1 Introduction

The oxidative environment generated around structural components in the core of a boiling water reactor (BWR) due to water radiolysis, reaches oxygen concentrations of about 200 and 10 ppb of hydrogen (Ref 1). These oxidative conditions are enough to induce stress corrosion cracking (SCC) susceptibility in the stainless steel 304 (304SS) reactor pressure vessel internal structural components (Ref 2). An approach to mitigate this SCC in BWR internal structures is to bubble hydrogen into the water reactor with the purpose of going from an oxidative to a reductive environment, allowing consequently to reach an electrochemical corrosion potential (ECPSHE) lower than − 230 mV. This ECPSHE value has been reported as appropriate to operate a BWR without SCC risk (Ref 3). However, a disadvantage of introducing high hydrogen concentrations in the BWR water feed is 304SS embrittlement and high radiation dose in the steam phase (Ref 4). In order to reduce the requirements of hydrogen in these systems, the NobleChem™ technology was applied, by employing platinum nanoparticles to catalyze the reaction between hydrogen and oxygen as reported by Kim et al. (Ref 5) who stated that they could decrease hydrogen injection while achieving SCC mitigation. Hettiarachchi et al. (Ref 6) applied the NobleChem™ technology at pilot scale, observing that it was possible to abate hydrogen concentration and radiation field in steam lines, while maintaining the SCC mitigation effectivity (Ref 7). Nevertheless, it has been seen that SCC in structural components in the core of a BWR can take place if an insufficient stoichiometric H2/O2 ratio is present around them (Yeh and Macdonald (Ref 8); Mei Ya Wang, et al. (Ref 9)).

Another approach to reduce the possibilities of an increasing corrosion process, is the assistance of a corrosion mitigation technique through the application of a coating (Ref 9). For this purpose, transition metal oxides have been proposed as insulating barriers to mitigate the SCC of 304SS components in a BWR. Advantages of this technique is that injection of hydrogen is not required anymore, also, it has been reported that the iron oxide film passivity increases besides restricting the mass transport of dissolved oxidants (H2O2, O2) to the metal surface, abating the respective oxidation–reduction reactions (Ref 10). Nanoparticles of transition metal oxides have been found to abate SCC of 304SS at BWR conditions (Ref 11). Among them, zirconium oxide (ZrO2) nanoparticles coatings based on oxides with mean particle sizes greater than 20 nm, have been extensively studied by Yeh et al. (Ref 12, 13); Ugas et al. (Ref 14); Kim and Andresen (Ref 15); Stellwag and Kilian (Ref 16); Zhou et al. (Ref 17) Suzuki et al. (Ref 18); Yehab et al. (Ref 19) The purpose of applying this inhibitive treatment is to use the ZrO2 nanoparticles coating characteristics as a physical barrier to restrict the redox reactions as mentioned above, by limiting the diffusion of oxidant species toward the metallic surface of the BWR structural components. In principle, the H2O2, and O2 reduction, the H2 oxidation and the oxidation of metals, could be deterred by the inhibitive treatment. The exchange current densities of these redox reactions would accordingly decrease, leading to slower reaction kinetics (Ref 12).

In the past few years, Yeh et al. (Ref 12, 13) have studied the impact of various ZrO2 treatments on the electrochemical characteristics of O2 and H2 over the 304SS surface in high temperature water. Beneficial effects in terms of abatement in corrosion current density have been reported (Ref 18). Open literature data also have demonstrated the inhibitive benefit of ZrO2 coatings in ECPSHE abatement and corrosion mitigation (Ref 15−17).

Atik and Aegeter (Ref 20) reported coatings of ZrO2 for this purpose, measuring corrosion resistance with a gravimetric method. Saleha et al. (Ref 21) found that coating carbon steel with ZrO2 nanoparticles with a size between 30 and 40 nm allowed them to reach an ECPSHE of −666.8 mV at 298 °C, in an aqueous saline environment. Elbasuney et al. (Ref 22) demonstrated that 10 nm ZrO2 nanoparticles effectively reduced the corrosion rate of AA2024 in the artificial sea water. Almomani et al. (Ref 23) obtained an ECPSHE of − 164 mV in 304SS coated with a 2% ZrO2 suspension.

Nevertheless, there is limited literature data about the impact of ZrO2 nanoparticle size and ZrO2 nanoparticles suspension concentration on the electrochemical and corrosion behavior of 304SS in high temperature pure water. In order to better understand this process, in this study we explored the influence of smaller ZrO2 nanoparticle size (3.75 nm) and their concentration in suspension, on the electrochemical corrosion potential (ECPSHE) of 304SS, under various water chemistry conditions at high temperature. The two parameters cited above have an impact on porosity and homogeneity of the coating formed. The surface morphologies and the ECPSHE of 304SS specimens before and after the ZrO2 coating were investigated and reported.

2 Experimental

2.1 ZrO2 Nanoparticles Synthesis

Under constant stirring and room temperature, 1.25 mL of acetic acid were added as catalyst to 2.5 mL of the precursor zirconium iso-propoxide. Immediately, 1.25 mL of acetyl acetone were incorporated into the reaction solution, and it turned yellow. Then, 4 mL of a propanol:water solution at a 7:1 molar ratio were added to the hydrolysis reaction. After 60 min of reaction, 9 mL of a 5.34 mM ZrO2 suspension was obtained. This result is compatible with that found by other authors (Ref 24,25,26,27). All glassware used was completely dried before starting the experiments. Reagents were from Sigma-Aldrich and used without further treatment.

2.2 Specimen Preparation and Oxidation

The alloy used in this study was 304SS. The chemical composition of this alloy is listed in Table 1. Specimens with a square plate shape of dimensions 10 × 10 × 3 mm were prepared. All specimens were manually polished with emery papers up to 600 grit, cleaned with acetone and with deionized water in an ultrasonic bath. Specimens underwent oxidation treatments for 2 weeks in 288 °C water containing 300 ppb O2 and electrical conductivity 0.6 µS/cm before the deposit of ZrO2 nanoparticles coating. After oxidation, samples were coated with ZrO2 nanoparticles from suspensions at different concentrations.

Table 1 Chemical composition (wt.%) of 304SS specimens with and without ZrO2 coating
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2.3 304SS Specimens Coated with ZrO2 Nanoparticles

ZrO2 coatings on 304SS oxidized specimens were prepared by hydrothermal deposit process at 150 °C for 3 days in autoclave (Ref 25). For this process, the 304SS specimens were introduced in vials with a suspension of a propanol:water mixture 7:1 with ZrO2 nanoparticles of 3.75 nm in diameter avg. Four ZrO2 concentrations were studied. In 6 mL of distilled water the following amount of ZrO2, in millimol (mmol), was added: 0.62, 1.23, 1.86 and 3.72 mmol.

For each of these three stages, samples were labeled as follows: 304SSpolished, specimen is only polished; 304SSoxidized is polished and oxidized for 2 weeks, and 304SSoxidizedZrO2 specimens underwent through polishing, oxidation and then coated with the four different concentrations of ZrO2 nanoparticles aqueous suspension as indicated in Table 1. Chemical composition reported in this table corresponds to the average value obtained from the analysis by EDS of five different sites on the sample surface.

2.4 Pure Water Circulation Loop and Water Chemistry Control

The diagram of the system for ECPSHE measurement and electrochemical polarization analysis is shown in Fig. 1. Test specimens (W) were placed in an autoclave that was in serial connection to a pure-water circulation loop. Ion-exchange resins and filters were used in the loop for water purification. The autoclave temperature was maintained at 288 °C and the coolant conductivity at the autoclave outlet, monitored by a WTW conductivity meter, was less than 0.8 µS/cm. Gaseous oxygen and a hydrogen–nitrogen mixture were independently pumped into a reservoir that was also in serial connection to the loop system through flow regulators located on the compressed gas bottles. Nitrogen acted as a purging gas in this work for fine-tuning the O2 and H2 concentrations. An Oakton low level dissolved oxygen monitor, model DO300 series, an Orbisphere hydrogen analyzer, model 510, A Cole Parmer pH meter, model Digi-Sense, and a WTW conductivity meter, model LF340 were used to measure dissolved O2, dissolved H2, pH, and electrical conductivity, respectively. An external Ag/AgCl reference electrode filled with saturated KCl electrolyte solution was inserted into the autoclave for ECPSHE measurement on the specimens.

Fig. 1
figure 1

Water cleaning loop and equipment to measure pH, electrical conductivity, dissolved oxygen, dissolved hydrogen, temperature, pressure and ECPSHE using tree electrodes: Pt, Ag/AgCl, 304SS(W)

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2.5 Surface Analysis

Surface morphologies of the specimens and ZrO2 particle distributions over the treated specimens were examined by a Jeol (Model JSM-6610LV) SEM operated at 20 kV, with a Thermo Scientific (Model K-Alpha) energy dispersive x-ray probe (EDX). The oxide structures of the oxidized specimens and of ZrO2 coatings were analyzed in a Siemens D5000 x-Ray diffractometer (XRD) and a TEM Jeol 2010HT at 200 kV.

2.6 Electrochemical Analyses

ECPSHE monitoring and polarization analyses on the specimens were performed in a Gill AC electrochemical analysis system, from ACM instruments, with the aid of a platinum counter electrode and an external Ag/AgCl reference electrode. Potentiodynamic polarization analyses were conducted in the forgoing high temperature loop with 288 °C pure water containing 100–1950 ppb of dissolved O2. The scan rate was set at 10 mV per 1 min and the scanned potentials ranged from −250 to + 250 mV. Experimental conditions for the polarization tests of 304SSoxidized and 304SSoxidizedZrO2 specimens are summarized in Table 2, which indicates the conditions that can be present in a BWR running process.

Table 2 Test conditions for polarization of 304SSoxidized and 304SSoxidizedZrO2 specimens, prepared with the ZrO2 nanoparticles suspensions with the four concentrations studied, at 288 °C and 8 MPa
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3 Results and Discussion

3.1 Characterization

3.1.1 304SS Surface Analyses by SEM

The surface morphologies of the studied specimens were analyzed by SEM, micrographs of representative specimens are shown in Fig. 2. Before oxidation, the surface is polished (Fig. 2a), where only low depth scratches were identified. After the oxidation process under dissolved O2 conditions, iron oxide micrometric particles are formed on the surface of the 304SSoxidized specimen, as shown in Fig. 2(b). It has been reported before that this iron oxide particles correspond to a magnetite phase (Ref 28) as discussed in the next section.

Fig. 2
figure 2

Surface morphology of 304SS specimens: (a) polished, (b) oxidized, (c) polished, oxidized and coated with ZrO2 nanoparticles from a 3.72 mmol suspension, (d) polished, oxidized and coated with ZrO2 nanoparticles from a ZrO2 1.86 mmol suspension

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The surface morphology of the 304SSoxidizedZrO2 specimen prepared with a suspension of 3.72 mmol can be seen in Fig. 2(c). At this concentration the ZrO2 coating on the 304SS specimen exhibits some cracks (indicated by a circle). Under the ZrO2 coating, the shape of the iron oxide microparticles can be distinguished as indicated (square). Figure 2(d) shows a typical micrograph of 304SS xidizedZrO2 coated with a suspension of 1.86 mmol of ZrO2 nanoparticles. In this case the obtained coating is more homogeneous and did not show the cracks observed at the higher concentration as shown in Fig. 2(c).

It has been reported that experimental conditions of 500 °C, (Ref 29) or the presence of a H2S environment, (Ref 30) cause cracking of the ZrO2 film due to bubble formation. In this research, the temperature of ZrO2 deposit to form the coating over 304SS was 150 °C, much lower than that reported for cracking formation due explosion of micro and nanobubbles. Also, under the study conditions reported in this research, there is no phase change of zirconia, hence a volume change that could generate microcracks in the coating, is not likely to occur. The cracks reported in this manuscript are generated during coating formation at 150 °C.

ZrO2 nanoparticles coatings on 304SS on all cases was performed at the same temperature of 150 °C, at which the formation of subcritical water is possible. However, only the sample prepared with the suspension of the highest amount of ZrO2 (3.72 mmol), presented fractures. The cracks are absent at the lower amounts of ZrO2 nanoparticles (0.63, 1.23, and 1.86 mmol) studied. So, these cracks could be attributed to the amount of ZrO2 used in this experiment and the thickness obtained with this higher concentration as reported by Liang et al. (Ref 31)

3.1.2 304SS Surface Analyses by X-Ray Diffraction

The 304SS xidizedZrO2 specimens were analyzed by XRD before and after Tafel test. Figure 3 shows a diffractogram of one of these specimens, the diffraction peaks of the monoclinic phase of zirconium oxide, baddeleyite, were identified at 28.175 and 31.468 69 2-theta, of planes (− 111) and (111) respectively (JCPDS 037–1484), along with the magnetite phase at 35.452 from plane (311) (JCPDS 01–1111). This result agrees with the sample processing, since from the oxidation stage, it was expected to obtain iron oxide, in this case, magnetite, which remained in the sample after the ZrO2 coating. The intensities of the diffraction peaks decreased after the Tafel test, suggesting that some of the iron oxide and the ZrO2 particles were lost during this process.

Fig. 3
figure 3

XRD pattern of 304SS specimens coated with ZrO2 nanoparticles, (a) before and (b) after 10-day exposed to Tafel test conditions

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The magnetite phase is maintained after polarization, which indicates that the monoclinic ZrO2 coating on magnetite particles protected it from oxidation, usually induced by the BWR operation conditions. The conservation of magnetite structure could be an indication that the ZrO2 coating is not allowing water to reach the magnetite particles, neither the specimen 304SS surface (Ref 28).

3.1.3 ZrO2 Nanoparticles TEM Characterization

TEM analysis of the ZrO2 nanoparticles deposited over oxidized 304SS for all the tests, allowed to determine an average particle size of 3.75 nm, as those shown in the micrograph in Fig. 4(a). Once the specimen with the ZrO2 nanoparticle coating went through the polarization test, they tend to irreversibly aggregate into larger particles, on the surface of the 304SS oxidized specimen (Fig. 4b). This aggregation yielded a thin ZrO2 film, which coats the surface of the oxidized 304SS, which could prevent water and the dissolved oxidizing species to reach the 304SS surface.

Fig. 4
figure 4

Micrograph of ZrO2 nanoparticles deposited on 304SS (a) before and (b) after Tafel test

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3.2 Polarization Tests

3.2.1 ZrO2 Coating Effect on ECPSHE of 304SS

The electrochemical study of 304SSoxidized and 304SSoxidizedZrO2 specimens was performed under the conditions presented in Table 2. These specimens were polarized under similar experimental conditions.

Suspensions with four different concentrations of ZrO2 nanoparticles, as indicated in Table 1, were prepared and deposited over the surface of 304SS specimens, in order to determine the effect of ZrO2 concentration on the ECPSHE of 304SSoxidizedZrO2 specimens.

It has been reported by Hettiarachchi et al. (Ref 6) that ECPSHE values of − 230 mV or more negative are desirable to avoid SSC in structural steel in BWR. In this study, it was possible to reduce the ECPSHE values even lower than -230 mV with the application of a ZrO2 nanoparticles coating.

Perdomo et al. (Ref 32) applied a ZrO2 coating on mild steel at 800 °C, reporting corrosion abatement behavior for 316 stainless steel on acid medium. They indicated that the ZrO2 film applied achieved a good adherence over iron oxide layers because they effectively interact, promoting the formation of Fe-O-Zr bonds. They reached an ECPSHE difference of 50 mV, between uncoated and coated 316 stainless steel with a ZrO2 film. Zhou et al. (Ref 17) studied a 304SS coated with a film made with ZrO2 monoclinic nanoparticles. The average thickness of the coating is 2 microns. They reported an ECPSHE of − 400 mV for the coated specimen and −200 mV for the uncoated stainless steel, at 265 °C.

In this study a difference of 375.27 mV was obtained between ZrO2 coated and uncoated 304SS samples.

With an oxygen concentration lower than 10 ppb, which exhibits already a reducing condition, the most significant electrochemical differences were observed in the 304SS specimens coated with the 3.72 and 1.86 mmol suspensions of ZrO2 nanoparticles. The 304SS specimens coated with ZrO2 nanoparticles suspension with 0.62 and 1.23 mmol, respectively, were also studied, however none of these samples reached ECPSHE values of − 230 mV or lower. The 304SS oxidizedZrO2 specimen treated with a 3.72 mmol ZrO2 suspension reached an ECPSHE of − 345 mV, while the 304SS oxidizedZrO2 specimen coated with a 1.86 mmol ZrO2 suspension attained an ECPSHE of − 400 mV (Fig. 5). For this reason, the study was centered on ZrO2 suspension with 1.86 and 3.72 mmol.

Fig. 5
figure 5

ECPSHE of 304SS oxidized specimens coated with two different concentrations of ZrO2 nanoparticle suspensions: 1.86 mmol and 3.72 mmol, and polarized under 8.5 MPa, 288 °C, at an oxygen concentration ≤ than 10 ppb in aqueous environment

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In the presence of oxygen at 110 ppb, the lowest ECPSHE value at the studied concentrations was reached when the ZrO2 nanoparticles were deposited from a 1.86 mmol ZrO2 nanoparticle suspension. Figure 6 shows the ECPSHE of 304SSoxidized and of 304SSoxidizedZrO2. From the polarization curves presented in this figure, it can be seen that the 304SSoxidized ZrO2 specimen has an ECPSHE of − 379.21 mV when a suspension with 1.86 mmol of ZrO2 nanoparticles was used, while 304SSoxidized has an ECPSHE to − 3.94 mV. This result indicates that the coating of ZrO2 on the oxidized 304SS specimen can protect it from SCC under BWR operation conditions, as observed in this case, when the coating is prepared with the 1.86 mmol suspension of 3.75 nm mean size ZrO2 nanoparticles and the experimental conditions indicated in Table 3. Despite the presence of 110 ppb of dissolved oxygen it was possible to reach an ECPSHE more negative than − 230 mV.

Fig. 6
figure 6

ECPSHE of 304SSoxidized, and 304SSoxidizedZrO2 using 1.86 mmol suspension

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Table 3 Electrochemical data obtained from the polarization analyses on 304SSoxidized and 304SSoxidizedZrO2 specimens (with 1.86 mmol of ZrO2) at the same concentration of dissolved O2
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ZrO2 nanoparticles suspension with 3.72 mmol produced a more positive ECPSHE, and this could be attributed to the fact that the suspension with the highest concentration generated coatings with cracks (see Fig. 3c). The cracks could have allowed infiltration of the surrounding media and thus an interaction between oxygen dissolved in water and 304SS, which could have increased the ECPSHE. ZrO2 nanoparticles suspension with 1.86 mmol concentration generates a continuous coating without cracks (Fig. 2d) and yields a more negative ECPSHE. 1.86 mmol ZrO2 nanoparticles suspension created continuous deposits, acting as an improved barrier between the oxidizing environment and the surface of 304SS, coating the 304SS specimen with a better protection against SCC, as can be seen in Fig. 6. However, the goal was accomplished with both concentrations, obtaining an ECPSHE lower than − 230 mV.

SEM analysis was performed on the surface of the oxidized samples without (Fig. 7a) and with coating (Fig. 7b and c). In Fig. 7(a), a micrograph of a typical sample of uncoated stainless-steel surface is shown. Observe the presence of the iron oxide particles distributed on the surface. Figure 7(b) shows a micrograph the 304SSoxidized ZrO2 sample (3.72 mmol of ZrO2), where the macro cracks formed on the ZrO2 film and the presence of the magnetite particles under the ZrO2 film can be noted. In Fig. 7(c), the surface of a 304SSoxidized ZrO2 sample (1.86 mmol of ZrO2), where the coating is less dense and the iron oxide particles are visible. Also, EDS analysis was done on both samples and presented in Table 4.

Fig. 7
figure 7

SEM micrograph of oxidized 304SS surface, after tafel test, (a) uncoated and coated with (b) 3.72 mmol of ZrO2 and (c) 1.86 mmol of ZrO2

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Table 4 Average concentration (wt.%) of main elements of uncoated and ZrO2 coated 304SS after Tafel test
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Also, analysis of the samples by SEM and EDS indicated that even after the Tafel test took place, ZrO2 coating remains attached to the surface of 304SS samples. Figure 8 shows elemental mapping from a cross section of a 304SS sample with ZrO2 coating, after the Tafel test, where the surface can be partially observed. SEM micrograph in Fig. 8(a) corresponds to the analyzed area. In Fig. 8(b), it can be observed that even after this Tafel analysis, Zr continues attached to the surface of the 304SS sample, where Oxygen is also present. The rest of the elements, correspond to 304SS and 304SS oxidized, including O from iron oxides.

Fig. 8
figure 8

Elemental mapping of a cross section of oxidized 304SS with a ZrO2 coating, after Tafel test. (a) SEM image of the analyzed zone; mapping of (b) Zr, (c) O, (d) Cr, (e) Fe, (f) Ni, (g) Mn

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As stated in previous reports by Yeh et al. (Ref 13), R. Ugas et al. (Ref 14), coatings of ZrO2 nanoparticles with sizes of 40, 100 nm have proven to reduce the ECPSHE values of 304 SS under BWR conditions, indicating a lower probability of SCC in the material. However, this study highlights the advantages of ZrO2 reduced particle size when preparing the 304SS surface coating. From the obtained results, the smaller nanoparticles in the appropriate concentration of particles play an important role in avoiding the formation of cracks that deteriorate the electrochemical properties of 304SS under BWR operating conditions.

4 Conclusions

A reduction of the average particle size to 3.75 nm in the ZrO2 nanoparticle suspensions used to form the coating over an oxidized surface of 304SS, allowed to reduce the ECPSHE to more negative values than those obtained without the coating or those with coarser particle size. Obtained results allowed proposing that a more compact coating was obtained with this size of ZrO2 particles in comparison with the larger nanoparticles reported by other authors elsewhere, reducing the contact of oxidizing species with the surface of stainless steel. In the presence of oxygen, values as low as − 379.21 mV were registered, which are favorable to avoid SCC in 304SS at BWR operating conditions.

In addition, the effect of ZrO2 nanoparticles concentration on the characteristics of the coatings over the oxidized 304SS was determined. It is important to use the appropriated concentration of the nanoparticles used to coat the surface of the 304 SS in order to avoid a detrimental performance due to either cracking of the coating or a partial coating of the surface, which consequently affects the electrochemical behavior of the 304SS at high temperature.