Eric Vermesse

Catherine Mabru

Laurent Arurault
email: arurault@chimie.ups-tlse.fr

Surface integrity after pickling and anodization of Ti-6Al-4V titanium alloy

Keywords:

The surface integrity of Ti-6Al-4V titanium alloy was studied at different stages of surface treatments, especially pickling and compact anodization, through surface characteristics potentially worsening fatigue resistance.

No significant changes of the equiaxe microstructure were detected between sample core and surface, or after the pickling and anodization steps. Surface hydrogen and oxygen superficial contents were found to remain unchanged. Roughness characteristics (i.e. R a , R z but also local K t factor) similarly showed only slight modifications, although SPM and SEM revealed certain random local surface defaults, i.e. pits about 400 nm in depth. Finally internal stresses, evaluated using X-ray diffraction, highlighted a significant decrease of the compressive internal stresses, potentially detrimental for fatigue resistance.

Introduction

Titanium alloys are increasingly used in the aeronautical industry due to their good mechanical properties and their low density. The most commonly used is Ti-6Al-4V because it is a good compromise between titanium's properties. However, additional surface treatment is usually required [START_REF] Zhecheva | Enhancing the microstructure and properties of titanium alloys through nitriding and other surface engineering methods[END_REF][START_REF] Critchlow | Review of surface pretreatments for titanium alloys[END_REF] to increase, for instance, superficial mechanical properties (i.e. tribological properties, wear resistance and superficial hardness), as well as to improve its behaviour with respect to corrosion by fluorided acidic solutions. Surface treatments on titanium alloys usually involve three main steps. The first is degreasing step which removes the oil and impurities on the surface, left by previous machining stages. The second is pickling commonly performed by chemically controlled corrosion typically in mixed hydrofluoric-nitric acid bath. The aim of this pre-treatment is to remove the natural passivating layer. Finally, the third stage is the main treatment, i.e. nowadays thermodiffusion treatments (nitriding, carburizing) or shot peening treatments [START_REF] Bansal | Surface engineering to improve the durability and lubricity of Ti-6Al-4V alloy[END_REF][START_REF] Yaskiv | Formation of oxynitrides on titanium alloys by gas diffusion treatment[END_REF][START_REF] Pohrelyuk | Corrosion resistance of Ti-6Al-4V alloy with nitride coatings in Ringer's solution[END_REF]. Another simple and cost-effective treatment is anodization, creating either a compact top film or a porous one, including numerous mesopores [START_REF] Macak | Self-organized nanotubular oxide layer on Ti-6Al-7Nb and Ti-6Al-4V formed by anodization in NH4F solution[END_REF][START_REF] Zwilling | Anodic oxidation of titanium and TA6V alloy in chromic media: an electrochemical approach[END_REF]. Porous anodic films are currently being widely studied to prepare innovating photovoltaic cells [START_REF] Lee | Effect of electrolyte conductivity on the formation of nanotubular TiO2 photoanode for a dye-sensitized solar cell[END_REF][START_REF] Cao | Double-layer structure photoanode with TiO2 nanotubes and nanoparticles for dye-sensitized solar cells[END_REF] or to enhance osteointegration for bioapplications [START_REF] Chiesa | Osteointegration of titanium and its alloys by anodic spark deposition and other electrochemical techniques: a review[END_REF]. In contrast, compact anodic films are used both to colour the titanium surface [START_REF] Diamanti | Interference colors of thin oxide layers on titanium[END_REF][START_REF] Van Gils | Colour properties of barrier anodic oxide films on aluminium and titanium studied with total reflectance and spectroscopic ellipsometry[END_REF][START_REF] Sul | The electrochemical oxide growth behaviour on titanium in acid and alkaline electrolytes[END_REF] and to improve paint adhesion for aeronautic parts [START_REF] Critchlow | Review of surface pretreatments for titanium alloys[END_REF][START_REF] Dalard | Adherence of paints on titanium: study and characterization of oxide films[END_REF].

However, previous industrial and academic studies have unfortunately shown, especially for aluminium alloys (AA), that the anodization process causes significant modifications of the AA surface integrity and a subsequent decrease of the fatigue resistance [START_REF] Lonyuk | The effect of oxide coatings on fatigue properties of 7475-T6 aluminum alloy[END_REF][START_REF] Shahzad | Influence of surface treatments on fatigue life of Al 7010 alloy[END_REF][START_REF] Shahzad | Surface characterization and influence of anodizing process on fatigue life of Al 7050 alloy[END_REF][START_REF] Shahzad | Effect of sealed anodic film on fatigue performance of 2214-T6 aluminium alloy[END_REF]. To our knowledge, no research works have previously studied and explained the possible influence of surface treatments, especially pickling or the anodization, on fatigue resistance in titanium alloys. In the case of AA, decreased fatigue resistance after anodization is under investigation and has been mainly attributed to influent surface parameters such as: microstructure [START_REF]Materials Properties Handbook: Titanium Alloys[END_REF][START_REF] Lütjering | Titanium[END_REF], uptake of embrittling chemical species [START_REF] Lütjering | Titanium[END_REF][START_REF] Nakasa | The effect of hydrogen-charging on the fatigue crack propagation behavior of ␤-titanium alloys[END_REF][START_REF] Evans | Hydrogen and fatigue behaviour in a near alpha titanium alloy[END_REF], roughness [START_REF] Haddad | Prediction of non propagating cracks[END_REF][START_REF] Chaussumier | A predictive fatigue life model for anodized 7050 aluminium alloy[END_REF] and internal stresses [START_REF] Wagner | Influence of shot peening on the fatigue behavior of titanium alloys[END_REF][START_REF] James | Residual stresses and fatigue performance[END_REF].

Microstructure has an important effect on the fatigue resistance of titanium alloys [START_REF]Materials Properties Handbook: Titanium Alloys[END_REF][START_REF] Lütjering | Titanium[END_REF]. The main microstructural parameters are ␣ grain size and the percentage and morphology of ␤ phase. For example, a lamellar microstructure does not have the same mechanical properties as equiaxe microstructure [START_REF] Lütjering | Titanium[END_REF]. Some surface treatments, such as nitriding or carburizing treatment, can also affect the microstructure and consequently the fatigue resistance.

Hydrogen, carbon, nitrogen and oxygen are the four main potential uptaking and embrittling species. Their respective effects are different but hydrogen embrittlement is well-known to affect mechanical properties of metal parts. This point is particularly critical as Ogawa et al. [START_REF] Ogawa | Hydrogen absorption behavior of beta titanium alloy in acid fluoride solutions[END_REF] clearly showed using hydrogen thermal desorption that the amount of absorbed hydrogen in a beta titanium alloy increased with immersion time in fluorided acid solutions.

The role of roughness on fatigue resistance of metal parts is well known and has been studied for a long time. Roughness can be significantly modified by surface treatments, such as for instance the pickling step. Thus, pickling of AA substrate often modifies the roughness through dissolution of microprecipitates initially included in the multiphase AA matrix [START_REF] Shahzad | Effect of sealed anodic film on fatigue performance of 2214-T6 aluminium alloy[END_REF]. On commercially pure titanium, pickling performed in a concentrated (48%) sulphuric acid bath is similarly used to increase roughness in order to promote osteo-integration [START_REF] Ban | Surface modification of titanium by pickling in concentrated sulfuric acid[END_REF].

Finally, internal stresses contribute to the average stress applied to the material and consequently its fatigue resistance. Internal stresses may be generated by mechanical treatment (shot peening, ball burnishing), thermal treatment (quenching, annealing) and/or (electro)chemical treatment (nitriding, anodization). For instance, anodization induces compressive or tensile internal stresses, whose intensities depend on the operational parameters of the electrochemical treatment [START_REF] Pilling | The oxidation of metals at high temperatures[END_REF][START_REF] Archibald | Internal stresses formed during the anodic oxidation of titanium[END_REF][START_REF] Nelson | Stress generation during anodic oxidation of titanium and aluminum[END_REF][START_REF] Kim | Effect of hydrogen on stresses in anodic oxide film on titanium[END_REF].

In the present work, the aim was to study the influence of surface treatment (pickling and anodization) on surface integrity of Ti-6Al-4V titanium alloy, widely used for aircraft parts such as for example engine pylons.

Surface integrity was studied through surface characteristics potentially influencing fatigue resistance. Firstly the microstructure of Ti-6Al-4V alloy was carefully studied through two types of substrates, i.e. rolled sheet and forged bar. Secondly, this work only focussed on hydrogen and oxygen uptakes because carbon and nitrogen are not involved, either in pickling treatment or in anodization treatment. Then roughness was studied through R a and R z values but also an approach based on the local stress concentration factor (local K t ). Erratic and punctual defaults were observed and characterised to complete the morphological analysis. Finally, internal stresses were measured using XRD methods. These different characteristics were deeply studied at both steps of surface treatment (pickling, anodization) while endurance limit was finally evaluated in order to both clarify the surface changes and predict their potential impact on fatigue resistance.

Experimental

Surface preparation

The substrate material was Ti-6Al-4V titanium alloy. Its chemical composition in weight percent was: 5.5 < Al < 6.5%, 3.5 < V < 4.5%, C ≤ 0.08%, O ≤ 0.20%, N ≤ 0.05%, Fe ≤ 0.30%, H ≤ 0.0125% with Ti accounting for the remainder. Two different substrates were used in this study to finally obtain three different surface states.

The first substrate was Ti-6Al-4V rolled sheet (45 mm × 60 mm × 1 mm). From this substrate, two surfaces were prepared: raw rolled sheet and polished sheet. The polished samples were obtained using 800, 1200, 2400 grade polishing paper discs and 6, 3 then 1 m diamond paste polishing pads.

The second substrate was Ti-6Al-4V forged bar (length: 105 mm; diameter: 16 mm) turned (feed rate: 0.1 mm/revolution; cutting speed: 25 m/min) to obtain a fatigue specimen (working length: 20 mm; working diameter: 8 mm), that made up a third surface state.

All samples were degreased with ethanol and then acetone. They were then pickled in aqueous 20 w% HNO 3 and 2 w% HF mixed solution at 20 • C for 200 s without stirring.

Finally, the anodization was performed in an electrochemical cell, where the titanium substrate was used as anode and a lead plate as counter-electrode. The anodization was run for 2 min in the direct voltage mode (5-80 V) using a sulfuric acid solution (1 M) thermally regulated at 20 • C. Samples were rinsed with distilled water after each step.

Characterizations

The thickness of the anodic film was measured using a Bentham PVE300 reflectometer, with a TMc300 monochromator in the 300-900 nm wavelength range. For the anodic film, the refractive index provided by Diamanti et al. [START_REF] Diamanti | Interference colors of thin oxide layers on titanium[END_REF] was used.

Secondary Ion Mass Spectrometry (SIMS) surface and crosssectional analysis were performed with a Cameca IMS 4F6 device. The area analysed was 150 m × 150 m using a 10 keV Cs + primary beam with 10-50 nA current range. It is important to note that this area of analysis was larger than the ␣ and ␤ grain sizes (maximum 40 m 2 ).

Prior to microstructural observations, samples were dipped for 10 s in a mixed acid solution (4% HF; 3%HNO 3 ) at ambient temperature. Sample microstructures were observed with a scanning electron microscope (XL30 ESEM), while ␣ and ␤ grain sizes were estimated by analyzing SEM views with the free software ImageJ [START_REF] Schneider | NIH Image to ImageJ: 25 years of image analysis[END_REF]. Additional observations of the sample microstructure were carried out using an optical microscope (OM) (Olympus GX 71).

Elementary chemical analyses were performed by energydispersive X-ray spectroscopy (Rondec-EDX) coupled with a SEM device, in order to obtain a semi-quantitative analysis of V, Ti, Al contents in both types of grains.

A Mahr perthometer PGK 120 (contact mode in ambient atmosphere, diamond tip with 2 m radius) was used to access to the amplitude parameters of the roughness, i.e. here the roughness average (R a ) and roughness height (R z ), defined by:

R a = 1 l l 0 |z(x)|dx (1)
with l the length of profile and z(x) the profile height distribution with respect to mean line

R z = 1 5   5 i=1 |(z i ) max | + 5 j=1 |(z j ) min |   (2)
where (z i ) max and (z j ) min are respectively the five higher local maxima and lower local minima of the profile height distribution (z).

The R a and R z values shown in this paper were the average values, resulting from 4 to 40 measurements. Corresponding accuracy was low (about 0.01 m) while the standard deviation was about 0.05 m for R a and 0.5 m for R z . Only R a and R z values were used in this paper but the final conclusion would remain the same with the other roughness indexes.

Unfortunately, roughness indexes do not always provide sufficient information to know the real effect of morphology on fatigue resistance [START_REF] Suraratchaï | Modelling the influence of machined surface roughness on the fatigue life of aluminium alloy[END_REF][START_REF] Ås | Surface roughness characterization for fatigue life predictions using finite element analysis[END_REF]. Consequently, Suraratchaï et al. [START_REF] Suraratchaï | Modelling the influence of machined surface roughness on the fatigue life of aluminium alloy[END_REF] and Shahzad et al. [START_REF] Shahzad | Influence of surface treatments on fatigue life of Al 7010 alloy[END_REF] proposed to use the local K t to determine the morphology impact on the fatigue resistance. The specificity of the K t parameter is that it takes depth and sharpness of the local defaults into account. Its calculation is based on roughness profile analysis and finite elements simulation [START_REF] Suraratchaï | Modelling the influence of machined surface roughness on the fatigue life of aluminium alloy[END_REF]. In this method, roughness profile was filtered to extract its "useful" part, the cut length, assimilated to a 0 [START_REF] Haddad | Prediction of non propagating cracks[END_REF], being here close to 10 m. Standard deviation for K t is estimated to be equal to 0.03 for machined samples and 0.5 for laminate samples.

A scanning probe microscope (SPM-Bruker) in contact mode in ambient atmosphere with a 10 nm radius and a 10 m long cantilever tip was used to characterise erratic local surface defaults at the micro-and nano-levels (50 m × 50 m).

The internal stresses were measured by X-ray diffraction (Xpert Philips device) using the well-known "sin 2 method" firstly introduced by Macherauch [START_REF] Tonshoff | Influence of stress distribution on adhesion strength of sputtered hard coatings[END_REF][START_REF] Jawahir | Surface integrity in material removal processes: recent advances[END_REF]. Measurements were only carried out on unpolished rolled samples because the XRD technique is unsuitable for cylindrical samples (like machined forged bar used for fatigue specimen) and polishing can induce additional uncontrolled stresses. All measurements were performed in the laminate direction using a 213 diffraction ray with scanning angle (2) from 136 • to 146 • . The measurement of internal stresses was performed in a material depth depending on the XRD analysis operating parameters (angle, X-ray tube), but depth is usually considered as about the first 14 m [START_REF] Velasquez | Sub-surface and surface analysis of high speed machined Ti-6Al-4V alloy[END_REF].

Preliminary fatigue tests were performed on a servo hydraulic machine MTS-810 at 10 Hz using a load ratio of 0.1. Specimen geometry, already used in previous works, was described for instance in Ref. [START_REF] Shahzad | Influence of surface treatments on fatigue life of Al 7010 alloy[END_REF]: gauge length was 20 mm and diameter was 8 mm. The specimens were machined from a unique bar. Three groups of seven specimens were tested: the first one without surface treatment, another one only pickled and the last one pickled and then anodized at 80 V.

Results and discussion

As mentioned in the introduction, surface integrity was studied through surface characteristics potentially influencing the fatigue resistance, i.e. microstructure, species uptake, roughness and internal stresses.

Microstructure

Ti-6Al-4V rolled sheet (First substrate) showed a fine isotropic equiaxe microstructure (Fig. 1), homogeneous through the whole thickness. The second substrate (Ti-6Al-4V forged bar) showed similar fine equiaxe-type microstructure in the radial direction (Fig. 2a) but also specifically long fine ␤ grains oriented along the longitudinal direction (Fig. 2b). The average diameter of ␤ grains was about 1 m while the diameter of ␣ grains was estimated to be about 5 m.

Image analysis showed that the surface area of ␤ grains is 10 ± 2% for both rolled samples and radial observation of the forged sample, and 7 ± 2% for the longitudinal direction of the forged sample. These results agree with the fact that (␣ + ␤) titanium alloys show ␤ phase in the 5-20% range, especially close to 12% for the Ti-6Al-4V alloy [START_REF] Elmer | In situ observations of lattice expansion and transformation rates of ␣ and ␤ phases in Ti-6Al-4V[END_REF].

There were no significant differences of microstructure between sample cores and their interfaces, i.e. in the last superficial one thousand microns. Moreover, no changes were experimentally observed after the pickling and anodization steps. These results were firstly explained by the similar chemical content and chemical distribution in both types of substrates. Secondly, both processes are indeed known to usually induce no changes of superficial microstructure because of the ambient or moderate operational temperatures used, contrary to thermodiffusion treatments (i.e. nitriding [START_REF] Pohrelyuk | Corrosion resistance of Ti-6Al-4V alloy with nitride coatings in Ringer's solution[END_REF]) or high energy treatments (i.e. micro-arc oxidation, laser treatment [START_REF] Yerokhin | Characterisation of oxide films produced by plasma electrolytic oxidation of a Ti-6Al-4V alloy[END_REF][START_REF] Murr | Microstructure and mechanical behavior of Ti-6Al-4V produced by rapid-layer manufacturing, for biomedical applications[END_REF]). So, using pickling and anodization, microstructure cannot be considered as a key parameter for potential fatigue resistance modification of Ti-6Al-4V titanium alloy.

Hydrogen and oxygen uptake

This work focussed on the uptake of hydrogen and oxygen both known to affect mechanical properties [START_REF] Judy | Effects of oxygen and iron on the environmental and mechanical properties of unalloyed titanium[END_REF][START_REF] Bache | The effects of environment and internal oxygen on fatigue crack propagation in Ti-6Al-4V[END_REF] and both involved in the pickling and the anodization processes, unlike carbon and nitrogen. To study the hydrogen storage capacity of the Ti-6Al-4V, temperature programmed desorption (TPD) and a volumetric sorption technique have been performed [START_REF] Blackburn | Measurement of the reversible hydrogen storage capacity of milligram Ti-6Al-4V alloy samples with temperature programmed desorption and volumetric techniques[END_REF]. The diffusion kinetics of oxygen into Ti-6Al-4V substrate was measured by weight gain measurements, oxygen diffusion zone (ODZ) depth and hardness measurements on the cross-sections of oxidised alloy [START_REF] Guleryuz | Oxidation of Ti-6Al-4V alloy[END_REF]. Using SIMS, Thair et al. [START_REF] Thair | Surface characterization of passive film formed on nitrogen ion implanted Ti-6Al-4V and Ti-6Al-7Nb alloys using SIMS[END_REF] obtained the elemental (Ti, Al, V, N, O) depth profiles of passive layers of (un)implanted specimens of Ti-6Al-4 V, while Lamolle et al. [START_REF] Lamolle | The effect of hydrofluoric acid treatment of titanium surface on nanostructural and chemical changes and the growth of MC3T3-E1 cells[END_REF] studied the impact of the pickling duration on the SIMS depth profiles of oxide, fluoride and hydride in the surface of commercially pure (cp) titanium.

In the present study, SIMS was selected as it is very sensitive and provides the elemental in-depth profiles of the passive layer and the beginning of the metal matrix. However, quantitative values were not obtained but the method was efficient to compare significant variations of the elements present.

Fig. 3 shows the oxygen profiles after each treatment, i.e. polishing, pickling (20 w% HNO 3 -2 w% HF solution for 200 s) and anodization (at 80 V). For polished and pickled samples, the initial peaks at the extreme surface (sputerring time lower than 200 s) were attributed to passive layers. Sittig et al. [START_REF] Sittig | Surface characterization of implant materials c.p. Ti, Ti-6Al-7Nb and Ti-6Al-4V with different pretreatment[END_REF] highlighted that the natural passive layer (before pickling) on Ti-6Al-4V alloy is usually made up of TiO 2 , but also includes Al 2 O 3 and V 2 O 5 . Lamolle et al. [START_REF] Lamolle | The effect of hydrofluoric acid treatment of titanium surface on nanostructural and chemical changes and the growth of MC3T3-E1 cells[END_REF] pointed out that prolonging the pickling time (up to 150 s) in 0.2%HF caused a higher oxide content at the cp titanium passivation layer. For Ti-6Al-4V alloy, our results did not show such an increase, may be due to the different titanium substrate and the lack of oxidising compound (i.e. HNO 3 ) in the pickling solution used by Lamolle et al. [START_REF] Lamolle | The effect of hydrofluoric acid treatment of titanium surface on nanostructural and chemical changes and the growth of MC3T3-E1 cells[END_REF].

For anodized samples, the profile first presents an increase and then reaches a plateau due to the formation of the anodic film, about 200 nm thick, according to:

Ti + 2H 2 O → TiO 2 + 4H + + 4e ( 3
)
Then there is a drastic decrease of the SIMS profile at about 1500 s of sputtering, clearly indicating the interface between the bottom of the anodic film and the top of the metal alloy.

After each treatment (polishing, pickling, anodizing), oxygen intensity in bulk material was very low, indicating that no oxygen uptake occurred in the bulk alloy. So, a potential decrease of fatigue resistance due to oxygen uptake [START_REF] Lütjering | Titanium[END_REF], usually attributed to titanium hardening [START_REF] Liu | Effects of oxygen and heat treatment on the mechanical properties of alpha and beta titanium alloys[END_REF], is not expected in these conditions. After pickling (20 w% HNO 3 -2 w% HF solution for 200 s), the interfacial peak was clearly lower than the previous one (after polishing). This result could be explained by considering the possible pickling reactions occurring in 20 w% HNO 3 and 2 w% HF aqueous mixed solution: Eq. ( 5) clearly shows that H 2 gas evolution can occur but also that titanium oxide reacts with fluoride anions. Thus, the decrease of the interfacial peak after pickling could perhaps be explained by the modification of the initial hydrated passive layer by the fluoride ions. Furthermore, Bijlmer [START_REF] Bijlmer | Pickling titanium in hydrofluoric-nitric acid[END_REF] demonstrated that hydrogen uptake decreases with increasing nitric acid concentration. He showed in particular that, when the nitric acid content is higher than 20% in the pickling bath, hydrogen uptake became close to zero, due to a predominant re-passivation phenomenon. So, our results, obtained using 20 w%HNO 3 -2 w%HF aqueous mixed pickling solution, are in good agreement with Bijlmer's work.

6HF + TiO 2 → 2H + + 2H 2 O + TiF 6 2- (4) 3HF + Ti → 3/2H 2 + 3F -+ Ti 3+ (5)
For an anodized sample (80 V, i.e. about 200 nm thick), the hydrogen profile increased from 750 s reaching a maximum at about 1500 s of sputtering. This maximum corresponded, as previously demonstrated for the oxygen depth profile, to the interface between the bottom of the anodic film and the top of the bulk metal alloy. Moreover, the area integration under the hydrogen profiles for pickled and anodized samples were the same, proving that anodization did not induce additional hydrogen uptake. Finally, hydrogen intensity in the bulk of the titanium alloy appeared low and identical after each treatment, confirming that no additional hydrogen inclusion was induced. In contrast, Lamolle et al. [START_REF] Lamolle | The effect of hydrofluoric acid treatment of titanium surface on nanostructural and chemical changes and the growth of MC3T3-E1 cells[END_REF] showed a significant penetration depth of hydride (deeper than one micron) but used 0.2%HF alone (without nitric acid) aqueous solution as pickling bath, i.e. without re-passivation phenomenon.

Additional SIMS surface mapping (Fig. 4) was then performed to clarify the hydrogen state, which is a key factor in hydrogen embrittlement [START_REF] Yokoyama | Hydrogen absorption of titanium and nickel-titanium alloys during long-term immersion in neutral fluoride solution[END_REF] Diffusive hydrogen indeed has a different effect than hydride precipitates [START_REF] Nagaoka | Evaluation of hydrogen absorption behaviour during acid pickling for surface modification of commercial pure Ti, Ti-6Al-4V and Ni-Ti superelastic alloys[END_REF][START_REF] Briant | Hydrogen embrittlement of commercial purity titanium[END_REF], that decrease the ductility of the material [START_REF] Hardie | Effect of hydrogen and strain rate upon the ductility of mill-annealed Ti-6Al-4V[END_REF]. The sample was in fact divided into two zones in order to compare the top surface and the "bulk" material (Fig. 4): in the middle was located a sputtered crater a few microns in depth and around the top surface of the anodic film. The colour homogeneity attested that hydrogen was uniformly distributed through the film, i.e. both on the surface and in the bulk of the anodic film. Considering that no chemical heterogeneity was detected, it is assumed that in the present case there were no hydride precipitates but only diffusive hydrogen. Nakasa and Satoh [START_REF] Nakasa | The effect of hydrogen-charging on the fatigue crack propagation behavior of ␤-titanium alloys[END_REF] claimed that diffusive hydrogen is usually incorporated into the ␤-phase but is less detrimental than the hydride from the ␣-phase, that can become a crack nucleation and propagation path under loading. Thus, the oxygen and hydrogen uptakes were very limited in Ti-6Al-4V alloy using such surface treatments (polishing, pickling, anodization) in our operating conditions. So, the effect of variations in the hydrogen and oxygen contents will be considered as negligible from the fatigue resistance point of view.

Average roughness values and local default measurements

The morphological modifications of the surface which can impact the fatigue resistance were then analysed. Average roughness values (R a and R z ) were firstly analysed to have a global description of the surface modifications. Considering that the real impact of surface morphology on fatigue resistance is incompletely described through roughness indexes [START_REF] Suraratchaï | Modelling the influence of machined surface roughness on the fatigue life of aluminium alloy[END_REF][START_REF] Ås | Surface roughness characterization for fatigue life predictions using finite element analysis[END_REF], local K t values were secondly obtained. However, these values do not directly take local and random defaults into account. That is why SEM and SPM analysis were performed (on polished surfaces) to determine the occurrence of any possible localised defaults.

On pickled samples

Roughness values (R a and R z ) were measured before and after pickling on three types of samples (rolled, rolled then polished, machined) with different initial roughness (Table 1).

For rolled then polished samples, R a and R z changed from 0.06 ± 0.01 m and 0.28 ± 0.01 m respectively before pickling to 0.08 ± 0.01 m and 0.58 ± 0.02 m after pickling. So, R a increased about 33% and R z about 107% (Table 1). These are high relative increases but the final roughness values stayed very low. When the initial roughness was higher, i.e. for rolled or machined samples, roughness indexes (measured perpendicularly to the roll direction and in the longitudinal direction respectively) did not increase or even showed a slight decrease (Table 1). For example, changes were close to 0% for rolled sample with initial R a of 0.42 ± 0.07 m, while R a and R z decreased, about -5.3% and -6.4% respectively, after pickling the machined sample. However, these roughness index changes were lower than standard deviation, i.e. SD > 15%.

Roughness changes (increase or decrease) can result from different phenomena: preferential morphological dissolution of specific zones (tip/hollow), preferential chemical dissolution of specific phases of the alloy matrix, formation of local corrosion pits, etc.

Firstly, preferential morphological dissolution of the peaks that were more exposed to the pickling bath can occur. Indeed, the dissolution rate would be probably higher on peaks compared with the rate observed in hollows since mass transport and bath renewal would be more difficult in hollows.

Secondly, there is preferential chemical dissolution of ␣ phase, as shown by SEM (Fig. 5) where ␤ grains are higher than the ␣ phase. SPM analysis (Fig. 6) provided an approximate height of about 800 nm for the ␤ phase, i.e. lower than the average diameter of the ␤-grain (≈1 m). This preferential chemical dissolution could be explained by considering the respective chemical compositions of both ␣ and ␤ phases. Indeed the ␤-phase has a higher concentration in vanadium and the ␣-phase a higher aluminium content (Table 2). Knowing that the standard potential of aluminium and vanadium are respectively -1.66 V/SHE and -1.13 V/SHE, the ␤-phase can be considered as more noble (containing more vanadium) than the ␣-phase with less vanadium and more aluminium.

Thirdly, another way to increase roughness is pitting. However, Fig. 6 shows that pits were local and less than 1 m in diameter, and the pit density was evaluated at about one pit for 200 m 2 . Pits can be found at the grain boundaries, i.e. the interface between ␣ and ␤ phases, or also in the middle of ␣ grains. SPM analysis revealed pit depth of about 400 nm (Fig. 6). So, preferential chemical dissolution and pitting phenomena resulting from the pickling step induced faults (respectively about 800 and 400 nm) lower than the roll roughness (R z = 2.70 m in Table 1). Both these types of defaults could explain the roughness changes (R z = 0.58 m in Table 1) in polished substrate.

Surprisingly, local K t remained unchanged in polished and machined samples, while rolled samples showed a K t decrease (-6.5%). Even though its variation seems to be significant, it is clearly lower than standard deviation on local K t analysis, which reached 30% for the rolled sample. The low influence of faults on local K t value can be explained by too shallow sharpness and depth of faults. For instance, in rolled sample, defaults had a lower density and/or a lower depth than the initial guide marks from the rolling process (Table 1).

3.3.2.

On anodized samples R a , R z , K t changes after anodization of rolled samples, performed at different constant voltages (5-80 V) are reported in Fig. 7, i.e. as function of the thickness of the compact anodic film in the 16-195 nm range (Table 3). R a values changed significantly, maximal variations being about 7% between the thinnest and the thickest films. Although significant, the variations stayed very low. In the case of R z values, the changes were larger (up to 8%) but they depended more on local faults. Local K t changes were higher than R z , but the corresponding standard deviation was large (30%) due to irregular roughness profiles.

So, no effect of the anodization voltage (i.e. the compact film thickness in the 16-195 nm range) was noted on the roughness or local K t .

Complementary SEM surface views showed that the morphology of the anodized surface was unchanged whatever the anodic film thickness. For instance, SEM observations (Fig. 8) of the top surface of the thickest anodic oxide film clearly showed pits similar to the previous ones resulting from pickling step (Fig. 5); besides the pit density remained the same. However, additional concretions appeared (Fig. 8) on the surface of this anodic film but only when prepared at the highest voltage (80 V). The concretions occurring at the high voltage required to obtain thick layers, probably resulted from the occurrence of micro-sparks during the anodization. To sum up, the growth of the anodic film on Ti-6Al-4V alloy induced no or slight modifications of the roughness (R a , R z ) and local default (K t ) parameters. They appeared almost unchanged, compared with those obtained before anodization, i.e. after pickling. So, the growth of the compact film could be considered as simply arising from the previous roughness and local default characteristics, and could have no additional influence on fatigue resistance from this point of view.

Internal stresses

There are many techniques to measure internal stress. Among them, the most widespread are X-ray diffraction (XRD) analysis [START_REF] Sridhar | Effect of recrystallisation on the residual stress pattern and fatigue behaviour of titanium alloy IMI-685[END_REF][START_REF] Lee | Evolution of residual stresses in a stress-free titanium alloy subjected to fretting fatigue[END_REF], incremental hole drilling analysis [START_REF] Cellard | Laser shock peening of Ti-17 titanium alloy: influence of process parameters[END_REF] or beam bending analysis [START_REF] Goueffon | Mechanical behavior of black anodic films on 7175 aluminium alloy for space applications[END_REF][START_REF] Peng | Stiffness, residual stresses and interfacial fracture energy of diamond films on titanium[END_REF]. The XRD analysis was selected in our study for three main reasons: it is the only non-destructive technique allowing analysis of the internal stress and then subsequent fatigue testing on the same sample; the second reason is that, unlike beam bending analysis, the measurement can be carried out on thick samples and thirdly the data for stress calculation are easily available.

Rolled sheets showed initial compressive internal stresses (-198 ± 50 MPa) in the laminate direction (Fig. 9) in agreement with Abdelkhalek et al. [START_REF] Abdelkhalek | Coupled approach for flatness prediction in cold rolling of thin strip[END_REF]. Then the pickling step halved the internal stresses (-100 ± 50 MPa). Knowing that in this case the pickling removed about 3 m thickness from the outer surface suggests that the internal stresses were mainly located at the outer surface and that a stress gradient could be located in the first microns of depth. Moreover, stress is seen to result from anodization, for the thinnest (16 nm at 5 V) and thickest (195 nm at 80 V) films, i.e. -50 ± 40 MPa and -39 ± 35 MPa respectively Fig. 9; the values can be considered as similar, and were clearly lower than the previous ones obtained after pickling (-100 ± 50 MPa). Moreover, the anodic film was mainly amorphous and consequently did not contribute to the XRD analysis. So, the measured remaining compressive stresses were probably located at the metal/oxide interface, in agreement with the mechanism proposed by Nelson and Oriani [START_REF] Nelson | Stress generation during anodic oxidation of titanium and aluminum[END_REF]. They explain that the anodization induces additional tensile stress (up to 50 MPa) due to vacancies generated by the formation and the migration of titanium ions at the metal/oxide interface. Such compressive stresses, detected at each step of the surface treatments (especially pickling and anodization) could clearly have a detrimental effect on the fatigue resistance [START_REF] Wagner | Influence of shot peening on the fatigue behavior of titanium alloys[END_REF][START_REF] Schwach | A fundamental study on the impact of surface integrity by hard turning on rolling contact fatigue[END_REF][START_REF] Webster | Residual stress distributions and their influence on fatigue lifetimes[END_REF].

Endurance limit

Fatigue life is divided in three parts: crack nucleation, crack propagation and ductile failure. Crack nucleation occurs principally at the surface. Thus, it is highly sensitive to surface integrity. Furthermore, it represents the main part of fatigue life when the number of cycles is high. That is why, endurance limits which corresponding to fatigue failure at 10 7 cycles are closely linked with surface integrity. The experimental values of endurance limit as a function of surface treatments are reported on Fig. 10. A decrease of endurance limit was observed after pickling and anodizing. The decrease was about 20 MPa after pickling and 10 MPa after anodizing. These variations were meaningful but appeared quite low in comparison with internal stresses modifications measured in rolled specimens. It must be noted that these results are preliminary results and that further work is needed to precisely investigate fatigue behaviour and fatigue models including the effect of residual stresses.

Nevertheless, this preliminary fatigue study is in good agreement with the previous results on surface integrity and confirms that in this case changes of internal stresses modify the endurance limit. These results should be taken into account to improve the surface treatments from a fatigue point of view.

Conclusion

The surface integrity of Ti-6Al-4V alloy was studied through surface characteristics (microstructure, hydrogen and oxygen uptakes, roughness and local surface defaults, internal stresses) potentially influencing fatigue resistance. These surface characteristics were studied in detail after both surface treatments (pickling, anodization). The main conclusions were the following:

Both rolled and forged Ti-6Al-4V substrates showed equiaxe microstructures, including about 10 ± 2% ␤ grains. No significant microstructural changes were detected between sample core and surface, or after the pickling and anodization steps.

SIMS analysis revealed then that no oxygen and hydrogen uptake occurred in the bulk alloy, irrespective of the surface treatment (pickling, anodizing). It was also assumed that no hydride precipitates were formed but only diffusive hydrogen was present in our experimental conditions.

The morphological modifications of the surface were then analysed through average roughness (R a , R z ) or local K t parameters, as well as using SEM and SPM techniques to detect some possible localised and random faults. After pickling, local K t was not affected while there were slight roughness changes, attributed to different phenomena: preferential morphological dissolution of peaks, preferential chemical dissolution of the less noble ␣-phase and formation of local pits. No effect of the anodization compact film thickness in the 16-195 nm range was found on either the roughness parameters or local K t .

In contrast, XRD analysis clearly showed that there were compressive internal stresses and that their values significantly decreased (from -198 ± 50 MPa to -39 ± 30 MPa) after each step of the surface treatment. The internal stresses were mainly located at the metal/oxide interface.

Moreover, preliminary fatigue tests revealed a significant decrease of endurance limit after each surface treatment. Finally, this surface integrity analysis showed that the modification of the initial compressive internal stresses could be a key-parameter, potentially explaining the decrease of the endurance limit after each surface treatment (pickling and anodization) on the Ti-6Al-4V titanium alloy.

Fig. 1 .

 1 Fig. 1. (a) SEM view and (b) OM view of rolled sheet microstructure, previously pickled in a mixed 4 w%HF-3 w%HNO3 solution for 10 s at ambient temperature.

Fig. 2 .

 2 Fig. 2. SEM views (a) in radial direction and (b) in longitudinal direction (arrow) of forged bar microstructure, previously pickled in a mixed 4 w%HF-3 w%HNO3 solution for 10 s at ambient temperature.

Fig. 3

 3 Fig.3also shows the hydrogen SIMS profile after each treatment, i.e. polishing, pickling and anodization. After polishing, the SIMS profile showed a high initial peak attributed to the hydration of the natural passive layer, mainly made up of TiO 2 .After pickling (20 w% HNO 3 -2 w% HF solution for 200 s), the interfacial peak was clearly lower than the previous one (after polishing). This result could be explained by considering the possible pickling reactions occurring in 20 w% HNO 3 and 2 w% HF aqueous mixed solution:

Fig. 3 .

 3 Fig. 3. (a) Hydrogen and (b) oxygen SIMS profiles obtained on rolled sample after three different steps: polishing then pickling (HNO3-HF (20-2 w%) solution, 20 • C, 200 s) and finally anodization (80 V, 1 M H2SO4, 20 • C, 2 min).

Fig. 4 .

 4 Fig. 4. SIMS surface mapping of hydrogen on a polished and then pickled sample (HNO3-HF (20-2 w%) solution, 20 • C, 200 s). In the middle of the sample appears a sputtered crater a few microns in depth.

Fig. 5 .

 5 Fig. 5. SEM surface view of rolled sample, polished and then pickled (HNO3-HF (20-2 w%) solution, 20 • C, 200 s) showing pits and ␤-phase (white parts).

Fig. 6 .

 6 Fig. 6. SPM surface mapping and profile of polished then pickled sample (HNO3-HF (20-2 w%) solution, 20 • C, 200 s) showing a pit.

Fig. 7 .

 7 Fig. 7. Changes of (a) Ra (b) Rz average roughness values and (c) local Kt values as a function of the anodization voltage (5-80 V), i.e. the thickness of the film (16-195 nm) prepared on rolled sample pickled (HNO3-HF (20-2 w%) solution, 20 • C, 200 s) then anodized (1 M H2SO4, 20 • C, 2 min). The dotted lines correspond to standard deviation.

Fig. 8 .

 8 Fig. 8. SEM surface view of rolled sample, polished and pickled (HNO3-HF (20-2 w%) solution, 20 • C, 200 s) then anodized (80 V, 1 M H2SO4, 20 • C, 2 min).

Fig. 9 .

 9 Fig. 9. Evolution of internal stresses obtained on four rolled samples after different surface treatments: rolled, rolled then pickled (HNO3-HF (20-2 w%) solution, 20 • C, 200 s), rolled then pickled and finally anodized at 5 V (1 M H2SO4, 20 • C, 2 min) and a rolled then pickled and finally anodized at 80 V (1 M H2SO4, 20 • C, 2 min).

Fig. 10 .

 10 Fig. 10. Endurance limit of three samples: turned, turned then pickled (HNO3-HF (20-2 w%) solution, 20 • C, 200 s), turned, pickled and finally anodized at 80 V (1 M H2SO4, 20 • C, 2 min).

Table 1

 1 Roughness and local Kt values for rolled and polished, rolled and machined samples before and after pickling (HNO3-HF (20-2 w%) solution, 20 • C, 200 s).

		Polished			Rolled			Machined
		Initial (m)	Etched (m)	Variation (%)	Initial (m)	Etched (m)	Variation (%)	Initial (m)	Etched (m)	Variation (%)
	Ra	0.06	0.08	33	0.42	0.42	0	1.01	0.96	-5.3
	Rz	0.28	0.58	107	2.72	2.70	-1.8	4.91	4.62	-6.4
	Kt	1	1	0	2.31	2.16	-6.5	1.15	1.16	0.9

Table 2

 2 EDX evaluation of aluminium and vanadium contents in the ␣ and ␤ phases in rolled Ti-6Al-4V titanium alloy.

	% Al	%V

Table 3

 3 Thickness variation as a function of direct anodization voltage (1 M H2SO4, 20 • C, 2 min.) on rolled then pickled samples (HNO3-HF (20-2 w%) solution, 20 • C, 200 s). Thicknesses were obtained using reflectometry analysis.

	U (V)	e (nm)
	5	16
	20	60
	35	94
	50	130
	65	164
	80	195

Acknowledgments

The authors thank Claude Armand for his help in performing the SIMS analysis and Pascal Lamesle for the internal stress measurements, as well as Peter Winterton for his helpful comments.


Sonia Stita

Marta Galera Martinez

Huynh Pham

Doan Pham

Ange Nzihou

Patrick Sharrock

Doan Pham Minh
email: doan.phammin@mines-albi.fr

Metal-doped apatitic calcium phosphates: preparation, characterization, and reactivity in the removal of hydrogen sulfide from gas phase

Keywords: Hydroxyapatite, Metal deposition, Inorganic composite, Sorption, H 2 S removal

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

Introduction

Hydrogen sulfide (H 2 S), a simple chemical molecule, is considered to be one of the most noxious gases. This malodorous compound exists in all environmental media: air, surface water, groundwater, soil, sediment etc. It can be found in different natural sources such as volcanic gas, geysers, sulphurous lakes etc. Nowadays, natural gas, biogas, and syngas being considered as important energy sources or raw materials (for syngas and hydrogen production), contain also H 2 S at various concentrations, and need pre-treatment before being processed. [START_REF] Marzouk | Simultaneous removal of CO 2 and H 2 S from pressurized CO 2 -H 2 S-CH 4 gas mixture using hollow fiber membrane contactors[END_REF][START_REF] Maghsoudi | Simultaneous separation of H 2 S and CO 2 from CH 4 by a high silica CHA-type zeolite membrane[END_REF][START_REF] Hao | Upgrading low-quality natural gas with H 2 S-and CO 2 -selective polymer membranes Part II. Process design, economics, and sensitivity study of membrane stages with recycle streams[END_REF][START_REF] Mondal | Syngas production through gasification and cleanup for downstream applications -Recent developments[END_REF][START_REF] Castellani | Hydrate-based removal of carbon dioxide and hydrogen sulphide from biogas mixtures: Experimental investigation and energy evaluations[END_REF] Different industrial processes and rural and agricultural activities generate also waste gases containing H 2 S, for examples, crude oil refineries, coal combustion and/or gasification, paper mills, food industries, wastewater treatment plants, animal farms, etc. At high concentration, the Claus process and its derivatives were developed for the recovery of sulphur. [START_REF] Rezazadeh | Investigation of inlet gas streams effect on the modified Claus reaction furnace[END_REF] At lower concentration, several methods for H 2 S treatment and/or purification were developed. Biological processes call for the use of bacteria, such as Thiobacillus and Sulfolobus, for the oxidation of H 2 S into elemental sulphur or sulphate. [START_REF] Sublette | Oxidation of hydrogen sulfide by continuous cultures of Thiobacillus denitrificans[END_REF][START_REF] Oyarzun | Biofiltration of high concentration of hydrogen sulphide using Thiobacillus thioparus[END_REF][START_REF] Morales | Bio-oxidation of H 2 S by Sulfolobus metallicus[END_REF] However, these processes require strict operational conditions (temperature, pressure, input composition for energy source…). [START_REF] Omri | Performance study of biofilter developed to treat H 2 S from wastewater odour[END_REF][START_REF] Lebrero | Comparative assessment of a biofilter, a biotrickling filter and a hollow fiber membrane bioreactor for odor treatment in wastewater treatment plants[END_REF] Catalytic processes usually consist of the oxidation of H 2 S over solid catalysts, in order to convert it to molecular sulphur, sulphide, thiosulfate, and sulphate. [START_REF] Goifman | Catalytic oxidation of hydrogen sulfide by dioxygen on CoN 4 type catalyst[END_REF][START_REF] Bineesh | Selective catalytic oxidation of H 2 S over V 2 O 5 supported on TiO 2 -pillared clay catalysts in the presence of water and ammonia[END_REF][START_REF] Duong-Viet | Nitrogen-doped carbon nanotubes decorated silicon carbide as ametal-free catalyst for partial oxidation of H 2 S[END_REF] Absorption and adsorption are also strongly developed for the elimination of H 2 S. Absorption refers to the process by which one substance (absorbent), such as a solid or a liquid, takes up H 2 S in liquid or gas phase. Amine solutions are commonly used for the removal of H 2 S from natural gas. They are attractive because of the potential for high removal efficiencies, their ability to be selective for H 2 S abatement, and their regeneration possibility. [START_REF] Astarita | Hydrogen sulphide absorption in aqueous monoethanolamine solutions[END_REF][START_REF] Kohl | Chapter 2 -alkanolamines for hydrogen sulfide and carbon dioxide removal[END_REF][START_REF] Haghtalab | Simultaneous measurement solubility of carbon dioxide + hydrogensulfide into aqueous blends of alkanolamines at high pressure[END_REF] Adsorption relates to the fixation of H 2 S on the surface of a solid by weak physical or physicochemical bonds. Different adsorbents having high reactivity, such as activated carbon, zeolites, modified alumina or other metal oxides, have been investigated for H 2 S removal process. [START_REF] Maghsoudi | Simultaneous separation of H 2 S and CO 2 from CH 4 by a high silica CHA-type zeolite membrane[END_REF][START_REF] Song | Enhanced adsorption of hydrogen sulfide and regeneration ability on the composites of zinc oxide with reduced graphite oxide[END_REF][START_REF] Arespacochaga | Biogas deep clean-up based on adsorption technologies for solid oxide fuel cell applications[END_REF][START_REF] Florent | Effects of surface heterogeneity of cobalt oxyhydroxide/graphite oxide composites on reactive adsorption of hydrogen sulphide[END_REF] As example, Florent and Bandosz investigated the performance of graphite oxide supported cobalt oxyhydroxyde (CoOOH/C) for the treatment of H 2 S in moist or dry air. [START_REF] Florent | Effects of surface heterogeneity of cobalt oxyhydroxide/graphite oxide composites on reactive adsorption of hydrogen sulphide[END_REF] The adsorption capacity of this adsorbent up to reached 69.1 mg g -1 . In addition to these standard methods for H 2 S treatment, hydrate technology has been developed for H 2 S separation. Details of this technology can be found elsewhere. [START_REF] Castellani | Hydrate-based removal of carbon dioxide and hydrogen sulphide from biogas mixtures: Experimental investigation and energy evaluations[END_REF] This technology was found to be efficient for cleaning and upgrading the biogas. However, because of their relatively high cost, currently, there has been an increasing interest in utilizing low cost adsorbents which leads to screening industrial wastes, agricultural byproducts and natural materials as alternative adsorbents. [START_REF] Ri | Effect of reduced graphite oxide as substrate for zinc oxide to hydrogen sulfide adsorption[END_REF] This study aims to investigate the reactivity of zinc-, lead-, and iron-doped hydroxyapatites for the removal of H 2 S from gas phase at room pressure and temperature.

Hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 , called thereafter CaP) can be easily synthesized from a calcium source and a phosphate source such as calcium hydroxide and phosphoric acid. [START_REF] Pham Minh | Carbonated hydroxyapatite starting from calcite and different orthophosphates under moderate hydrothermal conditions: Synthesis and surface reactivity in simulated body fluid[END_REF] But it can also be obtained from solid wastes such as egg shells, animal bones [START_REF] Chaudhuri | Preparation and characterization of nanocrystalline hydroxyapatite from egg shell and K 2 HPO 4 solution[END_REF][START_REF] Figueiredo | Effect of the calcination temperature on the composition and microstructure of hydroxyapatite derived from human and animal bone[END_REF][START_REF] Sasakin | Immobilization of Sr 2+ on naturally derived hydroxyapatite by calcinations of different species of fish bones and influence of calcinations on ionexchange efficiency[END_REF] allowing the access to low-cost materials. It has the advantage of being thermally stable and being adapted for the deposition of well-dispersed metal particles by different methods, including the conventional incipient wetness impregnation (IWI), and also the ionic exchange (CEX) thanks to its high affinity for metal cations such as Zn, Pb, Fe. [START_REF] Sheha | Sorption behavior of Zn(II) ions on synthesized hydroxyapatites[END_REF][START_REF] Pham Minh | Calcium phosphate based materials starting from calcium carbonate and orthophosphoric acid for the removal of lead(II) from an aqueous solution[END_REF][START_REF] Mobasherpour | Comparative of the removal of Pb 2+ , Cd 2+ and Ni 2+ by nano crystallite hydroxyapatite from aqueous solutions: adsorption isotherm study[END_REF][START_REF] Kramer | A comparative study of the sintering behavior of pure and iron-substituted hydroxyapatite[END_REF] The influence of the preparation method for the deposition of metals on CaP support, and the influence of the nature of the metals on the performance of functionalized solids for H 2 S removal in gas phase is the subject of this communication.

Materials and methods

Materials

A commercial apatitic calcium phosphate (specific surface area: 7 m 2 /g) powder was used as received for the preparation of metal-doped calcium phosphate sorbents. An activated carbon (L3S, from CECA, 800 m 2 g -1 ) was also used as reference for H 2 S removal. Iron(III) nitrate nonahydrate (Fe(NO 3 ) 3 •9H 2 O, 98.2%, Fisher Scientific), lead(II) nitrate (Pb(NO 3 ) 2 , 99.8%, Fisher Scientific) and zinc nitrate hexahydrate (Zn(NO 3 ) 2 •6H 2 O, >97%, Fisher Scientific) were purchased and used without further purification.

Synthesis of metal-doped sorbents

The synthesis of metal-doped sorbents was the same as described elsewhere by Galera Martinez et al. [30] Using CEX method (cation exchange), 39 g of CaP powder were put into 2 L of the metal nitrate solution of defined concentration under stirring at 500 rpm. After 4 h of contact, the suspension was filtered and the resulting solid was washed several times with permuted water and dried in ambient air. Using IWI method (incipient wetness impregnation), an aqueous solution of metal precursor was prepared. The volume and the concentration of this solution were calculated in order to just wet the powder, and the desired metal content (0.7 or 1 wt.%) in the final product. The impregnation of the CaP powder with the metal precursor solution resulted in the formation of a doughy mixture. This last one was dried at ambient conditions. For both methods used, the dried metal-doped solids were fired at 450-500 °C for 1 h. Samples were designated according to: (i) the method of preparation (CEX or IWI);

(ii) the metal doped. For example, CEX-Fe corresponds to Fe-doped CaP prepared by CEX method. Three metals (Fe 3+ , Pb 2+ , and Zn 2+ ) were selected to prepare the sorbents, because of their availability, their low cost and their high affinity for sulphur compounds.

Characterizations

Inductively coupled plasma-atomic emission spectrometry analysis (ICP-AES) was performed using a HORIBA Jobin Y von Ultima 2 for the analysis of metals contents of the prepared solids. Solid samples were dissolved in a mixture of HCl and HNO 3 acids before analysis. Scanning electron microscopy (SEM) observation was carried out with a Philips XL30 FEG microscope. TEM-EDS analysis was carried out using a FEG JEOL JEM 2100F, equipped with a HAADF detector. This TEM apparatus is coupled with an EDS BRUKER which is equipped with a SDD detector (SDD of 127 eV resolution) for the analysis of elements on the surface of the solids.

2.4. Removal of H 2 S from gas phase H 2 S removal experiments were carried out at room temperature and atmospheric pressure. For each experiment, 500 mg of sorbent sample was set into a glass fixed-bed reactor (i.d. of 15 mm). Figure 1 illustrates the configuration of the experimental setup.

Synthetic gas effluents were purchased from Air Liquide (France). These effluents consist of dried air containing 50 or 200 ppmv of H 2 S. For a given experiment, the synthetic gas effluent was passed through the sorbent bed at the flow rate of 50 mL/min. The concentration of H 2 S in the outlet gas stream was monitored using an electrochemical detector (GasAlertQuatro, BW Technologies), which recorded H 2 S concentration every 60 s, and which has the detection limit of 0.1 ppm.

Figure 1

In this work, the following terms were used in order to compare different sorbents and different experimental conditions:

• inlet S H m 2
(mg): the total quantity H 2 S injected to the reactor at a given reaction time.

• sorbent S H m 2 (mg): the total quantity H 2 S fixed on the sorbent at a given reaction time.

• t 100% (min): the reaction time wherein the removal of H 2 S was total. So,

theoretically inlet S H m 2 is equal to sorbent S H m 2 within t 100% .
• t bth (breakthrough time, min): the time from the beginning of the experiment to the point where the outlet concentration raised to 75% of the inlet concentration.

Theoretically, t bth is longer than t 100% .

The calculation of

      - = ∫ dt C t C RT PQM m t out sorbent S H 0 int 6 10 2 (2)
where Q: inlet flow rate (L/min), M: the molecular weight of H 2 S (34.06 mg/mmol), W: mass of sorbent used (g), V M : molar gas volume at 20 °C and 1 atm (L/mmol), C in : inlet concentration of H 2 S (50 or 200 ppm), C out : outlet concentration of H 2 S (ppm), t: time-onstream (min).

Results and discussion

Characterization of the sorbents

Figure 2 reports the ICP-AES analysis results of the metal contents of the sorbents.

For iron and zinc, their theoretical contents were 1 wt.%. On the other hand, the theoretical content of lead was 0.7 wt.%. As expected, IWI method was effective for the deposition of all three metals. The metal contents obtained by ICP-AES analysis were close to the theoretical values. In fact, the only metal loss during this deposition method might be due to metal deposition on the flask wall. For the deposition of iron using CEX method, high deposition level was also observed. When CaP was added into the iron(III) nitrate solution for cationic exchange, the pH increased immediately to about 6. In these conditions, iron(III) could be exchanged with Ca(II), [START_REF] Kousalya | Synthesis of nanohydroxyapatite chitin/chitosan hybrid biocomposites for the removal of Fe(III)[END_REF] and could be also precipitated on the surface of CaP particles. On the other hand, CEX method allowed only the partial deposition of lead(II) and zinc(II). This could be due to the low specific surface area of the commercial CaP used in this work, which was 7 m 2 /g. Figure 2 All metal-doped CaP samples were characterized by XRD method, after calcination under air atmosphere at 400-500 °C for 2 h. No diffraction of the metals-based products (iron, lead, zinc) was observed, which can be explained by their low contents (less than 1 wt.%) and/or their high dispersion or amorphous form (XRD results not shown). All these samples show similar patterns compared to the initial CaP support. So the metal deposition by CEX or IWI methods, followed by calcination step did not change the apatitic crystalline structure of the initial support. In order to identify the presence of metals on the surface of CaP support, SEM and TEM analyses were performed. Figure 3 illustrates SEM images of different solids obtained by CEX and IWI methods. Both lead-doped CaP solids (CEX-Pb and IWI-Pb) and zinc-doped CaP solid by IWI (IWI-Zn) showed lead and zinc based particles at micrometric scale. These particles had higher contrast, so higher brightness, compared to other elements present on CaP surface. In fact, IWI method is expected to deposit metal particles on the surface of the support. On the other hand, the formation of lead-based particle in CEX-Pb sample might be due to the agglomeration of lead during the thermal treatment.

Both iron-doped CaP solids (CEX-Fe and IWI-Fe) and zinc-doped CaP solid by CEX method did not show visible metals particles at micrometric scale. This could correspond to a high dispersion of these metals on the surface of CaP support and their thermal stability during the calcination under air atmosphere. To confirm it, TEM analysis of iron-doped CaP by IWI method was performed (Figure 4). The distribution of all elements present on this solid including P, O, Ca, Fe was also recorded. By contrast difference, TEM image shows the formation of iron clusters at nano-scale which had higher contrast than other elements of the CaP support (top, left-hand-side). By EDS analysis using SDD detector, P, O and Ca were homogeneously distributed on the surface of the particles. Iron could also be identified and it did not cover the entire surface of the support. The superposition of phosphorus, oxygen and iron demonstrates that iron was deposited on the surface of the CaP support (bottom, righthand-side). Figure 4 3.2. Removal of H 2 S diluted in air Figure 5 shows the results obtained for the synthetic effluent containing 200 ppmv of H 2 S in dry air atmosphere. As mentioned previously, for each sorbent, H 2 S, Accumulated inlet curve (Fig. 5 (a)) was superposed with the curve expressing the total quantity of H 2 S captured by the sorbent within t 100% period. The initial CaP support showed very low reactivity under the experimental conditions used, with t bth and t 100% equal to 4 and < 1 min, respectively. On the other hand, doping with metals increased strongly the reactivity of the initial CaP, with t bth and t 100% varying in the range of 22-168 and 2-15 min, respectively. The comparison of t bth for all metals-doped CaP reveals that, for each metal, the solids prepared by IWI method were systematically more active than those prepared by CEX method. This observation was also similar for the comparison of t 100% obtained with these sorbents. In fact, IWI led to the formation of dispersed metal-oxide particles on the surface of CaP support, which could react with H 2 S molecules. On the other hand, CEX method theoretically led to the insertion of metal cations into and inside the apatitic structure of CaP, which probably became inert and unavailable for the fixation of H 2 S, when not localized on the surface. Among the prepared sorbents, CaP doped with iron by IWI (IWI-Fe) was the most active for H 2 S removal, regarding both t 100% and t bth (Figure 5). After 260 min of time-on-stream, this sorbent was completely saturated and the sorption capacity could be calculated, which reached 3.9 mg of H 2 S per gram of IWI-Fe. The comparison of t 100% and t bth for each sorbent shows that, under the experimental conditions used, there were big gaps between t 100% and t bth for all the sorbents. This suggests slow kinetics for the H 2 S-sorbent interaction under the experimental conditions used.

Increasing contact time by decreasing gas flow rate or by increasing sorbent volume may reduce these gaps between t 100% and t bth .

From the results obtained in Figure 5, three sorbents prepared by IWI method were selected for testing with synthetic gas effluent containing lower H 2 S concentration. Their reactivity was compared with a commercial activated carbon (L3S powder, from CECA, France) as reference. Figure 6 confirms that the initial CaP had low reactivity for the abatement of H 2 S with negligible t 100% , even with lower H 2 S concentration of 50 ppmv. As expected, doping with metals made the sorbents much more active, with t 100% equal to 31, 67, and 85 min for the sorbents doped with Zn, Fe, and Pb, respectively. The reactivity of the CaP doped with metals was comparable to that of this activated carbon. This last one showed t 100% of 67 min, which was equal to the value obtained with IWI-Fe. Contrary to the results obtained with synthetic air containing 200 ppmv of H 2 S, IWI-Pb was found to be the most efficient sorbent for synthetic air containing 50 ppmv of H 2 S. Standard characterizations such as SEM-EDS, XRD, FTIR, TG-DSC were performed with sorbents recovered after sorption tests. However, since the content of sulphur compounds was low in the used sorbents, these characterization techniques did not allow explaining this observation. Further analysis is needed in order to understand these results.

Reactivation of saturated sorbent and sorption capacity

The saturated IWI-Fe sorbent recovered from the first test was thermally treated by calcination at 500 °C for 2 h. This temperature was chosen because it was the firing temperature for the preparation of the initial IWI-Fe sorbent. The objective was to release the sulfur species from the surface of the saturated sorbent. The regenerated sorbent was used again for hydrogen sulfide removal under the same experimental conditions (the 1 st recycling). This regeneration procedure was repeated for the 2 nd recycling. The results are presented in the Figure 7.

Figure 7

At this regeneration temperature (500 °C), IWI-Fe sorbent lost about one third of its reactivity at the first recycling. t 100% decreased from 67 to 44 min. The observation was similar at the second recycling wherein t 100% decreased from 44 to 30 min. So, about one third of available active sites (iron oxides) on the surface of HAP particles could not be reactivated at each recycling. This might be due to the formation of strong chemical bond between sulfur species and active sites, which could not be released by the calcination at 500 °C. This might be also due to the agglomeration of iron-based particles during the regeneration step at 500 °C, forming bigger iron-based particles and so reducing the dispersion of the active phase.

Figure 8 Reactivation study was also performed with the saturated activated carbon. After the first test with the fresh sorbent, the resulting saturated sorbent was regenerated by calcination at 400 °C under air atmosphere for 2 h. This regenerated sorbent was tested again under the same experimental conditions. The value of t 100% of 49 min was obtained at the 1 st recycling. This indicates the similar reactivity of the reactivated carbon to IWI-Fe. The reactivity loss of the activated carbon might be due to the presence of irreversible interaction between sulfurbased species with active sites, or the partial destruction of active sites on the surface of the sorbent during calcination step.

Conclusions

Metals-doped apatitic calcium phosphates (HAP) were prepared by cationic exchange (CEX) and incipient wetness impregnation (IWI) methods. Metals-based particles could be observed at micrometric scale using SEM technique for lead-and zinc-doped HAP. On the other hand, iron-doped HAP contained particles at nanometric scale and were observed by TEM technique. For the removal of H 2 S diluted in air atmosphere, the initial HAP showed very low reactivity. The addition of a metal such as lead, iron and zinc improved strongly the reactivity of this initial HAP. For each metal, sorbents prepared by IWI method were found to be more reactive than those prepared by CEX method. IWI preparation led to the formation of dispersed metal particles which have available and reactive surfaces for H 2 S fixation. On the other hand, CEX method led to the incorporation of metals in the CaP structure which makes the metal less available for the fixation of H 2 S. The sorbent containing only 0.77 wt.% of iron and prepared by IWI method (IWI-Fe) had the similar reactivity compared to an activated carbon under the same experimental conditions. The reactivation of this sorbent (IWI-Fe) by calcination at 500 °C, after H 2 S removal test, allowed recovering two thirds of its reactivity. This signifies the possible presence of irreversible interaction of sulfur-based species with the sorbent.

Future work will focus on the optimization of metal deposition for the formation of highly-dispersed metal particles at nanometric scale, and also on the influence of the contact time by changing the volume of sorbent's bed. Other regeneration methods may be also investigated. The removal of H 2 S at high temperature, for example for the purification of hot syngas generated from thermal gasification of biomass, seems to be particularly interesting because of high thermal stability of hydroxyapatite-based materials, as demonstrated previously. [30] Table and figure captions

Figure 3

 3 Figure 3

Figure 5

 5 Figure 5

Figure 6

 6 Figure 6

Figure 1 .

 1 Figure 1. Experimental setup for H 2 S removal from gas phase.

Figure 2 .

 2 Figure 2. Metals contents of the prepared sorbents obtained by ICP-AES analysis.

Figure 3 .

 3 Figure 3. SEM images of the prepared sorbents.

Figure 4 .

 4 Figure 4. TEM image of IWI-Fe (top, left-hand-side) and the corresponding distribution of O, P, Ca and Fe.

Figure 5 .

 5 Figure 5. Performance of the prepared sorbents for the removal of H 2 S (200 ppmv) diluted in air. Experimental conditions: room temperature and pressure, 500 mg of sorbent, 50 mL/min of gas effluent. (a): "H 2 S, Accumulated inlet" curve: total quantity of H 2 S injected in the

Figure 6 .

 6 Figure 6. Performance of the prepared sorbents and an activated carbon as reference for the removal of H 2 S (50 ppmv) diluted in air. Experimental conditions: room temperature and pressure, 500 mg of sorbent, 50 mL/min of gas effluent. (a): "H 2 S, Accumulated inlet" curve:

Figure 7 .

 7 Figure 7. Regeneration study of the saturated IWI-Fe sorbent for the removal of H 2 S (50 ppmv) diluted in air. Regeneration procedure: calcination of saturated sorbent in air atmosphere at 500 °C. Experimental conditions: room temperature and pressure, 500 mg of sorbent, 50 mL/min of gas effluent. "H 2 S, Accumulated inlet" curve: total quantity of H 2 S

Figure 8 .

 8 Figure 8. Regeneration study of the saturated activated carbon sorbent for the removal of H 2 S (50 ppmv) diluted in air. Regeneration procedure: calcination of saturated sorbent in air atmosphere at 500 °C. Experimental conditions: room temperature and pressure, 500 mg of sorbent, 50 mL/min of gas effluent. "H 2 S, Accumulated inlet" curve: total quantity of H 2 S

Figure 1 .Figure 2 .

 12 Figure 1. Experimental setup for H 2 S removal from gas phase.

Figure 3 .

 3 Figure 3. SEM images of the prepared sorbents.

Figure 4 .

 4 Figure 4. TEM image of IWI-Fe (top, left-hand-side) and the corresponding distribution of O, P, Ca and Fe.

Figure 6 .

 6 Figure 6. Performance of the prepared sorbents and an activated carbon as reference for the removal of H 2 S (50 ppmv) diluted in air. Experimental conditions: room temperature and pressure, 500 mg of sorbent, 50 mL/min of gas effluent. (a): "H 2 S, Accumulated inlet" curve:

Figure 7 .

 7 Figure 7. Regeneration study of the saturated IWI-Fe sorbent for the removal of H 2 S (50 ppmv) diluted in air. Regeneration procedure: calcination of saturated sorbent in air atmosphere at 500 °C. Experimental conditions: room temperature and pressure, 500 mg of sorbent, 50 mL/min of gas effluent. "H 2 S, Accumulated inlet" curve: total quantity of H 2 S

Figure 8 .

 8 Figure 8. Regeneration study of the saturated activated carbon sorbent for the removal of H 2 S (50 ppmv) diluted in air. Regeneration procedure: calcination of saturated sorbent in air atmosphere at 500 °C. Experimental conditions: room temperature and pressure, 500 mg of sorbent, 50 mL/min of gas effluent. "H 2 S, Accumulated inlet" curve: total quantity of H 2 S

  Performance of the prepared sorbents for the removal of H 2 S (200 ppmv) diluted in air. Experimental conditions: room temperature and pressure, 500 mg of sorbent, 50 mL/min of gas effluent. (a): "H 2 S, Accumulated inlet" curve: total quantity of H 2 S injected in the Times for the complete removal of H 2 S (t 100% ) and breakthrough time (t bth ).

	180 Figure 5. reactor (or t 100% or t bth , min (b)	CaP t B t 100% t bth <1 inlet S H m 2 ); other curves: total quantity of H 2 S captured by the sorbents (or CEX-Pb IWI-Pb CEX-Zn IWI-Zn CEX-Fe IWI-Fe Sorbent (b)	sorbent S H m 2	);

Acknowledgment

The authors gratefully acknowledge colleagues at RAPSODEE Center for technical help.