Kathleen Jaffré

Hiroshi Abe

Dr Benoît Ter-Ovanessian
email: benoit.ter-ovanessian@insa-lyon.fr

Nicolas Mary

Bernard Normand

Yutaka Watanabe

Influence of mechanical surface treatments on oxide properties formed on 304L stainless steel in simulated BWR and PWR primary water

Keywords: Oxides, Stainless steel, Surface treatment, BWR, PWR, Mott-Schottky analysis, Semiconductive properties Highligths

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Among the materials used in boiling water reactors (BWRs) and pressurized water reactors (PWRs), stainless steels (SS) are often used as in-core and out-of-core components of nuclear power plants for their good mechanical properties and corrosion resistance. For example, 304L SS is the constitutive material of components for reactor vessel internals in the primary circuit.

Even in light water reactor environment, its corrosion resistance is due to the formation of a protective oxide on the surface that limit the dissolution of the SS. In simulated BWR and PWR environments, a duplex structure is commonly considered for the oxide [START_REF] Cissé | Effect of surface preparation on the corrosion of austenitic stainless steel 304L in high temperature steam and simulated PWR primary water[END_REF], [START_REF] Chen | Characterization of interfacial reactions and oxide films on 316L stainless steel in various simulated PWR primary water environments[END_REF], [START_REF] Kuang | The mechanism of oxide film formation on Alloy 690 in oxygenated high temperature water[END_REF], [START_REF] Hemmi | General corrosion of materials under simulated BWR primary water conditions[END_REF], [START_REF] Ming | Effect of surface state on the oxidation behavior of welded 308L in simulated nominal primary water of PWR[END_REF]. It is composed of an outer oxide-hydroxide and an inner oxide. The outer oxide-hydroxide develops at the interface solution/inner oxide. It consists of non-uniformly distributed oxide precipitates, spread over the surface and composed mainly of iron, nickel, and a small amount of chromium. Generally, they are identified as magnetite (Fe3O4) or iron-chromium spinel oxide (FeCr2O4) for PWR and BWR environments [START_REF] Féron | Behavior of stainless steels in pressurized water reactor primary circuits[END_REF]. For oxides formed in simulated BWR environment, nickel-enriched spinel (NiFe2O4) is also found [START_REF] Takeda | In situ and ex situ characterisation of oxide films formed on strained stainless steel surfaces in high-temperature water[END_REF]. It is usually assumed that these precipitates are formed by a redeposition phenomenon after the dissolution of the elements of SS in solution due to generalized corrosion and the diffusion of this ionic species through the lattice of the dense oxide [START_REF] Féron | Behavior of stainless steels in pressurized water reactor primary circuits[END_REF], [START_REF] Stellwag | The mechanism of oxide film formation on austenitic stainless steels in high temperature water[END_REF]. As the inner oxide is compact and rich in chromium, it acts as a barrier that limits the material's dissolution. Indeed, the compact inner oxide slows down the inward diffusion of O 2-, OH -and aggressive elements and the outward diffusion of metal ions. According to Cissé et al. [START_REF] Cissé | Effect of surface preparation on the corrosion of austenitic stainless steel 304L in high temperature steam and simulated PWR primary water[END_REF], who listed three main mechanisms of diffusion responsible of the growth and steady-state behavior of the inner oxide at high temperatures, mechanism identification have to be made from adequate microstructure analysis and observations. Since it has been shown that the passive film formed on the 304L SS is not porous, the most common mechanism is the growth of inner oxide by the solid-state process even at relatively low temperatures. Furthermore, in an aqueous solution, the formation, growth, and failure of oxides are partly governed by ionic and electronic transports within the oxide. Consequently, the characterisation of the doping density and semiconductive behavior of the oxides provides information on their evolution over the lifetime of SS components [START_REF] Stellwag | The mechanism of oxide film formation on austenitic stainless steels in high temperature water[END_REF].

The oxides' properties depend on their conditioning environment, the alloy's composition, and the post-treatments applied to the alloy's surface. These mechanical surface treatments induce surface modifications that change the material's mechanical properties and the metallurgical state of the top surface. The surface machining of SS by grinding generates roughness [START_REF] Turnbull | Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure[END_REF], [START_REF] Daopiset | Effects of Surface Finishes on Corrosion Resistance of Welded Stainless Steels[END_REF], [START_REF] Lee | Influence of peening on the corrosion properties of AISI 304 stainless steel[END_REF], [START_REF] Hakiki | Influence of surface roughness on the semiconducting properties of oxide films formed on 304 stainless steel[END_REF] and defects as scratches, abrasion, and material removal [START_REF] Lopes | Behavior of hardened steel grinding using MQL under cold air and MQL CBN wheel cleaning[END_REF], [START_REF] Bianchi | Evaluating the effect of the compressed air wheel cleaning in grinding the AISI 4340 steel with CBN and MQL with water[END_REF]. It also induces residual stress [START_REF] Chang | Understanding the effect of surface finish on stress corrosion crack initiation in warm-forged stainless steel 304L in high-temperature water[END_REF], [START_REF] Ben Fredj | Fatigue life improvements of the AISI 304 stainless steel ground surfaces by wire brushing[END_REF], [START_REF] Silva | Environmentally friendly manufacturing: Behavior analysis of minimum quantity of lubricant -MQL in grinding process[END_REF] and an ultrafine grain layer resulting from recrystallization [START_REF] Lou | Surface nanocrystallization of metallic alloys with different stacking fault energy induced by laser shock processing[END_REF], [START_REF] Abe | Chracteristics of work hardened surface layer on austenitic stainless steels and its relation to SCC susceptibility in high temperature water[END_REF], [START_REF] Ben Rhouma | Effect of machining processes on the residual stress distribution heterogeneities and their consequences on the stress corrosion cracking resistance of AISI 316L SS in chloride medium[END_REF]. Several studies show that surface modification induced by surface treatments have an influence on the properties of oxides formed in aqueous environment [START_REF] Jaffré | Effect of mechanical surface treatments on the surface state and passive behavior of 304L stainless steel[END_REF], [START_REF] Luo | Effect of cold deformation on the corrosion behaviour of UNS S31803 duplex stainless steel in simulated concrete pore solution[END_REF], [START_REF] Hilbert | Influence of surface roughness of stainless steel on microbial adhesion and corrosion resistance[END_REF]. It has been shown that the surface treatment modifies the oxide characteristics in the simulated PWR environment in terms of composition, morphology, and microstructure [START_REF] Cissé | Effect of surface preparation on the corrosion of austenitic stainless steel 304L in high temperature steam and simulated PWR primary water[END_REF]. For example, it has been proposed that the difference in thickness of the ultrafine grain layer produced during the surface treatment implies that the inner oxide was thinner for the ground sample than for the polished one. Another work also highlighted that the ultrafine grain layer resulting from the mechanical surface treatment had a strong influence on the composition of the oxide film grown on a 304L SS in a PWR environment [START_REF] Bai | Electrochemical and spectroscopic characterization of oxide films formed on Alloy 182 in simulated boiling water reactor environment: Effect of dissolved hydrogen[END_REF].

Moreover, the surface modification due to grinding or mechanical polishing is also known to influence the morphology, number, and size of the outer oxide-hydroxide precipitates but not on their composition [START_REF] Ming | Effect of surface state on the oxidation behavior of welded 308L in simulated nominal primary water of PWR[END_REF]. Besides, the composition of the oxides and the thickness may vary but not significantly [START_REF] Takeda | In situ and ex situ characterisation of oxide films formed on strained stainless steel surfaces in high-temperature water[END_REF]. In addition, the surface modifications can induce harmful alterations of the oxides formed on SS, affecting their behavior significantly to stress corrosion cracking [START_REF] Lyon | Influence of milling on the development of stress corrosion cracks in austenitic stainless steel[END_REF], [START_REF] Ghosh | Microstructural changes in AISI 304L stainless steel due to surface machining: Effect on its susceptibility to chloride stress corrosion cracking[END_REF], [START_REF] Ben Rhouma | Effect of machining processes on the residual stress distribution heterogeneities and their consequences on the stress corrosion cracking resistance of AISI 316L SS in chloride medium[END_REF] and corrosion fatigue [START_REF] Sudesh | Electrochemical & optical characterisation of passive films on stainless steels[END_REF]. Thus, it is essential to understand and describe how mechanical surface treatments influence the oxidation behaviour of SS in simulated BWR and PWR environments. It is also interesting to determine whether the mechanical surface treatments similarly impacts SS's oxidation features in simulated PWR and BWR environments. Indeed, the results obtained for one environment should probably not be directly transposable to the other. Their main difference lies in the steam generation process (different temperatures and pressures). BWR generates steam by boiling directly in the primary circuit. In contrast, the water in the primary circuit is kept at a much higher pressure in PWR, preventing boiling, and steam is produced via a second circuit [START_REF] Liu | Corrosion behaviour of nanocrystallines 304 stainless steel in simulated secondary side environment[END_REF]. It is also worth noticing that the water chemistries are different.

Therefore, this study considers the effect of mechanical surface treatments, not only on the oxidation behavior of 304L SS in high-temperature water, but also on the electronic properties of the grown inner oxides by ex-situ measurement. Three surface treatments were selected: dry grinding, polishing down to 2400 with SiC, and with Colloidal Silca (CS). Compared to grinding, polishing down to 2400 with SiC reduces the roughness, scratches, microstructural changes, and residual compressive stresses, and, polishing with CS removes almost all of these modifications [START_REF] Jaffré | Effect of mechanical surface treatments on the surface state and passive behavior of 304L stainless steel[END_REF]. Samples with different surface treatments were then oxidized during 120 h of immersion in a simulated BWR and PWR environments. After this short-term exposure in high temperature water, the morphology and chemistry of the grown oxides were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) or X-Ray photoelectron spectrometry (XPS). Besides, the electronic properties of the oxides were also determined by using an advanced electrochemical methods mainly used on thin passive films formed at room temperature [START_REF] Jaffré | Effect of mechanical surface treatments on the surface state and passive behavior of 304L stainless steel[END_REF], [START_REF] Zhang | Investigation of the passive behavior of a Ni-Cr binary alloy using successive electrochemical impedance measurements[END_REF]. This ex-situ method is based on the coupling of electrochemical impendance spectroscopy investigation and analysis and Mott-Schottcky approach and allows to obtain frequency independent electronics properties. Therefore, this work proposes to establish a relationship between surface modifications, and, the physical, and, chemical properties of the oxides using surface characterization techniques and a recently developed electrochemical methodology.

Experimental part

Materials and mechanical surface treatments

Experiments were performed on a commercial 304L SS plate. Before immersion, samples were either ground in dry condition or mechanically polished. The dry grinding (G) treatment was applied to mimic the industrial surface state obtained after a repairing procedure. The detailed experimental steps was described elsewhere [START_REF] Abe | Chracteristics of work hardened surface layer on austenitic stainless steels and its relation to SCC susceptibility in high temperature water[END_REF], [START_REF] Jaffré | Effect of mechanical surface treatments on the surface state and passive behavior of 304L stainless steel[END_REF]. The second sample was polished down to 2400 with silicon carbide SiC paper (2400 SiC). The surface was prepared using emery papers from 320 grit to 2400 grit. In addition to the previous polishing steps, the third surface treatment consisted of a polished down 1 µm with diamond paste and with colloidal silica (CS) finishing suspensions. Colloidal silica finishing suspensions are widely used to eliminate subsurface and surface deformations and remove compressive residual stresses due to previous polishing steps.

The last two samples were prepared to discriminate the influence of mechanical surface treatments on corrosion behavior in the simulated PWR and BWR environments.

Coupons for exposures were machined by slow rate cutting with diamond blade in the form of square bars 15 x 15 mm 2 and 5 mm thick (approximatively the half thickness of the plate) to limit the heating transfer during the machining. The coupons were cut directly from the commercial plate and then polished down to 2400 SiC and with colloidal silica. While, they were directly cut from the grinding surface preparation after the dry grinding. Then, coupons were ultrasonically cleaned in acetone, ethanol and pure water.

Immersion of samples in high-temperature water (simulated BWR and PWR environments)

Samples were immersed for 120 h to either simulated PWR or BWR environments. PWR conditions were simulated in a 316L stainless steel autoclave, with a refreshing loop with a flow rate of 10 L.h -1 . The vessel volume is 7 L. The pressure is 15 MPa and the water contains 2 ppm Li, 500 ppm B and 2.7 ppm H2 at 325 °C. BWR ones were simulated in a 316L stainless steel autoclave, with a refreshing loop with a flow rate of 0.6 mL.h -1 . The vessel volume is 1.2 L. The pressure is 10 MPa, with ultrapure water at 288 °C. The dissolved oxygen was maintained at 8 ppm O2 and the outlet conductivity was 0.3 μS.cm - 1 .

Surface characterizations

Surface topographies and microstructures were observed by scanning electron microscopy (SEM, 10 kV, SE detector) (SU-70, Hitachi, Japan), and transmission electron microscopy (TEM, 200 kV) (HF2000, Hitachi, Japan) after focused ion beam (FIB) preparation [START_REF] Abe | Chracteristics of work hardened surface layer on austenitic stainless steels and its relation to SCC susceptibility in high temperature water[END_REF]. The sample roughness was determined using MICROMAP's 3D optical profilometer. The parameter chosen for the texture description is the root means square (RMS). Sample residual stresses were evaluated with the cosα-method (µ-X360s, Pulstec Company, Japan). It has been demonstrated that this method gives similar results to the classical sin²Ψ method [START_REF] Matsuda | Application of X-ray stress measurement for residual stress analysis by inherent strain method -Comparison of cosα and sin 2 Ψ method[END_REF]. A Cu X-ray source was selected to assess the austenite lattice deformation (311) . The average penetration depth of the X-ray beam was estimated to about 10 µm while the X-ray spot had a diameter of 2 mm. Chemical compositions of the oxide films formed in the simulated PWR and BWR environments were mainly determined by X-ray Photon Spectroscopy (XPS). Cr2p, Fe2p, Ni2p, O1s and C1s spectra were recorded by an Omicron Argus XPS equipped with a mono-chromatic spectrum was recorded by a Quantum 2000 apparatus (ULVAC-PHI, Inc, Japan) operated with a 100 µm spot monochromatic Al Kα source (1486.7 eV) in constant-analyzer-energy mode (pass energy 187.85 eV for wide scan and 58.7 eV for regions). Spectra were acquired at 90° (normal angle with respect to the surface plane) and referenced to the hydrocarbon C1s level set at 284.6 eV. To acquire the atomic percentage profile of the elements, sputtering was carried out with monomer Ar + ; the accelerating voltage was 2 kV, the sputtering area was 2 mm x 2 mm at 3.25 nm.min -1 for SiO2. And, the total sputtering time was 200 min.

Ex-situ electrochemical characterization

Electrochemical and electronic properties of the oxides were obtained from the electrochemical analysis. Just after the immersion in simulated PWR and BWR environments, the samples were immersed in an H3BO3 (0.05 M) and Na2B4O7 10H2O (0.075 M) buffer solution (pH = 9.2) at room temperature. The low solubility of the Cr oxides within this solution was one of the reasons for this choice. A three-electrode cell connected to potentiostat SP-300 (Biologic, France) was used.

The working electrode was the oxidized 304L SS with a surface of 0.57 cm 2 in contact with the electrolyte. The reference electrode was a mercury/mercurous sulfate electrode (EMSE = 0.658 V vs SHE), and the counter electrode was a large carbon rod. Open circuit potential has been measured for 24 h to achieve stationarity. Then potentiodynamic polarization tests were carried out from -0.76 V to 1.2 V vs. MSE with a scan rate of 0.5 mV.s -1 .

Electrochemical impedance spectroscopy (EIS), coupled with the Mott-Schottky (MS) approach, was proposed to evaluate the oxide films' semiconductive properties [START_REF] Marcelin | Electronic properties of passive films from the multi-frequency Mott-Schottky and power-law coupled approach[END_REF], [START_REF] Coelho | Mechanical and corrosion characterization of industrially treated 316L stainless steel surfaces[END_REF]. EIS diagrams were successively recorded at each potential (in the potential range selected) within a frequency range from 100 kHz to 100 mHz, with 9 points per decade and a sine wave amplitude of 15 mV. This technique's primary interest is to assess the capacitance and doping densities relying on the analysis of a complete impedance diagram and not a single frequency. The potential domain varies from -1.10 V toward 0.45 V vs. MSE with a potential step of 50 mV.

The MS method is relevant for determining the capacitance and doping density in oxides [START_REF] Hamadou | Impedance investigation of thermally formed oxide films on AISI 304L stainless steel[END_REF], [START_REF] Ferreira | Semiconducting properties of oxide and passive films formed on AISI 304 stainless steel and Alloy 600[END_REF]. A MS curve is obtained by plotting the space charge capacitance ( ) value as a function of the applied potential (Equation 1).

≈ = - + (1)
where q is the elementary charge (-e for electrons and +e for holes), is the vacuum permittivity (8.85 x 10 -14 F.cm -2 ), ε is the relative dielectric constant of the oxide ( = 12 for chromium or iron oxides [START_REF] Dewald | The charge distribution at the zinc oxide-electrolyte interface[END_REF], [START_REF] Chakri | Improved EIS Analysis of the Electrochemical Behaviour of Carbon Steel in Alkaline Solution[END_REF]), and Nq is the density of doping densities that can be acceptors NA or donors ND, is the flat band potential, T is the absolute temperature, is the Boltzmann constant.

In the present work, the capacitance values are innovatively determined by using the complexcapacitance plane representation from each obtained impedance diagram during the measurement. This methodology was validated by Jonscher [START_REF] Jonscher | Admittance spectroscopy of systems showing low-frequency dispersion[END_REF] and Benoit et al. [START_REF] Benoit | Comparison of different methods for measuring the passive film thickness on metals[END_REF] and mentioned in our previous work [START_REF] Jaffré | Effect of mechanical surface treatments on the surface state and passive behavior of 304L stainless steel[END_REF] but never used for thick oxide films.

The complex capacitance plane representation is obtained (Figure 1) by plotting the imaginary part of the capacitance (Equation 3) as a function of the real part of the capacitance (Equation 2) obtained from the impedance signal corrected by the electrolyte resistance. Figure 1 shows an example of a representation of a complex capacitance plane from an impedance measurement at one potential for an oxide formed on a 304L SS in PWR. To determine the capacitance of the system, it is necessary to draw the tangent to the curve from the origin and the first value that the tangent intercepts correspond to the capacitance C , which is equated to the space charge capacitance (C -. ). These results are a point on the Mott-Schottky curve (Equation 1). After several potential measurements and the determination of each capacitance through the complex capacitance plane representation, the MS curve is obtained.

= !" #" $$ %(" !' ( ) (2)
= " !' ( #* $$ %(" !' ( ) (3)
This curve allows to determine the type of semiconductivity and the doping densities (Nq). For a semi-conductivity of n-type, the slope is positive, whereas, for a semi-conductivity of p-type, the slope is negative. The doping densities can be determined from the linear part of the slope of the MS curve, which corresponds to the depletion domain where the MS approach is valid (Equation 4).

/ = 0 (4)
where a the slope of the straight line in the MS curves.

Results

Surface modifications after mechanical surface treatments

Before immersion in simulated PWR and BWR environments, the surfaces were observed by SEM after the three mechanical surface treatments. The ground surface (G) is presented in Figure 2 (a). Large scratches characteristic of abraded surfaces are observed as well as material detachments. It leads to numerous valleys and ridges that induce significant surface roughness of about 1300 nm.

Oxidation behavior of 304L SS in simulated BWR and PWR evironments following surface treatments

Morphology and nature of the oxides

After 120 h in PWR and BWR environments, the sample surfaces were observed with SEM (Figure 4). For all samples, white precipitates with broad sizes distribution smaller than a few microns were detected at the surfaces. Note that they may be isolated or forming agglomerates. At higher magnification, a thin nanograin layer microstructure could be observed on all the samples.

These first observations are consistent with the duplex structure of the oxides generally formed in simulated BWR or PWR environments: an outer oxide-hydroxide made of precipitates embedded in an inner oxide. Figure 4 (a) shows a representative image of the oxidized surface after immersion in the simulated BWR environment for G surface treatment. The initial scratches, valleys, and ridges are still observed, while dispersed precipitates with a maximum diameter of 1.7 µm (Table 2) are observed on the surface.

Chemical composition of oxides

The elemental depth profiles obtained by XPS after immersion in simulated PWR and BWR environments are reported in Figure 5. The concentrations of individual metallic constituents are normalized to the total concentration of the metallic elements. The concentration of oxygen is normalized to the total concentration of metallic elements plus oxygen. The atomic percentages of elements are reported to the sputtering times, which is more accurate, but an approximative thickness could be estimated by considering the sputtering depth rate of 3.25 nm.min -1 measured on SiO2 reference. Moreover, the thickness by XPS profiles may give some qualitative or comparative trends but it is not sufficient to get quantitative data. The dispersion of large precipitates corresponding to the outer oxide-hydroxide may affect the accuracy of the measurement.

As previously mentioned, the oxide grown on SS in either simulated PWR or BWR environments is generally duplex. Generally, the inner oxide is compact and rich in Cr while the outer oxidehydroxide is not compact and rich in Fe. Consequently, the inner oxide interface/outer oxidehydroxide is located where the Cr/(Cr + Ni + Fe) value reaches the highest value or becomes stable with the sputtering time. The SS/inner interface is considered where the value of the oxygen O/(O+Cr+Ni+Fe) falls to 50 % of the surface value [START_REF] Ming | Effect of surface state on the oxidation behavior of welded 308L in simulated nominal primary water of PWR[END_REF], [START_REF] Ghosh | High temperature oxidation behavior of AISI 304L stainless steel -Effect of surface working operations[END_REF], [START_REF] Takeda | In situ and ex situ characterisation of oxide films formed on strained stainless steel surfaces in high-temperature water[END_REF], [START_REF] Betova | Surface film electrochemistry of austenitic stainless steel and its main constituents in supercritical water[END_REF]. The description of the profiles is proposed from the SS bulk to the outer oxide-hydroxide. After immersion in simulated BWR environment, the total equivalent oxide thicknesses, determined with the analysis of the O/(O+Fe+Cr+Ni) profile, are 120 nm, 184 nm, and 218 nm for the G surface, the polished surface down to 2400 SiC, and with CS, respectively (Table 2). The Fe ratio decreases at the SS/inner oxide interface for all three surface conditions (Figure 5 For the samples exposed in the PWR environment, the oxide films' equivalent thicknesses are approximately 45 nm, 81 nm and 113 nm for G surface, polished surface down to 2400 SiC and with CS, respectively (Table 2). The Fe ratio decreases at the SS/inner oxide interface and decreases through the G surface inner oxide. For polished surface 2400 SiC and with CS, it remains constant and then drastically decreases at inner oxide/outer oxide-hydroxide interface.

For the G surface (Figure 5

Polarisation curves

Electrochemical reactivity of the oxides formed during 120 h in simulated BWR and PWR environments was studied ex-situ in a borate buffer solution.

Figure 6 (a) and Figure 6 (b) presents anodic polarization curves after immersion in simulated BWR and PWR environments, respectively. For all samples tested in the simulated BWR environment (Figure 6 (a)), the corrosion potential is approximately -0.58 V vs. MSE. The anodic currents are lower than 10 µA.cm -², proving the presence of a barrier oxide layer. A plateau in current density is observed between -0.58 and 0.00 V vs. MSE then the current density slightly increases. This variation suggests that some secondary oxidations probably take place in the oxide. Above 0.52 V vs. MSE, a significant increase in current density indicates that the transpassive reactions and water oxidation may occur. For oxide that have grown in simulated BWR environment, the potentials of OCP and the breakdown do not depend on the surface state.

The curves of the G surface and the polished surface down to 2400 SiC are overlapped. Their current at the plateau is approximately 0.95 µA.cm -2 while it is approximately 0.31 µA.cm -2 for polished surface with CS.

For the three samples oxidized in the simulated PWR environment (Figure 6 These characterizations confirm the presence of a barrier oxide at the surface of all the samples.

However, it is interesting to note that the oxide films' reactivity differs depending on the exposure environment. Whatever the exposure environment, the inner oxide is rich in Cr. Thus, two hypotheses can be put forward to explain this difference. The first one is that the outer oxide-hydroxide plays a role in the oxide films' reactivity. Indeed, the chemical compositions of the outer oxide are different, corrosion potential and the transpassive potential are modified.

For an outer oxide-hydroxide rich in Fe, Cr, and Ni (case of simulated BWR environment), the corrosion potential is more cathodic, and the transpassive potential appears for lower anodic values than for the Cr and Fe rich oxides (case of simulated PWR environment). The second one is related to the stoichiometry of the inner oxide and its defectiveness. To confirm this, the semiconductive properties were investigated in the following part.

Semiconductive properties

The electronic behavior of the oxides grown in the simulated BWR and PWR environments is discussed using the Mott-Schottky approach. The MS plots were shown in Figure 7 (a) and Figure 7 (b) for oxides grown after immersion in simulated BWR and PWR environments, respectively.

For samples immersed in simulated BWR environment (Figure 7 (a)), the MS curves have a positive slope at low potentials. This trend is associated with n-type semiconductor behavior with donor dopants (ND) [START_REF] Toor | Mott-Schottky Analysis of Passive Films on Si Containing Stainless Steel Alloys[END_REF]. At higher potential, an inversion of the main charge carrier could occur. In the present case, this phenomenon is related to modifying reactivity observed on the polarization curves (Figure 6 (a)). As the interface reactivity is modified, there is no direct relationship between semiconductor properties and doping densities [START_REF] Hamadou | Impedance investigation of thermally formed oxide films on AISI 304L stainless steel[END_REF]. In the MS approach's validity domain, the donor's density is in the range of 10 19 cm -3 , whatever the surface treatment (Table 3). Quantification for each material shows a decrease in the donor density with the degree of surface treatment (from 7.9 x 10 19 cm -3 for the G surface to 0.8 x 10 19 cm -3 for polished surface with CS). Moreover, the capacitance of the oxide formed on the G surface is higher than for the other two surfaces, and that of the polished surface down to 2400 SiC is higher than that the polished surface with CS. As the capacitance is inversely proportional to the thickness, these results are consistent with the trend observed with the XPS analysis. Here the donor density is in the range of 10 21 cm -3 . However, the donor density trend is similar to previous observations. The donor density decreases with the surface treatment (6.2 x 10 21 cm -3 , 4.5 x 10 21 cm -3, and 1.5 x 10 21 cm -3 for G surface, polished surface down to 2400 SiC and with CS, respectively). The oxides' capacitance values formed on the polished surface down to 2400 SiC and that of the G surface are very close, but the G surface slope is less steep. The capacity of the polished surface with CS has a lower value than the other two.

The oxides formed in simulated BWR and PWR environments behave as n-type semiconductors.

However, two orders of magnitude differences between the density of dopant, with a lower density for the oxides formed in simulated BWR environment. Moreover, it is worth mentioning that the MS theory's domain of validity seems to shift to a higher potential for oxides grown in simulated PWR environment. The consistency of these results indicates that the methodology used in this work is appropriate for characterizing the electronic properties of oxide films.

Discussion

Comparison of the oxidation features according to the environment

SEM observation (Figure 3) and XPS analysis (Figure 5) report similarities and differences in the tested environment's oxidation features.

The precipitates forming the outer oxide are larger and seems more numerous for samples immersed in the simulated PWR environment than in BWR one. This finding is completed by the complementary high-magnification TEM cross-sectional images for oxides grown on the ground surface for the simulated BWR environment (Figure 8) and for the simulated PWR environment (Figure 9). The displayed EDX mapping was performed with a 200 kV field-emission transmission electron microscopy. On Figure 8, the outer oxide precipitates are relatively small but seems to form an inhomogenous but dense layer while on Figure 9, the precipitates are larger but dispersed all over the surface. Note that Kuang et al. [START_REF] Kuang | The oxidation behaviour of 304 stainless steel in oxygenated high temperature water[END_REF] found that oxidized stainless steel's surface morphology evolved with increasing immersion time. In the present work, the same duration was used for PWR and BWR exposures, suggesting that the size and the density is related to the environment and the exposure conditions. Furthermore, the outer oxide-hydroxide composition changes with the environment. It is well known that the outer oxide-hydroxide precipitates can be composed of spinel rich in iron, chromium or nickel, or magnetite crystallites [START_REF] Cissé | Effect of surface preparation on the corrosion of austenitic stainless steel 304L in high temperature steam and simulated PWR primary water[END_REF], [START_REF] Ming | Effect of surface state on the oxidation behavior of welded 308L in simulated nominal primary water of PWR[END_REF], [START_REF] Cunha Belo | Composition, structure and properties of the oxide films formed on the stainless steel 316L in a primary type PWR environment[END_REF]. In our study, XPS profiles also show a lower Ni atomic percentage in all oxides formed in the simulated PWR environment than in the simulated BWR one. It is consistent with the literature that reports Ni-free outer oxide-hydroxide and inner oxide for PWR conditions. Moreover, Figure 8 and 9 evidence that the chemistry of the outer oxide is more homogeneous after BWR exposure than PWR one. Indeed, enriched zones in Ni or in Fe could be observed on Figure 9. From Figure 8 and9, it seems also that the outer oxide precipitates formed in simulated PWR environment are richer in Fe and Cr than in simulated BWR one. This finding shows that there is a difference in corrosion behavior between the simulated BWR and PWR environment, which may be due to the difference in temperature or a difference in solubility of anions in the environments.

For both environments, the inner oxide is rich in Cr compared to the SS bulk and the outer oxidehydroxide, whatever the surface treatment. According to XPS and electrochemical characterizations, the inner oxides formed in the simulated PWR environment are thinner than those formed in the BWR one. However, Figure 9 shows also that the thickness of the inner oxide is not homogenous for PWR exposure and that some internal oxidation may locally occur, especially for the ground sample. The inner oxides formed in the simulated BWR environment have a much lower donor density (ND) (oxygen vacancy) than in PWR one (in PWR ∼ 10 21 cm -3 and in BWR ~ 10 19 cm -3 ). It confirms that the higher the doping density, the thinner the oxide [START_REF] Jaffré | Effect of mechanical surface treatments on the surface state and passive behavior of 304L stainless steel[END_REF]. It suggests also that the stoechiometry deviation differs according to the environment. The chemical composition differences affect consequently the surfaces' electrochemical reactivity, as shown on the polarization curves (Figure 6). Indeed, the Ecorr is more anodic for simulated PWR environment than for the BWR one. Note that the inner oxide, which is rich in Cr behaves like an n-type semiconductor whatever the surface treatment. Note that, at room temperature, the oxide rich in Cr generally behaves as a p-type semiconductor. However, an oxides' semiconductivity transition from p-type to n-type is commonly admitted when the oxides are formed in high-temperature water [START_REF] Szklarska-Smialowska | The composition and properties of oxide films on type 304 stainless steel on exposure to lithiated water at 100-350°C[END_REF], which is the case in simulated BWR and PWR environments [START_REF] Marchetti-Sillans | Corrosion généralisée des alliages à base nickel en milieu aqueux à haute température : Apport à la compréhension des mécanismes To cite this version[END_REF].

It was found that water chemistry has a significant influence on the nature and properties of the oxides and is related to the oxidation kinetics [START_REF] Xu | The effects of dissolved hydrogen on the corrosion behavior of Alloy 182 in simulated primary water[END_REF], [START_REF] Qiu | Effect of dissolved hydrogen on the electrochemical behaviour of Alloy 600 in simulated PWR primary water at 290°C[END_REF], [START_REF] Montemor | Influence of pH on properties of oxide films formed on type 316L stainless steel, alloy 600, and alloy 690 in hightemperature aqueous environments[END_REF], [START_REF] Peng | Effect of dissolved hydrogen on corrosion of Inconel Alloy 600 in high temperature hydrogenated water[END_REF], [START_REF] Peng | Effect of dissolved hydrogen on corrosion of Inconel Alloy 600 in high temperature hydrogenated water[END_REF]. In our study, the chemistry of the two environments is different. The simulated BWR environment is made of pure water, and dissolved oxygen is monitored (8 ppm) leading to an oxidizing environment.

Conversely, the simulated PWR environment is composed of elements such as boron, which is an acid, and lithium hydroxide, which is added to buffer the solution. It also contains dissolved hydrogen (2.7 ppm) to monitor the redox potential of the solution. It has been found that dissolved hydrogen in the simulated PWR environment induces a reduction in the thickness of the inner oxide and a decrease in the transport of ionic defects in the inner oxide [START_REF] Peng | Effect of dissolved hydrogen on corrosion of Inconel Alloy 600 in high temperature hydrogenated water[END_REF]. Moreover, the dissolved hydrogen leads to an increase of chromium anion and a decrease of nickel concentrations in the inner oxide [START_REF] Peng | Effect of dissolved hydrogen on corrosion of Inconel Alloy 600 in high temperature hydrogenated water[END_REF]. Then, the presence of the dissolved gases monitoring the redox potential of the environment play a major role on the chemical composition of both outer and inner oxide layers. It seems also influence the nucleation or the initial growth rate. Indeed, the thickness of the inner oxide is thicker after 120 h of immersion in simulated BWR environment than in PWR. Note that there is a slight difference in temperature between simulated BWR and PWR environments (288 °C and 325 °C, respectively). It has been shown that the effect of a slight difference at this range of temperature seems negligible in terms of oxides thickness [START_REF] Cissé | Effect of surface preparation on the corrosion of austenitic stainless steel 304L in high temperature steam and simulated PWR primary water[END_REF]. However, this difference of temperature may affect the thermodynamic equilibrium and be responsible for the difference in terms of chemical species distribution within the oxide layers.

Effect of mechanical surface treatment on the surface of SS

In summary, the G surface has deep and intense scratches due to the degradation of the abrasion mechanism, resulting in high roughness. The ridges and valleys are less marked for polished surface down to 2400 SiC until a flat surface when polished with CS. In addition, the G process and polishing down to 2400 SiC induce different levels of residual compressive stress and generate an ultrafine grain layer. Both are annihilated for polished surface with CS. Indeed, the polished surface with CS does not have this ultrafine grain layer nor any residual stress.

Consequently, the polished surface CS can be considered free of surface defects, microstructure change, and residual compressive stress.

Effect of mechanical surface treatment on the properties of the oxides formed in simulated BWR and PWR enviroments

As oxides grow on different surface states, all the elementary processes associated with oxidation are expected to be affected (charge transfer, adsorption, nucleation, growth, and coalescence of the oxide islands). For samples exposed to the simulated PWR and BWR environments, the density of the outer oxide-hydroxide precipitates is higher for G surface than for the polished surface down to 2400

SiC. It is worth noticing that the G surface's outer oxide-hydroxide precipitates' size is the largest.

So, the G surface immersed in the solution oxidizes more rapidly than a polished surface. Indeed, the ultrafine grain layer, acting as a diffusion short circuit, promotes the dissolution of metallic elements and the formation of large precipitates in the outer oxide-hydroxide. While, on polished surface with CS, no large oxide precipitates were observed with SEM. Complimentary high magnification cross-section images (Figure 12) reveal small oxide precipitates rich in Fe and Ni embedded on the inner oxide's surface for the polished surface with CS. Their nanometric size may be related to a lower dissolution rate due to the absence of surface defects. Figure 13 shows that the oxide formed in simulated BWR on polished surface down to 2400 SiC exhibit some oxidation features close to the ones observed on G sample: larger oxide precipitates and more numerous than for CS sample. According to surface treatment, the evolution of inner oxide thickness follows the same trend for both environments. It increases following G surface, polished surface down to 2400 SiC, and, with CS. The inner oxide on the G surface is more evolved than on the polished surfaces. For samples immersed in simulated PWR and BWR environments, the donor density increases as the oxide thickness decreased. On the G surface for both simulated BWR and PWR environments the Cr enrichment in the inner oxide is higher than for polished surfaces.

Regarding the mechanical surface treatment, our results are consistent with the literature where thicker oxide grows on smoother and less deformed surfaces [START_REF] Cissé | Effect of surface preparation on the corrosion of austenitic stainless steel 304L in high temperature steam and simulated PWR primary water[END_REF], [START_REF] Ming | Effect of surface state on the oxidation behavior of welded 308L in simulated nominal primary water of PWR[END_REF], [START_REF] Hemmi | Electrochemical considerations regarding general corrosion of materials in a bwr primary circuit[END_REF], [START_REF] Warzee | Effect of Surface Treatment on the Corrosion of Stainless Steels in High-Temperature Water and Steam[END_REF]. It also corroborates the results of Coelho et al. [START_REF] Coelho | Mechanical and corrosion characterization of industrially treated 316L stainless steel surfaces[END_REF] who found that doping densities may depend on residual compressive stresses or surface roughness resulting from mechanical surface treatment.

Like the previous study [START_REF] Jaffré | Effect of mechanical surface treatments on the surface state and passive behavior of 304L stainless steel[END_REF], the present work confirms that stress, ultrafine grain layer, and roughness play a more critical role in corrosion kinetics. However, they don't significantly affect the electrochemical reactivity in the borate buffer solution. Indeed, polarization curves are similar for the three surface states, probably because all oxides have approximately the same composition. Therefore, surface treatment doesn't significantly impact the chemistry of the oxides.

In summary, the more disordered the surface (G), the faster the oxidation kinetics. Thus, the outer oxide-hydroxide precipitates are larger, the inner oxide is thinner with higher doping densities than for polished surfaces.

Conclusion

The influence of mechanical surface treatment on the oxidation behavior of 304L SS in simulated BWR and PWR environments was investigated in this work by a detailled ex-situ characterization including electronic miscroscopic observations, surface and electrochemical techniques. This work demonstrates the potentiality of the proposed electrochemical procedure to discriminate the impact of the surface treatment and the environment on the oxidation process.

For both test environments, the oxide formed has a duplex structure composed of an outer oxide-hydroxide precipitates rich in Fe and an inner oxide rich in Cr. Nevertheless, for simulated PWR environment, the outer oxide-hydroxide precipitates are larger, denser, and the Ecorr is more anodic than for the BWR one. Thus, simulated BWR environment is more oxidizing than the PWR one. Indeed, dissolved oxygen is oxidizing and dissolved hydrogen is reducing. Both have an

Figure 1 .

 1 Figure 1. Example of determination of capacitance at one potential by complex capacitance plane representation for an oxide formed on 304L stainless steel in PWR.

Figure 2 (

 2 b) shows the polished surface down to 2400 SiC. Abrasion and scratches are still observed, but within less extent than the G. Here, the roughness is lower with a value of 12 nm. For the polished surface with CS (Figure2 (c)), the surface looks smoother, as confirmed by a lower roughness (< 0.3 nm) and free of scratches confirming that CS is an appropriate procedure to obtain a mirror-polished state.

Figure 2 .

 2 Figure 2. SEM (10 kV, SE detector) observations of 304L, (a) G surface, (b) polishing down to 2400 SiC and (c) with CS.

Figure 3 .

 3 Figure 3. TEM (200 kV) observations of 304L cross-section, (a) G surface, (b) polishing down to 2400 SiC and (c) with CS.

Figure 4 (

 4 b) shows the polished surface down to 2400 SiC, the overall oxidation behavior is similar to the G sample. The initial roughness is still observed. However, precipitate sizes are smaller with a maximum diameter of about 1 µm. In contrast to the two previous conditions, there is almost no precipitation on the polished surface with CS (Figure4 (c)).Similarly, the surfaces after immersion, in the simulated PWR environment, were observed by SEM. In agreement with previous observations, higher magnification reveals an oxide covering the surface with some oxide precipitates embedded on it. For the G surface immersed in PWR environment (Figure4 (d)), scratches, valleys, and ridges are less visible than in the BWR condition. And, qualitatively, more outer oxide-hydroxide precipitates are observed. For both conditions, the precipitates are distributed over the entire surface (Figure4(a), (d)). Moreover, agglomerated particles may reach a size of 3µm. On the polished surface down to 2400 SiC, the initial roughness is no longer observed (Figure4 (b)). Agglomerated oxides and a uniform oxide cover the surface. These oxides are larger than the ones observed on G surface. It can also be noticed that the density of outer oxide-hydroxide precipitates is higher for this surface treatment in this condition. For the polished surface with CS (Figure4(f)), few oxide precipitates with an average size of 1 µm are spread all over the surface. These results indicate that the oxidation behavior is different as a function of the environment. There are more numerous and large precipitates dispersed on the surface after immersion in the simulated PWR environment.

Figure 4 .

 4 Figure 4. SEM observations (10 kV, SE detector) of 304L after 120 h in simulated BWR environment (10 MPa; 288 °C) : (a) G surface, (b) polished surface down to 2400 SiC, and, (c) with CS and in simulated PWR environment (15 MPa; 325 °C) : (d) G surface, (e) polished surface down to 2400 SiC, and, (f) with CS.

Figure 5 .

 5 Figure 5. XPS depth profile for 304L SS after 120 h of immersion in simulated, BWR environment (10 MPa; 288 °C): (a) G surface, (b) polished surface down to 2400 SiC, (c) with CS, and in PWR environment (15 MPa; 325 °C): (d) G surface, (e) polished surface down to 2400 SiC, (f) with CS.

  (a), (b), (c)). It gradually decreases in the inner oxide for the polished surface down to 2400 SiC and with CS to achieve a stable value in the outer oxide-hydroxide. It decreases much more abruptly for G surface. For the G surface (Figure5(a)), the Cr ratio increases very slightly at the SS/inner oxide interface, then continues to increase in the inner oxide from 24 % to 30 %, and then decreases to 15 % in the outer oxide-hydroxide. For the polished surface down to 2400 SiC (Figure5 (b)), the ratio is constant at 20 % at the SS/inner oxide interface, remains constant in the inner oxide, and decreases slightly in the outer oxide-hydroxide down to 15 %. For the polished surface with CS (Figure5 (c)), Cr increases very slightly at the SS/inner oxide interface, then continues to increase in the inner oxide from 22 % to 27 %, and decreases to 10 % in the outer oxide-hydroxide. The Ni ratio increases at the SS/inner oxide interface for all three conditions. Then it continues to increase in both the inner and outer oxide-hydroxide.

  (d)), the Cr ratio increases at the SS/inner oxide interface from 20 % to 28 %, then increases drastically to 39 % and stabilizes in the outer oxide-hydroxide at about 40 %. For polished surface down to 2400 SiC (Figure5(e)), the Cr ratio increases at the SS/inner oxide interface from 20 to 26 % and in the inner oxide up to 38 % and 39 % in the outer oxidehydroxide. For polished surface with CS (Figure5(f)), the Cr ratio decreases slightly at the SS/inner oxide interface, increases by 20 to 30 % in the inner oxide, and increases up to 40 % in the outer oxide-hydroxide. The Ni ratio increases slightly at the SS/inner oxide interface for G surface, decreases for polished surface down to 2400 SiC and remains constant for polished surface with CS. It then decreases in the inner oxide for all three conditions. It remains at 5 % in the outer oxide-hydroxide for G surface and drops to about 0 % for polished surface down to 2400 SiC and with CS.

  (b)), the corrosion potential is approximately -0.27 V vs. MSE. The three oxidized surfaces show protective behavior due to the oxide's presence until 0.85 V vs. MSE. The trend is different from samples immersed in the simulated BWR environment, indeed, in this case, the current slightly increases from OCP to 0.85 V vs. MSE. The CS sample presents the lowest current density values on a wide range of potential, whereas the grinding sample has the highest. The transpassive reaction and water oxidation take place at 0.85 V vs. MSE.

Figure 6 .

 6 Figure 6. Polarization curve performed ex-situ in borate buffer solution (pH = 9.2) for oxides formed in simulated (a) BWR and (b) PWR environments (120 h), on 304L SS for three differents surfaces treatments: G, polished down to 2400 SiC and with CS.

Figure 7 (

 7 Figure 7 (b) shows that all MS curves obtained for samples immersed in PWR environment. They exhibit a positive slope from -0.27 V to 0.25 V vs. MSE, corresponding to an n-type semiconductor behavior of oxide. Here the donor density is in the range of 10 21 cm -3 . However,

Figure 7 .

 7 Figure 7. Mott-Schottky plot obtained ex-situ in borate buffer solution (pH = 9.2) for oxides formed in simulated (a) BWR and (b) PWR environments (120 h), on 304L SS for three differents surfaces treatments: G, polished down to 2400 SiC and with CS.

Figure 8 .

 8 Figure 8. Oxide formed in simulated BWR on ground surface, (a) cross-sectional view performed by TEM, (b) O, Ni, Fe, Cr mapping of the surface performed by TEM-EDX (200 kV).

Figure 9 .

 9 Figure 9. Oxide formed in simulated PWR on ground surface, (a) cross-sectional view performed by TEM, (b) O, Ni, Fe, Cr mapping of the surface performed by TEM-EDX (200 kV).

Figure 10 and

 10 Figure 10 and Figure 11 summarize the oxidation features considering the different surface states after immersion for 120 h in simulated BWR and PWR environments, respectively. Those schematics have been inspired by the complimentary high magnification cross-section images (Figure 8, 9, 12, 13).

Figure 10 .

 10 Figure 10. Schematic representation of the samples after 120 h of immersion in the simulated BWR environment: (a) G surface, (b) polished surface down to 2400 SiC, and, (c) with CS.

Figure 11 .

 11 Figure 11. Schematic representation of the samples after 120 h of immersion in the simulated PWR environment: (a) G surface, (b) polished surface down to 2400 SiC, and, (c) with CS.

Figure 12 .

 12 Figure 12. Oxide formed in simulated BWR on polished surface with CS, (a) cross-sectional view performed by TEM, (b) O, Ni, Fe, Cr mapping of the surface performed by TEM-EDX (200 kV).

Figure 13 .

 13 Figure 13. Oxide formed in simulated BWR on polished surface down to 2400 SiC, (a) cross-sectional view performed by TEM, (b) O, Ni, Fe, Cr mapping of the surface performed by TEM-EDX (200 kV).

Table 1 .

 1 Size and density of outer oxide-hydroxide precipitates, formed on G surface, polished surface down to 2400 SiC and with CS in simulated BWR and PWR environments.

	Environment		BWR			PWR
	Surface treatment	G	2400 SiC	CS	G	2400 SiC	CS
	Max. Precipitate sizes (µm)	1.7	1	0	(Aggl.) 3	(Aggl.) 3	1
	Precipitate density	Medium	Low	----	Medium	High	Very low

Table 2 .

 2 Total equivalent oxide thickness, measured by XPS, formed on the G surface, polished surface down to 2400 SiC and with CS in simulated BWR and PWR environments.

	Thickness inner oxide (nm)	BWR	PWR
	Grinding	120	45
	2400 SiC	184	81
	CS	218	113

Table 3 .

 3 Donors densities (ND) measured ex-situ in borate buffer solution (pH = 9.2) for oxides formed in simulated BWR and PWR environments (120 h), on 304L SS for three differents surfaces treatments: G, polished down to 2400 SiC and with CS.

	Parameters	ND (cm -3 )
	Environment	BWR	PWR
	Grinding	7.9 x 10 19 6.2 x 10 21
	2400 SiC	1.5 x 10 19 4.5 x 10 21
	CS	0.8 x10 19 1.5 x 10 21

Acknowledgements

The authors would like to thank Takamichi Miyazaki, TEM specialist at Tohoku University, for his contribution to the TEM experiment and electron diffraction pattern analysis. Masatoshi Tanno for the XPS measurements. Atsushi Hashimoto for help in performing the FIB procedure, and Xiong Yida for immersion of the samples in the simulated PWR autoclave.

effect on the chemical species and defect distribution within the oxide layers and kinetics of the oxidation and consequently on the reactivity of the oxide. While the surface state after mechanical surface treatment only impacts the oxidation kinetics due to the modifications of the surface. As a result, the density of the outer oxide-hydroxide precipitates is higher for the G surface than for the polished surface down to 2400 SiC. The G surface immersed in the solution oxidizes more rapidly than a polished surface. Indeed, the ultrafine grain layer, acting as a diffusion short circuit, promotes the dissolution of metallic elements.

The capacitance of the inner oxide formed on the G surface is higher than for the other two surfaces, and, that of the polished surface down to 2400 SiC is higher than that with CS.

Moreover, there is a decrease in the donor density with the degree of surface treatment. And, the lower the doping density, the thicker the inner oxide.

To conclude, the oxide grown's electrochemical reactivity is more affected by the environment chemistry than by the mechanical surface treatment. This last finding indicates that a technical solution potentially interresting for one environment is not directly transposable to the other. It could be decisive for SCC issue.

CRediT authorship contribution statement

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.