Influence of Zn Injection on Corrosion of 304SS Under PWR Primary Side Conditions

The structural materials in nuclear power plants experience corrosion under high temperature water chemistry environments, which could result in serious safety issues. Zinc injection to the primary side chemistry has been demonstrated to reduce corrosion rate of the structural materials and radiation dose rate by modifying the oxide film formed on the structural materials. The purpose of this work is to investigate the effect of Zn addition at different concentrations on 304SS under PWR primary side conditions by SEM, GIXRD, Raman spectrum, XPS, electrochemical method and thermodynamic calculation. When Zn concentration is increased, the number and size of Fe-based spinel oxide particles in the outer layer decreases while Cr-based spinel oxide in the inner layer remains unchanged. The corrosion current density of 304SS and defect density of the oxide film decrease with increasing Zn concentration. These results conclude that corrosion resistance of 304SS is enhanced in the presence of Zn. In addition, the thermodynamic analyses on the spinel oxides of surface were performed and the results are in good agreement with the experimental observations presented in this work.

Introduction

The structural materials, such as stainless steel (SS) and Ni-based alloys, used in nuclear power plants (NPPs) have demonstrated excellent performance against corrosion. This is due to formation of a protective oxide layer on the surfaces can effectively separate the underlying alloy from the surrounding environment and protect the alloy from corrosion (Lu et al., 2009; Pandey et al., 2009; Hoffelner, 2013; Chen et al., 2019; Yang et al., 2021; Wang et al., 2022). However, when these structural materials are exposed to the aggressive environments, such as the high temperature, water chemistry and high stress, the oxide layer can be damaged, leading to occurrence of localized corrosion (Landolt, 2007). Therefore, in the last decades, a number of studies have been conducted on zinc addition to the primary side water chemistry to mitigate corrosion of the structural materials and to reduce radiation dose rate during shut down (Ziemniak and Hanson, 2005; Liu et al., 2011; Liu et al., 2014; Jeon et al., 2017; Holdsworth et al., 2018). The oxide films formed on the structural materials in the aqueous environment have a duplex structure, which typically comprises of an inner layer of Cr-rich oxides and an outer layer of Fe-rich oxides (Sennour et al., 2010; Liu et al., 2014). Zn is able to modify the structure and composition of the oxide films on the structural materials by replacing Ni²⁺ or Fe²⁺ in the tetrahedral site of the spinel oxides. Kawamura et al. (1998) and Ziemniak and Hanson (2005) revealed that when Zn²⁺ was present at 10 ppb or higher, the outer layer in the oxide film formed on Alloy 600 and 304SS became thinner while zinc chromite was formed in the inner layer, which increased the oxide film stabilization and PWSCC (primary side water chemistry stress corrosion cracking) resistance. The test on zinc injection in nuclear power plant has also been reported. It reveals that zinc addition can thin the oxide film and mitigate corrosion of metal materials such as stainless steel or nickel-based alloy (Wu et al., 2015). On the contrary, Zhang et al. (2013) observed that the oxide film of Alloy 600 becomes thicker in the presence of Zn at 650 ppb. Furthermore, Kim and Andresen (1997) found that the low content of zinc had little effect on the electrochemical behavior of 304SS. The difference in the results obtained from different research groups are likely due to the experimental conditions. In order to explore the discrepancies between these results, the electrochemical behavior and oxide film of 304SS in the presence of Zn under the high temperature and high pressure conditions are studied in this paper.

Experimental

Materials and Methods

Main chemical composition of 304SS is listed below (wt.%): Si 0.43, Mo 0.07, Mn 1.02, Ni 9.45, Cr 17.71 and Fe balance. The specimens with a dimension of 20 mm × 20 mm × 2 mm were machined and grounded on a polishing mill with 240, 320, 800, 1,200 and 2,000 sandpaper in order. The specimens were then ultrasonically cleaned with ethanol, followed by drying with soft paper. Samples were weighed before and after corrosion.

High Temperature Corrosion Tests

The high temperature corrosion tests were performed in a 2 L autoclave (GCF-2L, Dalian) at a temperature of 553.15 K and a pressure of 6.3 MPa for 336 h. The aqueous solution contained 2.5 ppm Li as LiOH·H₂O, 1,500 ppm B as H₃BO₃, and different Zn concentrations (0 ppb, 100 ppb, 400 ppb) as Zn(CH3COO)₂. During the corrosion tests, the dissolved oxygen in the solution was controlled less than 10 ppb.

Characterization Methods

Surface morphology on the specimens was examined by an SU-8010 Field Emission Scanning Electron Microscope (SEM). The oxide phases on the surfaces were analyzed using Raman Spectrum (Invia Reflex) and Grazing Incidence X-ray diffraction methods (GIXRD, Empyrean). Electrochemical workstation (Chi660e, Chenhua) and standard three electrode system were used for the electrochemical tests. The boric acid buffer solution containing 0.15 mol/L boric acid and 0.0375 mol/L sodium was used as an electrolyte. The three-electrode system uses the alloy sample as working electrode, the platinum sheet as auxiliary electrode, and the saturated calomel electrode as reference electrode. The chemical state of the elements and composition in oxide film were performed with ESCALAB 250Xi X-ray photoelectron spectrometer (XPS). Sputtering rate of 0.2 nm/s was used with reference to SiO₂ layer. XPS data were corrected with Ni 2p3/2 peak at 852.8 eV (Machet et al., 2004).

Results and Discussion

Scanning Electron Microscope Observation and Corrosion Rate

Figure 1 presents surface morphology of the 304SS specimens after exposure to the solutions with different zinc concentrations at 553.15 K for 336 h. It is shown that Zn addition has a significant effect on morphology of the oxide film. Figure 1A shows that the oxide particles on the surface are not uniform in size and there are some large oxide particles. The particle sizes were measured by Nano Measurer and range from 69.74 to 557.42 nm. It has been widely though that large oxide particles are located in the outer layer of the oxide film and small oxide particles in the inner layer. When Zn was added to the solution at a concentration of 100 ppb, compared to Figure 1A, the number and size of the large particles were reduced as shown in Figure 1B while the small particles remained unchanged. When Zn concentration was increased to 400 ppb, the large particles in the outer layer was continually reduced to the size with a range of 42.65–112.94 nm, similar to those in the inner layer, as shown in Figure 1C. These results are consistent with observed by Liu et al. (2012).

FIGURE 1

FIGURE 1. The surface morphology of 304SS after exposure to different zinc concentrations at: (A) 0 ppb (B) 100 ppb (C) 400 ppb.

Figure 2 is the corrosion rate of the 304SS specimens at different zinc concentrations. The data were best fitted to the equation y = a*exp (−x/b) + c. It is shown that with increasing Zn concentration, reduction in weight of the sample decreases and thus the corrosion rate decreases.

FIGURE 2

FIGURE 2. Corrosion rate of 304SS under different zinc solutions.

Grazing Incidence X-ray diffraction and Raman Spectrum

Figure 3A presents the grazing incidence XRD pattern of the 304SS specimen corroded in the zinc-free solution for 336 h. The measured diffraction peaks match well those of spinel-type NiFe₂O₄. However, for the GIXRD of the sample corroded in the Zn solution, there was no diffraction peak observed. It is likely due to forming of the thin oxide film because when Zn is present, the oxide particle size decreases to less than 100 nm, as observed in Figure 1C. The similar behavior was observed after Alloy 600 was corroded in the high temperature water (Tapping et al., 1986; Zhang et al., 2013).

FIGURE 3

FIGURE 3. (A) GIXRD of the oxide film on 304SS after exposure to the Zn-free solution (B) Raman spectrum of the oxide film on 304SS after exposure to the solutions with 0, 100 and 400 ppb Zn.

Figure 3B presents Raman spectrum of the oxide film on 304SS after exposure to the solutions with 0, 100 and 400 ppb Zn. In the absence of Zn, the strongest Raman peak is located at 686 cm⁻¹, which corresponds to the Raman-active mode A1g resulting from the A-O vibration in the tetrahedral group AO₄. The other peaks located at 336 cm⁻¹, 483 cm⁻¹ and 590 cm⁻¹ correspond to the other Raman-active modes, which represent the B-O vibration characteristics in the octahedral group BO₆ (Wang et al., 2002 and 2003). In comparison with the Raman spectra measured by Hosterman (2011) for NiFe_xCr_2-xO₄ (0 ≦ x ≦ 2), the spectrum measured in this work is best fitted into the composition NiFe_xCr_2-xO₄ (1.2 ≦ x ≦ 1.6) as the F2g (3) mode becomes detectable only at x ≧ 1. The non-stoichiometric oxide in the oxide film formed under the high temperature water chemistry environment without Zn has been also identified in the other studies (Stellwag, 1998; Huang et al., 2011; Liu et al., 2012). NiCr₂O₄ is a spinel structure while NiFe₂O₄ is an inverse spinel structure. It has been demonstrated that NiFe_xCr_2-xO₄ is a solid solution, in which inversion of the cations Ni²⁺ and Fe³⁺ between the tetrahedral and octahedral sites occurs (Allen et al., 1988). When x is between 0 and 1, the entering Fe³⁺ has a preference to occupy the tetrahedral site, in which Ni²⁺ is forced to move to the octahedral site (Park and Suito, 1992). When x is greater than 1, the trivalent iron substitutes for chromium in the remaining octahedral sites. Thus, when adding to the solution, Zn occupied the octahedral site. Therefore, the oxide formed on the sample surface in the Zn-containing solution can be expressed in the form of Zn_xNi_1-xFe_yCr_2-yO₄. Figure 3B shows that when Zn was added to the solution, intensity of the main peak at 686 cm⁻¹ decreased more dramatically than the other peaks. Weakening of the Raman peak could be also attributed to forming of fine oxide particles and thin oxide film in the presence Zn. This is consistent with the observations by SEM and GIXRD. Thus Raman spectrum is a useful tool to characterize the oxide film composition in the presence of Zn.

X-ray Photoelectron Spectrometer Analysis

Depth profiles of the oxide films formed on 304SS in the zinc-free and zinc solutions at 553.15 k for 336 h were measured using the XPS technique and are presented in Figure 4. The interface between the oxide film and metal substrate is usually defined at the point where the intensity of oxygen level is 50% of its initial value (Machet et al., 2002). It can be seen from Figure 4 that the sputtering time of XPS on the metal substrate formed at different Zn concentrations is 2,600, 1,500 and 850 s, respectively. As the XPS sputtering rate of 0.2 nm/s was used, the thicknesses of the oxide films formed at three Zn concentrations are 520, 300 and 170 nm, respectively. Thus, Zn addition significantly leads to thinning of the oxide film, which is consistent with the SEM observations.

FIGURE 4

FIGURE 4. XPS depth analysis of oxide film on 304SS in non-zinc and zinc solutions at 553.15 k for 336 h: (A) 0 ppb (B) 100 ppb (C) 400 ppb.

Figure 4A shows that in the absence of zinc, the outer layer of the oxide film was rich in Fe and the inner layer was rich in Cr, which is a typical duplex structure of the oxide film (The outer layer between the first dotted line and the y axis and the inner layer between the two dotted lines). The oxide film is formed by re-deposition of dissolved metal ions during corrosion process. As diffusion rates of Fe and Ni cross the interface between metal and oxide film are faster than that of Cr; Fe and Ni are enriched in the outer layer, and Cr is enriched in the inner layer (Stellwag, 1998; Lee et al., 2021). When zinc was present, Ni and Fe in the oxide layer were depleted while Cr in the inner layer was enriched up to 20% at., as shown in Figures 4B,C. In addition, Zn with a concentration up to 8% at was incorporated to the oxide film. This is due to the fact that Zn²⁺ can replace Fe²⁺ and Ni²⁺ in the spinel oxide of the oxide film (Ziemniak and Hanson, 2005; Huang et al., 2011; Liu et al., 2011). The formed oxide film is dense with less cation vacancies and has low solubility, which prevents metal ions from movement and oxygen from diffusion. Therefore, ZnCr₂O₄ is dominant in the inner layer formed in the presence of Zn while Fe-based spinel oxide in the outer layer was recrystallized to form small size particles.

After the 304SS specimens were exposed to the simulated primary water chemistry with 0, 100 and 400 ppb zinc for 336 h, the spectra of Ni 2p3/2, Fe 2p3/2, Cr 2p3/2, O 1s and Zn 2p3/2 were measured and presented in Figure 5. The XPS spectra of Ni 2p3/2 in the oxide film formed in the absence of Zn can be deconvoluted to the Ni⁰ peak at 852.59 eV, NiO at 854.31 eV and satellite peak of the Ni⁰ at 858.67 eV, with Ni⁰ accounting for 46.87% (Dickinson et al., 1997; Lee et al., 2021). Among these peaks, the strongest peak of Ni⁰ could be attributed to reduction of Ni²⁺ during the Ar ion beam sputtering (Machet et al., 2002). Three peaks of 852.82, 854.23 and 855.88 eV, corresponding to Ni⁰, NiO and Ni²⁺, respectively, were observed after adding 100 ppb zinc into the solution, with Ni⁰ accounting for 69.34%. After 400 ppb zinc was added into the solution, two peaks appeared at 852.72 and 855.28 eV, corresponding to Ni⁰ and Ni²⁺, respectively, with Ni⁰ accounting for 74.56%. Ni²⁺ may be in the form of Ni(OH)₂, NiFe₂O₄ and NiCr₂O₄ (Lee et al., 2021). Comparison of the percentage of Ni⁰ in the three Zn solutions reveals that more Ni⁰ can be detected after Zn addition. This implies that the oxide film becomes thinner after Zn addition.

FIGURE 5

FIGURE 5. The XPS spectra of oxide films on 304SS exposed to 0, 100 and 400 ppb Zn solutions after sputtering 500 s: (A) Ni 2p3/2 (B) Fe 2p3/2 (C) Cr 2p3/2 (D) O 1s (E) Zn 2p3/2.

The XPS spectrum of Fe 2p3/2 in the oxide film formed in the Zn-free primary water chemistry can be deconvoluted to Fe³⁺ at 710.99 eV, Fe²⁺ at 708.71 eV and Fe⁰ at 706.69 eV. The Fe²⁺ and Fe³⁺ peaks could be attributed to presence of FeCr₂O₄, NiFe₂O₄ and Fe₃O₄ (Lee et al., 2021). The XPS spectrum of Cr 2p3/2 can be deconvoluted to Cr⁰ at 574.05 eV, and Cr³⁺ at 576.52 eV. The Cr³⁺ peak could be assigned to Cr₂O₃, NiCr₂O₄, FeCr₂O₄ and ZnCr₂O₄ (Lee et al., 2021). The XPS spectrum of O1s can be deconvoluted to O²⁻ at 530.25 eV, and OH⁻ at 531.25 eV (Lee et al., 2021). The XPS peak of Zn 2p3/2 can be deconvoluted to ZnFe₂O₄ at 1,021.83 eV and ZnCr₂O₄ at 1,022.12 eV (Liu et al., 2011). When Zn is added at a concentration of 400 ppb, the main peak is assigned to ZnCr₂O₄ with no observation of ZnFe₂O₄.

Effect of Zn on Electrochemical Properties of 304SS

Figure 6 shows the potentiodynamic polarization curves and Mott-Schottky curves of 304SS after exposure to the simulated primary water environments with different Zn concentrations for 336 h. The corrosion current densities of 304SS in the presence of Zn at 0, 100 and 400 ppb are 3.33, 1.81 and 0.87 µA/cm², respectively. The change in corrosion potential with increase in the Zn concentration is insignificant. Decrease in the current density with increasing the Zn concentration indicates that Zn incorporation into the oxide film improves the resistance of the 304SS material against corrosion.

FIGURE 6

FIGURE 6. Potentiodynamic polarization curve and Mott-Schottky curve of 304SS after exposure to different Zn solutions at 553.15 K: (A) Potentiodynamic polarization curve of 304SS with 0, 100 and 400 ppb Zn (B) Mott-Schottky curve of 304SS with 0, 100 and 400 ppb Zn (C) Mott-Schottky curve of 304SS with 0 and 100 ppb Zn.

Since the passive films formed on 304SS contain large number of defects and holes, they exhibit characteristic of semiconducting property. Therefore, they can be characterized by the Mott-Schottky equation as follows.

$\frac{1}{{\mathit{C}}^2}=\frac{2}{\mathit{qN\epsilon }{\mathit{\epsilon }}_0}\left(\mathit{E}-{\mathit{E}}_{\mathit{FB}}-\frac{\mathit{kT}}{\mathit{q}}\right)$

where, ε is the dielectric constant of the passive film; ε₀ is the vacuum permittivity (8.854 × 10⁻¹⁴ F/cm); q is elementary charge of electrons (1.602 × 10⁻¹⁹ C, -q for holes); N is number density of acceptors or donors; E and E_FB are the applied and flat band potentials (V), respectively; k is Boltzmann’s constant (1.38 × 10⁻²³ J/K) and T is the absolute temperature (K).

The capacitance data of the passive films formed on 304SS after exposed to the Zn solutions were measured as a function of applied potential and are presented in Figures 6B,C. According to the slope of the linear part in the three regions of the curve, defect density of the oxide film can be calculated by the Mott-Schottky equation. The results are shown in Table 1. When the slope is positive, the oxide film exhibits n-type semiconductor. When the slope is negative, the oxide film exhibits p-type semiconductor. According to Feng et al. (2010), the passivation films composed of Cr₂O₃ and NiO are generally p-type semiconductors, while the passivation films composed of Fe oxides are generally n-type semiconductors. The results in Table 1 show that in the regions I and region II, the oxide film exhibits n-type semiconductor, and in the regions III, the oxide film exhibits P-type semiconductor, indicating that the surface oxide film is a composite structure. In addition, it is found that the defect densities of the sample after zinc addition all decrease in the three potential regions. Decrease in carrier concentration of the passive film formed in the presence of Zn indicates improvement in the corrosion resistance of 304SS. Similar result was also observed in other study (Montemor et al., 2000; Lim et al., 2021).

TABLE 1

TABLE 1. Calculated defect densities in oxide layers formed in different Zn solutions.

Solubilities of the Related Spinel Oxides

The SEM and XPS studies reveal that the oxide films formed on 304SS under simulated primary circuit environments are a duplex structure composed of Fe-based spinel oxide and Cr-based spinel oxide. The order of standard Gibbs free energies of various spinels is as follows (Liu et al., 2011):ΔG (ZnCr₂O₄)＜ΔG (FeCr₂O₄)＜ΔG (NiCr₂O₄) and ΔG (ZnFe₂O₄)＜ΔG (Fe₃O₄)＜ΔG (NiFe₂O₄). According to the thermodynamic law, stability of these spinel oxides is in the reverse order. Therefore, when Zn is added to the solution, Zn can replace Ni²⁺ in NiFe₂O₄ or Fe²⁺ in Fe₃O₄ to form more stable ZnFe₂O₄ in the outer layer as shown in Eq. 1, while Zn can replace Ni²⁺ in NiCr₂O₄ or Fe²⁺ in FeCr₂O₄ in the inner layer to form more stable ZnCr₂O₄ as shown in Eq. 2. The thermodynamic analysis is in full agreement with the observations from the SEM and XPS data.

$\mathit{NiF}{\mathit{e}}_2{\mathit{O}}_4\mathit{ }+\mathit{Z}{\mathit{n}}^{2+}=\mathit{ZnF}{\mathit{e}}_2{\mathit{O}}_4+\mathit{N}{\mathit{i}}^{2+}(1)$

$\mathit{NiC}{\mathit{r}}_2{\mathit{O}}_4\mathit{ }+\mathit{Z}{\mathit{n}}^{2+}=\mathit{ZnC}{\mathit{r}}_2{\mathit{O}}_4+\mathit{N}{\mathit{i}}^{2+}(2)$

On the other hand, the solubilities of the related spinel oxides in the oxide films at 553.15 k are calculated using the equation and the thermodynamic data in Ref. (Liu et al., 2011) and presented in Figure 7. The SEM and XPS analyses reveal that the oxide films are composed of Cr-rich spinel and Fe-rich spinel oxides. Figure 7 shows that the solubilites of Zn spinel oxides ZnCr₂O₄ and ZnFe₂O₄ under the primary side water conditions at pH 7.0 and at 553.15 k (between two dash lines) are lower compared to the corresponding Ni and Fe spinel oxides. Therefore, when Zn is added to the solution, the Zn spinel oxides prefer to form due to their lower solubility, which makes the material more resistant to corrosion.

FIGURE 7

FIGURE 7. Solubilities of spinel oxides formed on 304SS in the high temperature water.

Conclusion

The oxide films formed on 304SS exposed to the different Zn solutions at 553.15 K are characterized by SEM, GIXRD, Raman spectrum, XPS, polarization and Mott-Schottky curves.

1) When Zn is present in the solution, the oxide particles in the oxide film formed on 304SS becomes smaller, leading to the oxide film being thinner. The corrosion rate of 304SS was reduced after zinc addition.

2) With increasing Zn concentration, the corrosion current density of 304SS and carrier concentration of the oxide film decrease. This indicates that corrosion resistance of 304SS is enhanced in the presence of Zn. In addition, intensity of the main Raman peak for NiFe_xCr_2-xO₄ (1.2 ≦ x ≦ 1.6) decreases with increasing Zn concentration.

3) Thermodynamic calculations show that the solubilities of the related spinel oxides (like NiFe₂O₄, FeCr₂O₄) are in good agreement the experimental observations. Compared to other spinel oxides, the Zn-containing spinel oxides i.e., ZnFe₂O₄ and ZnCr₂O₄ have lower solubilities, leading to better corrosion resistance of the material.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author Contributions

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

Funding

This work was supported by Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China (2021ZR108) and the National Natural Science Foundation of China (Grant No. 52001302).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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