# Corrosion of titanium alloys in pressurised water at 350 °C and 17.5 MPa

Sabrina Selva, Quentin Bignon, Amandine Raynal, Frantz Martin, Quentin

Auzoux, Gilles Renou, Laurence Latu-Romain, Yves Wouters

► To cite this version:

Sabrina Selva, Quentin Bignon, Amandine Raynal, Frantz Martin, Quentin Auzoux, et al. Corrosion of titanium alloys in pressurised water at 350 °C and 17.5 MPa. Corrosion Science, 2022, 201, 10.1016/j.corsci.2022.110268. hal-04889623

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# Corrosion of titanium alloys in pressurised water at 350 °C and 17.5 MPa

<figure><img src="image_3.png" /></figure>

Sabrina Selva<sup>a,b,*</sup>, Quentin Bignon<sup>a,b</sup>, Amandine Raynal<sup>a</sup>, Frantz Martin<sup>a</sup>, Quentin Auzoux<sup>a</sup>, Gilles Renou<sup>b</sup>, Laurence Latu-Romain<sup>b,c</sup>, Yves Wouters<sup>b</sup>

## ARTICLE INFO

**Keywords:**
Titanium
Corrosion
Dissolution
Kinetics
Primary water

## ABSTRACT

The corrosion of titanium alloys in pressurised water at 350 °C and 17.5 MPa was studied in a loop made of stainless steel. The corrosion mechanism involved simultaneous TiO₂ formation and dissolution, which led to surface precipitation of TiO₂ anatase and FeTiO₃ ilmenite crystallites. Based on image analyses made on cross-sections, a minimum corrosion rate, for all tested titanium alloys, was estimated at 4 µm year⁻¹ at 350 °C. Comparison with a previous study indicated an increase of the corrosion rate by an approximate factor of 4 between 300 °C and 350 °C.

## 1. Introduction

Titanium alloys could replace some stainless steel parts as primary circuit structure components for nuclear Pressurised Water Reactor (PWR). They have a lower neutron activation and also good mechanical properties [1]. To date, very few studies have been dedicated to the corrosion behaviour of such alloys in PWR primary water environment, i.e. at nominal temperature and pressure 300 °C and 15 MPa respectively. The work of Bignon et al. [2] and Selva et al. [3] contributed to the identification of the corrosion mechanisms and kinetics in such nominal conditions. They found that the oxidation of commercially pure titanium (CP Ti), Ti-6Al-4V (also called Ti64) and Ti10-2-3 during exposure led to the growth of a continuous TiO₂ layer made of rutile for CP Ti and of anatase for Ti64 and Ti10-2-3. This difference was assigned to the participation of alloying elements to the oxide formation. For all materials, the evolution of this continuous layer's thickness with exposure duration could be modelled by a Loriers-Haycock's equation, reaching rapidly – within 200 h – a steady thickness of about 30 nm. Its contribution to the global mass uptake appeared negligible over long durations. The constancy of this thin layer thickness attested of its simultaneous growth and dissolution. Thus, the corrosion rate depends on the dissolution rate of this thin oxide layer. Its dissolution and the one of the stainless steel experimental device caused the formation of titanium and iron hydroxides in the medium, which promoted the precipitation of small – TiO₂ anatase – and large – FeTiO₃ ilmenite – crystallites

at the surface of the sample, leading to a significant mass gain. A Ti precipitation ratio was introduced to take into account the fact that a part of the oxidised then dissolved Ti species did not take part to precipitation at the surface and could be evacuated away from the surface by the media. Following the proposed mechanisms, these authors linked the corrosion rate to the weight variation and to the Ti precipitation ratio. The deduced corrosion rate of titanium alloys in primary water at 300 °C was estimated between 2 and 6.6 µm year⁻¹ with a Ti precipitation ratio ranging from 0.7 to 1. For autoclave experiments without renewal of the media, and for long exposure durations these authors also highlighted the full coverage of the surface by FeTiO₃ crystallites and the very significant decrease in the corrosion rate once the surface became fully covered [3]. Conversely, in the corrosion loop experiments, the surface was far to be totally covered: the coverage ratio was only around 0.25 after 3499 h of exposure.

In order to accelerate the corrosion processes kinetics, which are usually thermally activated, the experimental temperature was increased in the present study, as is done for zirconium alloys in the nuclear industry dedicated studies – i.e. typically 360 °C and 18.8 MPa instead of 330 °C and 15 MPa [4]. The aim was to access to corrosion states that could not be achieved within reasonable laboratory time scales and to extrapolate more accurately the corrosion kinetics behaviour for very long exposure durations, provided the involved mechanisms identified at 300 °C are still valid at higher temperatures. The present study was therefore conducted in PWR pressurised water at

<footnote><sup>a</sup> Université Paris-Saclay, CEA, Service de la Corrosion et du Comportement des Matériaux dans leur Environnement, 91191 Gif-sur-Yvette, France</footnote>

<footnote><sup>b</sup> Université Grenoble Alpes, CNRS, SIMaP, 38402 Saint Martin d'Hères, France</footnote>

<footnote><sup>c</sup> Ugitech, Centre de recherches, Av. Paul Girod, 73400 Ugine, France</footnote>

<footnote>* Corresponding author at: Université Paris-Saclay, CEA, Service de la Corrosion et du Comportement des Matériaux dans leur Environnement, 91191 Gif-sur-Yvette, France.</footnote>

<footnote>E-mail address: Sabrinaselva@yahoo.fr (S. Selva).</footnote>
350 °C and 17.5 MPa, conditions for which no data was available in literature.

Nonetheless, few studies carried out in close – but different – conditions can provide some information on temperature effect on titanium alloys corrosion in high temperature water. Kaneda *et al.* [5] exposed Ti-3Al-2.5V, Ti-15V-3Al-3Sn-3Cr, and Ti64 during 500 h to pure supercritical water (25 MPa, 290–550 °C, 8 ppm of dissolved oxygen) in a device composed of aluminium parts. They highlighted, after 500 h exposure, a slight increase in the mass variation despite large measurement uncertainties of the specimen between 290 °C and 380 °C for Ti-3Al-2.5V and Ti64. The mass variations of the first material were of $12 \pm 2$ mg dm$^{-2}$ at 290 °C and of $30 \pm 35$ mg dm$^{-2}$ at 380 °C. For Ti64, they measured a mass variation of $10$ mg dm$^{-2}$ at 290 °C and of $14 \pm 52$ mg dm$^{-2}$ at 380 °C [5]. However, a significant increase of the mass variation was noticed at 550 °C to reach $396 \pm 25$ mg dm$^{-2}$ for Ti-3Al-2.5V and $285 \pm 12$ mg dm$^{-2}$ for Ti64. This seems to demonstrate that there was a thermal activation of the corrosion phenomenon. Then, they also showed an effect of temperature on the morphology of the continuous oxide layer of all alloys. Its thickness was 30–300 times greater depending on the studied alloys after exposure to 550 °C than after exposure to 290 °C. Moreover, this layer became porous at 550 °C, albeit only for Ti-3Al-2.5V and Ti-15V-3Al-3Sn-3Cr. For Ti64, no porosity was observed at the micrometric scale in the continuous oxide layer after exposure at 290 °C and at 550 °C in supercritical water [5] as well as at the nanometric scale after exposure at 300 °C in a primary medium [2]. The continuous oxide layers of these alloys exposed to 550 °C in supercritical water were rich in O and Ti while the particles on the surface were rich in O, Al and Ti. The presence of Cr in particles in addition of O, Al and Ti was only detected in Ti-15V-3Al-3Sn-3Cr. These crystallites could originate from the corrosion of the experimental device and the studied samples. Bignon *et al.* [2] did not observe the formation of aluminium-rich crystallites after exposure in a primary medium at 300 °C of CP Ti, Ti64 and Ti10-2-3 in a stainless steel device. In this case, there was no precipitation of the alloying elements such as aluminium and vanadium for Ti64 and Ti10-2-3 on the sample surface. These microstructural studies of the corrosion products suggest that some similarities exist between the corrosion mechanisms involved at the different investigated temperatures, but also some differences, hence needing some clarification.

The work published in [2,3], carried out at 300 °C, constituted a solid base for comparison of the corrosion mechanisms and kinetics because the same materials cut from the same billets were used in the present study at 350 °C, 17.5 MPa and $pH_{25}$ °C = 10.1. The flat polished specimens were exposed to these conditions for different durations in a corrosion loop with a dynamic medium. After that, the oxides formed at 350 °C were characterised. This includes thin oxide film extensive characterisation, surface crystallites identification and precipitation kinetics study as well as metal dissolution kinetics estimation. These results will then be discussed.

## 2. Materials and procedures

### 2.1. Materials

Three materials belonging to different metallurgical families of titanium alloys were studied. They originate from the same billets, supplied by TIMET Savoie, than in the previous works [2,3]. There were the commercially pure titanium (CP Ti – grade 2), a single-phase α, Ti64, a two-phase α + β alloy with 4 vol% of β phase and the Ti10-2-3, a β-metastable alloy with 38 vol% of β phase. Ti64 is composed of 6 wt% Al and 4 wt% V and Ti10-2-3 is composed of 2 wt% Fe, 3 wt% Al and 10 wt% V. More detailed information about the materials and microstructures are available in [2] and [3]. The samples, with dimensions of $20 \times 10 \times 1.2$ mm$^3$, were ground with SiC paper from grit 500 to grit 2400 then polished with diamond suspensions (down to 1 µm granulometry). Finish polishing was made with colloidal silica suspension (pH = 9). The

final thickness after polishing both sides of the samples was approximately 1 mm.

### 2.2. Primary water exposure

Fig. 1 presents a schematic of the stainless steel experimental device used in this present study to expose the samples to primary water at 350 °C, 17.5 MPa, $pH_{350}$ °C = 7.7 under dynamic conditions. It was the same as the one described in [3] for the exposure at 300 °C, 15 MPa, $pH_{300}$ °C = 7.2. The same parameters were set, except for temperature and pressure: the dissolved hydrogen concentration was $25 \times 10^{-6}$ m$^3$ kg$^{-1}$ H$_2$O NTP (normal temperature and pressure), which was equivalent to 2.25 wt. ppm, the water velocity was of $4.7$ m h$^{-1}$ at the samples' surface and 0.9 wt. ppm of $Li^+$ were added in the form of LiOH for both cases. This concentration induced a pH at 25 °C of 10.1. The difference of pH values at high temperatures was due to the $pK_{water}$ value evolution with temperature. The pH of a neutral solution was estimated to be 5.78 at 350 °C in pressurised water at 25 MPa [6]. Assuming that there is no significant effect of pressure between 15 and 25 MPa on the $pK_{water}$ value, the theoretical pH values are 7.2 at 300 °C and 7.7 at 350 °C, both for a pH equal to 10.1 at 25 °C.

There were no re-exposure of samples. For statistical concerns, three samples per material were immersed for each exposure duration, which were 100 h, 296 h and 1750 h. The short duration tests – 100 h and 296 h – took place in the downstream reactor (R2) and the longest duration test – 1750 h – was conducted simultaneously in the upstream reactor (R1).

The medium that enters the upstream reactor (R1) was well purified by the ion exchange resin located in the cold part. However, solute corrosion products from R1 may have enriched in Fe and Ti species the medium entering the downstream reactor (R2). An associated effect on corrosion kinetics of the samples due to the reactors set-up was suspected in a previous work [3]: the solubility limit in the downstream reactor would be reached earlier than in the upstream reactor leading to a higher precipitation of corrosion products in the downstream reactor. This hypothesis was further investigated in the present study. For that reason, for each sample, the {reactor; exposure duration} couple has to be taken into consideration for further interpretations.

### 2.3. Characterisation techniques

All the weight measurements were performed on a XPE26/A Mettler Toledo balance with an accuracy of $10^{-6}$ g. The macroscopic characterisations of the oxides (analysed surface of $7 \times 7$ mm$^2$) were made with a PANalytical X'Pert PRO MPD X-Ray Diffractometer (XRD) in θ/2θ scanning and then in grazing incidence (0.8°) after exposure to media. The oxides were observed from above the surface and on cross-sections with an Ultra 55 Zeiss Field Emission Gun Scanning Electron Microscopy (FEG-SEM) at 5 kV. Before cutting the samples for cross-sections observations, the oxide layers were protected by Au sputtering and Ni electroplating. The post-treatment and image analyses were done with the software ImageJ™. Microscopic characterisations of the oxides were first carried out with an XFlash Detector 4010 Bruker Energy Dispersive Xray Spectrometer (EDS) coupled with the SEM-FEG at 20 kV. Then, the identification of the nature of these oxides were performed by a Horiba XploRA™PLUS μ-Raman spectrometer with a laser wavelength of 532 nm. The analysed volume was around 1–2 μm$^3$. Finally, nanoscopic characterisations of the oxides required the preparation of 50-nm-thick foils cut from the samples with a dual-focused ion beam scanning electron microscope (FIB-SEM, Zeiss Cross Beam 750 for the foil extraction and a FEI Strata DB400 for its thinning). Identification of the nature of the nanoscopic oxides was carried out with the precession-assisted crystal orientation mapping technique, ASTAR (a TEM-based diffraction spot recognition technique [7]) coupled with STEM-EDS realised on a FEG-TEM Jeol 2100F at 200 keV.
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Schematical representation of the corrosion loop used for the exposure of CP Ti, Ti64 and Ti10-2-3 samples located in reactor 2 (downstream) for short exposure durations (100 h and 296 h) and in reactor 1 (upstream) for long exposure durations (1750 h). The primary water circulation direction is indicated by arrows. The same experimental device was used and presented in [3].</figcaption></figure>

# 3. Results

## 3.1. Oxides formation during exposure at 350 °C

XRD analyses at grazing incidence of 0.8° were carried out on CP Ti, Ti64 and Ti10-2-3 exposed during 100 h at 350 °C to primary water. The phase volume fraction detection limit was around 1% and the analysed depth was a few micrometres. The phase indexation with the references (International Centre for Diffraction Data) are shown in the XRD diffractograms in Fig. 2. As attested by the detection for all materials of the Ti-α phase of the metal (Ref: 00-044), all the oxide layers were analysed. For all materials, anatase TiO₂ with tetragonal structure (Ref: 00-021) and ilmenite FeTiO₃ with trigonal structure (Ref: 04-007) were identified. For CP Ti only, an additional oxide was detected at 2θ = 27.5°, d₁₁₀ = 0.325 nm: the tetragonal structure of rutile TiO₂ (Ref: 00-021). For all three materials, the peak at 2θ = 43.3°, d₁₁₁ = 0.209 nm would correspond to the presence of copper impurities with cubic structure. They could stem from the dissolution of the machining residues of the specimen edges after their manual polishing.

After 100 h exposure to primary water at 350 °C, the samples' surfaces were observed by SEM (secondary electrons) as shown in Fig. 3. Two different crystallites populations were identified: large and scarce 'round' polyhedral crystallites of 1–5 µm in diameter and small numerous parallelepipedic or prismatic crystallites of the order of a few hundred nanometres in size. Focusing on the surface of CP Ti, the low coverage of small crystallites at this exposure duration permitted the

observation of the crystallite-free surface of the sample (medium grey on Fig. 3.a, showing some surface porosity or localised dissolution. The metal dissolution seemed to be dependent on the crystal orientation (different grains are visible on Fig. 3.a, and more pronounced around large crystallites (e.g. around the large crystallite on the right). A dependence of the dissolution rate to the crystal orientation was already observed for other metallic materials and environments [8,9].

The EDS qualitative mappings performed on these samples, not presented here, showed that the large crystallites contained mainly iron, oxygen and titanium at a lower extent. The small crystallites as well as the surface oxide were mainly made of titanium and oxygen as already observed by Bignon and Selva [2,3].

The nature of the large crystallites was identified by micro-Raman analysis on the three materials exposed in the primary environment at 350 °C for 296 h and 1750 h. This technique allows the analysis of a surface of 1 µm² with an analysed depth of 1–2 µm. Therefore, like for EDS, it did not allow a distinction to be made between small crystallites and the surface continuous oxide. Fig. 4.a shows the Raman spectrum obtained after analysis of the large crystallites on CP Ti exposed for 1750 h at 350 °C, compared with a reference spectrum for ilmenite (on RRUFF website, ID = R130214 done by Doell [10]). On all materials and for all investigated exposure durations, the large crystallites were identified as ilmenite FeTiO₃. The spectra obtained after combined analysis of the surface oxide and small crystallites formed after 1750 h exposure at 350 °C are shown in Fig. 4.b for CP Ti and Ti64. For these microstructural features, rutile TiO₂ and anatase TiO₂ were identified for CP Ti while only anatase TiO₂ was identified for Ti64 and Ti10-2-3 (not shown here for clarity). This technique unambiguously confirmed that the large crystallites were in FeTiO₃ ilmenite whereas the oxide surface and the small crystallites were in TiO₂. Furthermore, it ensured that no additional amorphous phase was present. The reference spectra for rutile and anatase were found on RRUFF website with ID = R110109 done by the Gemological Institute of America and ID = R070582 done by Shigley respectively [10]. The same results and trends were obtained on the samples exposed 296 h at 350 °C.

The presence of FeTiO₃ crystallites in such a large quantity on the surface of the three materials, even on the iron-free alloys, shows that this element comes in part from the dissolution of the stainless steel experimental device. In contrast, titanium can only come from the samples. Thus, a mechanism of dissolution then co-precipitation on the surface would be at the origin of the formation of the FeTiO₃ crystallites. It is assumed that, as at 300 °C [2,3], the micrometric TiO₂ crystallites were also formed by a dissolution/precipitation mechanism.

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** XRD diffractograms (grazing incidence 0.8°) on CP Ti, Ti64 and Ti10-2-3 exposed for 100 h in primary water at 350 °C.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 3. Surface SEM (secondary electrons) observations of crystallites formed on CP Ti (a), Ti64 (b) and Ti10-2-3 (c) after 100 h exposure at 350 °C in primary water.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. Raman spectra performed on CP Ti and Ti64 surfaces after 1750 h exposure at 350 °C aiming at identifying the nature of the different crystallites. a) Raman bands obtained on a large crystallite on CP Ti, and b) Raman bands obtained ahead of the small crystallites and surface oxides on CP Ti and Ti64, compared with Ilmenite, rutile and anatase reference spectra [10].</figcaption></figure>

CP Ti, Ti64 and Ti10-2-3 samples exposed for 100 h, 296 h and 1750 h at 350 °C in primary water were cut into cross-sections that were observed by SEM (secondary electrons) and shown in Figs. 5 and 6. Only the evolution of the cross-section of CP Ti with exposure duration is presented in Fig. 5, showing the $FeTiO_3$ (large) and the $TiO_2$ (small and characterised by EDS in cross-section) crystallites. They were located on top of a nanometric oxide layer, which covered continuously all the metal surface. These observations were also noticed on Ti64 and Ti10-2-3 as illustrated in Fig. 6.

As only anatase was detected in Raman and XRD analyses performed on Ti64 and Ti10-2-3 samples, both the $TiO_2$ crystallites and the

continuous oxide layer were assigned to be anatase phase. Conversely, for CP Ti, since the resolution of the techniques used was not high enough, the phase of the continuous oxide layer and that of the crystallites could not be differentiated.

As in the study performed at 300 °C, for all materials, local zones where the metal was more consumed was evidenced. The presence of these local attacks around ilmenite crystallites suggested by surface observations in Fig. 3.a was confirmed (Fig. 5). The relatively flat interface and the lack of significant thickening of the continuous oxide layer, both underneath the ilmenite crystallites seemed to indicate that the crystallites protect the metal from additional corrosion. These

<figure><img src="image_8.png" /><figcaption>Fig. 5. Cross-section SEM (secondary electrons) observations of CP Ti samples after a) 100 h, b and d) 296 h and c) 1750 h of exposure at 350 °C. d) Magnification of a local attack zone showing the continuous oxide layer.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Cross-section SEM (secondary electrons) observations of Ti64 samples – a), b), c) – and Ti10-2-3 samples – d), e), f) – after a and d) 100 h, b and e) 296 h and c and f) 1750 h of exposure at 350 °C.</figcaption></figure>

behaviours conducted to an increase in the tortuosity of the interface with exposure duration as shown in Fig. 5 for CP Ti and in Fig. 6 for Ti64 and Ti10-2-3. These figures also illustrate the increasing size of the crystallites as the exposure duration increased. After 1750 h at 350 °C, the mean diameter of TiO₂ and FeTiO₃ crystallites were around 1 µm and 11 µm, respectively.

Another interesting point is the presence of spongy oxide detected by SEM observations only in the local attacks zones of around 400–500 nm deep after 296 h exposure at 350 °C as pointed by black arrows in Fig. 5. c for 1750 h exposure. Note that this was also observed on CP Ti at 300 °C, but only after 3499 h of exposure [3]. SEM observations of Ti64 and Ti10-2-3 cross sections did not reveal this spongy oxide. For these materials, the morphology of the oxides in the local attack zones as shown in Fig. 6 appeared more to be like a gathering of nanocrystallites, larger than in the spongy oxide areas observed for CP Ti. These spongy oxide areas were therefore specific to CP Ti and will be thereafter referred to as “zone 1”. In addition to those, SEM cross-sectional observations highlighted another feature: in some areas on the surface, the continuous oxide layer seems to be much thicker than elsewhere, as shown in Fig. 7. These thickened oxide areas will be referred to as “zone 2”, and were observed on CP Ti only, also beyond 296 h of exposure. Observations made on SEM images show that these “zones 2” correspond to about 9% of the total studied length of the CP Ti exposed for 296 h at 350 °C and about 72% after 1750 h at 350 °C. Note that they were not observed at 300 °C, even after 3499 h exposure [2,3].

A TEM thin foil was extracted from CP Ti exposed 1750 h at 350 °C to study more precisely these two types of zones. The zones are shown in Fig. 8.a. This duration was chosen because these zones were more

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Cross-section SEM (secondary electrons) observations of the two zones of interest (zone 1, surrounded in yellow and zone 2, pointed by black arrows) of CP Ti samples after 296 h of exposure at 350 °C.</figcaption></figure>

numerous and more visible as the exposure duration increased.

The nature of the spongy oxide layer and of this thickened oxide was determined by TEM-ASTAR. Fig. 8.b illustrates that, in zone 1 (yellow squares), rutile was exclusively detected, while a mixture of rutile and anatase was detected in zone 2. The spongy oxide layer was composed of rutile nanograins as shown in Fig. 8.b. The zones of local thickening of the oxide were composed of grains of several hundred nanometres of anatase, which were in direct contact with the metal and covered by a thin layer of rutile nanograins.

In conclusion, the same elements were observed for all three studied materials after exposure at 350 °C, i.e. large FeTiO₃ and small TiO₂ crystallites on the top of a nanometric TiO₂ oxide layer, which covered the substrate. The interface between the metal and the oxide became all the more tortuous as the exposure duration increased, and it seems that no significant dissolution occurred beneath the FeTiO₃ crystallites. However, there were some differences between the materials as sketched in Fig. 9. On the one hand, a remarkable rutile spongy oxide layer covered the zones resulting from the local metal consumption on CP Ti beyond 296 h of exposure at 350 °C and beyond 3499 h at 300 °C.

For Ti64 and Ti10-2-3, no such spongy oxide layer was observed in these local attack zones: the morphology of the oxides looked like more to a gathering of nanocrystallites rather than a spongy oxide layer (see Fig. 6). Moreover, the only titanium-containing oxide detected was anatase. The continuous oxide layer and the small crystallites of these materials were therefore made of anatase.

On the other hand, only CP Ti presented – beyond 296 h of exposure and only at 350 °C – thicker oxide zones identified as coarse anatase grains, which were covered by a thin nanosized grains rutile layer. As the exposure duration increased, the proportion of such zones increased.

Thus, the important result highlighted by the TEM-ASTAR study is that, for CP Ti at 350 °C – and only for this material at this temperature –, anatase was detected not only in the micrometric crystallites but also under the nanometric rutile layer. Furthermore, the thickness of the nanometric oxide layer depended on the exposure durations, temperature and material. The corrosion mechanism of CP Ti therefore seems to differ slightly at 300 °C and 350 °C and will later be further discussed.

## 3.2. Corrosion kinetics at 350 °C

Fig. 10 presents the positive mean specific mass variation of the samples of the three materials for each exposure duration at 300 °C (data from [3]) and 350 °C.

At 350 °C as shown in Fig. 10.b), the mass uptakes of all three materials were similar, and on average three times higher than at 300 °C for
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Observations of the zones of interest on a cross-sectional TEM slide of the CP Ti exposed 1750 h at 350 °C. a) show bright field TEM images of areas of interest, showing zone 1 in yellow and zone 2 in black. b) TEM-ASTAR phase map combined with the index showing Ti metal, rutile and/or anatase oxide phases.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Schematic representation of the corrosion product layer grown at 350 °C in primary water over a) CP Ti and b) Ti64 and Ti10-2-3. In a) so-called zones 1 and 2 are sketched. TiO₂ anatase and TiO₂ rutile are represented in red and bright pink respectively. FeTiO₃ ilmenite crystallite is hatched in green and red.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Mean specific mass variation of CP Ti, Ti64 and Ti10-2-3 samples after exposure to each exposition duration in upstream reactor (R1) and downstream reactor (R2) a) at 300 °C (data from [3]) and b) at 350 °C in the corrosion loop.</figcaption></figure>

a given exposure duration. The reactor effect suspected in [3] at 300 °C appeared clearer at 350 °C. Indeed, the mass uptakes of the samples exposed in the upstream reactor (R1) for both temperatures were lower than the ones expected according to the data obtained in the downstream reactor (R2). Concerning the data in downstream reactor only, the weight gain kinetics was approximately linear for all three materials at both temperatures.

The evolution of the interface corrugation along with exposure duration (Fig. 5) was used to estimate the amount of corroded metal following the method fully described in [3] and illustrated in Fig. 11. This method is based on SEM cross-section image analysis. It consists in

tracing the initial interface along the bottom of all ilmenite crystallites, then selecting the corroded metal areas, and finally dividing this total corroded metal area (hatched in red in Fig. 11) by the length of the image. The main uncertainties of this method stem from the estimation of the initial surface of the sample, here taken as the bottom of ilmenite crystallites. This method does not take into account the uniform corrosion occurring before the onset of the ilmenite precipitation, thus leading to the estimation of a minimum corrosion rate. This image analysis procedure was performed on 20 SEM images for each exposure duration and material for statistical purpose.

Fig. 12 shows the as-estimated mean depth of corroded metal
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Illustration of the method used to estimate the mean corroded metal depth, here for CP Ti exposed 1750 h at 350 °C in primary water. The black line represents the reference line, corresponding to the assumed initial position of the sample's surface. The red hatched areas represent the corroded metal areas.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Evolution of the mean corroded metal depth as a function of the exposure duration for the three materials exposed in the upstream reactor (R1) and downstream reactor (R2) at 300 °C (full symbols, data from [3]) and 350 °C (empty symbols).</figcaption></figure>

evolution with exposure duration for all three alloys at 350 °C and at 300 °C for comparison with data provided in [3]. The kinetics of the mean corroded metal depth evolution was approximately linear for all three materials at both temperatures regardless of the reactor position (upstream or downstream). The corroded metal depths at 350 °C were,

as already mentioned, four-to-five times higher than those at 300 °C [3] for each exposure duration and for all materials.

The observation of the surfaces of the samples by optical microscopy with polarised light revealed the evolution of the large ilmenite crystallites with exposure duration, as illustrated in Fig. 13 for Ti64. Similar pictures were obtained for CP Ti and Ti10-2-3. This observation technique produced a high contrast between the ilmenite crystallites (in dark grey in Fig. 13) and the surface of the samples. The coverage ratio of the surface by FeTiO₃ crystallites was then quantified by image analyses. For each material and exposure duration, analyses were performed on at least 0.2 mm², i.e. 0.1% of the total sample surface. In order to carry on the comparison between the corrosion kinetics at 300 °C and at 350 °C, the coverage ratio of the surface by ilmenite crystallites during exposure at both temperatures is presented in Fig. 14. The coverage ratio increased, for all three materials, at least twice quicker at 350 °C than at 300 °C. Note that in Fig. 14.a-300 °C the abscissa ends at 4000 h whereas it ends at 2000 h in Fig. 14.b-350 °C. The coverage ratio remained nevertheless far below 1 in both reactors at both studied temperatures.

Fig. 14.a was taken from [3] and did not show any effect of the reactor position (upstream or downstream) at 300 °C contrary to the present results obtained after exposure at 350 °C and shown in Fig. 14.b. At 350 °C, the coverage ratio of the samples exposed in the upstream reactor R1 was lower than that expected by following the trend obtained from the samples exposed in the downstream reactor R2. The evolution as exposure progressed seemed to be approximatively linear in reactor R2, whatever the material and temperature.

<figure><img src="image_16.png" /><figcaption>**Fig. 13.** Surface optical microscope observations of the evolution of the ilmenite coverage with exposure duration for Ti64 exposed a) 100 h, b) 296 h and c) 1750 h at 350 °C. The oxide layer and the small crystallites, both in TiO₂, are in blue and the large ilmenite crystallites are in dark grey.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. 14.** Evolution of the mean coverage surface ratio of ilmenite FeTiO₃ as a function of the exposure duration of CP Ti, Ti64 and Ti10-2-3 in upstream reactor (R1) and downstream reactor (R2) at a) 300 °C (data from [3]) and b) 350 °C measured from 10 optical microscope observations for each duration and material.</figcaption></figure>

# 4. Discussion

## 4.1. On the influence of temperature on the corrosion mechanisms

All the results presented in previous studies at 300 °C [2,3] and in the present study at 350 °C indicate that, whatever the investigated temperature at pH25 = 10.1 and for all three materials, micrometric crystallites of ilmenite FeTiO3 and anatase TiO2 have grown on top of a nanometric continuous TiO2 layer during exposure. As mentioned in the introduction, the difference of TiO2 structure between CP Ti and the α/β titanium alloys, seen at both temperatures, could result from the presence in the continuous oxide layer of the alloying elements (Al, V mainly) and of the incorporated hydrogen (during corrosion but not during the precipitation processes) [2]. These elements act like dopants that favour or not the anatase formation. The hydrogen incorporated in CP Ti favours the rutile formation [12]. For Ti64 and Ti10-2-3, the aluminium effect compensates for that of hydrogen, promoting the anatase formation instead of the rutile one [13]. Trends observed at 300 °C seem still valid at 350 °C.

As both the metal consumption and the surface coverage by ilmenite crystallites increased almost linearly with exposure duration, it was inferred that the precipitation of these crystallites most probably originated from the simultaneous dissolution and growth of the continuous TiO₂ layer. The estimation of the metal consumption rate from the measure of the tortuosity of the metal/oxide interface, which is dependent on the presence of FeTiO₃ crystallites, therefore remains valid at 350 °C (see Section 4.2). The acceleration of those kinetics by increasing the temperature allows the extrapolation of the minimum corrosion rate of these materials to longer exposure durations.

Regarding Ti64 and Ti10-2-3, only anatase was evidenced in the continuous TiO₂ layer at 300 °C [2,3] and at 350 °C. No difference was noticed in the morphology of the surface or the oxides formed at these two temperatures. There was only a clear acceleration of corrosion with increasing temperature, discussed in the next section. Consequently, the corrosion mechanism of these two titanium alloys did not seem to be affected by the rise of exposure temperature from 300 °C to 350 °C in pressurised water.

The case of CP Ti required more attention. A spongy rutile layer for CP Ti was observed after 296 h and 1750 h at 350 °C in the local attacks zones of about 400 nm in depth (Figs. 5, 7 and 8) as well as after 3499 h at 300 °C [3]. This time-shift (towards shorter times with increasing temperature) of the observation of such spongy oxides in local attacks sites was consistent with a temperature-induced acceleration of corrosion. However, it did not indicate a temperature-induced change in the corrosion mechanism. The spongy rutile layer observed at both studied temperatures probably resulted from the heterogeneous dissolution of the nanometric oxide layer. However, an important local increase in the thickness of the TiO₂ oxide layer was noticed on CP Ti samples exposed at 350 °C (40 nm to several hundreds of nanometres) in comparison to

the one exposed at 300 °C (~ 30 nm) [2,3]. As shown in Fig. 8, this thick oxide layer included both anatase relatively large nanograins below a continuous rutile layer composed of relatively small nanograins. The morphology of these grains and of the metal/oxide interface in such zones shown in Fig. 7 and Fig. 8 suggested that the corrosion mechanism of CP Ti in water at 350 °C differs from the one at 300 °C. Such thickening may result from two different mechanisms: either the precipitation in anatase crystallites under the rutile continuous layer or the higher growth rate of the continuous oxide layer compared to the dissolution rate. The first mechanism would mean that the rutile layer was not fully continuous, allowing some anatase to precipitate and then to grow under the layer. It could be consistent with Fig. 8. However, the morphology at the metal/oxide interface in the zone 2 shown in Fig. 7 looked more like an internal corrosion front as would be expected from the second mechanism. In this case, a lack of understanding remained concerning the cause of the local increased growth rate and the change due to kinetics and/or thermodynamics considerations from rutile to anatase during the growth process of the continuous oxide layer. Be that as it may, concerning CP Ti, using higher temperature results should not be relevant for an accurate extrapolation of the lower temperature corrosion behaviour.

## 4.2. On the influence of temperature on the corrosion rate

As shown in Figs. 10, 12 and 14, all the kinetics obtained in R2 were accelerated at 350 °C compared to the ones at 300 °C. The mass uptake and the mean corroded metal depth kinetics were accelerated by a factor of around 4, while those of ilmenite coverage ratio were accelerated by a factor of around 2. For the kinetics determination, more exposure durations would have been necessary to be more accurate. Nevertheless, some temperature effects can be highlighted. Thanks to that acceleration, the reactor effect on the precipitation, already noticed at 300 °C, was emphasised at 350 °C. The parameters impacted by this reactor effect (coverage ratio and mass variation) were the same for both temperatures, supporting the hypothesis emitted in [3] of an enriched media in cations or soluble iron and titanium species in downstream reactor R1 due to the corrosion of samples in the upstream reactor R2. The presence of the soluble iron and titanium species was consistent with the solubility of Fe(OH)₂ and Ti(OH)₄ at this temperature and this pH [14–16]. For all materials, the linear evolution with exposure duration of the mean corroded metal depth indicated that the corrosion rate of the metal was constant with the exposure duration in both reactors and independent of the reactor. Due to the constancy of the corrosion rate of the metal for these exposure durations, it was possible to estimate a lower bound of this mean corrosion rate by knowing the mean depth of corroded metal after long exposure duration as seen in Fig. 12. For all materials, it was around 4 µm year⁻¹ at 350 °C (present study) and around 0.9 µm year⁻¹ at 300 °C [3]. However, the presence of anatase grains under the rutile layer discussed in Section 4.1, which expanded
with the exposure duration prevented a more accurate estimation of the corrosion rate with the method proposed in [3] at 300 °C, especially a maximum corrosion rate.

In the work carried out at 300 °C, Selva et al. [3] suspected that the corrosion rate reached a very low value once the surface was fully covered by ilmenite crystallites, as was observed for specimen exposed in autoclave, contrary to those exposed in the corrosion loop, where the ilmenite coverage was still below 1 after 3499 h exposure. Increasing the temperature by 50 °C was expected to favour ilmenite precipitation, by increasing the global reactions rates. Experiments carried out in the present study aimed also to tackle the validity of this assumption. Thus, the kinetics of surface coverage by these crystallites can be extrapolated for longer exposure durations. However, as shown in Fig. 14, the acceleration caused by the temperature increase was insufficient to reach a coverage ratio of 1 within the studied exposure durations. No slowing down of the corrosion rate was observed, with less than 30% of the surface covered by ilmenite crystallites at 350 °C within the investigated test durations. It showed that the effect of ilmenite coverage might be visible only for higher coverage ratios. This observation was consistent with the one made at 300 °C [3]. In order to completely cover the surface with these crystallites, the exposure durations extrapolated from the data of samples in each reactor would be 2000 h in R2 and 7000–12, 000 h in R1 depending on the materials. Thus, increasing the temperature accelerated the corrosion process and thus led to corrosion states similar to long exposure durations in a shorter time. However, 50 °C was not sufficient to achieve full ilmenite coverage in 1750 h. Moreover, since in an industrial environment, fluid speeds are in the order of m s⁻¹, work is underway to study the influence of increasing fluid speed on the corrosion kinetics.

## 5. Conclusions

Titanium and its alloys corrosion behaviours were investigated in 350 °C PWR primary water-type conditions. Main results are as follows:

- For the two-phase alloys Ti64 and Ti10-2-3, no effect of the temperature between 300 °C and 350 °C was noticed on the nature and morphology of the formed oxides. For commercially pure titanium CP Ti, from an average of 200 h of exposure, the thickness of the nanometric oxide layer varied locally from 40 nm to hundreds of nanometres at 350 °C due to the presence of anatase grains under the thin rutile layer, whereas it remained constant at 30 nm and only in rutile at 300 °C [3]. The oxide layer thickening phenomenon extended after 1750 h of exposure at 350 °C and proved a change in the corrosion mechanism compared to 300 °C.

- For all materials, the morphology at the metal/oxide and oxide/medium interfaces was consistent with a mixed regime of growth and dissolution of the continuous oxide layer. Furthermore, the precipitation mechanisms at the oxide/medium interface were not influenced by temperature.

- Although the corrosion mechanisms of the nanometric oxide layer can evolve with increasing temperature, as illustrated in the case of CP Ti, it was still possible to use a higher temperature to access data inaccessible in reasonable durations at the operating temperature of 300 °C. Exposure to higher temperatures was reliable for extrapolating the behaviour of FeTiO₃ crystallite precipitation and interface tortuosity, allowing the estimation of a minimum corrosion rate.

- Based on the measurement of the mean corroded metal depth, the corrosion rate was found approximately constant during the exposure at 350 °C, and about 4 times larger than at 300 °C: for the three materials, it was, on average, superior to 4 µm year⁻¹ at 350 °C and superior to 0.9 µm year⁻¹ at 300 °C.

- The ilmenite crystallites did not cover the entire surface of the materials and no decrease in the metal consumption rate was observed with increasing exposure duration.

- Further understanding of the phenomenon of the thickening of the oxide layer at 350 °C and more exposure would be required to estimate a range of corrosion rates at this temperature based on the method proposed in [3].

## CRediT authorship contribution statement

**Sabrina Selva:** Conceptualization, Investigation, Validation, Writing – original draft. **Quentin Bignon:** Conceptualization, Investigation. **Amandine Raynal:** Investigation. **Frantz Martin:** Conceptualization, Writing – review & editing, Validation. **Quentin Auzoux:** Conceptualization, Writing – review & editing, Validation. **Gilles Renou:** Investigation. **Laurence Latu-romain:** Conceptualization, Investigation, Writing – review & editing, Supervision. **Yves Wouters:** Conceptualization, Writing – review & editing, Supervision.

## Declaration of Competing Interest

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.

## Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.

## Acknowledgements

The authors wish to thank the CEA for funding the study. Technical support of Frederic Datcharry from CEA was crucial for the set-up of the high temperature and pressure exposures. Benoist Muzeau from CEA provided access and training in Raman spectroscopy. Many thanks go to Grenoble INP - CMTC platform supported by the Centre of Excellence of Multifunctional Architectured Materials "CEMAM" no. ANR-10-LABX-44-01 funded by the Investments for the Future programme, which allowed the use of their characterisation equipment. To finish, the authors thank Stéphane Coindeau and Thierry Encinas, from the CMTC for the XRD analyses and their treatment.

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