Xuejie Li

Jeffrey D Henderson

Fraser P Filice

Dmitrij Zagidulin

Mark C Biesinger

Fan Sun

Bingnan Qian

David W Shoesmith

James J Noël

Kevin Ogle

The contribution of Cr and Mo to the passivation of Ni22Cr and Ni22Cr10Mo alloys in sulfuric acid

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

Introduction

Ni-Cr-Mo alloys are a class of corrosion resistant alloys commonly used in the petrochemical, nuclear, and chemical processing industries and have been suggested as one of the corrosion resistant engineering barriers in the proposed Yucca Mountain spent nuclear fuel repository [START_REF] Carranza | Crevice corrosion of nickel-based alloys considered as engineering barriers of geological repositories[END_REF]. The alloying elements, Cr and Mo, play an important role in establishing the corrosion resistance of the alloy. Cr is known to promote the formation of a corrosion resistant surface oxide, known also as a passive film. That Cr is the majority species in the passive film has been confirmed by various techniques [START_REF] Boudin | Analytical and electrochemical study of passive films formed on nickel-chromium alloys: Influence of the chromium bulk concentration[END_REF][START_REF] Ogle | Atomic emission spectroelectrochemistry applied to dealloying phenomena II. Selective dissolution of iron and chromium during active-passive cycles of an austenitic stainless steel[END_REF][START_REF] Zhang | Characterization of film properties on the NiCrMo Alloy C-2000[END_REF]. Additions of Mo to the alloy have been shown to stabilize the passive film, especially when there is a risk of localized corrosion [START_REF] Ilevbare | The role of alloyed molybdenum in the inhibition of pitting corrosion in stainless steels[END_REF][START_REF] Zadorozne | Crevice corrosion kinetics of nickel alloys bearing chromium and molybdenum[END_REF][START_REF] Bocher | Prediction of Critical Crevice Potentials for Ni-Cr-Mo Alloys in Simulated Crevice Solutions as a Function of Molybdenum Content[END_REF]. The benefits of alloyed Mo are empirically known, yet the underlying mechanisms are not well understood [START_REF] Lutton Cwalina | Revisiting the effects of molybdenum and tungsten alloying on corrosion behavior of nickel-chromium alloys in aqueous corrosion[END_REF]. Many studies have been carried out to clarify the role of Mo and yet even the conditions under which Mo may be accumulate in the passive film remain uncertain. Some studies suggest Mo enriches in the film [START_REF] Maurice | Effects of molybdenum on the composition and nanoscale morphology of passivated austenitic stainless steel surfaces[END_REF][START_REF] Olefjord | The Composition of the Surface during Passivation of Stainless Steels[END_REF][START_REF] Zagidulin | Characterization of surface composition on Alloy 22 in neutral chloride solutions[END_REF] while some others suggest the opposite [START_REF] Cahoon | Auger Electron Spectroscopic Studies on Oxide Films of Some Austenitic Stainless Steels[END_REF][START_REF] Hashimoto | An X-ray photo-electron spectroscopic study on the role of molybdenum in increasing the corrosion resistance of ferritic stainless steels in HC1[END_REF][START_REF] Yaniv | The Composition of Passive Films on Ferritic Stainless Steels[END_REF][START_REF] Ogawa | Auger Electron Spectroscopic and Electrochemical Analysis of the Effect of Alloying Elements on the Passivation Behavior of Stainless Steels[END_REF]. The proposed mechanisms by which Mo may promote passivation include a beneficial influence on repassivation, in which case Mo may have only a transitory existence in the film [START_REF] Bocher | Prediction of Critical Crevice Potentials for Ni-Cr-Mo Alloys in Simulated Crevice Solutions as a Function of Molybdenum Content[END_REF][START_REF] Gusieva | Repassivation Behavior of Individual Grain Facets on Dilute Ni-Cr and Ni-Cr-Mo Alloys in Acidified Chloride Solution[END_REF][START_REF] Ebrahimi | The influence of chromium and molybdenum on the repassivation of nickel-chromium-molybdenum alloys in saline solutions[END_REF], and/or the formation of an ion-selective layer of Mo species, reinforcing the protectiveness of the Cr-based film [START_REF] Sakashita | The effect of molybdate anion on the ion-selectivity of hydrous ferric oxide films in chloride solutions[END_REF][START_REF] Lloyd | Cr, Mo and W alloying additions in Ni and their effect on passivity[END_REF].

Zhang et al. [START_REF] Zhang | Chloride attack on the passive film of duplex alloy[END_REF] recently proposed a theory that the enrichment or depletion of Mo in the passive film is strongly correlated with the nature of the solution, based on their aberration-corrected transmission electron microscopy results. Solutions containing Cl -induced the enrichment of Mo. They proposed that Cl -ions could extract Mo from the matrix into the electrolyte solution by forming a soluble MoClx complex, then the MoClx could be further oxidized to Mo oxide and enriched in the outer region of the passive film. However, other researchers have reported Mo depletion in the passive film in solutions containing Cl - [START_REF] Hashimoto | An X-ray photo-electron spectroscopic study on the role of molybdenum in increasing the corrosion resistance of ferritic stainless steels in HC1[END_REF][START_REF] Habazaki | The corrosion behavior of amorphous Fe-Cr-Mo-P-C and Fe-Cr-W-P-C alloys in 6 M HCl solution[END_REF]. Inline ICP-MS, coupled with other in situ and ex situ techniques, was recently employed by Cwalina et al. [START_REF] Cwalina | In Operando Analysis of Passive Film Growth on Ni-Cr and Ni-Cr-Mo Alloys in Chloride Solutions[END_REF] to investigate the kinetics of oxide formation and growth on Ni22Cr and Ni22Cr6Mo (wt.%) alloys in 0.1 M NaCl solutions with the pH adjusted to 4 and 10. It was found that the addition of Mo to the Ni22Cr alloy resulted in the increased enrichment of Cr in the surface film.

We have recently demonstrated that the accumulation of Cr and Mo could be monitored quantitatively as a function of time during electrochemical experiments with a commercial Ni22Cr10Mo alloy, Hastelloy C-22, in sulfuric acid, using atomic emission spectroelectrochemistry (AESEC) [START_REF] Li | The Passivation of Ni-Cr-Mo Alloys: Time Resolved Enrichment and Dissolution of Cr and Mo during Passive-Active Cycles[END_REF]. Besides having the ability to passivate in sulfuric acid when an anodic polarization was applied, it also spontaneously passivated at the open-circuit (or corrosion) potential, Eoc. We demonstrated that the accumulation of surface Cr occurred during both spontaneous passivation and anodic passivation, attributed to the build-up of Cr oxides as a passive film. The film was removed by a subsequent cathodic activation. By contrast, Mo accumulation only occurred during spontaneous passivation at open circuit following cathodic activation. This excess Mo partially dissolved during a subsequent anodic passivation step (E = 0.3 VSCE) and almost completely dissolved during the following cathodic activation (E = -0.8 VSCE). Therefore, it was proposed that Mo accumulation in the oxide film was strongly influenced by the history of the material and the specific conditions of passivation. Based on thermodynamic considerations, the accumulation of Mo in the film during spontaneous passivation at lower potential was attributed to the formation of insoluble Mo(IV) oxides; their subsequent dissolution at higher potential was attributed to the oxidation of these oxides to the more soluble Mo(VI) species.

The goal of this work is to further clarify the role of Mo on the kinetics of passive film formation and dissolution. To this end we present new results with a simplified ternary alloy (Ni22Cr10Mo). To highlight the role of Mo, we compare the behavior of this material with the binary alloy (Ni22Cr) and with pure Mo and Ni using a combination of in situ AESEC and ex situ XPS for the alloys.

Experimental

Sample preparation

The Ni22Cr and Ni22Cr10Mo (wt.%) alloy ingots were fabricated using an Edmund Bühler MIM1 arc-melting system operated under an Ar atmosphere. Pure Ni (99.99%), Cr (99.99%), and Mo (99.99%) metals were used to obtain the desired compositions. The melting process was repeated at least three times for each alloy, with ingots turned over after each melting process. The as-fabricated ingots were annealed at 1000°C for 24 h under vacuum (10 -5 mbar). The annealed ingots were then cold-rolled into sheets. The as-rolled sheets were solution-treated at 1000°C under vacuum (10 -5 mbar) for 24 hours and then quenched in water. The as-quenched samples were confirmed to be single fcc phase using X-ray diffraction (XRD), not shown. Samples used for AESEC and XPS experiments were cut from the sheet, mechanically ground by a series of SiC papers up to P4000 then polished to a mirror finish using 1 μm diamond suspension. Before each experiment, samples were rinsed with deionized (DI) water.

Atomic emission spectroelectrochemistry

Atomic emission spectroelectrochemistry (AESEC) is based on the combination of an electrochemistry flow cell with an inductively coupled plasma atomic emission spectrometer (ICP-AES). The AESEC technique has been comprehensively reviewed in the recently published article by Ogle [START_REF] Ogle | Atomic Emission Spectroelectrochemistry: Real-Time Rate Measurements of Dissolution, Corrosion, and Passivation[END_REF]. The principle of AESEC is to collect and analyze species released from a sample surface in real-time while conducting electrochemical measurements. To achieve this, a specially designed flow cell is used to carry out the electrochemistry. Meanwhile, the ICP-AES is used to detect the dissolved species downstream. As a result, the dissolution rate of each element as a function of time can be obtained. Additionally, by assuming the oxidation state of each element, the dissolution rates can be converted into elemental currents to compare with the external electrical current measured by the potentiostat. The emission wavelengths selected for analysis of Ni, Cr, and Mo are shown in Table 1, as well as the detection limits defined as (C3σ= 3σ/κ) where σ is the standard deviation of background intensity in the absence of the element and κ is the sensitivity (i.e. slope) of the intensity vs. concentration curve. Note that detection limits may vary considerably from one experiment to the next depending on plasma conditions. In all cases, the polychromator of the ICP-AES spectrometer was used for both Ni and Cr, and the monochromator was used for Mo. Additionally, the equivalent detection limits expressed in dissolution rates and electrical current densities are shown. The calculation of νM and jM are described in the data treatment section.

Electrochemical setup

Immediately after surface preparation, samples were assembled with the homemade electrochemical flow cell, exposing either 0.5 or 1 cm 2 of the surface to the electrolyte. A typical three electrode system was used during all electrochemical measurements. Electrochemical experiments were performed using a Gamry Instruments Reference 600 for AESEC experiments or a Solartron 1287 potentiostat for XPS sample preparation. The sample served as the working electrode (WE). A Pt foil serving as the counter electrode (CE) and a saturated calomel electrode (SCE) as the reference electrode (RE) were placed in a second compartment separated from the WE by a conductive membrane. Solutions of 2 M H2SO4 were made by mixing DI water and 95% H2SO4. The electrolyte was pumped into the cell at a speed of ~3 mL per minute (cyclic potentiostatic experiments) or ~1.5 mL per minute (potentiodynamic polarization). Deaeration was achieved by sparging Ar gas into the solution for 30 mins before the experiment and continuously until the experiment finished. Aerated solution refers to the solution exposed to air.

Data treatment

The emission intensity of the element M at the specific wavelength (Iλ) obtained from ICP-AES (Horiba, Ultima 2C) was converted into a dissolution rate (νM) according to eq. ( 1).

ν = f (I -I ∘ )/κ A (1)
where Iλ ° is the background intensity, and κλ is the sensitivity factor of element M at the chosen wavelength, determined by conventional ICP-AES calibration methods using commercially available element standards (PlasmaCAL, SCP Science). The flow rate, f, was measured to a relative error of ≤ 1% for each experiment. The surface area, A, is the geometric surface of the WE exposed to the electrolyte, defined by the O-ring of the electrochemical flow cell.

The dissolution rate of element M (νM) may be normalized to the bulk composition relative to a chosen element (N) by eq. ( 2).

ν = ν (2)
where XM and XN are the mass fractions of elements M and N, respectively.

The accumulated surface excess (relative to Ni) of an element M, ΘM, from time A to time B, may be determined by eq. ( 3)

Θ = ( ν -ν ) (3)
An increase of ΘM indicates M is enriching on the surface while a decrease of ΘM indicates excess dissolution of M.

To compare the elemental dissolution rate (νM) with the external electrical current density (je) measured by the potentiostat, νM may be transformed to an equivalent elemental current density, jM, by equation ( 4):

j = (n Fν ) m ( 4
)
where n is the presumed number of electron(s) involved in the electrochemical dissolution reaction, mM is the atomic mass of M, and F is the Faraday constant.

During transient experiments at short times, the comparison of je with the sum of the elemental currents (ΣjM) requires a correction of the residence time distribution, h(t), of dissolved elements in the flow cell and capillaries of the system [START_REF] Shkirskiy | On The Time Resolution of the Atomic Emission Spectroelectrochemistry Method[END_REF]. It has been demonstrated that h(t) follows a log-normal function:

h(t) = " # $ %& ' ⋅ e * + ,-⋅ e *$ ./ 0 1 ' if t > 0 0 if t = 0 (5)
where the values of τ and β are empirically determined parameters, 0.99 and 10.23 respectively, obtained by applying a short anodic pulse to Cu in HCl solution [START_REF] Shkirskiy | On The Time Resolution of the Atomic Emission Spectroelectrochemistry Method[END_REF][START_REF] Ogle | Anodic Dissolution of 304 Stainless Steel Using Atomic Emission Spectroelectrochemistry[END_REF]. It was demonstrated that the correction of the current density may be realized by convoluting je using eq. ( 6) [START_REF] Ogle | Anodic Dissolution of 304 Stainless Steel Using Atomic Emission Spectroelectrochemistry[END_REF].

j 6 * (t) = j 6 (τ)h(t - 9 : τ)dτ (6)

XPS characterization

Specimens considered for XPS characterization were prepared using the identical electrochemical flow cell setup enclosed in a N2-filled glove box to minimize the influence of oxygen exposure. The oxygen level in the glovebox was controlled in the range of 50 ppm to 100 ppm. After treatment, each sample was rinsed with DI water, dried, and stored in a separate Ar-filled glove box with oxygen levels continuously maintained at 0.1 ppm until XPS analysis.

XPS measurements were carried out using a Kratos AXIS Supra spectrometer. All spectra were collected using the monochromatic Al Kα X-ray source (photon energy = 1486.6 eV) operating at 12 mA and 15 kV (180 W). During analysis, pressures inside the analysis chamber were maintained at, or below, 10 -8 Torr. Calibration of the instrument work function was done using the binding energy (B.E.) of a standard metallic Au sample (4f7/2 at 83.95 eV). In all spectra, photoelectrons were collected from an area of 700 µm x 400 µm. For all samples, spectra were collected at multiple take-off angles including 90, 50, 30, and 15°; however, only the 90° data set is described here due to the high noise level associated with the lower angles.

Survey spectra were recorded over a B.E. of 0 -1200 eV using a pass energy of 160 eV at a step size of 1 eV. High-resolution spectra of the C 1s, O 1s, Ni 2p, Cr 2p, Mo 3d, and S 2p lines were collected using a pass energy of 20 eV at a step size of 0.1 eV. All spectra were charge-corrected by setting the aliphatic (C-C) adventitious carbon signal to 284.8 eV. All signal processing and deconvolution were performed with CasaXPS software (v2.3.19) using a Shirley background subtraction. The deconvolution of high-resolution spectra was done using previously determined fitting parameters and constraints collected from standard metallic and oxide samples of Ni [START_REF] Biesinger | Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn[END_REF][START_REF] Biesinger | X-ray photoelectron spectroscopic chemical state quantification of mixed nickel metal, oxide and hydroxide systems[END_REF], Cr [START_REF] Biesinger | Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn[END_REF][START_REF] Biesinger | X-ray photoelectron spectroscopy studies of chromium compounds[END_REF], and Mo [START_REF] Spevack | Thermal reduction of molybdenum trioxide[END_REF].

Results

Overview of Electrochemical Behavior

Fig. 1 shows conventional polarization curves for Ni22Cr and Ni22Cr10Mo alloys in aerated and deaerated 2 M H2SO4. Both alloys showed an anodic peak denoting the active to passive transition and a well-defined passive domain. For convenience, these two anodic domains will be labelled A1 ( ~ -0.25 VSCE < E < ~ 0.1 VSCE) and A2 ( E > ~ 0.1 VSCE) respectively as indicated. For Ni22Cr10Mo, current densities were lower in magnitude in both the cathodic domain (C, E < -0.25 VSCE) and the active to passive transition (A1) as compared to Ni22Cr. The effect was particularly marked for the critical current (maximum of A1) for which approximately an order of magnitude decrease was observed. Within the passive domain (A2), current densities were comparable for both Ni22Cr and Ni22Cr10Mo. By comparing the potentiodynamic polarization curves, we conclude that Mo promotes the formation of the passive film during the active to passive transition in the low potential domain (A1) but does not seem to alter the protective nature of the film once formed in the high potential domain (A2). This is consistent with what has been previously reported on Ni alloys [START_REF] Lloyd | Cr, Mo and W alloying additions in Ni and their effect on passivity[END_REF][START_REF] Hayes | Influence of Chromium and Molybdenum on the Corrosion of Nickel-Based Alloys[END_REF][START_REF] Mishra | Materials Selection for Use in Concentrated Hydrochloric Acid[END_REF] and for stainless steel [START_REF] Rockel | The Effect of Molybdenum on the Corrosion Behavior of Iron-Chromium Alloys[END_REF].

Deaeration of the electrolyte had little influence on the electrochemistry (Fig. 1) for either alloy. The slight increase in the critical current density after deaeration may be attributed to the elimination of oxygen reduction, which also occurs in this potential range, the total current being the sum of the cathodic and anodic partial reactions. This same effect also accounts for the slight shift of the peak position and Ecorr to a lower potential in both cases. Although the effect was minimal, deaeration was important for this study so as to have the maximum resolution of the anodic reactions in the domain A1, where the effect of Mo is most clearly visible.

Elemental electrochemistry of Ni22Cr and Ni22Cr10Mo

Elementally resolved polarization curves in aerated and deaerated 2 M H2SO4 solutions are shown in Fig. 2 and Fig. 3 for alloys Ni22Cr and Ni22Cr10Mo, respectively. The conventional polarization curve, identical to Fig. 1, is shown in the upper part of each figure. The lower part shows the elemental currents for Ni (jNi), Cr (jCr), and Mo (jMo). Elemental currents were calculated from the elemental dissolution rate according to eq. ( 4) assuming nNi = 2, nCr = 3 and nMo = 6. The green dashed lines overlaid with jCr and jMo are the expected elemental currents, j'Cr and j'Mo, assuming congruent dissolution with Ni, based on the following equation:

j = < < / / j ( 7
)
where χM is the mole ratio of element M (Cr or Mo). The sum of the elemental dissolution currents, ΣjM, is also compared with the total electrical current, je.

During all polarization measurements in Fig. 2 and Fig. 3, an excess dissolution of Cr (jCr >> j'Cr) was seen at the onset of the cathodic polarization (-0.6 VSCE). For Ni22Cr10Mo, excess Mo dissolution (jMo >> j'Mo) was also observed. This may be attributed to the dissolution of the air-formed oxide. As the applied potential was increased, elemental currents stabilized at low values, around the detection limit. This continued until the active to passive transition, where both jNi and jCr showed a peak. Deaeration had little to no effect on the elemental dissolution rates.

Unlike the case for a 304 stainless steel [START_REF] Ogle | Atomic emission spectroelectrochemistry applied to dealloying phenomena: I. The formation and dissolution of residual copper films on stainless steel[END_REF], anodic dissolution of Ni22Cr was not faradaic.

Considering the active peak for the deaerated solution, a faradaic dissolution yield of 56.7% was obtained based on the assumption of nNi = 2 and nCr = 3. The electrical current that did not lead to dissolution (j∆ = je -ΣjM, shown as the blue dotted line in Fig. 2b) may be attributed to the formation of insoluble oxides for the deaerated solution, as the cathodic current is negligible.

Integration of j∆ yielded 14 mC cm -2 corresponding to a film thickness of 7 nm if only the formation of Cr2O3 is considered.

The accumulation of Cr in the film was observed in the active to passive transition domain (A1) for the Ni22Cr alloy. This is indicated by calculating the partial rate of Cr accumulation, jΔCr = j'Cr -jCr, shown in the insets of Fig. 2a andb. A broad, secondary dissolution peak (labelled "α") was detected during the descending edge of the active dissolution peak, (ΣjM > je) implying that non-faradaic dissolution occurred. This supports the idea that excess oxide formation occurred in the active domain, and this excess oxide subsequently dissolved to leave the final passive film. For Ni22Cr10Mo (Fig. 3), je > ΣjM in the active to passive transition domain (A1) and the passive domain (A2), indicating the formation of insoluble species. However, the contributions of Cr and Mo changed markedly with potential. During the active to passive transition, a significant Mo accumulation (jNi >> jMo ≈ 0) and a less significant Cr accumulation were observed. In the passive domain, Mo dissolution approached a congruent level and Cr dissolution was below the detection limit.

Electrochemistry of pure Mo and Ni

To further clarify the behavior of Mo as a function of potential, the polarization curve of nominally pure Mo was obtained, Fig.

Passive film formation and dissolution

Spontaneous Passivation (potential domain A1)

The growth of the oxide film in the potential domain A1, where the effect of Mo was most significant, was further investigated. The spontaneous passivation of the alloys was monitored as a function of time via a cycle of cathodic activationopen circuit potential, referred to as an SP cycle, shown in Fig. 5a and b for Ni22Cr and Ni22Cr10Mo, respectively. Cathodic activation (CA) was achieved by polarizing the alloy to -0.8 VSCE for 200 s. This resulted in a sharp dissolution peak involving all the alloying elements. The excess dissolution of Cr was indicated for both alloys (ν'Cr >> νNi) and excess Mo for the Ni22Cr10Mo alloy (ν'Mo >> νNi). We refer to this as cathodic activation, as it corresponds to the dissolution of the passive film. Spontaneous passivation was then observed on the release of the potential to Eoc (see inset). This is indicated by the excess Ni dissolution (νNi > ν'Cr) observed during this period, and the increase in Eoc, which quickly stabilized at ~ -0.2 VSCE for both alloys. Mo accumulation was also indicated for the Ni22Cr10Mo alloy where νNi > ν'Mo.

Spontaneous passivation, Fig. 5, showed not only the accumulation of Cr and Mo in the oxide film, but also indicated the effect of the enriched oxide film on the corrosion rate. For the Ni22Cr alloy, the Ni dissolution rate reached a steady-state value (νNi = 3 ng cm -2 s -1 , Eoc = -0.24 VSCE) with Cr dissolving congruently (ν'Cr = νNi). This is a factor of 4 lower than the dissolution rate observed for pure Ni under nominally identical conditions [START_REF] Li | The Passivation of Ni-Cr-Mo Alloys: Time Resolved Enrichment and Dissolution of Cr and Mo during Passive-Active Cycles[END_REF] (νNi = 12 ng cm -2 s -1 , Eoc = -0.31 VSCE) and illustrates the effect of Cr on spontaneous passivation. However, this was still a 10-fold increase

relative to the open circuit dissolution rate before cathodic activation (νNi = 0.3 ng cm -2 s -1 , Eoc = -0.003 VSCE), when the material was protected by the air-formed oxide film. This demonstrates that the air-formed passive film on Ni22Cr was more efficient at lowering the corrosion rate than the film formed spontaneously in H2SO4.

For the Ni22Cr10Mo alloy, the spontaneous dissolution rate decreased to nearly the detection limit The surface excess of Cr (ΘCr) and Mo (ΘMo) during spontaneous passivation of Ni22Cr10Mo (Fig. 5b) may be calculated according to Eq. 3, assuming Ni completely dissolved into the solution. The calculation yielded ΘCr ≈ 25 ng cm -2 and ΘMo ≈ 16 ng cm -2 , while for the Ni22Cr alloy, ΘCr ≈ 40 ng cm -2 , although the error was large.

Anodic passivation

Passivation at higher potential in the passive domain 1 (A2) was investigated for both alloys by stepping to a more positive potential, E = 0.3 VSCE, (Fig. 1) following the SP cycle. The resulting dissolution profiles for the anodic passivation are shown in Fig. 6a During the anodic potential step, Ni was selectively removed from the surface (νNi >> ν'Cr) for both Ni22Cr and Ni22Cr10Mo alloys. This is consistent with the growth of the Cr-enriched passive film. A Mo dissolution peak was observed for Ni22Cr10Mo which is nearly congruent with Ni dissolution (see below) and may be attributed to the formation of slightly soluble Mo(VI). The following open circuit corrosion rate for t > 200 s was somewhat higher for the binary alloy as compared to the ternary alloy but the difference was less marked than following spontaneous passivation (t < 0). That the post-AP corrosion rate of the ternary alloy was similar to that of the binary alloy is not surprising since Cr accumulation occurred simultaneously with Mo dissolution, partially eliminating the Mo-oxides formed during spontaneous passivation.

By mass balance, the surface accumulation of Cr was ΘCr = 56 ng cm -2 and ΘCr = 81 ng cm -2 for 1 We use the term " passive " as referring to the potential domain were the current is relatively independent of potential in the polarization curve. As spontaneous passivation was demonstrated to occur at potentials below the passive domain, the term is used simply as a convention.

Ni22Cr and Ni22Cr10Mo, respectively, corresponding to estimated film thicknesses of 0.16 nm and 0.23 nm, assuming a uniform Cr2O3 layer with the usual density of 5.22 g cm -3 . These extremely low values suggest that passivation may not be uniform on the surface and/or the oxides formed are much less dense than the standard densities used in the calculation. Similarly low values have been determined in previous studies for stainless steel [START_REF] Ogle | Atomic emission spectroelectrochemistry applied to dealloying phenomena II. Selective dissolution of iron and chromium during active-passive cycles of an austenitic stainless steel[END_REF][START_REF] Laurent | Dissolution and Passivation of a Silicon-Rich Austenitic Stainless Steel during Active-Passive Cycles in Sulfuric and Nitric Acid[END_REF], C-22 Ni alloy [START_REF] Li | The Passivation of Ni-Cr-Mo Alloys: Time Resolved Enrichment and Dissolution of Cr and Mo during Passive-Active Cycles[END_REF], and a Ni-based high entropy alloy [START_REF] Li | Communication-Dissolution and Passivation of a Ni-Cr-Fe-Ru-Mo-W High Entropy Alloy by Elementally Resolved Electrochemistry[END_REF]. The faradaic yield of dissolution during anodic passivation can be determined by comparing the sum of the elemental current, ΣjM, with the convoluted external electrical current, je * . This comparison for Ni22Cr and Ni22Cr10Mo is shown in Fig. 7. For the binary alloy, the decreasing background dissolution from the open circuit corrosion (jBG) preceding the polarization was calculated and subtracted [START_REF] Ogle | Atomic emission spectroelectrochemistry applied to dealloying phenomena II. Selective dissolution of iron and chromium during active-passive cycles of an austenitic stainless steel[END_REF]. The resulting difference between ΣjM and jBG was compared with je * , and a faradaic yield of 80% was obtained. The quantity of excess charge calculated by integrating jΔ = je * -ΣjM -jBG was 450 μC cm -2 , corresponding to an estimated uniform Cr2O3 film thickness of ≈ 0.23 nm. Again, the low value suggests that the film is not uniform but the good agreement between the coulometric analysis and the elemental mass balance indicates the quantitative nature of the approach.

The faradaic yield of dissolution for Ni22Cr10Mo is shown in Fig. 7b. Two separate curves are shown in which n = 2 (lower) and n = 6 (upper) were assumed for Mo dissolution, as there is some ambiguity as to the precise dissolution reaction, Mo(IV) Mo(VI) or Mo(0) Mo(VI). In either case, with only 10% Mo, the assumption of the Mo oxidation state did not significantly affect the calculated faradaic yield which was near 100%. The quantity of excess charge, Θe, was obtained by integrating the difference, j∆ = je * -ΣjMo(2e-). The value of Θe = 269 μC cm -2 , similar in magnitude to the value obtained by mass balance. This result confirms the hypothesis that the oxidation of Mo in the high potential domain results in a predominately soluble form of Mo oxide.

Potentiostatic cycles

The interplay between the different elements may be apprehended by their behavior over several active-passive cycles. The surface excess of Cr and Mo, ΘCr and ΘMo, were determined as a function of time during a sequence of activation-passivation cycles as shown in Fig. 8. The cycles consisted of cathodic activation (200 s at E = -0.8 VSCE), OCP (300 s), anodic passivation (200 s at E = 0.3 VSCE), and OCP (300 s) referred to as an AP cycle. For both alloys, the formation of the passive film was demonstrated by the increase of ΘCr during spontaneous passivation and more significantly during anodic passivation. The removal of the passive film was detected by the rapid decrease of ΘCr during cathodic activation. The cycles demonstrate that the passive film was formed and removed, however, the slow increase in the background may suggest some non-reversibility in the process and/or changes in the near-surface alloy composition. The Mo surface excess, ΘMo, for the Ni22Cr10Mo alloy showed a similar trend, increasing during spontaneous passivation, decreasing slightly during anodic passivation, and decreasing during cathodic activation. Overall, the results indicate that, for AP cycles, passivation and activation were reasonably reversible within experimental error.

The reversibility of spontaneous passivation during repeated SP cycles was investigated for Ni22Cr10Mo in Fig. 9, where the surface excess of Cr and Mo, ΘCr and ΘMo during four SP cycles are shown. The accumulation and subsequent dissolution of surface Cr oxide was clear in the variation of ΘCr with time, however, ΘMo increased steadily, indicating that Mo enriched more in the spontaneous passivation (SP) period than was removed during the cathodic activation period, resulting in a net increase in Mo accumulation with every cycle. The maximum amount of Cr decreased with successive cycles such that by the fourth cycle Cr accumulation and dissolution were considerably attenuated. This result suggests a cumulative action of the Mo build-up: as the Mo content of the surface was enhanced during successive SP cycles, the formation of Cr oxide was hindered.

These results for both Cr and Mo are very similar to previous results for alloy C-22, a commercial Ni-Cr-Mo alloy [START_REF] Li | The Passivation of Ni-Cr-Mo Alloys: Time Resolved Enrichment and Dissolution of Cr and Mo during Passive-Active Cycles[END_REF]. The maximum accumulation of Cr throughout the AP cycles, ΘCr(max), was approximately 114 ng cm -2 , similar to that of the C-22 alloy, ΘCr(max) = 133 ng cm -2 . During spontaneous passivation, ΘCr from the first spontaneous passivation was around 15 ng cm -2 , similar to that of the commercial alloy.

XPS characterization

To gain further insight into the chemical nature of Cr and Mo in the oxide film, as observed by AESEC, XPS was performed at various stages in the cyclic experiments. To avoid atmospheric contamination, the electrochemical experiments were performed in an inert atmosphere glovebox and the specimens transferred into the XPS vacuum system without exposure to the ambient atmosphere. The parameters for deconvoluting the high-resolution spectra were described in the experimental section. The fitting of the Mo 3d signal for samples exposed to H2SO4 required the consideration of sulfate and sulfide signals, due to the overlap of S 2s and Mo 3d bands. The presence of sulfate and sulfide was confirmed by the S 2p band in the survey spectrum, and their ratio was obtained from the high-resolution spectrum of the S 2p region (neither shown here). The ratio of different Mo species was then calculated after subtracting the contribution of sulfate and sulfide signals (S 2s).

XPS results for the air-formed oxide on Ni22Cr10Mo alloy are given in Fig. 10 and may be considered a reference for the electrochemical experiments. For Ni and Mo, the metallic peak dominated the spectra. Small quantities of Mo and Ni oxide were detected with 8% NiO and 14% Ni(OH)2 and 15% Mo(IV) and 20%Mo(VI). The Cr spectrum showed oxide as the predominate species, with Cr(III) oxide and hydroxide accounting for 25% and 28% respectively with no detectable Cr(VI). XPS spectra obtained at different points within a single anodic passivation (AP) cycle for Ni22Cr10Mo are shown in Fig. 11. The spectra represent the surfaces obtained after cathodic activation (CA), spontaneous passivation (SP), and anodic passivation (AP) as indicated in Fig. 8. Fig. 12 shows the spectra obtained after four cycles of spontaneous passivation (SP) as indicated in Fig. 9. The relative intensities of the integrated peaks for the different species of each element for each phase of the cycles are presented in Fig. 13.

Cathodic activation (Fig. 11a) led to an increase in the fraction of Cr(0) (76%) as compared to the same material with air-formed oxides (48%), consistent with the partial removal of the passive film by cathodic activation. The Ni(0) and Mo(0) showed only a slight increase (9% and 3%, respectively). This is consistent with the intense Cr dissolution peaks observed in Fig. 5b. It should be noted that even under the nitrogen atmosphere in the glove box, it is impossible to obtain spectra of the surface immediately after cathodic activation, as the surface is exposed to the electrolyte for several seconds, and some degree of reoxidation most certainly occurs during this period.

Spontaneous passivation occurred during the open circuit exposure following the cathodic activation. The corresponding XPS spectra are shown in Fig. 11b. A decrease of ~10% was observed in Ni(0), Cr(0) and Mo(0) fractions and an increase of the respective oxidized species, as shown in Fig. 12. The passivating effect of this film on the corrosion rate was indicated in Fig. 5b where the elemental dissolution rates were reduced to around the detection limit after film formation.

Anodic passivation was investigated in Fig. 11c after a single complete AP cycle. The ratio of Cr(III)/Cr(0) was further enhanced, consistent with the AESEC results shown in Fig. 6b. The relative amount of Mo(VI) increased while that of Mo(IV) decreased. These trends are also indicated in Fig. 12. This implies that the passive potential (0.3 VSCE) favored the formation of Mo(VI), in agreement with thermodynamic considerations [START_REF] Li | The Passivation of Ni-Cr-Mo Alloys: Time Resolved Enrichment and Dissolution of Cr and Mo during Passive-Active Cycles[END_REF]. The enhanced Mo dissolution observed in the anodic domain of the polarization curve (Fig. 3) and the anodic passivation (Fig. 6b) can, therefore, be attributed to the formation of Mo(VI) in the passive domain which, in an acid solution, is more soluble than the Mo(IV) species. The accumulation of Mo was observed to increase with successive SP cycles in the AESEC profiles of Fig. 9. To confirm this result and identify the oxidations states of Mo present in the passive film, XPS spectra (Fig. 12) were obtained after a series of four SP cycles. The Mo(0) contribution decreased by nearly 50% while Cr(0) actually increased slightly by 6% (Fig. 12). These results are qualitatively consistent with the AESEC results of Fig. 9.

The total elemental analysis was determined by combining the elemental analysis from the survey with the oxidation state analysis from the high-resolution spectra. The total composition is shown in Fig. 14a which includes the metallic species. As previously mentioned, the M(0) oxidation states will decrease in percentage as the oxide film thickens because the metallic substrate is masked by the oxide film. These results demonstrate that the oxide film increased in thickness CA < SP < AP as we would expect. The SPx4 was intermediate between CA and SP.

The relative composition of the surface oxides, Fig. 14b, was determined by removing the metallic states from the analysis and normalizing to 100%. CA and SP have similar composition, although the results of Fig. 14a indicate that the passive film thickness had increased following SP.

Although cathodic activation should remove much of the passive film, some degree of spontaneous passivation may happen, due to the inevitable contact with the electrolyte before the sample was taken out of the flow cell, resulting in the re-formation of the oxide. The spontaneously passivated film showed a Cr/Mo oxide ratio of 1.5. This value increased to 4.2 following anodic passivation. Again, this result demonstrates the significant accumulation of Cr during anodic passivation. The ratio of Mo(IV)/Mo(VI) increased from 1.0 (SP) to 2.1 (AP) with a slight appearance of Mo(V) species. A series of four SP cycles led to a marked enhancement of Mo (Mo/Cr = 1.3) and a Mo(VI)/Mo(IV) ratio = 0.36. There is consistency between the XPS results (Fig. 14) and the surface excess profiles from AESEC (Fig. 8b and Fig. 9). The ratio of Mo/Cr in the surface oxides following spontaneous passivation was determined to be 1.2 by XPS compared with 1.4 by AESEC. After anodic passivation, this ratio decreased to 0.4 for XPS and 0.2 for AESEC. The results were less consistent after four SP cycles with a Mo/Cr ratio of 2.4 for XPS as compared to 12 for AESEC although the trend of Mo accumulating on the surface was consistent in both experiments.

The origin of the discrepancies in quantification between XPS and AESEC is difficult to assess, however the local nature of XPS analysis compared to the global nature of AESEC may play a role. For XPS, only a small surface area (0.028 mm 2 ) in the center of the dissolution crater is measured while AESEC gives an average value over the entire working electrode area. Add to this the fact that the active-passive cycle experiment may be difficult to reproduce after several cycles, a problem which is further exacerbated due to the transfer protocol to XPS.

The Ni22Cr10Mo alloy showed a significantly lower spontaneous dissolution rate after open circuit passivation than did the Ni22Cr alloy. To gain insight into the origin of this difference, the spontaneously formed passive film on Ni22Cr was also analyzed by XPS, as shown in Fig. 15.

The sample was cathodically activated and then released to OCP for 300 seconds. Both NiO and Cr2O3 fitting parameters were included in the deconvolution; however, they were not required to model the experimental spectra, suggesting the absence of these species at the surface. Instead, both spectra were dominated by signals of the metallic state, with small amounts of hydrated oxides (i.e. Ni(OH)2 and Cr(OH)3). Taking the AESEC results into account, we can attribute the enriched Cr observed during spontaneous passivation to the formation of Cr(OH)3. This is in contrast to the Ni22Cr10Mo alloy where the Cr 2p spectra showed a Cr2O3/Cr(OH)3 ratio of 1.8 (Fig. 11).

These results suggest that Mo oxides formed in the A1 potential domain facilitate the oxidation of Cr to Cr2O3 rather than Cr(OH)3 which may be considered a step in the nucleation of the passive film as a high Cr2O3/Cr(OH)3 ratio has been associated with a more protective oxide, at least for one alloy C2000 (Ni23Cr16Mo1.6Cu) [START_REF] Zhang | Characterization of film properties on the NiCrMo Alloy C-2000[END_REF].

It is well known that in sulfuric acid, the critical current for passivation decreases markedly with increasing %Cr, as less Ni dissolution is necessary to produce an equivalent Cr(III) film [START_REF] Myers | Anodic Polarization Behavior Of Nickel-Chromium Alloys In Sulfuric Acid Solutions[END_REF].

How are these reactions affected by Mo alloying? In the experimental conditions of this work, it appears that Mo will affect the film when passivation occurs at low potentials (A1) because, unlike Ni and Cr, Mo oxidation yields completely insoluble oxidation products in this potential domain. Therefore Mo(IV)-oxides build-up on the surface. This Mo oxide accumulation apparently promotes Eq. 8 over Eq. 9 thereby enhancing the ratio of Cr2O3/Cr(OH)3 resulting in a film with improved barrier properties.

As the potential increases, the excess Mo will dissolve such that the high potential passive film shows a similar composition and corrosion resistance properties for both alloys. The spontaneously formed passive film probably does not correspond to the stable film formed at longer times and/or higher potential which has been the object of most ex situ spectroscopic studies to date. More likely it represents a "pre-passive" film [START_REF] Kirchheim | Kinetics of Film Formation on Fe-Cr Alloys[END_REF] that may be considered as an unstable film during the early stages of growth.

The low-potential accumulation of Mo during spontaneous passivation was previously attributed to the oxidation of Mo(0) to MoO2 (Eq. 10) in the low potential domain, based on thermodynamic considerations [START_REF] Li | The Passivation of Ni-Cr-Mo Alloys: Time Resolved Enrichment and Dissolution of Cr and Mo during Passive-Active Cycles[END_REF]. Its subsequent dissolution at higher potential was attributed to the oxidation of Mo and partial oxidation of preformed MoO2 to MoO4 2-, Eq. 11. In acid solution, molybdate exists in equilibrium with precipitated molybdates, indicated as MoO3•H2O(s) (Eq. 12), which would explain the partial dissolution of Mo observed at high potential. The solid molybdate species may also be linked to the series of polymeric species such as Mo7O24 6-and Mo8O26 4-known to form under these conditions [START_REF] Jakupi | Corrosion product analysis on crevice corroded Alloy-22 specimens[END_REF].

Mo + 2H2O MoO2 + 4H + + 4e - (10) MoO2 + 2H2O MoO3•H2O (s) + 2H + + 2e - (11) MoO3•H2O (s) MoO4 2-+ 2H + (12)
The dissolution of molybdic acid (Eq. 12) is known to be slow and the kinetics and mechanism of this reaction have been investigated in sulfuric acid in the framework of Mo extraction from ores [START_REF] Nevskaya | Kinetics of Molybdenum(VI) Oxide Dissolution in Sulfuric Acid Solutions[END_REF]. Alloys containing Mo have frequently been associated with an enhanced self-healing ability of the passive film [START_REF] Maurice | Effects of molybdenum on the composition and nanoscale morphology of passivated austenitic stainless steel surfaces[END_REF][START_REF] Hayes | Influence of Chromium and Molybdenum on the Corrosion of Nickel-Based Alloys[END_REF]. A defect in the passive film results in the underlying alloy being exposed to the electrolyte, followed by the re-formation of the passive film. The formation of a defect is to some extent simulated in this paper by the removal of the passive film by cathodic activation, and its subsequent re-formation initiated by the spontaneous passivation. Similar ideas have been proposed previously [START_REF] Henderson | The Complementarity of Cr and Mo in Reinforcing Oxide Protectiveness on Ni-Cr-Mo Alloys[END_REF]. Newman [START_REF] Newman | The dissolution and passivation kinetics of stainless alloys containing molybdenum -1. Coulometric studies of Fe  Cr and Fe  Cr  Mo alloys[END_REF] investigated repassivation of Fe-19Cr and Fe-19Cr-2.5Mo (wt.%) in 1 M HCl using a scratching electrode device. During repassivation, Mo significantly inhibited dissolution after the removal of 1-2 atom layers of Fe-Cr alloy. Thus, he concluded that Mo took effect at a sub-monolayer level, e.g. the Mo accumulated at kink or step sites. Scanning tunneling microscopy results given by Maurice et al. [START_REF] Maurice | Effects of molybdenum on the composition and nanoscale morphology of passivated austenitic stainless steel surfaces[END_REF] suggest that, for an austenitic stainless steel single crystal surface in 0.05 M H2SO4, Mo nanostructures would form selectively in local defects in the passive film. In this work, the accumulation of Mo measured from mass balance during spontaneous passivation yielded an extremely low value (16 ng cm -2 , less than a monolayer), which would suggest that Mo was not homogeneously distributed but enriched at local sites.

Spontaneous passivation was also observed for the binary Ni22Cr alloy; however, the spontaneously formed passive film on Ni22Cr was less efficient than the film formed on Ni22Cr10Mo under identical conditions. The XPS results suggested that the formation of Mo(IV)-oxides on the surface favored the formation of Cr2O3 during spontaneous passivation. Recent publications have suggested similar effects. A first-principles investigation by Samin et al. [START_REF] Samin | First-principles investigation of surface properties and adsorption of oxygen on Ni-22Cr and the role of molybdenum[END_REF] showed that the addition of Mo to the alloy stabilized oxygen adsorption, thus making the oxide more favorable. Yu et al. [START_REF] Yu | In Situ Observations of Early Stage Oxidation of Ni-Cr and Ni-Cr-Mo Alloys[END_REF] reported that the addition of 6 wt.% Mo inhibited Kirkendall void formation by promoting the nucleation of a metastable Ni2-xCrxO3 corundum structure.

There have been some discussions recently regarding the role of NiCr2O3 in the passivation of Ni-Cr and Ni-Cr-Mo alloys [START_REF] Cwalina | In Operando Analysis of Passive Film Growth on Ni-Cr and Ni-Cr-Mo Alloys in Chloride Solutions[END_REF]. In the data presented here, the mass balance calculations assume that oxidized Ni is nearly 100% soluble. Therefore, it is impossible to detect the formation of Ni oxide independent of the other alloying components. The near 100% faradaic efficiency for anodic passivation (Fig. 7) suggests that it makes a very small contribution at best. At higher pH however, Ni(II)-oxides are much less soluble than in 2M H2SO4, and very different results might be expected [START_REF] Cwalina | In Operando Analysis of Passive Film Growth on Ni-Cr and Ni-Cr-Mo Alloys in Chloride Solutions[END_REF].

The relatively complex chemistry of Mo and its transitory existence in the passive film may explain some of the disagreement in the literature concerning its presence and nature in the passive film. It has been reported for stainless steels in HCl solutions that during active dissolution, the oxidation state of Mo was Mo(IV) according to Olefjord et al. [START_REF] Olefjord | The Composition of the Surface during Passivation of Stainless Steels[END_REF][START_REF] Olefjord | Surface Composition of Stainless Steels during Anodic Dissolution and Passivation Studied by ESCA[END_REF]; however, it was Mo(VI) according to Hashimoto et al [START_REF] Hashimoto | An X-ray photo-electron spectroscopic study on the role of molybdenum in increasing the corrosion resistance of ferritic stainless steels in HC1[END_REF]. Our results would support the Mo(IV) theory, though both oxidation states were present after spontaneous passivation. Passivation at 0.3 VSCE resulted in the increase of Mo(VI) (the thermodynamically predominant oxidation state of Mo [START_REF] Ogle | Atomic Emission Spectroelectrochemistry: Real-Time Rate Measurements of Dissolution, Corrosion, and Passivation[END_REF] at 0.3 VSCE) and the decrease of Mo(IV). To our knowledge, there are no data in the literature concerning Ni22Cr10Mo alloys in this electrolyte to confirm this attribution. In acidic chloride solution, Olefjord et al. [START_REF] Olefjord | The Composition of the Surface during Passivation of Stainless Steels[END_REF] also reported that Mo was significantly enriched as Mo(VI) in the passive film in a Mo-alloyed austenitic stainless steel during anodic passivation. The effect of Mo for anodic passivation was less obvious in this work, however the low pH may have masked the effect. Local breakdown of the passive film in Cl -containing solutions may be due in part to the recombination of metal cation vacancies, e.g. V"Ni and V'''Cr, expressed in the conventional Kröger-Vink notation [START_REF] Yu | In Situ Observations of Early Stage Oxidation of Ni-Cr and Ni-Cr-Mo Alloys[END_REF][START_REF] Yu | Nonequilibrium Solute Capture in Passivating Oxide Films[END_REF][START_REF] Urquidi | Solute-Vacancy Interaction Model and the Effect of Minor Alloying Elements on the Initiation of Pitting Corrosion[END_REF]. It has been proposed that doping Ni and Cr oxides with Mo(VI) may create defects in the film by replacing Ni or Cr in the oxide lattice (e.g. Mo 4• Ni and Mo 3• Cr) thereby eliminating the metal cation vacancies and thereby hindering local film breakdown.

These results confirm the mechanisms proposed in the previous AESEC study of commercial alloy C-22, which also contained Fe (3.7 wt.%), W (2.8 wt.%), and Mn (0.3 wt.%). The critical and passive current densities were quasi-identical in this work and the previous study, indicating that the other alloying elements did not play a significant role in the passivation mechanisms. Therefore, we may generalize the results obtained with a ternary Ni-Cr-Mo alloy to similar commercial systems.

Conclusions

The electrochemical behaviors of Ni22Cr and Ni22Cr10Mo alloys and pure Mo were investigated by elemental resolved polarization curves in aerated and deaerated sulfuric acid solutions. Alloying with Mo improved the active to passive transition probably by favoring the nucleation and growth of Cr2O3 with respect to Cr(OH)3 by a factor of 10 less decrease in the critical current density. This was associated with a significant Mo accumulation. In the passive domain, the current density was not significantly different between the two alloys suggesting that the passive film formed in the high potential domain (E > 0 VSCE) was independent of Mo.

These results were consistent with the electrochemistry of pure Mo which oxidized to form insoluble Mo oxides in the low potential domain (A1, E < ~0 VSCE) but formed soluble Mo species at higher potential. A similar behavior was observed for Mo in the alloys with preferential Ni and Cr dissolution in the low potential domain and the accumulation oxidized Mo in the oxide film.

The accumulation of Cr (on Ni22Cr) or Cr and Mo (on Ni22Cr10Mo) were observed in real-time during spontaneous passivation in the low potential domain (E ≈ -0.2 VSCE) under free corrosion conditions. The Mo enriched passive film formed on the ternary alloy showed a markedly reduced free corrosion rate as compared to the binary alloy and a higher Cr2O3/Cr(OH)3 ratio. It is reasonable to infer that the Mo enrichment in this potential domain favors the nucleation and growth of Cr2O3 thereby improving the corrosion resistance of the film. Repeated spontaneous passivation -cathodic activation cycles showed a continuous accumulation of Mo in the oxide and a decrease in the quantity of Cr.

Anodic passivation resulted in a further increase of Cr on the surfaces of both Ni22Cr and Ni22Cr10Mo. However, Mo dissolved nearly congruently with Ni at this step for the ternary alloy and the resulting passive film was depleted in Mo as compared to the film formed spontaneously at lower potential. The ratio of Mo(VI)/Mo(IV) increased demonstrating that Mo dissolution was due to the formation of partly soluble Mo(VI) species.

Figure 1 .

 1 Figure 1. The potentiodynamic polarization curves of Ni22Cr (red) and Ni22Cr10Mo (blue) alloys in aerated (solid line) and deaerated (dotted lines) 2 M H2SO4, at ambient temperature, from -0.6 VSCE to 0.9 VSCE at 0.5 mV s -1 .

  Lutton et al. also observed a delayed dissolution of excess oxide during potentiostatic passivation experiments on Ni22Cr and Ni22Cr6Mo alloys [35].

Figure 2 .

 2 Figure 2. Elemental resolved potentiodynamic polarization curve for Ni22Cr recorded in aerated (a) and deaerated (b) 2 M H2SO4. For je in the bottom panels, only positive values are shown. ΣjM indicates the sum of the elemental currents. Elemental currents of Ni(II) and Cr(III) are offset for

Figure 3 .

 3 Figure 3. Elemental resolved potentiodynamic polarization curve for Ni22Cr10Mo in aerated (a) and deaerated (b) 2 M H2SO4. For je in the bottom panels, only positive values are shown. ΣjM indicates the sum of the elemental currents. Elemental currents of Ni(II), Cr(III), and Mo(VI) are offset for clarity. The green dashed lines are the assumed congruent elemental currents for Cr and Mo.

  [START_REF] Zhang | Characterization of film properties on the NiCrMo Alloy C-2000[END_REF]. Also shown for comparison is a similar curve for nominally pure Ni, replotted from reference[START_REF] Li | The Passivation of Ni-Cr-Mo Alloys: Time Resolved Enrichment and Dissolution of Cr and Mo during Passive-Active Cycles[END_REF]. It is observed that the onsets of Ni and Mo oxidation occur at nearly the same potential (≈ -0.25 VSCE) however the dissolution of Mo does not occur until a more positive potential (≈ 0 to 0.1 VSCE) most likely associated with the oxidation of Mo(IV) species to the more soluble Mo(VI) species. This is accompanied by a transition in the polarization curve from a passive plateau to an increasing currentpotential curve, characteristic of transpassive behavior. This confirms the hypothesis that the oxides of Mo formed in the potential range of the active to passive transition are not soluble while oxidized Ni is soluble. Thus, we would expect a Ni-Mo alloy to exhibit selective Ni dissolution in this potential domain, leaving behind a residual Mo oxide film. For E > 0 VSCE, Mo dissolution is significantly enhanced.The potential range in which Mo oxidation occurs without measurable dissolution corresponds well with the domain of the active to passive transition (A1) of the ternary alloy. It is therefore reasonable to attribute the lower critical current measured for the ternary alloy in this domain to the build-up of Mo oxides. These results suggest that Mo oxides formed during active to passive transition facilitates the nucleation and growth of the passive film.

Figure 4 .

 4 Figure 4. The elemental resolved polarization curves of pure Ni and Mo in 2 M H2SO4. The Ni data are replotted from reference [23].

(

  νNi ≈ 0.4 ng cm -2 s -1 , Eoc = -0.22 VSCE) following spontaneous passivation, comparable to the open circuit dissolution rate before cathodic activation. This demonstrated the effect of Mo on reinforcing spontaneous passivation of Ni22Cr10Mo alloy as compared to the binary alloy.

Figure 5 .

 5 Figure 5. Normalized elemental dissolution rates (ν'Cr and ν'Mo) recorded during cathodic activation for Ni22Cr (a) and Ni22Cr10Mo (b) in deaerated 2 M H2SO. Insets show the first 15 seconds of the open circuit potential after cathodic activation.

  and b for Ni22Cr and Ni22Cr10Mo, respectively. The anodic step occurs at t = 0 as indicated. Prior to this, the alloy was allowed to corrode freely at open circuit. Again, the efficiency of the spontaneously formed passive film on the ternary alloy is indicated by the decreased open circuit dissolution rate for t < 0.

Figure 6 .

 6 Figure 6. Normalized elemental dissolution rates (ν'Cr andν'Mo) recorded during anodic passivation (0.3 VSCE) for Ni22Cr (a) and Ni22Cr10Mo (b) in deaerated 2 M H2SO4. Surface acumulations were determined by integration and mass balance, exemplified in yellow for ΘCr in Fig. 6a.

Figure 7 .

 7 Figure 7. Comparison of the convoluted electrical current, je * , and the sum of the elemental currents, ΣjM, for (a) Ni22Cr and (b) Ni22Cr10Mo alloys. Mo(6e -) and Mo(2e -) indicate the number of electrons contributed by Mo is chosen as 6 and 2 respectively.

Figure 8 .

 8 Figure 8. The surface excess of Cr (and Mo) for Ni22Cr (a) and Ni22Cr10Mo (b) during AP cycles in deaerated 2 M H2SO4. The upper part of the figure shows the corresponding potential. The activation cycle with spontaneous passivation is highlighted in light yellow, the anodic passivation in light blue. AFO: air-formed oxide.

Figure 9 .

 9 Figure 9. The surface excess of Cr (ΘCr) and Mo (ΘMo) during SP cycles for Ni22Cr10Mo in deaerated 2 M H2SO4. The upper part of the figure shows the corresponding potential. AFO: air-formed oxide.

Figure 10 .

 10 Figure 10. Deconvoluted XPS high-resolution spectrum of Ni(2p3/2), Cr(2p3/2), and Mo(3d) regions recorded for the air-formed oxide on Ni22Cr10Mo alloy.

Figure 11 .

 11 Figure 11. Deconvoluted XPS high-resolution spectrum of Ni(2p3/2), Cr(2p3/2), and Mo(3d) recorded at various steps within the AP cycle. (a): cathodic activation. (b): spontaneous passivation. (c): anodic passivation.

Figure 12 .

 12 Figure 12. Deconvoluted XPS high-resolution spectrum of Ni(2p3/2), Cr(2p3/2) and Mo(3d) recorded after a sequence of four SP cycles.

Figure 13 .

 13 Figure 13. Comparison of relative composition for Ni, Cr, and Mo species obtained at various stages of the experiment cycle.

Figure 14 (

 14 Figure 14 (a) Total XPS elemental analysis including the zero oxidation state elements and (b) Normalized surface composition of the oxides on Ni22Cr10Mo following CA, SP, AP, and SP x4 (fractions given in at.%). The species are arranged in the legend in the same order (indicated by the arrow) as thin the graph.

Figure 15 .

 15 Figure 15. Spontaneous passivation: Deconvoluted XPS high-resolution spectra of Ni(2p3/2) and Cr(2p3/2) from the Ni22Cr alloy after a CA-OCP cycle.

  of Cr containing alloys (Fe-Cr, Ni-Cr), Cr is the major element enriched in the passive film. This occurs via a mechanism of selective dissolution of Ni or Fe, leaving behind a residual Cr(III) oxide/hydroxide film, i.e. 2NixCr + 3H2O 2xNi 2+ (aq) + Cr2O3(s) + 6H + + (6+ 4x)e - (8) NixCr + 3H2O xNi 2+ (aq) + Cr(OH)3(s) + 3H + + (3+ 2x)e -

Figure captions:Figure 1 .

 1 Figure captions:

Figure 2 .

 2 Figure 2. Elemental resolved potentiodynamic polarization curve for Ni22Cr recorded in aerated (a) and deaerated (b) 2 M H2SO4. For je in the bottom panels, only positive values are shown. ΣjM indicates the sum of the elemental currents. Elemental currents of Ni(II) and Cr(III) are offset for clarity. The green dashed lines are assumed congruent elemental currents for Cr (relative to Ni). Shown in insets are jΔCr=j'Cr -jCr, showing the accumulation of Cr during the active to passive transition.

Figure 3 .

 3 Figure 3. Elemental resolved potentiodynamic polarization curve for Ni22Cr10Mo in aerated (a) and deaerated (b) 2 M H2SO4. For je in the bottom panels, only positive values are shown. ΣjM indicates the sum of the elemental currents. Elemental currents of Ni(II), Cr(III), and Mo(VI) are offset for clarity. The green dashed lines are the assumed congruent elemental currents for Cr and Mo.

Figure 4 .

 4 Figure 4. The elemental resolved polarization curves of pure Ni and Mo in 2 M H2SO4. The Ni data are replotted from reference [23].

Figure 5 .

 5 Figure 5. Normalized elemental dissolution rates (ν'Cr and ν'Mo) recorded during cathodic activation for Ni22Cr (a) and Ni22Cr10Mo (b) in deaerated 2 M H2SO. Insets show the first 15 seconds of the open circuit potential after cathodic activation.

Figure 6 .

 6 Figure 6. Normalized elemental dissolution rates (ν'Cr andν'Mo) recorded during anodic passivation (0.3 VSCE) for Ni22Cr (a) and Ni22Cr10Mo (b) in deaerated 2 M H2SO4. Surface accumulations were determined by integration and mass balance, exemplified in yellow for ΘCr in Fig. 6a.

Figure 7 .

 7 Figure 7. Comparison of the convoluted electrical current, je * , and the sum of the elemental currents, ΣjM, for (a) Ni22Cr and (b) Ni22Cr10Mo alloys. Mo(6e -) and Mo(2e -) indicate the number of electrons contributed by Mo is chosen as 6 and 2 respectively.

Figure 8 .

 8 Figure 8. The surface excess of Cr (and Mo) for Ni22Cr (a) and Ni22Cr10Mo (b) during AP cycles in deaerated 2 M H2SO4. The upper part of the figure shows the corresponding potential. The activation cycle with spontaneous passivation is highlighted in light yellow, the anodic passivation in light blue. AFO: air-formed oxide.

Figure 9 .

 9 Figure 9. The surface excess of Cr (ΘCr) and Mo (ΘMo) during SP cycles for Ni22Cr10Mo in deaerated 2 M H2SO4. The upper part of the figure shows the corresponding potential. AFO: air-formed oxide.

Figure 10 .

 10 Figure 10. Deconvoluted XPS high-resolution spectrum of Ni(2p3/2), Cr(2p3/2), and Mo(3d) regions recorded for the air-formed oxide on Ni22Cr10Mo alloy.

Figure 11 .

 11 Figure 11. Deconvoluted XPS high-resolution spectrum of Ni(2p3/2), Cr(2p3/2), and Mo(3d) recorded at various steps within the AP cycle. (a): cathodic activation. (b): spontaneous passivation. (c): anodic passivation.

Figure 12 .

 12 Figure 12. Deconvoluted XPS high-resolution spectrum of Ni(2p3/2), Cr(2p3/2) and Mo(3d) recorded after a sequence of four SP cycles.

Figure 13 .

 13 Figure 13. Comparison of relative composition for Ni, Cr, and Mo species obtained at various stages of the experiment cycle.

Figure 14 (

 14 Figure 14 (a) Total XPS elemental analysis including the zero oxidation state elements and (b) Normalized surface composition of the oxides on Ni22Cr10Mo following CA, SP, AP, and SP x4 (fractions given in at.%).

Figure 15 .

 15 Figure 15. Spontaneous passivation: Deconvoluted XPS high-resolution spectra of Ni(2p3/2) and Cr(2p3/2) from the Ni22Cr alloy after a CA-OCP cycle.

Table 1 .

 1 Wavelengths

	Element	Ni	Cr	Mo
	Wavelength / nm	231.604	267.716	202.032
	Detection limit / ng cm -3	12	6.2	1.4
	νM / ng cm -2 s -1	0.30	0.16	0.035
	jM / µA cm -2	0.99	0.89	0.21

and typical experimental detection limits for Ni and Cr (polychromator), and Mo (monochromator). Equivalent dissolution rates and current densities at the flow rate of 1.5 mL/min are also shown, the latter were calculated with the oxidation states of Ni(II), Cr(III), and Mo(VI).

Table Element

 Element

		Ni	Cr	Mo
	Wavelength / nm	231.604	267.716	202.032
	Detection limit / ng cm -3	12	6.2	1.4
	νM / ng cm -2 s -1	0.30	0.16	0.035
	jM / µA cm -2	0.99	0.89	0.21

Table 1 .

 1 Wavelengths and typical experimental detection limits for Ni and Cr (polychromator), and Mo (monochromator). Equivalent dissolution rates and current densities at the flow rate of 1.5 mL/min are also shown, the latter were calculated with the oxidation states of Ni(II), Cr(III), and Mo(VI).

Acknowledgments

Author X.L. would like to acknowledge the Chinese Scholarship Council (CSC) for financial support. Author J.D.H. is grateful to be the recipient of an Alexander Graham Bell Canada Graduate Scholarship (NSERC). Author J.D.H. also acknowledges the financial support of the ECS H. H. Uhlig Summer Fellowship.


Martin Behr

Pierrick Le Borgne
email: pierrick_med@yahoo.fr

Florent Baicry

Charles-Eric Lavoignet

Lise Berard

Nicolas Tuzin

Mathieu Oberlin

Pascal Bilbault

- P Le Borgne

-F Baicry

-Ce Lavoignet

- L Berard 4 -N

Tuzin

-M Oberlin

- P Bilbault

Crise nationale des urgences : le résultat d'un déséquilibre croissant entre offre et demande de soins ?

Keywords: Urgences, engorgement, offre de soins, coût de la santé Emergency Department, overcrowding, health resources, health costs

Introduction

Depuis 2010, une aggravation de l'engorgement (overcrowding) des structures des urgences (SU) a été constatée dans les pays occidentaux. Celle-ci est d'origine multifactorielle mais le manque de lits d'aval semble en être la cause principale [START_REF] Pines | Emergency department crowding in California: a silent killer?[END_REF]. Cette stagnation de patients est corrélée à une hausse de la morbi-mortalité [START_REF] Richardson | Increase in patient mortality at 10 days associated with emergency department overcrowding[END_REF][START_REF] Sprivulis | The association between hospital overcrowding and mortality among patients admitted via Western Australian emergency departments[END_REF][START_REF] Sun | Effect of emergency department crowding on outcomes of admitted patients[END_REF][START_REF] Singer | The association between length of emergency department boarding and mortality[END_REF]. En 2019, c'est un état de rupture dans de nombreux SU en France, suivi d'un grand mouvement de contestation à l'hôpital qui ont fait l'actualité. Les SU sont pris en étau entre le manque de lits d'aval et l'augmentation croissante des recours pour des soins non programmés, aboutissant à des « hospitalisationsbrancards ». En 20 ans, le nombre de passages en SU a doublé (de 10 millions à 21 millions de passages entre 1996 et 2016) [START_REF] Drees | Les établissements de santé : édition 2018[END_REF] et l'activité de régulation médicale a presque triplé (de 11 millions à 29 millions d'appels entre 1997 et 2017) [START_REF] Chanteloup | Les appels d'urgence au centre 15 en 1997[END_REF][START_REF]Statistique annuelle des établissements de santé : Période 2000-2012[END_REF]. Beaucoup de soignants attestent d'une dégradation de leurs conditions de travail [START_REF] Juven | La casse du siècle : A propos des réformes de l'hôpital public[END_REF][START_REF] Benallah | Réforme de l'hôpital. Quels enjeux en termes de travail et de santé des personnels ?[END_REF], dont l'impact est probablement négatif sur leur propre santé et sur la qualité des soins [START_REF] Yates | Physician Stress and Burnout[END_REF].

Ces symptômes semblent être le résultat d'une inadéquation entre l'offre et la demande de soins. L'objectif principal de ce travail était d'exposer l'évolution des principaux paramètres de capacité hospitalière, de ressources humaines, d'activité et de financement (offre) en parallèle de l'évolution de la structure de la population (demande) puis de revenir sur les prévisions à l'horizon 2030. L'analyse concerne principalement les acteurs intervenant fréquemment avec les SU. Les inégalités de territoire ne seront pas développées dans ce travail. La comparaison entre ces différents paramètres d'unités différentes a été réalisée en les exprimant selon leurs taux de croissance annuels moyens (TCAM), puisque chaque paramètre suit une évolution relativement homogène sur des séries de données contemporaines issues de divers organismes publics.

Capacité et activité hospitalière

Capacité en Court séjour de Médecine, Chirurgie, Obstétrique (MCO) Entre 2000 et 2018, le nombre de lits d'hospitalisation complète MCO dans les établissements publics, privés non lucratif et privés lucratifs est passé de 247000 à 204000 lits [START_REF]Statistique annuelle des établissements de santé : Période 2000-2012[END_REF]. En rapportant cette diminution à la population générale qui a augmenté de six millions d'habitants, la capacité hospitalière en MCO a diminué de 4,08 à 3,05 lits pour 1000 habitants (-25%). Cette baisse de capacité concerne surtout les secteurs de chirurgie (-44%) et d'obstétrique (-36%) (Fig. 1), et dans une moindre mesure le secteur de médecine (-7%). Des alternatives à l'hospitalisation complète ont certes été développées. Ainsi, les places d'hospitalisation partielle (ambulatoire) sont passées de 16000 à 33500 entre 2000 et 2018.

Elles correspondent surtout au développement de la chirurgie ambulatoire et des hôpitaux de jour. Toutefois, les pertes de lits d'hospitalisation en court séjour n'ont pas été compensées par les gains de places d'hospitalisations partielles ou d'hospitalisation à domicile (HAD) (Fig. 1).

Activité MCO

Le suivi de l'activité hospitalière et de son financement est assuré par l'Agence technique de l'information sur l'hospitalisation (ATIH) [START_REF] Atih | Panorama national de l'activité hospitalière[END_REF]. La valeur économique de la consommation hospitalière varie d'une année à l'autre selon un effet volume (activité) et selon un effet prix (exemple : salaires des personnels, prix des médicaments, etc.). L'activité en MCO peut ainsi être mesurée par sa valeur économique à prix constant ou par les nombres de séjours en hospitalisation complète, de séjours en hospitalisation partielle (aussi appelés

Le virage ambulatoire

Alors que le développement des alternatives à l'hospitalisation complète était déjà bien engagé (Fig. 1), le terme « virage ambulatoire » n'est apparu qu'en 2013. L'objectif était d'économiser le prix des nuitées passées en hospitalisation complète tout en maintenant la valorisation des actes. Ainsi, les séjours ambulatoires et de courte durée représentaient en 2013 respectivement 23% et 13% de l'ensemble des séjours contre 27% et 15% en 2018 [START_REF] Atih | Panorama national de l'activité hospitalière[END_REF].

Ces transferts d'hospitalisation complète vers l'ambulatoire concernaient majoritairement la chirurgie programmée. Parallèlement, il est important de noter la croissance importante du nombre de séances (Fig. 2), qui correspondent surtout à des actes itératifs de médecine en hôpitaux de jour mais qui ne sont pas considérées comme des séjours. De même, les activités de consultations et actes en externe représentent une activité hospitalière importante très hétérogène pour lesquels il n'existe pas d'analyse permettant d'identifier son impact sur le virage ambulatoire. D'après un récent rapport de la Cour des comptes, la marge de manoeuvre restante repose néanmoins sur la médecine ambulatoire, à la condition de renforcer qualitativement et quantitativement l'offre de soins de ville et le décloisonnement villehôpital. Le bilan économique du virage ambulatoire n'a pas pu être évalué précisément [START_REF]Rapport sur l'application des lois de financement de la sécurité sociale[END_REF].

Moyen séjour, psychiatrie et hospitalisation à domicile

Le nombre de lits de Soins de suite et de réadaptation (SSR) a augmenté de 90000 à 101000 lits entre 2000 et 2010, et est depuis resté stable. Néanmoins, en tenant compte de l'augmentation de la population gériatrique, la capacité effective en SSR a en fait diminué de 21 à 16,6 lits pour 1000 seniors de plus de 75 ans. Cette chute de capacité risque de s'aggraver avec l'augmentation des patients de plus de 75 ans prévue dans les prochaines années. Entre 2000 et 2018, le nombre de lits d'hospitalisation complète en psychiatrie a diminué de 63000 à 55000 lits, ce qui porte la capacité hospitalière de 1,04 à 0,83 lits pour 1000 habitants (-21%). Le nombre de places d'hospitalisation à domicile (HAD) est passé de 14000 à 18000 entre 2013 et 2018, l'activité y est variée et concerne surtout les soins palliatifs, les soins liés à la prise en charge des cancers, les pansements complexes et soins de nursing ainsi que la prise en charge de la périnatalité [START_REF] Drees | Les établissements de santé : édition 2018[END_REF]. avec de fortes disparités régionales (50 jours pour un ophtalmologue ou un dermatologue) [START_REF] Millien | La moitié des rendez-vous sont obtenus en 2 jours chez le généraliste, en 52 jours chez l'ophtalmologiste[END_REF]. En revanche, les effectifs et les volumes de soins infirmiers libéraux et salariés en ville sont en très forte hausse selon des TCAM d'environ 5% depuis 2009, probablement liés au vieillissement de la population et au virage ambulatoire [START_REF] Drees | Les dépenses de santé en 2018[END_REF].

Hébergement des personnes âgées dépendantes

Financement et contrôle des dépenses de santé

La construction de l'objectif national des dépenses d'assurance maladie (ONDAM) pour l'année n est réalisée selon la formule :

ONDAM n = (ODAM n-1) + (hausse tendancielle des dépenses de santé) -(mesures d'économies) Ou ONDAM n = (ONDAM n-1) + [(ONDAM n-1) x (taux tendanciel)] -(mesures d'économies)

La hausse tendancielle des dépenses est difficile à estimer (méthodes hétérogènes, branches par branches). Elle correspondrait à un taux de croissance tendanciel de 4,5% en prévision d'une activité en croissance ralentie, a été jugée insuffisante par les fédérations hospitalières [START_REF] Fhf | Pour l'ensemble des fédérations : Une hausse des tarifs hospitaliers de 1% est vitale ! 2019[END_REF].

Démographie, pathologies chroniques et projections d'activité hospitalière Population gériatrique, maladies chroniques

Les projections démographiques de l'institut national de la statistique et des études économiques (INSEE) prévoient une croissance et un vieillissement de la population jusqu'en 2070, marqué actuellement par l'arrivée de la génération du baby-boom dans les 70 ans [START_REF]Pyramide des âges interactive[END_REF][START_REF] Blanpain | Projections de population à l'horizon 2070[END_REF]. Or, le recours à l'hospitalisation complète ou partielle en MCO augmente presque exponentiellement avec l'âge. Ainsi, en 2016, on comptait environ 1 séjour en MCO pour 2 séniors entre 65 et 69 ans (Fig. 3). Par ailleurs, le nombre total d'affections longue durée (ALD) exonérantes du régime général a augmenté de 50% entre 2008 et 2017, soit un TCAM de 4,6% [START_REF] Cnam | Prévalence des ALD de 2008 à 2017[END_REF]. Entre 2011 et 2016, les dépenses au titre des ALD ont augmenté selon un TCAM de 3,8%, dont 2,1% s'expliqueraient simplement par la croissance et le vieillissement de la population et 1,6% par la progression des prévalences d'ALD dans chaque classe d'âge [START_REF] Granger | Le vieillissement de la population entraîne une hausse des dépenses de santé liées aux affections de longue durée[END_REF]. En 2016, les dépenses au titre des ALD exonérantes représentaient 57% des dépenses de santé remboursées [START_REF] Granger | Le vieillissement de la population entraîne une hausse des dépenses de santé liées aux affections de longue durée[END_REF], soit 87 milliards d'euros (47% de l'ONDAM 2016).

Ces constats amènent à penser que l'activité hospitalière va croître inexorablement dans les prochaines années. Si l'on applique les taux de recours à l'hospitalisation par tranches d'âge de 2016 aux données et projections démographiques de l'INSEE, on obtient une courbe proche de celle des séjours mesurés jusqu'en 2018 (Fig. 4) et qui prévoit une augmentation linéaire du nombre de séjours jusqu'en 2040.

Les projections d'activité hospitalière de la DREES

La DREES a publié en 2008 un travail de projection de l'activité hospitalière prenant en compte le vieillissement de la population [START_REF] Mouquet | Impact du vieillissement sur les structures de soins à l'horizon 2010, 2020 et 2030[END_REF]. Pour chaque groupe de pathologies, des experts médicaux proposaient une évolution attendue de l'activité hospitalière d'après Dès 2017, un rapport de la Cour des comptes évoquait ces travaux pour appeler à une accélération de la baisse de capacité hospitalière en court séjour et à la restructuration globale de l'offre de soins [START_REF]assurance maladie : Assurer l'efficience des dépenses, responsabiliser les acteurs[END_REF]. Par ailleurs, l'évolution des eqJ et du nombre de lits depuis 2004 semble suivre voire dépasser les projections des experts de 2008 (Fig. 5). Cette évolution peut s'expliquer par une modernisation de certaines prises en charge ainsi qu'une demande de soins plus orientée vers l'ambulatoire (hypothèses des experts) mais pourrait aussi être contrainte par le pilotage administratif ou le manque de ressources. Dans les faits, ce virage ambulatoire prôné par ces études malgré le vieillissement de la population, a bien fonctionné pour une partie de l'activité chirurgicale. Mais il ne répond pas à la hausse d'activité d'hospitalisation complète en médecine, mise en tension par les décompensations itératives de pathologies chroniques.

Historique de la restructuration de l'hôpital et comparaisons internationales

La restructuration de l'hôpital en vue de maîtriser les coûts de santé a commencé dès les années 1970 et s'est accélérée depuis 1990 [START_REF] Safon | Les réformes hospitalières en France : aspects historiques et réglementaires[END_REF]. Une logique de centralisation contraignante et standardisée s'est développée avec la mise en place des schémas régionaux d'organisation sanitaire à partir de 1991, puis avec les ordonnances Juppé de 1996 qui ont créé les agences régionales de l'hospitalisation avec un objectif assumé de fermeture d'un tiers des lits d'hospitalisation et de diminution des DMS. Ainsi, entre 1980 et 2000, le nombre de lits avait déjà diminué de 30% en MCO et de 50% en psychiatrie [START_REF]assurance maladie : Assurer l'efficience des dépenses, responsabiliser les acteurs[END_REF]. C'est aussi avec ces ordonnances qu'a été instauré l'ONDAM qui est devenu un élément-clé des projets de loi de financement de la sécurité sociale (PLFSS) et que le numerus clausus médical a connu ses plus bas niveaux. Par la suite, ce sont les alternatives à l'hospitalisation complète qui ont été développées et l'instauration en 2004 de la T2A en remplacement de la dotation globale de fonctionnement.

La construction et l'encadrement de l'ONDAM sont devenus plus stricts, avec des ONDAM effectifs en sous-exécution depuis 2010 et dont la croissance annuelle a été ralentie entre 2 et 2,5% (Fig. 6). Depuis 1980, l'ensemble de ces efforts s'est effectivement traduit par un infléchissement de la croissance de la consommation de soins et de bien médicaux (CSBM). Depuis 1995, malgré la baisse progressive de la croissance du PIB, on note un infléchissement puis une stabilisation du rapport CSBM/PIB à 8,6% [START_REF] Soual | Les dépenses de santé depuis[END_REF].

Plusieurs rapports ont mis en avant la résilience de notre système de santé au regard des transformations qui lui ont été imposées [START_REF] Mouquet | Impact du vieillissement sur les structures de soins à l'horizon 2010, 2020 et 2030[END_REF]34). C'est par exemple le cas du rapport de la commission européenne « State of Health in the EU » de 2017 qui plaçait le système de santé français parmi les meilleurs indicateurs de santé et de qualité hospitalière de l'Union européenne (UE) (34). Ces bons résultats sont atteints avec une dépense courante de santé proche de 11,5 % du PIB. Les pays ayant les plus grandes dépenses de santé rapportées au PIB sont les Etats-Unis et la Suisse (ces 2 pays ayant fait le choix d'une solidarité plus limitée) (Fig. 7). Cependant, en coût par habitant à parité de pouvoir d'achat (en essayant de limiter l'effet des différences de niveaux de vie), la France reste dans la fourchette haute mais est dorénavant dépassée par de nombreux pays. La consommation de soins y est exécutée avec des moyens capacitaires et humains qui semblent légèrement inférieurs aux pays comparables : moins de médecins, un peu au-dessus de la moyenne en infirmières, moins de lits de court-séjour et une DMS assez courte (35) (Fig. 8).

Discussion

Les paramètres exposés dans ce travail nous renseignent sur l'évolution du système de santé, mais leurs unités de mesure différentes ne permettent pas de les comparer directement.

Toutefois, il est possible de comparer leurs TCAM sur la dernière décennie. Ainsi, nous avons pu synthétiser l'évolution de notre système de santé sur la décennie 2009-2019 (Fig. 9 Les premiers éléments du PLFSS 2020 prévoyaient de nouveau un ONDAM en croissance (2,3%) déconnectée de la hausse tendancielle (4,4%) impliquant des efforts d'économie historiquement hauts de 4,2 milliards d'euros dont 830 millions sur l'amélioration de la performance des établissements de santé et 215 millions d'euros sur les parcours de soins (notamment la fermeture de lits au profit de l'ambulatoire) [START_REF]Projet de loi de financement de la sécurité sociale pour 2020[END_REF]. Le récent plan « investir pour l'hôpital », prévoit un desserrement modeste de l'étau budgétaire, mais aucune mesure n'est prise pour sanctuariser les lits restants, ni pour relancer quantitativement l'offre de soins [START_REF][END_REF]. Ainsi, les conditions de travail et la qualité des soins ne peuvent que continuer à se dégrader, alors que les soignants resteront tenus à leur obligation de soins et de moyens dans un système de santé qui au contraire ne se donne plus les moyens de ses ambitions. En comparaison, après des années de crise et de restrictions budgétaires, le Royaume-Uni s'est engagé début 2019 sur une augmentation du budget du National Health Service de 3,4% par an pour les 5 prochaines années, la priorité étant donnée à l'augmentation du nombre de soignants et à la construction de nouveaux hôpitaux [START_REF]long term plan[END_REF].

Les projections récentes de la DREES prévoient que le nombre d'eqJ devrait encore légèrement diminuer et recommandent une poursuite des fermetures de lits jusqu'en 2030.

Cette baisse de capacité hospitalière se fait actuellement en totale contradiction avec la réalité du terrain. Un quart des hospitalisations complètes MCO résultent d'un passage par un SU [START_REF] Atih | Panorama national de l'activité hospitalière[END_REF], et les chiffres du « no bed challenge » sur l'année 2018 démontrent une insuffisance très préoccupante de la disponibilité des lits de médecine pour les hospitalisations non programmées [START_REF] Braun | Le no bed challenge et la maltraitance[END_REF]. Une étude française récente a retrouvé une association indépendante entre l'attente prolongée d'un lit d'hospitalisation en SU et la mortalité ainsi que la DMS, faisant de cette problématique un enjeu majeur de santé publique [START_REF] Thibon | Association entre mortalité et attente aux urgences chez les adultes à hospitaliser pour étiologies médicales[END_REF]. De plus, le postulat selon lequel l'aval ou les alternatives de l'hospitalisation complète MCO s'adapteraient naturellement aux hypothèses volontaristes avancées dans ces projections est loin de se vérifier. Les patients poly-pathologiques présentant des décompensations aigües itératives nécessiteront inexorablement des soins hospitaliers.

Les prévisions de volume d'activité hospitalière intègrent dorénavant le tassement de la croissance d'activité observé depuis 2017. D'après l'ATIH, ce déficit s'expliquerait par une sous-consommation conjoncturelle en médecine liée aux classes creuses de la 2 ème guerre mondiale [START_REF]Avis du comité d'alerte sur le respect de l'ONDAM n°2019-2[END_REF]. Cette explication va à contre-courant de la crise actuelle des SU dans lesquelles de nombreux patients âgés stagnent parfois plusieurs heures dans un couloir.

L'hypothèse d'une saturation des lits de médecine impactant l'activité nécessaire à la demande de soins, pourrait également être envisagée ; le taux d'occupation des lits de médecine a progressivement augmenté de 82% en 2000 à 87% en 2018 d'après le nombre de journées d'hospitalisation en médecine issu de la SAE [START_REF]Statistique annuelle des établissements de santé : Période 2000-2012[END_REF]. Or, il est documenté qu'un taux d'occupation moyen annuel supérieur à 85% est à haut risque d'épisodes de saturation itératifs en dehors de services regroupés en pôles polyvalents approchant les 100 lits [START_REF] Proudlove | The 85% bed occupancy fallacy: The use, misuse and insights of queuing theory[END_REF]. Cette baisse inattendue de l'activité de médecine devrait donc plutôt être interprétée comme un signe inquiétant de saturation capacitaire impactant de facto la qualité des soins.

Conclusion

Au cours de la dernière décennie, l'évolution du système de santé est marquée par une stagnation des paramètres de capacité et de ressources humaines (offre de soins) en parallèle d'une hausse d'activité, elle-même stimulée principalement par un essor rapide des pathologies chroniques (demande de soins). Malgré les efforts d'efficience déjà consentis, cette situation instable est à l'origine d'une saturation dangereuse des structures d'urgence.

Alors que cette inadéquation risque de s'accélérer dans les prochaines années, il apparaît urgent de renforcer ambitieusement l'offre de soins aussi bien en ville qu'à l'hôpital. Le vieillissement de la population et le poids des pathologies chroniques doivent nous amener à redéfinir les objectifs fondamentaux de notre système de santé.

FIGURES

  De nombreuses lois et plans d'actions ont accompagné l'évolution des structures d'hébergement des personnes âgées. Entre 2006 et 2011, 60000 lits d'USLD ont été transformés en places d'établissement d'hébergement pour personnes âgées dépendantes (EHPAD) et en SSR, les 30000 lits d'USLD restants ont été réservés aux patients nécessitant des soins médico-techniques importants. Des efforts ont également été mobilisés pour maintenir l'autonomie au domicile avec l'augmentation des capacités des services de soins infirmiers à domicile (SSIAD) et des services polyvalents d'aide à domicile. D'après les données de la caisse nationale de la solidarité pour l'autonomie (CNSA), le financement par la solidarité nationale de l'autonomie des personnes âgées a nettement augmenté ces dernières années : 23,6 milliards d'euros en 2017, contre 20,8 milliards d'euros en 2014 (19). Entre 2008 et 2018, la capacité d'hébergement permanente des personnes âgées est restée stable à 140 places pour 1000 seniors de plus de 75 ans, mais elle contient deux fois moins de moins de places d'USLD et plus de places en EHPAD et SSIAD (19). Concernant les EHPAD, l'âge moyen d'entrée est passé de 83 à 85 ans entre 2007 et 2015, les scores de dépendance des résidents ont augmenté, la durée moyenne de séjour s'est raccourcie et le taux d'occupation est resté très élevé à 98% (20).

  plusieurs paramètres (prévention, nouvelles techniques de prise en charge, alternatives ambulatoires, le raccourcissement des DMS et l'amélioration de la gestion des lits). L'activité y était représentée en nombre d'équivalent-journées d'hospitalisation (eqJ = nombre de journées d'hospitalisation complète + nombre de séjours ambulatoires). Le scénario se basant sur ces hypothèses volontaristes prévoyait que le nombre de séjours augmenterait régulièrement mais que le nombre d'eqJ allait légèrement diminuer puis se stabiliser en 2030. Les besoins en lits de court séjour MCO seraient donc inférieurs de 20% entre 2004 et 2030, mais nécessiteraient de poursuivre le développement des places d'hospitalisation ambulatoire. Une mise à jour de ce travail a été effectuée en 2017 (30) ; elle est arrivée à un résultat similaire, soit une stabilisation du nombre d'eqJ dont la part ambulatoire continuerait de croître permettant une diminution de 20000 lits de court séjour MCO entre 2012 et 2030, c'est-à-dire une poursuite de la tendance actuelle. Il est important de noter que ces travaux postulaient que l'aval du MCO (HAD, SSR, USLD) et les relais avec le secteur médico-social ainsi qu'avec les soins de ville s'adapteraient de façon optimale.

  ) selon les données disponibles (qui vont de 10 ans à 5 ans selon les paramètres). Les effectifs d'IDE libérales sont le seul paramètre d'offre de soins en nette croissance, mais cela semble aussi être la conséquence d'une perte d'attractivité de l'exercice hospitalier. Il existait une bonne corrélation entre les TCAM en croissance des paramètres d'activité et l'ONDAM qui en découle dans un système de tarification à l'activité, mais qui se module également selon les recettes disponibles des caisses d'assurance maladie. Plus globalement, l'offre de soins a stagné alors que l'activité et les dépenses ont augmenté conjointement, stimulées par le vieillissement démographique et surtout par les pathologies chroniques, ce qui a mécaniquement alourdi la charge de travail des soignants. Ceci est probablement le résultat d'efforts d'économie excessifs qui stérilisent l'investissement, d'une formation insuffisante des soignants et de politiques de réduction de capacité hospitalière sans toutefois renforcer suffisamment les alternatives ambulatoires et les soins de ville. En ajoutant aux données du problème une croissance plus soutenue de la population des plus de 75 ans dans les prochaines années, il devient peu probable que les seuls gains d'efficacité encore réalisables suffiront à résoudre la crise actuelle. L'objectif de stabiliser le rapport CSBM/PIB alors que la demande de soins augmente rapidement, tout en espérant maintenir la qualité des soins est théoriquement intenable sur le long terme. La saturation des SU serait donc le résultat logique de cette inadéquation croissante entre offre et demande de soins, obligeant de plus en plus de patients à recourir aux SU. Paradoxalement, cette situation est aussi un effet secondaire indissociable du progrès médical et de sa large accessibilité qui ont prolongé l'espérance de vie de plus de 15 ans depuis les années 50, mais avec une stagnation de l'espérance de vie sans incapacité depuis les années 2000. Aussi, il apparaît impossible d'établir des critères objectifs de limitation des soins, ou de demander à un médecin de le faire dans l'intérêt supposé de la collectivité plutôt que dans l'intérêt de son patient. Par ailleurs, l'extension des droits des patients s'est également accompagnée d'un risque judiciaire favorisant la réalisation trop systématique d'actes non pertinents. Concernant le financement de la santé, le principe même de tarification à l'activité favorise la multiplication d'actes excessifs, d'où les projets récents de financement par forfaitisations reposant sur les besoins de santé du territoire, mais qui complexifient encore plus le financement des structures de soins. En parallèle d'un investissement dans l'offre de soins, une réflexion sur le fonctionnement et sur les objectifs fondamentaux de notre système de santé semble également nécessaire.

Figure 1 :Figure 2 :

 12 Figure 1: Evolution du nombre de lits d'hospitalisation complète et de places d'hospitalisation partielle pour 1000 habitants en MCO, selon la spécialité

Figure 3 :Figure 4 :

 34 Figure 3: Taux de recours à l'hospitalisation complète ou partielle MCO par classe d'âge en 2016, exprimé en nombre de séjours pour 1000 habitants

Figure 5 :

 5 Figure 5: Comparaison de l'évolution mesurée des équivalents-journées d'hospitalisation et des lits en MCO avec les projections de DREES (en haut 2008, en bas 2017)

Figure 6 :

 6 Figure 6: Taux de croissance (abscisses), valeur (ordonnées), et sur-exécution (taille de la bulle rouge) ou sous-exécution (taille de la bulle verte) de l'ONDAM depuis 2004

Figure 7 :

 7 Figure 7: Dépense courante de santé par pays en 2017 exprimé en part de PIB et en coût par habitant à parité de pouvoir d'achat exprimée en euros avec la France comme référence

Figure 8 :

 8 Figure 8 : Nombre de lits en soins aigus pour mille habitants, et DMS, par pays

Figure 9 :

 9 Figure 9: Taux de croissance annuel moyen des différents paramètres de capacité, de ressources humaines, d'activité, de financement et de population, au cours de la dernière décennie (2008-2018)

séjours ambulatoires) et de séances (chimiothérapies, hémodialyses, etc.). Le nombre total de séjours et séances a augmenté d'environ 15% sur la période 2011-2018 soit un taux de croissance annuel moyen (TCAM) de 2% (Fig.2). On retrouve le même ordre de grandeur pour la croissance de la valeur économique MCO à prix constant entre 2009 et 2018, avec un TCAM de 2,1%[START_REF] Drees | Les dépenses de santé en 2018[END_REF]. Néanmoins, on note depuis 2017 un infléchissement inattendu de cette croissance d'activité MCO ; les croissances en volumes économiques n'étaient que de 0,9% en 2017 et 0,8% en 2018. Cette sous-consommation se concentre dans les séjours d'hospitalisation complète de médecine, et serait d'origine conjoncturelle liée à un rythme de croissance de la population gériatrique momentanément ralenti du fait de l'arrivée récente des classes creuses de la 2 ème guerre mondiale dans le 4 ème âge[START_REF]Avis du comité d'alerte sur le respect de l'ONDAM n°2019-2[END_REF].En hospitalisation complète, la relative stabilité du nombre de séjours masque en fait une augmentation de 25% des séjours de médecine contrebalancée par une baisse de 30% des séjours de chirurgie et d'obstétrique entre 2000 et 2018. La baisse du nombre de lits et de journées d'hospitalisation s'est accompagnée d'une baisse de la durée moyenne de séjour (DMS) de 6,2 jours en 1998 à 5,5 jours en 2017. Par ailleurs, le taux d'occupation des lits en MCO est resté stable depuis plusieurs années à 77% (en moyenne 80% dans les pays de l'Organisation de coopération et de développement économiques), mais avec des disparités selon les régions et les spécialités : 87% en médecine, 59% en chirurgie et 69% en obstétrique en 2018[START_REF]Statistique annuelle des établissements de santé : Période 2000-2012[END_REF].

d'urgence et de réanimation (SMUR)Entre 2013 et 2018, les paramètres d'activité ont régulièrement augmenté, notamment les passages en SU (+17,1%), les séjours en unité d'hospitalisation de courte durée (+23,6%), les hospitalisations via les SU (+6,8%), les appels au centre 15 (+15%), les dossiers de régulation médicale (+22%) et les sorties primaires ou secondaires d'unités mobiles d'urgence et de réanimation (+4,9%). Concernant les ressources humaines, entre 2013 et 2018, les ETP ont augmenté chez les médecins (+10,9%), chez les infirmières (+16,3%) et chez les aidessoignants (+13,5%)[START_REF]Statistique annuelle des établissements de santé : Période 2000-2012[END_REF].Soins de ville et secteur médico-socialSoins de villeEn ville, on observe une baisse continue de la démographie des médecins généralistes en activité selon un TCAM de -0,9% pour un effectif de 87800 en 2018, dont la répartition géographique reste hétérogène (16). Ainsi, 5,4 millions de français seraient sans médecin traitant[START_REF] Revel | directeur général de l'Assurance maladie : « L'accès au médecin traitant me préoccupe[END_REF]. Parallèlement, les volumes de soins de médecine générale ont très légèrement augmenté selon un TCAM de 0,49% depuis 2009[START_REF] Drees | Les dépenses de santé en 2018[END_REF]. Pour les autres spécialités exerçant en libéral, les effectifs sont restés stables, mais les volumes de soins ont nettement augmenté selon un TCAM de 1,7% depuis 2009, et les délais de rendez-vous peuvent être assez longs

[START_REF]Projet de loi de financement de la sécurité sociale pour[END_REF]. De même, l'estimation des mesures d'économies réalisables est un exercice complexe dont les résultats servent à piloter l'évolution des budgets des différentes branches de l'ONDAM[START_REF]Projet de loi de financement de la sécurité sociale pour[END_REF]. Ces efforts d'économies représentent 3,8 milliards d'euros en 2019[START_REF]Rapport économique, social et financier pour le projet de loi de finances 2019[END_REF], afin de ramener l'évolution tendancielle de l'ONDAM de 4,5% à environ 2,5%. Depuis 2011, le cumul des efforts d'économies demandé était d'environ 25 milliards d'euros. En l'absence de ces mesures d'économies, l'ONDAM aurait évolué « naturellement » selon un TCAM de 3,9% au lieu de 2,35%. Autrement dit, chaque année, le législateur demande au système de santé d'augmenter son efficacité pour limiter fortement une croissance tendancielle des dépenses de santé qui serait bien plus élevée que la croissance du produit intérieur brut (PIB) et des recettes des caisses d'assurance maladie.En MCO, ces efforts d'économies reposent en partie sur l'évolution des tarifs des groupes homogènes de séjours (GHS), base de la tarification à l'activité (T2A). Ainsi, entre 2010 et 2019, l'évolution globale des tarifs a presque systématiquement baissé[START_REF]Circulaires du mois de mars de la DGOS relatives à la campagne tarifaire des établissements de santé 2011 à 2019[END_REF] et évolué selon un TCAM de -0,37% dans le secteur public et de -0,32% dans le secteur privé. En 2017 et 2018, l'activité étant moindre que prévue avec des tarifs à la baisse, le déficit des hôpitaux a nécessité le déblocage de plusieurs centaines de millions d'euros sans pour autant dépasser l'ONDAM[START_REF]Rapport sur l'application des lois de financement de la sécurité sociale[END_REF]. En 2019, la faible augmentation des tarifs[START_REF]Circulaires du mois de mars de la DGOS relatives à la campagne tarifaire des établissements de santé 2011 à 2019[END_REF], en partie permise par la

Ressources humaines

L'emploi hospitalier (regroupant MCO, psychiatrie, SSR et unités de soins de longue durée : USLD) est mesuré par le nombre d'équivalents-temps-pleins (ETP). Entre 2013 et 2018, on assiste à une stagnation du nombre d'ETP administratifs et à une baisse du nombre d'ETP d'agents des services hospitaliers (-6,1%) en contrepartie d'une hausse des ETP des aides-soignants (+0,7%), des infirmières (+3,8%) et des médecins (+6,1%) (8).

Structures des urgences, Services d'aide médicale urgente (SAMU) et services mobiles