anti-corrosion performance regeneration in aged austenitic

ISIJ International, Vol. 60 (2020), No. 5

© 2020 ISIJ 1022

ISIJ International, Vol. 60 (2020), No. 5, pp. 1022–1029

* Corresponding author: E-mail: [email protected]: https://doi.org/10.2355/isijinternational.ISIJINT-2019-309

Anti-corrosion Performance Regeneration in Aged Austenitic Stainless Steel by Precipitate Dissolution Below Critical Temperature Using Electropulsing

Xuehao CHENG and Xinfang ZHANG*

State Key Laboratory of Advanced Metallurgy, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, 100083 P.R. China.

(Received on May 20, 2019; accepted on November 14, 2019)

Precipitates that form at high temperatures detrimentally affect the mechanical properties and corrosion resistance of engineering structural materials. Different from the typical solution treatment, a method that incorporates electropulsing is proposed in this study. Electropulsing is applied to aged 316LN austenitic stainless steel, which undergoes the aging process at 650°C for 2 000 h, microstructure characterization by SEM and TEM shows that electropulsing can aid in dissolving precipitates. Both the immersion and electrochemical tests showed a positive shift at corrosion potential and a decrease in the corrosion current density caused by electropulsing, and the corrosion resistance was improved. The change in the system’s free energy caused by the difference in the electrical conductivity between precipitates and matrix results in precipitate dissolution. This finding provides a technical reference for engineering applications, i.e., the on-line repair of properties in aged steels.

KEY WORDS: precipitates; corrosion resistance; electropulsing; dissolution; regeneration.

1. Introduction

Precipitates that are formed at relatively high tempera-tures are observed in supersaturated metallic materials.1) One of the causes of deterioration, such as strength loss, creep, and reductions in corrosion resistance and toughness of material properties, is precipitate coarsening.2–6) This coarsening typically occurs in engineering materials that are exposed to harsh environments for extended periods;7) these materials include boiler steel,8) turbine blade, and par-ticularly steels used for nuclear power plants.9,10) Nowadays, climate warming has become a global problem caused by carbon emissions. As a green energy, nuclear energy has a great potential in alleviating climate change.11) Hence, the development and safety of nuclear power plants are vital. Stainless steels are widely utilized for nuclear engineering structural components because of their excellent corrosion resistance, weldability, and mechanical properties.12–14) In the pressurized water reactor nuclear power plant, the nuclear island is regarded as the most important part; it is basically composed of pressure vessel, primary circuit main pipe, pressurizer, and main coolant pump, which are primar-ily linked by stainless steel components. In third generation AP1000 nuclear reactors, 316LN austenitic stainless steel is typically used as a material for the primary circuit main pipe. It has an operating temperature of 290–320°C and a

service life of up to 60 years; these conditions provide a conducive environment for coarsening and precipitate for-mation.15) As a result, stainless steel components in nuclear reactors also undergo precipitation. The distribution of brittle precipitates (M23C6, sigma, χ, and copper nanoclus-ters) at the grain boundaries leads to severe property losses; it also causes the intergranular corrosion crack of austenitic stainless steel used as loop pipeline system, pressure vessel brittleness caused by copper nanoclusters, and stress cor-rosion crack in the safety end of welded joints.16–18) Even worse, nuclear leakage may occur and severely threaten the safe operation of nuclear reactors that can cause significant economic losses and environmental pollution.19) The high-temperature solution treatment, which is a conventional treatment method, is typically used for aged materials.20–22) However, this ex-situ treatment cannot be easily applied to difficult-to-disassemble components, such as the pressure vessel and primary circuit main pipe of nuclear reactors. Therefore, it is of considerable importance to determine an on-line method to repair the properties of aged structural steels.

Electropulsing, which is an instantaneous high-energy input method with low energy consumption and high effi-ciency, is applied to promote and induce the microstructure evolution of metallic materials below the critical heat temperature.23–25) For example, electropulsing can induce the precipitation of Ni3(Ti, Mo)-type intermetallic phase in maraging steels.23) The recrystallization time of AZ31 mag-nesium alloy can be reduced by pulsed electric current;24)


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electropulsing can also refine the microstructure of castings during the solidification process.25) Moreover, pulsed electric current can also improve the corrosion resistance of metallic materials. For example, the corrosion of Ti-6Al-7Nb alloy can be retarded by inducing a re-solidified passivation layer using a large-pulsed electron beam.26) Electropulsing can improve the corrosion resistance of plain carbon steels by reducing the fraction of pearlite.27) Furthermore, pulsed elec-tric current can enhance α-Fe content and chemistry homog-enization and thereby improve the corrosion resistance of 2205 duplex stainless steels.28) The effects of electropulsing have been demonstrated by the microstructural evolutions of metallic materials and the change in corrosion resistance caused by such evolutions. However, research investigations related to the use of electropulsing for precipitate dissolu-tion and improvement of corrosion resistance in nuclear engineering materials remain limited.

In this study, the primary objective is to explore the pos-sibility of using electropulsing for precipitate dissolution and improve the corrosion resistance of aged austenitic stainless steels. These include the quantitative characteriza-tion of the size and number distribution of the precipitates in the aged and pulsed steels by scanning electron microscopy and transmission electron microscopy, and the corrosion rate detected by immersion and electrochemical tests. If precipi-tates can be dissolved, the microstructure and deteriorated properties of aged stainless steels can be recovered in situ rather than replacing components. Therefore, this technique provides a new pathway to in-situ improve the corrosion resistance of pipeline steel for nuclear power plants. It also offers certain reference value for the performance recovery of engineering materials in the fields.

2. Materials and Experiments

In this work, nuclear grade 316LN austenitic stainless steel with 0.01C, 0.112N, 16.98Cr, 12.93Ni, and 2.16Mo was employed in this investigation. The experiment was performed according to the following steps. First, the mate-rial was cut to a size of 24 mm × 5 mm × 0.5 mm using an electrical discharge wire. To ensure that the austenitic grain boundary was precipitate-free, a solid solution treat-ment was introduced to homogenize the microstructure and composition (Fig. 1); the material was heated at 1 050°C for 3 h, and thereafter water-quenched to room temperature. After the solid solution treatment, an aging treatment was immediately applied at 650°C for different durations (400,

700, 1 300, and 2 000 h). Thereafter, using a copper wire, the aged sample was connected to an electropulse generator. The pulse generator provided pulsed electric current to the sample; each pulse lasted 180 μs with a 200 Hz frequency and an 80 A/mm2 current density. In addition, the electro-pulsing equipment had a 24 V voltage. Each experiment was performed for 2 h; during this period, the temperature increased to 770°C.

After the pulse treatment, the microstructure characteris-tics of the material were observed using scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) and transmission electron microscopy (TEM). For the SEM, the samples were firstly prepared using a silicon carbide abrasive paper with a grit size of 180–2000#; there-after, it was mechanically polished by a 3 μm diamond polishing paste. Before the SEM observation, 316LN was electro-etched in a 60% nitric acid solution for 30 s. For the TEM, the samples were sectioned into 3 mm diameter discs, which were mechanically ground to approximately 100 μm using 180# SiC papers; these were further manually ground to approximately 60 μm with 2000# SiC papers. The thin foils were prepared by an RL-I type electrolytic double spray thinner until they were perforated. The electrolyte consisted of a mixture of 5% perchloric acid and 95% etha-nol; polishing was performed at an applied voltage of 30 V and at a temperature between –30°C and –40°C.

The size and number of precipitates were expressed by numerical density, which was the number of particles per unit area. It was calculated as follows: number density = n/M, where n is the number of particles at a certain size, and M is the as-calculated area. In this study, the size and number were determined using Image J software.

For corrosion resistance evaluation, the sample surfaces were ground with 2000# SiC paper; thereafter, the samples were completely immersed in a test solution with a 10% nitric acid and 3% hydrofluoric acid. Each experiment was performed for 2 h at a constant water bath temperature of 70°C; the test was conducted twice. The corrosion rate was calculated as the difference between the weights before and after the experiment, as follows:

RW W

S t�

��

1 2 ................................ (1)

where R is the corrosion rate, W1 and W2 represent the weights before and after the corrosion test, respectively; S is the surface area; t is the corrosion time.

In order to assess the electrochemical corrosion property, electrochemical potentiodynamic polarization tests were conducted by a three-electrode system on an electrochemi-cal workstation (VersaSTAT 3F). This system consisted of a saturated calomel electrode (SCE) as the reference electrode, a platinum tablet as the counter electrode, and the specimen as the working electrode. In this study, all the potentials were referred to the SCE electrode. Potentiody-namic polarization experiments were performed at a voltage range between –0.6 V and 0.8 V with a scan rate of 1 mV/s in the 3.5% NaCl solution at room temperature.

3. Results and Discussion

Figure 2 shows the SEM images of different heat-treated Fig. 1. SEM image of solution-treated sample.


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and pulsed samples. The aging treatment was performed at 650°C for up to 2 000 h (Figs. 2(a)–2(d)). At the early aging stage, the grain boundaries were decorated by a low number of precipitates with short rod forms. As the aging time advanced, the precipitates along the grain boundaries became chainlike and coarse, and the volume fraction of precipitates increased. However, when electropulsing was applied to the aged steel, precipitate dissolution occurred (Figs. 2(e)–2(h)); the same result was observed in the TEM images (Figs. 4 and 5). Moreover, Joule heating caused the temperature to rise up to 770°C during electropulsing. Heat treatment was further conducted for the aged samples at 770°C for 2 h. A considerable number of bright precipitates were observed in the grain boundary (Figs. 3(a)–3(d)). This indicates that no precipitate dissolution occurs at 770°C. Usually, 316LN austenitic stainless steel exhibits a dif-ferent precipitation behavior when it is exposed to high temperatures or long-term thermal aging process.29–31) As shown in Figs. 4(a)–4(c), the effective electron diffraction pattern of precipitates cannot be observed on the samples aged for relatively short times. Accordingly, after aging for 2 000 h, one of the precipitates was randomly selected

for diffraction examination. Based on the lattice parameters and crystal structure illustrated in Fig. 4(d), the precipitate is found to be χ-phase, which is an intermetallic compound with a body-centered cubic structure. Its molecular formula is generally written as Fe36Cr12Mo10. Moreover, χ-phase easily forms when austenitic stainless steels are exposed to a moderate temperature for an extended period.

To better characterize the size and number distributions of precipitates with and without electropulsing, individual precipitate particles with equivalent circular diameter were measured, presented in Figs. 6(a) and 6(b), respectively. The precipitate sizes in the steel aged for 400 h and 700 h were below 200 nm, the size distribution was mainly concentrated at 100 nm (Fig. 6(a)), and their number densities were 7.2×105/mm2 and 8.5×105/mm2, respectively. The precipi-tates in the steel aged for 1 300 h and 2 000 h had a size variation range of 50–250 nm, and the size distribution was mainly concentrated at 150 nm (Fig. 6(a)). Moreover, when electropulsing was applied to the steel aged for 400, 700, and 1 300 h, the precipitates completely dissolved. When electropulsing was applied to the steel aged for 2 000 h, the precipitates below 250 nm were dissolved, and distribution

Fig. 2. Distribution of bright precipitates under different treatment conditions. The aging treatment for (a) 400 h, (b) 700 h, (c) 1 300 h, and (d) 2 000 h durations. Electropulsing is applied to the aged samples with (e) 400 h, (f) 700 h, (g) 1 300 h, and (h) 2 000 h durations. (Online version in color.)


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was mainly concentrated at 100 nm (Fig. 6(b)). Further-more, the number density of particles in the pulsed steel (~4×104/mm2) was 12.5 times smaller than that of the aged steel (~5.0×105/mm2). Therefore, both the size and number density of precipitates sharply decrease when electropulsing is applied. This demonstrates that electropulsing promotes the dissolution of precipitates.

During the long thermal aging, the grain boundaries become considerably susceptible to intergranular corrosion due to precipitation. An immersion test was conducted to characterize the corrosion resistance of samples under dif-ferent treatment conditions. The results are presented in Fig. 7, where the yellow line indicates the central line along the cross-section. As shown in Figs. 7(a) and 7(d), only local corrosion with a depth of 5 μm and average width of 35 μm

was observed in the solution-treated sample. However, the corrosion crack of the aged sample penetrated the whole cross-section with an average width of 150 μm (Figs. 7(b) and 7(e)). After electropulsing treatment, the corrosion morphology became similar to that of the solution-treated sample (Figs. 7(c) and 7(f)). The corrosion rates of samples with and without electropulsing treatment were further calculated using Eq. (1) and illustrated in Fig. 8. The corrosion rate of the pulsed sample (78.9 g m–2 h–1) was reduced significantly compared with that of the aged sample (175.5 g m–2 h–1), which was slightly lower than that of the

Fig. 3. Distribution of bright precipitates in samples with (a) 400 h, (b) 700 h, (c) 1 300 h, and (d) 2 000 h aging dura-tions, followed by annealing at 770°C for 2 h. (Online version in color.)

Fig. 4. TEM bright field images of precipitate distribution sam-ples with (a) 400 h, (b) 700 h, (c) 1 300 h, and (d) 2 000 h aging durations. (Online version in color.)

Fig. 5. TEM bright field images of precipitate distribution in pulsed samples. Electropulsing is applied to samples with (a) 400 h, (b) 700 h, (c) 1 300 h, and (d) 2 000 h aging dura-tions; similar electropulsing parameters are used in (a)–(d). The precipitates in (a)–(c) are completely dissolved, and a small amount of precipitate in (d) remains at the grain boundary. (Online version in color.)


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solution-treated sample (79.9 g m–2 h–1). Again, these results indicate that electropulsing aids in dissolving intergranular precipitates and thereby improves the intergranular corro-sion resistance of steel.

Figure 9 illustrates the potentiodynamic polarization curve of 316LN steels under various treatment conditions. According to the Tafel extrapolation method, the corrosion current density (icorr) and corrosion potential (Ecorr) of dif-ferent conditions were determined based on the dynamic polarization curve, as listed in Table 1. It can be observed that the solid solution-treated samples exhibited an outstand-ing local corrosion resistance (Ecorr = –134 mV; icorr = 0.12 μA/cm2). When electrical pulses were applied to the steels aged at different durations (400, 700, 1 300, and 2 000 h), the corrosion current densities changed from 0.19, 0.29, 0.43, 1.1 μA/cm2 at aged states to 0.17, 0.23, 0.20, 0.16 μA/cm2 after pulse treatment, respectively. Furthermore, the corrosion potential for the steel aged at different durations (400, 700, 1 300, and 2 000 h) by pulsed treatment also changed from –254, –278, –298, –339 mV to –251, –255, –252, –246 mV, respectively. The positive shift amounts of corrosion potential were 3, 23, 46, and 93 mV for different durations (400, 700, 1 300, and 2 000 h), respectively. The positive shift at corrosion potential and the decrease in the corrosion current density for the pulsed samples indicate that the corrosion resistance is improved. But, the corrosion potential of the pulsed specimen remains lower than that of the solution-treated sample. Apart from corrosion potential and current density, the thickness loss (μm per year) is computed as

dW I

n Fcorr corr�

��

��

87 600 000 ................... (2)

where d is the thickness loss per year; Wcorr is the atomic weight; Icorr is the corrosion current density; n is the chemi-cal valence of metal; F is the Faraday constant (F = 26.8 Ah); ρ is the metal density (8.04 g/cm3). As summarized in Table 1, the thickness loss of the solution-treated sample was 1.3 μm per year, and those of the samples with dif-ferent aging durations (i.e., 400, 700, 1 300, and 2 000 h) were 2.2, 3.3, 4.9, and 12.3 μm per year, respectively. After electropulsing treatment, the thickness losses became 2.0,

Fig. 6. Number and size distributions of precipitates under differ-ent treatment conditions: (a) aging treatment with different durations and (b) electropulsing treatment. (Online version in color.)

Fig. 7. Corrosion micro-morphologies of steels under [(a) and (d)] solution treatment, [(b) and (e)] aging treatment for 2 000 h, and [(c) and (f)] electropulsing treatment. (Online version in color.)

Fig. 8. Corrosion rates of aged and pulsed samples in immersion test. (Online version in color.)


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Fig. 9. Potentiodynamic polarization curves of aged and pulsed samples. (Online version in color.)

Table 1. Corrosion potential and corrosion current density of samples under different treatment conditions.

Sample Corrosion potential (mV)

Corrosion current density (μA/cm2)

Corrosion rate (μm/y)

0 –134 0.12 1.3

1 –254 0.19 2.2

2 –278 0.29 3.3

3 –298 0.43 4.9

4 –339 1.10 12.3

5 –251 0.17 2.0

6 –255 0.23 2.6

7 –252 0.20 2.3

8 –246 0.16 1.9

2.6, 2.3, and 1.9 μm per year, respectively. These results are similar to those of the previously mentioned immersion test; they also demonstrate that electropulsing treatment can contribute to relatively improve the electrochemical corro-sion resistance and extended service life of aged steel. Both immersion test and electrochemical measurements indicate that electropulsing can significantly increase the corrosion resistance of aged steel.

Chromium and molybdenum atoms are dominant in the χ-phase structure. The detection of Cr and Mo distributions between the matrix and precipitates can aid in understanding

the effect of electropulsing on solute migrates. Figure 10 shows the Cr and Mo distributions in the samples with and without electropulsing treatment using SEM-EDS line scan-ning. In the aged samples, the number of atoms of Cr in the zone between the matrix and precipitates firstly decreased and thereafter sharply increased, as illustrated in Figs. 10(a) and 10(b). Peaks of element (Cr and Mo) enrichment are evident in the aged samples. A chromium-depleted zone existed near the precipitates for the aged samples. After electropulsing treatment, element distributions (Cr and Mo) were observed to be practically homogenous in the matrix, as shown in Figs. 10(c) and 10(d). Uniform distribution of Cr and Mo indicates the precipitate dissolution occurs with electropulsing treatment. In general, after solution treatment, the substitutional atoms (Cr and Mo) are under a supersatu-rated and unstable condition. The supersaturated Cr and Mo aggregate toward grain boundaries in long thermal aging, leading to precipitation. However, after electropulsing treat-ment, the precipitates are dissolved and the substitutional atoms (Cr and Mo) move into the matrix. The motions of Cr and Mo are related to the diffusion of the substitutional atoms during electropulsing. Furthermore, the diffusion coefficient of a substitutional atom is given by

D DE

RTi

T� ��

��

��0 exp ......................... (3)

where D0 is the frequency factor determined by the hop-ping frequency of an atom and lattice distance; ΔET is the diffusion activation energy. Generally, electropulsing aids the migration of atoms to the metallic material. The drift-ing electrons driven by the applied electrical potential are unevenly distributed around the defects; this reduces the interaction force between the atoms and their neighbors.32) Consequently, the diffusion activation energy of the substi-tutional atoms (Cr and Mo) is reduced; this increases the diffusion coefficient of atoms. In this manner, the enhanced atomic diffusion ability facilitates the evolution of precipi-tates towards either coarsening or dissolution, depending on thermodynamics.

Thermodynamically, phase dissolution is an evolution process, from equilibrium to non-equilibrium. It is widely known that this evolution process is closely related to the change in the free energy of the system; such a change is generally induced by precipitate dissolution, as expressed by33,34)

� � � � �G G G G Gc i s e� � � � .................. (4)

where ΔGc is the change in chemical free energy; ΔGi is the change in interface free energy; ΔGs is the change in stress–train free energy; ΔGe is the change in electric cur-rent free energy; ΔGi and ΔGs inhibit the dissolution of the precipitated phase, whereas ΔGc and ΔGe represent the driving force that promotes particle dissolution. For the steel samples, ΔGs is extremely small; hence, it is negligible. For the aged samples, ΔGe=0, and phase precipitation occurs when ΔG ≤ 0, i.e., ΔGc + ΔGi ≤ 0. However, for the pulsed samples, phase dissolution occurs when ΔG > 0, i.e., |ΔGe | > |ΔGc + ΔGi |. The free energy of electric current associated with the dissolution of precipitates can be obtained by the following:35)


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�Gj r j r j r j r

r rdrdre �

� � ��

��8

1 1 2 2

( ) ( ) ( ) ( ) ......... (5)

where μ is the magnetic permeability of the precipitate; j is the current density; r and r′ are two different positions inside the samples;

j r1( ) and

j r2 ( ) are the current density distribu-tions before and after electropulsing. The calculation of the ΔGe is complex; nevertheless, it can be simplified as36–38)

�G k j r Vem p

m p

� � ��

�2

2( )

� ��

................... (6)

where σm and σp are the electrical conductivities of the matrix and precipitates, respectively; k represents the geo-metric factor; V is the volume of the material. In the austen-itic steel, the electrical resistivity of the precipitate is higher than that of the matrix, i.e., σm > σp and ΔGe > 0. Particle dissolution can occur provided that the free energy of the electric current is increased by adjusting the current density. As illustrated in Fig. 10, chromium and molybdenum are redissolved into the matrix. Consequently, the corrosion resistance of the aged steel is restored by electropulsing.

In general, the corrosion potential is primarily dependent on the solute concentration in the matrix, that is, the higher the concentration, the higher the corrosion potential, and vice versa. During aging process, the solute atoms (such as Cr and Mo) were precipitated to be χ-phase, whose growth reduced the solute concentration and decreased the corro-sion resistance. As shown in Fig. 9, the corrosion potential of the steels aged at different durations (400, 700, 1 300, and 2 000 h) were –254, –278, –298, –339 mV, respectively. As the aging time prolongs, the corrosion potential is greatly reduced due to the consumption of solute atoms, which are used for nucleation and growth of the χ-phase (Figs. 2 and 6). After the pulsed treatment, the χ-phase at different aged

durations (400, 700, and 1 300 h) is dissolved, and for the 2 000 h aged sample, only a small amount of nano-sized precipitates is remained in the matrix, because the dissolu-tion requires sufficient time for diffusion to complete. It can be seen from the data in Fig. 9 and Table 1 that after the precipitates are dissolved, the corrosion potential (400 h aged sample after pulse: –251 mV; 700 h aged sample after pulse: –255 mV; 1 300 h aged sample after pulse: –252 mV; 2 000 h aged sample after pulse: –246 mV) of the samples are not much different. This means that once the precipitates dissolve, the corrosion resistance is similar when the atom concentration does not differ much. However, the greater recovery by electropulsing is obtained on aged samples for longer durations. This is because the long-term aging causes the precipitates to grow and coarsen, resulting in the consumption of a large number of solute atoms and a small atom concentration in the matrix. In other words, the positive shift amounts of corrosion potential at 3 mV and 93 mV for 400 h and 2 000 h durations are caused by the difference in the number and size of the precipitates in the different aged states.

4. Conclusion

In summary, electropulsing treatment can aid in the precipitate dissolution of aged steels. The results obtained by the immersion test and electrochemical experiment indi-cate that electropulsing treatment increases the resistance of intergranular corrosion and electrochemical corrosion. Compared with the annealing treatment, electropulsing pro-vides a new approach for the precipitate dissolution of aged metal materials below a critical heat treatment temperature. In addition, the electropulsing treatment can be performed in situ, which is particularly convenient for large equipment that is undetachable; this can yield a considerable number

Fig. 10. SEM-EDS line scanning map of (a, b) aged and (c, d) pulsed samples. (Online version in color.)


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of economic benefits.

AcknowledgementsThe work was financially supported by the National Natu-

ral Science Foundation of China (51601011, 51874023, and U1860206), Fundamental Research Funds for the Central Universities (FRF-TP-18-003B1), and Recruitment Program of Global Experts.

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anti-corrosion performance regeneration in aged austenitic

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