FeOOH þ Hþ þ e ¼ Fe3O4 þ 2H2O

(9)

Ni2þ þ 2OH ¼ Ni(OH)2

Fig. 12. The calculated metal amounts (a) oxidized and (b) dissolved after exposed to 350  C water at different exposure times.

(10)

X.Y. Cao et al. / Journal of Nuclear Materials 467 (2015) 32e41

Ni(OH)2 ¼ NiO þ H2O

(11)

In the meanwhile, the spinel structure oxides form as follows. Cr2O3þFe2þþ2OH ¼ FeCr2O4þ2H2O

41

phase interface due to formation of carbides after PWHT, which resulted in increase of metal amounts oxidized and dissolved, and were responsible for formation of more Fe-rich oxides and higher weight changes.

(12)

Apparently, the less release of metal ions caused by high Cr content (Fig. 12(a)) lead to less formation of Fe-rich oxides (Reactions (8) and (9)). Besides, high Cr content existed in the oxide film promotes more formation of FeeCr oxides (Reaction (12)), which is confirmed by XPS results as shown in Fig. 7, resulting the difference of element component especially in the outer layer (Fig. 8). For stainless steel including austenite and ferrite phases, the change of local composition and potential discrepancy resulting from d-g phase transformation and precipitates formation along the phase interface cause the local corrosion such as pitting [40] and preferential corrosion of ferrite (Fig. 11). As indicated before, PWHT contributes to the formation of carbides along phase interface, as shown in Fig. 3, which is the main cause for the high pitting corrosion susceptibility along the phase interface, as shown in Fig. 11, the similar work has been also reported by other researchers [7]. An increase in pitting depth and number along the phase interface not only promotes the oxidation by reducing the protectiveness of the oxide film, but also results in plenty of metal ions (Fe, Ni) dissolved (Fig. 12) into solution, which contributes to more Fe-rich oxides formation on the surface of the cladding materials (Figs. 4 and 7). Base on present results, it is apparent that high Cr content is favorable to the improvement of oxidation resistance since high Cr content decreases the pitting depth and metal ions release, as shown in Figs. 4, 11 and 12. On the other hand, PWHT increases the pitting corrosion susceptibility to cause more dissolution and oxidation of metal ions by forming carbides along the phase interface and promotes the oxidation of the cladding materials, as shown in Figs. 4, 11 and 12. 5. Conclusions Influences of Cr content and PWHT on microstructure and oxidation behavior of stainless steel weld overlay cladding materials in high temperature water were investigated. SEM, TEM and XPS were used to examine the microstructure and phase configuration of oxide film formed on the surface. The amounts of metal oxidized and metal dissolved were estimated to compare the oxidation behavior of the cladding materials with different Cr contents and PWHT. The main results can be gotten as follows: 1. The worm-like or island-like ferrites were distributed uniformly in the austenitic matrix. High Cr content resulted in formation of more ferrite content, while post weld heat treatment (PWHT) resulted in formation of carbides at d/g phase interface. 2. Cr content greatly affected the oxidation behavior of the cladding materials. High Cr content decreased the metal amount oxidized and dissolved by forming more compact Cr-rich inner layer, hence lowered the susceptible of the pitting and general corrosion, and produced less Fe-rich oxides and weight changes. 3. PWHT had influence on oxidation behavior of cladding materials. More and deeper pitting were promoted along the d/g

Acknowledgments The authors acknowledge the financial support for the present work from National Energy Application Technology Research and Engineering Demonstration Project (NY20111201-1), Important National Science & Technology Specific Projects of China (2011ZX06004-009) and the 111 Project (Grant No. B12012).

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