C Nkoua

C Josse

A Proietti

R Basseguy

C Blanc

Corrosion behaviour of the microbially modified surface of 5083 aluminium alloy

Keywords: A. aluminium, A. intermetallics, B. polarization, C. pitting corrosion, C. microbiological corrosion

The influence of microorganism-induced surface modifications on the corrosion behaviour of 5083 H11 aluminium alloy was studied. Pre-immersion in natural seawater (between 15 days and 2 months) led to the growth of a dense oxy-hydroxyde layer on the AA5083 surface. This layer led to a decrease in the cathodic and corrosion current densities, and an increase in the passivity domain on the polarisation curves plotted in chloride-containing sulphate solution for pre-immersed samples compared to bare samples. This improvement in the corrosion behaviour of AA5083 depended on the alloy microstructure, and the sunlight exposure during the pre-immersion in natural seawater.

Introduction

Al-Mg alloys of the 5xxx series are widely used for marine industry because of their high corrosion resistance, lightness and excellent processing performance [1 4]. However, they still remain susceptible to pitting and intergranular corrosion (IGC) when exposed to chloride media, despite the spontaneous formation of a thin protective oxide film on their surface [2, 5 8]. It is therefore necessary to improve their corrosion resistance, and several strategies can be used.

From a global point of view, conversion coatings can be considered as one of the most successful and cost-effective alternatives to efficiently increase the corrosion resistance of these metals and abundant literature exists on this topic [9 14]. However, towards a more constraining legislation as REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals), the surface treatment industry looks for more innovative and environmentally friendly alternatives and nothing is greener than Nature, which inspires new concepts to improve corrosion resistance performance.

Natural seawater is a complex corrosive medium composed of salts, dissolved gases and organic matter related to living organisms. The presence of microorganisms may cause either corrosion acceleration or inhibition, processes known as Microbiologically Influenced Corrosion (MIC) or Microbiologically Influenced Corrosion inhibition (MICI), respectively [15 19]. MIC in seawater has been largely studied for stainless steels, and the ennoblement of the open circuit potential (OCP) observed in all seawaters was proved to be the consequence of the biofilm formation [START_REF] Scotto | Correlation between marine biofilm structure and corrosion behaviour of stainless steels in sea water[END_REF]. Indeed, Scotto et al. followed the OCP of stainless steel during immersion in seawater in Genoa Harbour, and showed that the introduction of sodium azide (NaN3), a chemical that destroyed microorganisms, stopped the ennoblement process (OCP drops down) [START_REF] Scotto | The ennoblement of stainless steels in seawater: a likely explanation coming from the field[END_REF]. To explain the OCP ennoblement that increases the risks of pitting corrosion, the most proposed mechanism is the catalysis of oxygen reduction reaction caused by the biofilm [START_REF] Basseguy | Electroactive biofilms[END_REF]. MICI process has also been studied for steels in presence of marine bacteria [23 25]. The microorganisms that adhere to the surface of steels can promote a bio-mineralisation process: the layer formed can provide a barrier against corrosive media and inhibits corrosion.

Contrary to steels, only few studies of MIC/MICI in marine environment can be found for Al alloys. Some of them are carried out in natural seawater [START_REF] Marques | AA 5083 Al Alloy Corrosion in Estuarine Environment[END_REF][START_REF] Marques | Marine Microorganisms and Metal inter Eurocorr[END_REF], and some in laboratory [ 28 30].

Guan et al. found that sulphate-reducing bacteria (SRB) could accelerate the uniform and localised corrosion of 5052 Al alloy leading to surface modification due to SRB biofilm formation on the sample surface [START_REF] Guan | Interaction between sulfatereducing bacteria and aluminum alloys Corrosion mechanisms of 5052 and Al-Zn-In-Cd aluminum alloys[END_REF]. On the contrary, Shen et al. studied the impact of Bacillus subtilis on the corrosion processes of Al alloy 2A14, and showed that the corrosion rate was strongly reduced in the presence of microorganisms [START_REF] Shen | Study of pitting corrosion inhibition effect on aluminum alloy in seawater by biomineralized film[END_REF]. This short review showed that the corrosion behaviour of Al alloys in the presence of marine microorganisms depends on the type of microorganisms, some inhibiting the corrosion processes, whereas others lead to an acceleration of the damage. However, few works have studied the impact of bio-mineralization on Al alloy. Marques et al. studied the surface of 5083 H11 Al alloy samples after a two-year exposure in natural seawater [START_REF] Marques | AA 5083 Al Alloy Corrosion in Estuarine Environment[END_REF]. They showed that a layer formed in the immersed zone and inhibited corrosion, whereas the non-immersed zone was covered by pits. In the same way, Jaume et al. studied the influence of the marine microbial population on the corrosion behaviour of AA5083 H11 specimen [START_REF] Jaume | Surface modification of 5083 aluminum-magnesium induced by marine microorganisms[END_REF]. They showed that the microbial activity led to the formation of a double-structure layer: a dense, amorphous inner layer and a more porous outer layer on the Al surface, which ultimately protected it against further corrosion. These latter results suggested that it could be interesting to exploit the surface modifications due to exposure to the marine environment as a protective treatment against corrosion.

In this framework, the aim of the present work is to analyse how surface modifications, induced by a pre-immersion in natural marine environment of an AA5083 Al-Mg alloy, impact the subsequent corrosion behaviour of the alloy. This study was carried out on two Al-Mg alloy sheets of the same grade, but of different thicknesses associated with differences in microstructure. The surface modifications of the alloy after different immersion times in natural seawater were observed using scanning electron microscopy (SEM). The composition of the layer formed on the alloy surface was investigated by energy dispersive spectroscopy (EDS) and Glow Discharge Optical Emission Spectroscopy (GDOES). Then, polarisation tests were carried out on pre-immersed samples to further analyse the protective effect of the layer formed on the alloy surface. The mechanisms of layer formation and growth during pre-immersion in natural seawater were not the focus of the present study; in particular, understanding the action of the microorganisms on layer growth was not the focus of the study. However, control tests were also performed with samples pre-immersed in sterilised seawater to clearly demonstrate that the improvement in corrosion resistance resulted from exposure to seawater under biotic conditions.

Materials and methods

Materials

The material studied is an Al-Mg 5083 H11 alloy. It was provided as hot-rolled plates with two thicknesses: 1 mm and 7 mm. The weight percentage of alloying elements is shown in Table 1. Some slight differences are observed between the 7 mm plate and the 1 mm plate, with higher Mg, Fe and Si amounts for the 7 mm plate.

The corrosion behaviour of the plates (100 x 200 mm 2 rectangles in the rolling plane) was studied in the as-received state (samples referred to as bare samples), and after pre-immersion in natural Genoa seawater (biotic conditions, samples referred to as samples pre-immersed in Genoa seawater). Some experiments were also performed with samples pre-immersed in sterilised seawater (abiotic conditions, samples referred to as samples pre-immersed in sterilised seawater) for comparison. Prior to pre-immersion (in sterilised and Genoa seawater), the asreceived plates were cleaned with 99% purity ethanol. For samples pre-immersed in sterilised seawater, the as-received plates were immersed vertically for 1 month in a closed container containing 6 L of sterilised seawater. The seawater was collected near the coast at Gruissan (France), and was sterilised through a 0.2 µm filter. An air bubbling system was installed to ensure constant oxygenation and to create agitation similar to that in real conditions, but without microorganisms. After pre-immersion in sterilised seawater, the samples were washed with tap water and then distilled water with a vegetable sponge before being stored in airtight boxes with silica gel. For samples pre-immersed in Genoa seawater, the as-received plates were immersed vertically in seawater at the Genoa Experimental Marine Station (GEMS) -CNR-IAS for different periods of time: 15 days, 1 month, and 2 months from September to November. In this configuration, where the plate is immersed vertically, one side of the plate is exposed to the direct light of the sun and the other one faces the shadow: the sides are called light side and dark side, respectively. The seawater temperature, salinity, conductivity and pH were recorded once a week; the mean values were 22.1 ± 2.4 °C for temperature, 37.5 ± 0.9 % for salinity, 54 ± 4 mS cm -1 for conductivity, and 8.1 ± 0.1 for pH. After pre-immersion in Genoa seawater, the samples were rinsed with tap water, then air-dried and wrapped in absorbent paper before being stored in airtight boxes with silica gel and sent to our laboratory. Prior to use, they were washed in tap and then distilled water with a vegetable sponge and a spatula to remove the macrofouling on the sample surface. They were then rinsed with distilled water and dried in hot air. They were again stored in airtight boxes with silica gel until used for testing. The first laboratory tests started about 2 months after the pre-immersion stage was completed.

In the following, for both bare and samples pre-immersed in Genoa seawater, different planes were studied: the rolling plane for 1 mm and 7 mm plates, and a transverse plane for the 7 mm plate only, referred to as 1 mm L-LT, 7 mm L-LT and 7 mm LT-ST, respectively, where L, LT and ST are the rolling, long transverse and short tranverse directions, respectively. For samples pre-immersed in sterilised seawater, only the rolling plane of the 1 mm plate was studied. It was assumed that this set of experiments was sufficient to assess the effect of microorganisms on surface changes during pre-immersion in natural seawater. Test specimens with a cylindric shape (disc with a 45-mm diameter in the rolling plane) for 7 mm and 1 mm L-LT, and parallelepipeds (20 x 7 mm 2 rectangular surface in the LT-ST plane) for the 7 mm LT-ST plane were machined from the plates. For the samples pre-immersed in Genoa seawater, specific attention was paid to the surface during machining in order not to damage the microbially-modified surface. The same precautions were taken for the samples pre-immersed in sterilised seawater.

Analysis of the microstructure of AA5083 alloy

To identify the type, the global distribution and the morphology of coarse intermetallic particles (IMCs) of AA5083 Al alloy, bare samples were mounted in an epoxy resin, ground using SiC abrasive paper down to 2400, then polished with diamond paste down to 1 µm and finally using a colloidal silica (OPS). An optical microscope (OM, MA200 Nikon) and a Scanning Electron Microscope (SEM, FEI Quanta450) coupled with Energy Dispersive X-ray Spectroscopy (EDS Bruker) were used to characterise IMCs. For each sample, approximately 25 optical images were analysed using ImageJ software. A cumulative average of the fraction of surface area covered by IMCs was calculated.

The grain structure was analysed using OM. Before observations, samples were polished as before, then electrolytically etched with a 3.5% vol. HBF4 reagent.

Characterisation of the samples pre-immersed in Genoa seawater and in sterilised seawater

A SEM (FEI Quanta450) coupled with EDS (EDS Bruker) was used to analyse the surface of the samples pre-immersed in Genoa seawater and in sterilised seawater, and to determine the thickness of the layer formed during pre-immersion. For the latter, observations were made using cross-section samples polished following the procedure used for the analysis of IMCs.

In addition, in order to identify the composition of the layer, EDS mapping on cross-sections (SEM/FIB FEI HELIOS 600i coupled with EDS, Raimond Castaing Microanalysis Center, Toulouse, France) and Glow Discharge Optical Emission Spectroscopy analyses (GDOES GDprofiler 2 of the brand Horiba, Raimond Castaing Microanalysis Center, Toulouse, France) were performed. The GDOES depth profiles were performed at 30 W and 650 Pa. Concerning the samples pre-immersed in Genoa seawater and in sterilised seawater, no specified preparation was made before electrochemical tests, except for the washing step described in section 2.1.

The corrosion behaviour was studied by plotting polarisation curves in a 0.001 M NaCl + 0.1 M Na2SO4 solution using a three-electrode cell. In the present study, our objective was to evaluate the electrochemical response of samples covered with the layer formed during preimmersion in natural seawater: for this purpose, an electrolyte of moderate aggressiveness had to be chosen. The durability of this layer could have been tested in a common 3.5 % NaCl solution.

However, such an aggressive electrolyte would have masked interesting data regarding the electrochemical response of bare specimens compared to samples pre-immersed in Genoa seawater with a modified surface. An Ag/AgCl/KCl (saturated) electrode and a platinum foil were used as reference and counter electrodes, respectively. Nevertherless, for easy comparison with literature data, in the following, all potentials are quoted in relation to the saturated calomel electrode (SCE). Samples were vertically positioned in the electrochemical cell; as indicated before, for the 1 mm and 7 mm L-LT samples, the surface exposed to the electrolyte was a 45mm disc, whereas, for the 7 mm LT-ST sample, it was a 7 x 20 mm 2 rectangle. Due to their shape and/or thickness, 7 mm LT-ST and 1 mm L-LT samples were mounted into an epoxy resin with a Cu wire for electrical connection, before electrochemical tests. Before polarisation tests, the samples were immersed at their open circuit potential (OCP) for 30 minutes to figure out a near stationary state. The polarisation curves were plotted at a scanning rate of 0.139 mV/s from for the cathodic curves, and from OCP -20mV to a potential corresponding to a current density of 1 mA cm -² for the anodic curves. Each test was reproduced at least two times for reproducibility. After the electrochemical measurements, surface analyses were conducted by using OM (MA200 Nikon) and SEM (FEI Quanta450). Furthermore, pit propagation was studied in the same electrolyte, i.e. in a 0.001 M NaCl + 0.1 M Na2SO4 solution: samples were maintained at OCP for 5min or 24 hours, then polarised in the anodic domain at a fixed current density of 0.1 mA cm -² for 10 min to have a similar charge passed through all samples. The pit size and density were determined by using a confocal microscope (Sensofar S Neox), and Electron Backscatter Diffraction (EBSD, Oxford Instruments HKL EBSD System mounted on a Field-Emission Gun (FEG) SEM

) was used to characterise pit propagation.

Results and discussion

Study of the microstructure of AA5083 H11 Al alloy

Firstly, the IMCs present in the different samples of AA5083 H11 alloy were observed by OM (Fig. 1). Two types of IMCs were observed on the surface having a dark and a light grey appearance. EDS analyses were performed for IMC identification: the dark particles were identified as Mg-rich particles, containing also Si, and the light grey were identified as Fe-rich particles, corresponding to Al-Fe-Mn phases. The composition of these IMCs is in agreement with that observed for the same alloy by various other authors [START_REF] Engler | Control of second-phase particles in the Al-Mg-Mn alloy AA 5083[END_REF][START_REF] Sheppard | Modification pf cast structures in Al Mg alloys by terminal treatments[END_REF] who have reported that these phases are likely Al6(Mn,Fe) and Mg2Si. The fraction of surface area covered by the particles was also determined (Fig. 2). For the 7 mm LT-ST samples, the analysis was performed both at the surface (referred to as skin, near the L-LT plane) and in the core of the plate. For all samples, Al-Fe-Mn particles were found to be more abundant than Mg-rich particles. Moreover, whatever the particles, a higher fraction of surface area covered by the particles was determined for the 7 mm plate by comparison to the 1 mm plate, which is in agreement with the chemical composition of the two plates. Regarding the 7 mm plate, differences were observed depending on the plane, with more particles in the LT-ST plane compared to the rolling plane (L-LT). The results thus showed differences in the microstructure between the 1 mm and the 7 mm plates, and differences between the different planes of the 7 mm plate. Therefore, it was relevant to determine whether those differences could have a significant influence on the behaviour of the samples during pre-immersion in seawater and on their resulting corrosion behaviour.

Characterisation of the modified surfaces after pre-immersion in Genoa seawater and sterilised seawater

Fig. 3 shows OM images of the 1 mm L-LT sample surface before (bare sample, Fig. 3 (a))

and after pre-immersion in Genoa seawater for 15 days (Fig. 3 (c) and (f)), 1 month (Fig. 3 (d) and (g)) and 2 months (Fig. 3 (e) and (h)). Similar observations were obtained for 7 mm L-LT and LT-ST samples, but the results are not given for brevity. For all samples, first visual observations showed differences between the surfaces of bare samples and samples preimmersed in Genoa seawater.

The modifications occurring on the sample surface during pre-immersion in Genoa seawater were associated with the growth of a layer that could be observed from 15 days of exposure (Fig. 4). For all samples, the layer was found inhomogeneous for both the light and dark sides, but this was more true for the dark side. Furthermore, when the duration of pre-immersion in Genoa seawater was increased, the layer was more homogeneous for both dark and light sides.

However, a few pits were observed after 2 months of pre-immersion in Genoa seawater for the dark side (arrows in Fig. 3 (h)). An OM image of the surface of the 1 mm L-LT sample after a one-month pre-immersion in sterilised seawater is also given for comparison in Figure 3 (b). A layer was also observed on the sample surface, but first visual inspection suggested that this layer was thinner than after a one-month pre-immersion in Genoa seawater. In addition, the sample surface was covered with numerous pits, suggesting that the layer formed in sterilised seawater was less protective than that formed in Genoa seawater for the same pre-immersion time.

To go further in the characterisation of the modified surfaces, the composition of the layers formed during pre-immersion in Genoa seawater was determined. The distribution of the main chemical elements detected in the layers was analysed by plotting, from the layer to the substrate, SEM EDS maps using cross-section samples (Fig. 4), and GDOES profiles (Figs. 5, 6 and 7). For example, Fig. 4 shows the results obtained for the layer formed on the light side of the 1 mm L-LT sample (Fig. 4 to distinguish between the layer formed during pre-immersion and the interfacial zone on SEM images. GDOES profiles also showed the presence of H inside the layer with H enrichment at the extreme surface as well as in the interfacial zone. The results thus showed that the layer was an Al oxy-hydroxide with a Mg enrichment at the extreme surface probably linked to the formation of a Mg oxide or hydroxide, in agreement with the SEM EDS maps described above (Fig. 4).

Other elements were also detected during GDOES analyses, i.e. Ca and S, with Ca enrichment in the interfacial zone and S enrichment both in the extreme surface and in the interfacial zone. months of pre-immersion in Genoa seawater, which allows to determine the influence of the preimmersion duration time on the composition and thickness of the layer formed during preimmersion. The same elements were detected, and the global shape of the profiles for all the elements was similar whatever the pre-immersion time in Genoa seawater, so that the layer formed during pre-immersion, an interfacial zone and the substrate were identified for all profiles as previously done. However, the abrupt increase in the Al signal was shifted from 2 to 2.4 and 2.7 µm of sputtering depth, and the plateau corresponding to high Al amount was reached after 3.5, 4.5 and 5 µm for layers formed after 15 days, 1 month and 2 months, respectively. In parallel, the abrupt decrease in O signal was shifted from 2 to 2.4 and 2.7 µm of sputtering depth for layers formed after 15 days, 1 month and 2 months, respectively, whereas the intensity of the O signal on the first plateau increased when the duration time of preimmersion increased. The results thus showed a thickening of the layer when the duration time of pre-immersion in Genoa seawater was increased. Moreover, it was of interest to note the significant O enrichment at the extreme surface after 1 and 2 months. This could be related to the Mg enrichment at the extreme surface of the layer formed during pre-immersion, which was also found to increase when the layer thickened. The shift of the GDOES profiles was also observed for the other elements; moreover, the intensity of the S, H and Ca peaks in the interfacial zone increased when the pre-immersion duration time was increased. All the observations strengthened the conclusion that the thickness of the layer increased when the duration of preimmersion in Genoa seawater was increased.

Then, the results obtained for the 7 mm L-LT sample were compared with those obtained for the 1 mm L-LT sample. Mg GDOES profiles plotted for the 1 mm L-LT sample was added in an insert in Fig. 5 (Mg signal). Comparison with the Mg profiles plotted for the 7 mm L-LT sample showed not only that the Mg amount in the extreme surface was lower, but also that the Mg enrichment occurred for longer time of pre-immersion (1 month) for 1 mm L-LT sample compared to 7 mm L-LT sample. This result was consistent with the composition of the 2 plates (Table 1, lower Mg content in 1 mm sample) and showed an influence of the alloy microstructure on layer growth during pre-immersion in Genoa seawater. To go further in the analysis of the influence of the alloy microstructure on the thickness and composition of the layer formed during pre-immersion, GDOES profiles of the other elements were also plotted for the 1 mm L-LT sample. Results are given after 15 days of pre-immersion in Genoa seawater in Figure 6. They allowed to distinguish between the layer formed during pre-immersion, an interfacial zone and the substrate for the 1 mm L-LT sample as for the 7 mm L-LT sample.

However, as previously noted for the Mg signal, results showed that the amounts of S and H

were globally more important in the layer formed on the 7mm L-LT sample than on the 1 mm L-LT sample. Furthermore, the analysis of all the signals, except this of O, suggested that the layer was thicker on the 7 mm sample than on the 1 mm sample. When the pre-immersion in Genoa seawater was longer, i.e. 1 month (Fig. 7), a thickening of the layer formed on the 1 mm L-LT sample compared to the sample pre-immersed for 15 days (Fig. 6) was observed, as it was for the 7 mm L-LT sample. It was also interesting to note that, after 1 month of pre-immersion in Genoa seawater, the abrupt variations of the Al and O signals were observed for similar sputtering depths for the 1 mm and 7 mm samples, suggesting that, for this pre-immersion duration time, the layers formed on the 1 mm L-LT and 7 mm L-LT samples had a similar thickness. However, as observed in Fig. 4, the heterogeneity of the layer thickness could not be neglected. Therefore, a statistical analysis of the layer thickness had to be performed to strengthen the conclusions (see comments on Fig. 8

below).

Analyses were also performed for the 7 mm LT-ST samples: globally, the results were similar to those observed for the 7 mm L-LT samples. They are not given for brevity.

The results described above for the samples after pre-immersion in Genoa seawater were in accordance with other studies [START_REF] Jaume | Surface modification of 5083 aluminum-magnesium induced by marine microorganisms[END_REF][START_REF] Canepa | Corrosion behaviour of aluminium alloys in deep-sea environment: A review and the KM3NeT test results[END_REF]. Jaume et al. [START_REF] Jaume | Surface modification of 5083 aluminum-magnesium induced by marine microorganisms[END_REF] observed a double-structure layer for the same AA 5083 Al-Mg alloy pre-immersed in natural seawater with salt marsh used as a source of microorganisms: a dense and amorphous internal layer rich in O, Si and Ca and a porous external layer rich in Mg. Shen et al. [START_REF] Shen | Study of pitting corrosion inhibition effect on aluminum alloy in seawater by biomineralized film[END_REF] showed the formation of a protective layer of CaMg(CO3)2 on 2A14 Al alloy in seawater in presence of the bacteria B. subtilis: the formation of this chemical compound could be due to interactions of microbial activities and seawater containing Ca and Mg ions. Other authors have also observed the formation of an organicinorganic hybrid film composed of organic matter (characterised by C-H and O-H bond peaks)

and calcite ((Mg0.064Ca0.936)CO3) on steel after 14 days of immersion in marine broth medium containing different strains of bacteria Pseudoalteromonas lipolytica [START_REF] Liu | Marine Bacteria Provide Lasting Anticorrosion Activity for Steel via Biofilm-Induced Mineralization[END_REF]. Moreover, Dupraz et al. [START_REF] Dupraz | Processes of carbonate precipitation in modern microbial mats[END_REF] also showed the precipitation of CaCO3 due to the activity of microorganisms, producing CO2 at concentrations allowing the formation of CaCO3. In addition, the presence of S in our samples after pre-immersion in Genoa seawater could also be due to the metabolic activity of a specific microorganism SRB, which is conventionally attributed to the corrosiveness of H2S [START_REF] Guan | Influence of sulfate-reducing bacteria on the corrosion behavior of 5052 aluminum alloy[END_REF]. However, our analyses did not distinguish between sulphate and sulphide species; therefore, the detection of S could also result solely from the incorporation of sulphate from Genoa seawater.

To support the above explanations, and although the mechanisms of layer growth during preimmersion in Genoa seawater and the influence of microorganisms on this process were not the scope of this study, GDOES profiles were also plotted for the 1 mm L-LT sample after 1 month of pre-immersion in sterilised seawater for comparison (Fig. 7). Overall, the same elements (Al, O, S, H, Ca and Mg) were detected. The Al and O signals showed a global shape similar to that observed for the samples pre-immersed in Genoa seawater, which allowed the layer, an interfacial zone and the substrate to be distinguished. The H enrichment at the sample surface showed that the layer formed in sterilised seawater was also an oxy-hydroxide layer. However, the abrupt increase in the Al signal and abrupt decrease in O signal occurred after only 1 µm of sputtering depth for the sample pre-immersed for 1 month in sterilised seawater compared to 2.5 µm for the 1 mm L-LT sample pre-immersed for 1 month in Genoa seawater: this showed that the layer formed in sterilised seawater was significantly thinner than that formed during preimmersion in Genoa seawater.

In addition, the layer formed in sterilised seawater also contained S, but, in this case, this could not be attributed to the activity of microorganisms. This result therefore showed that S, probably from sulphate ions contained in sterilised seawater, could be incorporated into the layer during its growth. This suggested, as mentioned earlier, that the amount of S detected for the sample pre-immersed in Genoa seawater was probably related, at least in part, to S incorporated as sulphate ions from Genoa seawater. However, the presence of S in the form of sulphide ions in the layer formed in Genoa seawater, due to the activity of microorganisms could not be excluded; this hypothesis was strengthened by the higher amount of S in the layer formed in Genoa seawater compared to that formed in sterilised seawater. Similarly, the amount of Ca was significantly lower in the layer formed in sterilised seawater compared to that formed in Genoa seawater. Again, this could be explained by considering that, for the sample immersed in sterilised seawater, the amount of Ca was only related to the presence of Ca species in the seawater, Ca being simply incorporated into the layer during its growth. On the contrary, the high amount of Ca in the layer formed on the sample pre-immersed in Genoa seawater suggested that Ca was incorporated into the layer from the Ca species present in seawater, but its presence inside the layer was also the result of the activity of the microorganisms. Finally, it was also important to note the very high amount of Mg within the layer formed in sterilised seawater, whereas Mg was only found to be segregated at the extreme surface for the samples preimmersed in Genoa seawater. The result confirmed that the sample was much more reactive in the absence of microorganisms, and that the layer formed was significantly less protective than that formed in Genoa seawater.

In order to obtain consolidated data concerning the thickness of the layer formed during preimmersion, this structural parameter was determined by SEM observations carried out on crosssections prepared for the samples pre-immersed in Genoa seawater and sterilised seawater. The results are given in Fig. 8 for the light and dark sides of 1 mm L-LT, 7 mm L-LT and 7 mm LT-ST samples after 15 days, 1 month and 2 months of pre-immersion in Genoa seawater. For comparison, the results were also presented for the 1 mm L-LT sample pre-immersed for 1 month in sterilised seawater. For each condition, e.g. the 1 mm L-LT sample pre-immersed in Genoa seawater for 1 month, measurements were made on two specimens prepared at the same time with exactly the same procedure. For each specimen, the thickness of the layer was measured over a length of about 1.4 cm, which corresponded to about 20 measurements per specimen. This means that, for each condition, 40 measurements were made. This statistical analysis is illustrated by a boxplot representation. Considering that the layer thickness was not homogeneous (Fig. 3), this led to a large dispersion on the values given in Figure 8. However,

given this statistical analysis of the thickness, we assumed that the average values obtained for thickness were relevant.

First, concerning the light side, Fig. 8 (a) showed that a thicker layer formed on the 7 mm plate compared to the 1 mm plate regardless of the pre-immersion duration time, which confirmed the previous results (Fig. 6). Furthermore, for the 7mm plates, differences in the layer thickness were observed between the both L-LT and LT-ST planes. Indeed, for long preimmersion times, i.e. after 1 and 2 months of immersion, the layer was thicker on the 7 mm LT-ST plane than the 7 mm L-LT plane. An influence of the sunlight exposure (direct or shadow) on the growth of the layer was observed, with in particular a stronger scattering in the thickness values for the dark side compared to the light side, in agreement with previous OM observations (Fig. 3). However, globally, even for the dark side, the layer was thicker for the 7 mm LT-ST samples compared to both 1 mm L-LT and 7 mm L-LT samples, for the longest pre-immersion times. Finally, statistical analysis of the layer thickness confirmed that the layer formed in sterilised seawater was significantly thinner than that formed in Genoa seawater, as previously

shown by the GDOES profiles (Fig. 7). For all bare samples, the cathodic curves presented a globally similar shape; the main cathodic reaction, at least for potentials between the corrosion potential Ecorr and Ecorr 200 mV, corresponded to the reduction reaction of dissolved oxygen. The anodic curves showed a passivity plateau followed by a sharp increase in the anodic current densities. This reflected corrosion kinetics controlled by the properties of the passive film. However, since the current measured in the passive region is a function of the scan rate and will only approach the corrosion current for an infinitesimally slow scan rate, the corrosion current density, Jcorr, was calculated by extrapolating the cathodic currents in Tafel plots (Fig. 9 and10).

Corrosion resistance

OM observations (not shown) performed after the potentiodynamic polarisation tests in the cathodic domain showed dissolution phenomena around Al-Fe-Mn IMCs due to oxygen reduction at these sites, in agreement with literature showing that the cathodic reactions take place preferentially on those particles which are more noble than the surrounding matrix [41 43]. After polarisation in the anodic domain, numerous pits were observed on the sample surface, which evidenced that the passivity breakdown was related to pitting corrosion. Therefore, the breakdown potential observed in the anodic domain was referred to as Epit, and the width of the passivity plateau, referred to as E where E = Epit -Ecorr, was representative of the resistance to pitting corrosion of the sample. Due to their nobility, the Al-Fe-Mn particles were preferential pit initiation sites with, as said previously, dissolution of the surrounding matrix.

Characteristic values of Jcorr, Jcath, Ecorr and E are reported in Fig. 11. For all bare samples (dark grey bars), Ecorr values were similar and varied between -0.71 and -0.75 VSCE. Jcath values correspond to the cathodic current densities measured at a potential equal to Ecorr 120 mV.

Higher Jcath values were measured for the 7 mm L-LT and 7 mm LT-ST samples compared to the 1 mm L-LT sample. This was in agreement with the higher fraction of surface area covered by Al-Fe-Mn IMCs in the 7 mm samples compared to the 1 mm L-LT sample (Fig. 2). Moreover, despite the large standard deviation observed, Jcorr values were significantly higher for the 7 mm samples compared to the 1 mm sample, which was also in agreement with the higher density of IMCs in the 7 mm samples (Fig. 2), those particles promoting the destabilisation of the passive film. Surprisingly, this did not lead to significant variation in Epit, and thus E values.

Nevertheless, considering Jcorr values, it could be concluded that the 7 mm samples had a lower corrosion resistance than the 1 mm L-LT sample, which was correlated with higher fraction of surface area covered by IMCs.

For the samples pre-immersed in Genoa seawater, current-potential curves plotted (Fig. 9 and 10)

were similar to those of the bare samples, indicating that the presence of the layer formed during the pre-immersion in Genoa seawater did not change significantly the cathodic and anodic processes that occurred during the polarisation in the chloride-containing sulphate solution. Therefore, the same electrochemical parameters were extracted from the current-potential curves for the samples preimmersed in Genoa seawater (Fig. 11).

First, attention was paid to the light side for which the layer formed during pre-immersion was previously observed as being more homogeneous than that formed on the dark side. Results showed that no clear tendency was noted concerning Ecorr values for all the samples pre-immersed in Genoa seawater compared to the corresponding bare samples. However, for the samples pre-immersed in Genoa seawater, the current densities were lower compared to the bare samples, both in the cathodic and anodic domains. Indeed, the samples pre-immersed in Genoa seawater were associated with significantly lower Jcath values compared to the corresponding bare samples, for both 1 mm and 7 mm samples, and whatever the duration time of pre-immersion in seawater. Globally, the pre-immersed 1 mm L-LT samples showed the lowest Jcath values, whereas the pre-immersed 7 mm LT-ST samples had higher Jcath values compared to both the pre-immersed 1 mm L-LT and 7 mm L-LT samples.

Moreover, lower Jcorr values were measured for all the samples pre-immersed in Genoa seawater compared to the corresponding bare samples. Finally, for all the samples pre-immersed in Genoa seawater (except for the 7 mm L-LT sample pre-immersed for 2 months), higher E values associated with an extended passivation domain were calculated whatever the pre-immersion duration time compared to the corresponding bare samples. Therefore, the results showed that the layer formed during the pre-immersion in Genoa seawater could be considered as an efficient protection against corrosion for the AA5083 samples, for both 1 mm and 7 mm samples. No clear tendency was observed concerning the influence of the pre-immersion duration time on the electrochemical parameters values, but this was assumed to be related to the lack of homogeneity of the layer formed during pre-immersion. Besides, when attention was paid to the dark sides, it could be observed that globally similar conclusions as for the light side could be drawn. From a global point of view, the standard deviation calculated for each parameter, i.e Ecorr, corr and Jcath, was quite high for the samples pre-immersed in Genoa seawater. This might be a marker of the strong heterogeneity of the layer which could be explained by referring to the heterogeneity of the microstructure of the substrate, i.e. AA5083, with probably a significant influence of microstructural features of the substrate on the local properties of the layer formed during pre-immersion. Therefore, it was quite difficult to deduce, from the current-potential curves, a significant influence of lightness or microstructure on the protection efficiency of the layer formed during pre-immersion. However, as indicated previously, the comparison between bare and samples pre-immersed in Genoa seawater clearly indicated the positive role of the layer formed during pre-immersion on the corrosion behaviour of AA5083, for both 1 mm and 7 mm samples.

In addition, the results showed that the Jcorr and Jcath values were higher for the 1 mm L-LT sample pre-immersed in sterilised seawater for 1 month than for the 1 mm L-LT sample pre-immersed in Genoa seawater for the same duration time. Furthermore, the passivity plateau was less extended -immersed in sterilised seawater compared to the sample preimmersed in Genoa seawater. This clearly showed that the layer formed in sterilised seawater was less protective than that formed in Genoa seawater. This conclusion was in agreement with various studies and confirmed that the anticorrosion properties resulted from the layer formed during preimmersion under the action of the microorganisms [START_REF] Ezuber | A study on the corrosion behavior of aluminum alloys in seawater[END_REF][START_REF] Marques | AA 5083 Al Alloy Corrosion in Estuarine Environment[END_REF][START_REF] Jaume | Surface modification of 5083 aluminum-magnesium induced by marine microorganisms[END_REF]. This was also relevant to the previously observed differences in chemical composition and thickness between the layer formed in sterilised seawater and that formed in Genoa seawater.

To go further with the analysis of the anticorrosion properties of the layer formed during preimmersion in Genoa seawater, samples were polarised in the anodic domain at a fixed current density of 0.1 mA cm -² for 10 minutes after holding them at the OCP; a confocal microscope was then used to analyse pit propagation.

First, attention was paid to the bare samples. Figure 12 gives average values of pit density, pit depth and pit diameter measured for these samples when they were maintained at the OCP for 5 min before polarisation. Results showed that the standard deviation was very high for both pit depth and pit diameter values. Nevertheless, Fig. 12 (a) shows that the pit density is significantly higher for the 7 mm L-LT sample compared to the 1 mm L-LT sample. This result was associated with lower pit depth values (Fig. 12 (b)) for the 7 mm L-LT samples than for the 1 mm L-LT samples, whereas similar pit diameter values (Fig. 12 (c)) were obtained for both samples. For the 7 mm LT-ST samples, a decrease in both pit density and pit diameter was observed compared to the 1 mm L-LT sample, whereas the pit depth value was significantly increased. From all these results, the main conclusion was that the 7 mm LT-ST samples seemed to promote the in-depth propagation of pits compared to the other samples, which could be associated with the grain size and morphology (Fig. 1). Indeed, for the 7 mm LT-ST sample, pit propagation occurred in the L direction, which was perpendicular to the LT-ST plane. This led to deeper pits because the grains were elongated in this specific direction. This suggested an influence of the grain boundaries on the pit propagation, with the dissolution occurring initially at the grain boundaries, with an extension of the corrosion damage inside the grains. In order to check this hypothesis, EBSD observations were performed for a 7 mm L-LT sample after plotting the anodic polarisation curve in the chloride-containing sulphate solution. Fig. 13 (a)

shows a SEM observation of a pit formed on the sample ; Fig. 13 (b) shows the EBSD map of the same region, with the black lines corresponding to the high-angle grain boundaries HAGBs (misorientation higher than 10°), whereas the grey level referred to the quality of diffraction pattern (band contrast). Superimposition of the SEM image of the pit on the EBSD map (Fig. 13 (c)) clearly showed that the pit propagated following the HAGBs (white circles). Such an analysis was repeated for 20 pits: the same observations were done which strengthened the conclusion concerning the influence of the HAGBs on the pit propagation.

For the samples pre-immersed in Genoa seawater, galvanostatic tests were also performed by applying a fixed current density of 0.1 mA cm -² for 10 minutes after holding the samples at the OCP for 5 min. Only a few large and deep pits were observed on almost all the 1 mm L-LT samples pre-immersed in Genoa seawater, whereas numerous pits were observed for the bare samples for a similar test (Table 2, Fig. 14). It should be noted that, with these experimental conditions, within the limits of the experiments performed, no pit was observed for the 1 mm L-LT sample pre-immersed for 2 months for the light side and 15 days for the dark side. Finally, for the 7 mm L-LT and 7 mm LT-ST samples pre-immersed in Genoa seawater, in most cases, no or only a few pits were observed after similar galvanostatic tests. These results clearly confirmed the efficient anticorrosion properties of the layer formed during pre-immersion in Genoa seawater, as previously demonstrated with the polarisation curves (Figs. 9 and 10).

Furthermore, galvanostatic tests performed on the 1 mm L-LT sample pre-immersed for 1 month in sterilised seawater showed more numerous pits than for the 1 mm sample pre-immersed in Genoa seawater (Table 2), with a similar size (Fig. 14 (b)). This again showed the role of microorganisms in the mechanisms of layer growth and the subsequent anticorrosion properties of the layer.

Nevertheless, the absence of pits after the galvanostatic tests for many pre-immersed samples in Genoa seawater raised some questions. To explain the results, the potential was recorded as a function of time during the tests. Figure 15 shows the results obtained for the 1 mm L-LT sample pre-immersed for 1 month in Genoa seawater. Results obtained for the 1 mm L-LT bare sample and for the 1 mm L-LT sample pre-immersed in sterilised seawater are also given for comparison. Similar conclusions were obtained for the other pre-immersed samples, but the results are not shown for brevity. For the sample pre-immersed in Genoa seawater, the potential increased sharply to a potential of about 4.2 VSCE, and then stabilised at about 4.5 VSCE during the galvanostatic test, showing that the layer formed in Genoa seawater was quite insulating after 5 min of immersion at OCP in the chloride-containing sulphate solution, and retained these properties during the galvanostatic test. This could explain the absence of pits observed after some galvanostatic tests, considering that, depending on the samples, the electrical properties of the layer could vary in accordance with the heterogeneity mentioned earlier. In addition, the results showed that the layer formed in sterilised seawater did not have the same electrical properties, which was relevant with the differences in chemical composition, thickness and electrochemical response described previously. The sample pre-immersed in sterilised seawater rapidly showed similar behaviour to the bare sample, although, during the first 2 minutes of the galvanostatic test, an insulating behaviour was observed. Indeed, a potential peak was first observed, with a maximum potential value of about 2.4 VSCE and 0 VSCE for the sample preimmersed in sterilised seawater and the bare sample, respectively, and then the potential stabilised at a value of about 0.5 VSCE for both samples. Such a variation in potential for the sample pre-immersed in sterilised seawater should be due to hydration of the layer during the first few minutes of the galvanostatic test.

To strengthen the analysis, another set of galvanostatic experiments was performed after holding the samples at the OCP for 24 hours. The results showed that the peak was no longer observed for the sample pre-immersed in sterilised seawater, because the longer holding time at OCP allowed hydration of the layer prior to the galvanostatic test. The hydration of the layer formed on the sample pre-immersed in Genoa seawater was also evidenced for a holding step at OCP of 24 hours, with a lower potential value at the end of the galvanostatic test than in the first galvanostatic test (holding step at OCP of 5 min); however, the potential at the beginning of the galvanostatic test, of about 2.5 VSCE, was higher than that for the layer formed in sterilised seawater, confirming the differences in properties between the two layers. Furthermore, the analysis of the pits after these galvanostatic tests showed again more numerous pits on the bare sample and sample pre-immersed in sterilised seawater than on the samples pre-immersed in Genoa seawater (Table 2), confirming the better anticorrosion properties of the layer formed under biotic conditions (Genoa seawater). The increase in pit density for the sample preimmersed in Genoa seawater when the holding time at OCP was increased from 5 min to 24 hours prior to galvanostatic test was related to changes in the electrical properties of the layer due to hydration in chloride-containing sulphate solution.

Conclusion

The layer formed in the presence of microorganisms for different times of pre-immersion in natural seawater of a 5083 Al alloy was characterised using SEM-EDS and GDOES, and the influence of the layer growth on the corrosion behaviour of the alloy was investigated. Samples were taken from two different plates, a 1-mm plate and a 7-mm plate: L-LT plane was studied for both plates, and the LT-ST plane was also studied for the 7-mm plate. These samples differed by their microstructures, i.e. grain size and fraction of surface area covered by coarse intermetallics. The results obtained with samples pre-immersed in natural seawater were compared to those obtained after pre-immersion in sterilised seawater in order to assess the influence of the microorganisms. The main conclusions are described as followed:

1. A layer formed during pre-immersion in natural seawater (in Genoa) on the surface of AA 5083 Al-Mg alloy. The layer had a similar structure and composition for 1 mm L-LT, 7 mm L-LT and 7 mm LT-ST samples: it corresponded to an oxy-hydroxide layer separated from the substrate by an interfacial zone. The layer was mainly composed of O and Al, which were homogeneously distributed inside the layer, and also contained Mg, which was found to be segregated in the external part of the layer. S was also found both in the external part of the layer and in the interfacial zone, and Ca was mainly present in the interfacial zone. The layer was thicker for the 7 mm LT-ST samples compared to both 1 mm L-LT and 7 mm L-LT samples, which showed an influence of the substrate microstructure on the formation and growth of the layer.

2. When the 1 mm L-LT sample was immersed in sterilised seawater, i.e. in the absence of microorganisms, a thinner layer formed on the sample surface. This layer was also an oxyhydroxide layer, but with a lower amount of S and Ca, and a significantly higher amount of Mg.

This result demonstrated the influence of microorganisms on the layer growth. However, the mechanisms of layer growth under biotic conditions were not the focus of the study; they will be analysed in future work.

3. As expected, the influence of the substrate microstructure was also observed when the corrosion behaviour of the different bare samples was studied by plotting polarisation curves in 0.001 M NaCl + 0.1 M Na2SO4 solution. The 7 mm samples showed a significantly lower corrosion resistance than the 1 mm L-LT sample. These differences were correlated with the higher fraction of surface area covered by IMCs for the 7 mm samples compared to the 1 mm L-LT sample. Furthermore, pits were found to be deeper for the 7 mm LT-ST sample, which was explained by the influence of HAGBs on the pit propagation: for this plane, pits propagated in the L direction which was the direction of grain elongation.

4. Polarisation curves plotted in the same electrolyte for samples pre-immersed in Genoa seawater, i.e. samples pre-immersed in natural seawater and thus covered by the layer formed during pre-immersion, showed that this layer improved the corrosion resistance of AA 5083 for all the studied microstructures. The corrosion mechanisms were not modified, but it was found that the layer formed during pre-immersion improved the pitting corrosion resistance of all samples pre-immersed in Genoa seawater compared to bare samples. In comparison, samples pre-immersed in sterilised seawater showed significantly lower corrosion resistance, confirming the influence of microorganisms on the formation of the layer with good anticorrosion properties. The analysis of the role of the microorganisms on the layer growth will be the subject of future work.

2. 4 .

 4 Electrochemical behaviour of bare samples, samples pre-immersed in Genoa seawater and samples pre-immersed in sterilised seawater Prior to any electrochemical tests, bare samples were ground with SiC paper down to 240.

Fig. 1 (

 1 Fig. 1 (e) to (h) show OM images of the grain morphology after chemical etching. Similar

  (a) to (d)) and the 7 mm L-LT sample (Fig.4(e) to (h)) after 15 days of pre-immersion in Genoa seawater. Similar conclusions are obtained for the light side of the other samples and for the dark side of all the samples. The layer was cracked, which could be due to vacuum in SEM, but seemed to be adherent with the substrate (Figs.4 (a) and (e)). It was found to contain mainly O, Al and Mg, with a homogeneous distribution of O and Al inside the layer (Figs. 4 (b) and (c) for 1 mm L-LT sample, and 4 (f) and (g) for 7 mm L-LT sample), whereas Mg was mainly present in a thin film on the external part of the layer (Figs. 4 (d) and (h) for 1 mm and 7 mm samples, respectively).GDOES profiles plotted for the light side of samples pre-immersed in Genoa seawater (Figs.5, 6 and 7) confirmed these results. Considering first the results obtained for the 7 mm L-LT sample pre-immersed for 15 days in Genoa seawater (Figs.5 and 6), for the Al signal, from the sample surface to the substrate, a plateau corresponding to a low amount of Al was first observed, followed by an increase in the Al signal after 2 µm of sputtering depth before stabilisation at a higher amount of Al after 3.5 µm of sputtering depth. In parallel, for the O signal, a plateau showing a high amount of O was observed for [0 2] µm of sputtering depth, followed by a decrease in the O signal and a stabilisation at a lower intensity after 3 µm of sputtering depth. The analysis of the signals corresponding to these 2 elements led to identify 3 different areas: a first area identified as the layer formed during pre-immersion, a second area named interfacial zone, and a third area which corresponded to the substrate. It was not possible

Fig. 5

 5 Fig. 5 also shows GDOES profiles plotted for the 7 mm L-LT sample after 1 month and 2

  of the samples pre-immersed in Genoa seawater and in sterilised seawater Current-potential curves plotted in chloride-containing sulphate solution for the samples preimmersed in Genoa seawater are presented in Figs. 9 and 10 for the light and dark sides, respectively. The curves plotted for the bare samples and for the 1 mm L-LT sample preimmersed for 1 month in sterilised seawater are also shown for comparison.

Acknowledgments

This work was financially supported by the ANR, in the framework of the MICOATEC project (ANR-19-CE08-0018).

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Credit authorship contribution statement C. Nkoua: Investigation, Formal analysis, Validation, Visualization, Methodology, Data curation, Writing -original draft, Writing -review & editing. C. Josse: Investigation for GDOES


Frédéric Gruy

Gerd Hilpmann

Jean-Pierre Poyet

Frank Babick

Mesure de la force de traînée sur un agrégat

Keywords: Traînée, sphère, agrégat, mesure

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

n n a a a g g g r r r é é é g g g a a a t t t FRÉDÉRIC GRUY (1)(*) , GERD HILPMANN (2) , JEAN-PIERRE POYET (1) , FRANK BABICK (2)

(1) Ecole Nationale Supérieure des Mines de Saint Etienne, Centre SPIN -Département GENERIC-LPMG -UMR CNRS 5148, 158 Cours Fauriel -42023 Saint-Étienne Cedex 2, France (2) Technische Universität Dresden, MVT Münchner Platz, 3, 01187 D-Dresden (*) gruy@emse.fr

Introduction

La force de traînée sur une particule poreuse et un agrégat constitué de particules primaires identiques en particulier est une grandeur d'intérêt quand on s'intéresse à la trajectoire d'une particule dans un fluide. Elle intervient aussi indirectement lors de la collision entre deux particules conduisant à un agrégat. Nous nous intéressons à la force de traînée sur les agrégats quand l'écoulement relatif est de type stokien. En effet, les particules interagissant dans un phénomène d'agrégation sont submicroniques ou microniques ; le nombre de Reynolds faisant intervenir la vitesse relative entre la particule et le fluide est alors beaucoup plus petit que 1. L'évaluation de la force de traînée dans le cas des agrégats a déjà fait l'objet d'un certain nombre de travaux, la plupart théoriques (Vanni, 2000). Nous ferons un rappel succinct de quelques travaux réalisés dans notre laboratoire. Une première étude reposant sur la sédimentation a été réalisée [START_REF] Gruy | [END_REF]. Pour respecter les nombres de Reynolds, nous avons construit des agrégats de particules primaires millimétriques (et donc faciles à construire) à morphologie contrôlée et avons étudié leur sédimentation dans le glycérol (liquide newtonien dont la viscosité dynamique est mille fois celle de l'eau). La vitesse de sédimentation de 40 agrégats contenant moins de 100 particules primaires et de morphologies variées (de l'agrégat linéaire à l'agrégat compact, i.e. de l'agrégat à enveloppe cylindrique à l'agrégat à enveloppe sphérique) a ainsi été mesurée. Cette mesure facile présente, néanmoins, un inconvénient : l'orientation de l'agrégat lors de la sédimentation n'est pas contrôlée ; on observe parfois, en effet, une rotation de l'objet pendant sa descente, laquelle a une influence sur la valeur mesurée du temps de sédimentation, surtout pour des agrégats à symétrie non sphérique. Il a ainsi été décidé de changer la méthode de mesure. Un nouveau montage a été réalisé, lequel devrait permettre la mesure de la traînée pour une orientation donnée de l'agrégat. Ce dispositif doit obligatoirement permettre la mesure dans un liquide, car il est prévu, entre autres, de quantifier l'effet de la mouillabilité sur la force de traînée. vidange de la cuve sans circulation du glycérol : le robinet est ouvert ; la force et le niveau de glycérol sont mesurés en fonction du temps. La vitesse de baisse de niveau est comprise entre 0,01 cm/s et 0,2 cm/s. circulation à l'aide de la pompe : le glycérol circule du bas vers le haut ; le niveau de glycérol est constant. La force et la vitesse de circulation sont enregistrées en fonction du temps.

Montage expérimental et mode opératoire

Dans les deux cas, la distance entre le niveau initial de glycérol et l'objet est un paramètre important.

Analyse critique de l'expérience

3 3. .1 1 I
In nt tr ro od du uc ct ti io on n Le dispositif expérimental a été testé en mesurant la force de traînée sur une sphère. L'objet choisi est une bille en verre de 1cm de diamètre. La Figure 2 représente l'enregistrement de la force perçue par la microbalance lors de la vidange (mode opératoire n°1) en fonction du temps. Lors de cette expérience, la vanne de vidange est ouverte à t=155s. La surface du liquide rencontre la sphère à t=425s. (

0 3 / 2 D L p L B T T F V dU dt F ρ -= + + (2) Où D F ,
2 , / 4
G fil fil L air d h g F π ρ ρ = Δ - (3)
g est l'accélération de la pesanteur.

Si le diamètre d fil du fil est de 100µm, la variation totale de poussée d'Archimède sera égale à 10 -5 N. Cette valeur a été confirmée en remplaçant le glycérol par l'eau (les forces liées à la viscosité étant alors négligeables, l'effet de la poussée d'Archimède est bien observé). Le retrait du liquide le long du fil est la cause d'une force de démouillage (Karakashev, 2005), dont le point d'action est la jonction entre le fil et l'interface liquide-air. Son expression est :

3 fil L démouillage ext d KU F π μ θ = (4)
K est une constante comprise entre 11 et 15. L'angle (dynamique) ext θ de démouillage est une fonction de la vitesse de retrait. Si la valeur de ce dernier est 0,5 rad, la force de démouillage est égale à 10 -5 N.

Le liquide exerce une force de frottement (traînée) sur le fil lui-même. Il suffit de rappeler qu'un fil de diamètre 100µm et de longueur 0,1m a une surface qui est 10% celle d'une sphère de diamètre 1cm. Assimilant l'écoulement le long du fil à celui entre deux cylindres (fil et récipient), la force de frottement obéit à l'équation [START_REF] Bird | Transport Phenomena[END_REF] :

( , 8 D fil fil L ) / F LU G d D πμ = (5a) Avec ( ) ( ) ( ) ( ) ( ) 2 2 2 2 1 / 2ln 1 1 / l n x x x G x x x x + - = + + - (5b)
Où L est la longueur du fil. Si L est égale à 0,1m, la force de frottement sur le fil est égale à 2,35 10 -4 N. Il apparaît clairement que plus le fil est fin, plus faibles seront les forces exercées par le liquide. Cependant, si la variation de poussée d'Archimède et la force de démouillage peuvent devenir négligeables devant la force de traînée, cela n'est pas le cas du frottement du liquide sur le fil. 3 3. .5 5 I In nt te er rp pr ré ét ta at ti io on n d de e l l' 'e ex xp pé ér ri ie en nc ce e La variation initiale du signal est liée à la force exercée par le liquide en écoulement sur le système associant le fil et l'objet. Forces de traînée sur le fil et l'objet sont prépondérantes. La traînée sur le fil est du même ordre de grandeur que celle (compte tenu de l'effet de la paroi) sur l'objet. Au cours de la vidange, la longueur de fil immergée et, dans une moindre mesure, la vitesse du liquide diminuent ayant pour conséquence la diminution des forces de traînée sur le fil et l'objet. La force totale mesurée est proche de la traînée sur l'objet quand la surface du liquide est proche de la sphère. Cependant, l'écoulement devient, dans cette dernière situation, complexe ; la force de traînée sur l'objet a alors une valeur bien différente de celle donnée par la loi de Stokes. Il est donc plus judicieux d'analyser les premiers instants de la vidange. La variation initiale du signal dure quelques dizaines de secondes, dont une partie est due au temps de réponse de l'appareil et l'autre partie aux conséquences de l'établissement d'un régime quasi-stationnaire d'écoulement ( 2 / L t D ρ μ <

). Le maximum de la force mesurée (en valeur absolue) correspond au début du régime quasi-stationnaire. Nous nous sommes concentrés sur la valeur maximale de la force, sachant qu'elle contient les contributions de la traînée de l'objet étudié et du fil.

Quelques résultats

La force mesurée est la somme des forces de traînée sur le fil et l'objet et, dans une moindre mesure, de la force de démouillage. Ces trois forces sont proportionnelles à µU L . Le coefficient de proportionnalité dépend essentiellement de la géométrie du système. Un premier résultat (non présenté dans ce texte) est la comparaison de la force mesurée pour deux objets différents : une sphère en verre (d=1cm) et un cylindre en acier (0,65mmx4cm). Celle-ci est une fonction linéaire de µU L pour les deux objets. La valeur de la force mesurée pour le cylindre est 1,83 plus petite que celle pour la sphère. Ceci montre que la force mesurée dépend de l'objet. La Figure 3 représente la force mesurée F m en fonction de la force théorique F th donnée par la loi de Stokes pour un objet nécessite donc un grand nombre de mesures (N>50). La force mesurée est 4,39 fois plus importante que la force théorique. En utilisant l'expression 5a (en prenant pour L la longueur de la partie du fil immergé quand la force est maximale ; la longueur initiale est 15cm), la force de traînée sur le fil est 3,0 fois celle obtenue pour la sphère à partir de la loi de Stokes. On en déduit que la force sur la sphère est 1,39 fois celle de Stokes. L'écart correspond à l'effet de paroi pour D/d=6. Des essais ont été réalisés en diminuant le diamètre du fil (d=10 et 25µm) sans amélioration quant à la qualité des mesures. Le mode opératoire, qui met en oeuvre une circulation du liquide à l'aide d'une pompe, maintient constant le niveau du liquide. La longueur de fil immergé et la vitesse du liquide sont constantes, mais les lignes de courant dans le contenant ont une structure complexe rendant l'analyse difficile.

3 th L F d U π μ = . Fm = 4,39Fth -5E-06 R 2 = 0,90 0,0E+00

Conclusions

La mesure de la force exercée sur un objet fixe par un écoulement liquide a été réalisée de façon à maitriser l'orientation de l'objet par rapport à l'écoulement. L'interprétation de la mesure s'est avérée plus compliquée que prévue. Un mode opératoire a été proposé et une analyse des résultats a montré que le fil suspendant l'objet contribuait de façon non négligeable à la force mesurée, par frottement visqueux et résistance au démouillage. L'obtention d'une valeur (à 1% près) de la force de traînée exige la mesure de celle-ci pour de nombreuses valeurs du paramètre µU L .

  cr ri ip pt ti io on n d du u m mo on nt ta ag ge e Le montage (Fig.1) est constitué de différents éléments : une cuve cylindrique (à section carrée ou circulaire) contenant le liquide, muni, à sa base, d'un robinet de vidange, d'une entrée en bas et d'une sortie (deux tiers de hauteur) une pompe pour la circulation du liquide une microbalance, pour la mesure de la traînée un fil très fin liant l'agrégat à la microbalance La microbalance MB 60 (Setaram) est équipée d'un système de compensation magnétique qui permet de maintenir horizontal le fléau de la balance et donc l'objet suspendu, quelle que soit la sollicitation appliquée à celui-ci. ta ag ge e e ex xp pé ér ri im me en nt ta al l 2 2. .2 2. . M Mo od de e o op pé ér ra at to oi ir re e Le liquide est soit du glycérol pur, soit une solution aqueuse de glycérol. La température de travail est notée à 0,1°C près. La viscosité du glycérol ou de la solution est mesurée avant chaque expérience au moyen d'un rhéomètre DV-III (Brookfield). Cette dernière mesure est importante car la viscosité du glycérol pur varie beaucoup entre 18°C et 25°C. Deux modes opératoires ont été utilisés :

Fh

  Fi ig gu ur re e 2 2 : : F Fo or rc ce e m me es su ur ré ée e p pa ar r l la a m mi ic cr ro ob ba al la an nc ce e l lo or rs s d de e l la a v vi id da an ng ge e ( (v vi it te es ss se e d de e b ba ai is ss se e d du u n ni iv ve ea au u : : co on nt te en na an nt t. . L La a f fo or rc ce e e es st t e ex xp pr ri im mé ée e e en n m ma as ss se e. . L La a v vi is sc co os si it té é d du u l li iq qu ui id de e e es st t é ég ga al le e à à 0,335 . µ Pa s =On pourrait s'attendre, lors de l'ouverture de la vanne de vidange, à une variation brutale du signal (force ou masse équivalente) suivie d'une période où le signal est constant, jusqu'au moment où la surface du liquide rencontre la sphère. On constate en fait qu'après la variation initiale du signal, celui-ci diminue lentement avec le niveau de liquide. La force de traînée calculée en appliquant la loi de Stokes avec les données correspondant à la figure2est égale à 1,83 10 -5 N (soit 1,87mg). Il apparaît ainsi que la valeur mesurée n'obéit pas à la loi de Stokes. Cette constatation est vraie pour toutes les mesures. La valeur mesurée est plusieurs fois plus grande que la valeur théorique. Une analyse fine des mesures s'avère donc nécessaire, car de nombreux phénomènes, hormis la traînée sur l'objet, doivent être pris en compte : Vitesse de vidange non constante Forces liées à l'accélération du fluide Effet de paroi sur la traînée Force de démouillage sur le fil Film résiduel ou gouttes de liquide sur le fil Frottement du liquide sur le fil Poussée d'Archimède du fil Accrochage de bulle pendant la vidange Nous examinerons les plus pertinents avant de présenter les mesures. est la distance entre la surface du fluide et l'orifice (vanne) de vidange. a est une constante pour une expérience donnée ; elle dépend de la viscosité du liquide et des caractéristiques de la vanne. o ob bj je et t ( (s sp ph hè èr re e) ) Le bilan des forces appliquées à la sphère conduit à (Kim et Lawrence, 1988) :

  , , Dans ces conditions, les forces de traînée, de masse ajoutée et de Basset sont respectivement égales à 10 -4 N, 3 10 -9 N et 5 10 -7 N. Les forces de masse ajoutée et de Basset sont donc négligeables devant la force de traînée. Il est bien connu que les parois du récipient ont une influence sur la valeur de la traînée. Celle-ci est fonction du rapport entre le diamètre d de la sphère et le diamètre D du récipient. Ainsi, des simulations réalisées à l'aide du logiciel Comsol montrent que la traînée est multipliée par 1,074 et 1,876 quand le rapport des diamètres du récipient et de la sphère est égal à 10 et 3 respectivement. Ce rapport dimensionnel étant égal à 6 dans le montage, l'écart observé ne peut donc pas être expliqué uniquement, loin s'en faut, par un effet de paroi.

	0 T T	B F , L ρ et	V	p	sont la force de traînée, la tension du fil avant vidange, la tension
	du fil pendant la vidange, la force de Basset, la masse volum ique du	liquide et le volume de
	l'objet. La grandeur mesurée est la variation de tension	0 T T -. L'écart prévisible entre
	grandeur mesurée et force de traînée provient des forces de masse ajoutée et de Basset. Ces
	deux forces sont liées à l'accélération du fluide.
	Considérons une sphère de diamètre d=1cm dans du glycérol pur de viscosité dynamique µ=1
	Pa.s s'écoulant à 10 -3 m/s. Les hauteurs initiale et finale de liquide sont respectivement 0,4m
	et 0,25m. 3 3. .4 4 B Bi il la an n d de es s f fo or rc ce es s s su ur r l l' 'e en ns se em mb bl le e f fi il l+ +o ob bj je et t

)

Le fil, même tendu, exerce une influence sur la mesure. Au cours de la vidange, la contribution du fil à la poussée d'Archimède évolue suivant :

Remerciements

Les auteurs remercient l'Agence Nationale de la Recherche (programme CIREM) pour le financement partiel de cette étude.


Ph.D Stéphane Lambert
email: stephane.lambert@inrae.fr

Ph.D Maddalena Marchelli
email: maddalena.marchelli@polito.it

Reza Mohamadi
email: rmohamadi@eaupkeep.ca

Ragavendra Pavan

PGDBM B Tech

Pietro Rimoldi
email: pietro.rimoldi@gmail.com

Applications of Geosynthetics for Building Passive Protective Structures against Rockfall, Debris Flows and Snow Avalanches

This paper addresses the use of geosynthetics for building structures such as embankments, dams and mounds with the aim of intercepting, deviating and containing rockfall, snow avalanches and debris flows. In this paper some real cases are presented to illustrate the variety in structure designs and functions. Advantages in using geosynthetics for these structures are discussed. Some design considerations that are specific to these applications are raised, with a particular focus on the dynamic loading.

INTRODUCTION

Inhabited areas, transport corridors and other infrastructure types in mountain and hilly zones are often threatened by rapid gravity driven natural hazards such as rockfall, debris flows and snow avalanches. These phenomena initiate up on the slope, propagate downward along a path mainly defined by the topography and progressively stop with decreasing gradient. Any element at risk, that is to say on the hazard propagation path, may be damaged or destroyed as a result of the large volume and/or velocity of the moving mass. The protection of the elements at risk may be based on active techniques, aiming at preventing the hazard initiation and on passive ones which aim is to deviate, slow down or stop the rockfall, debris flows or snow avalanches before they reach the critical areas.

Passive protection structures have been used for centuries, employing conventional materials such as earth, concrete, rip-rap and steel structural components. With the advent of geosynthetics, new design alternative with increasing complexity have been developed and implemented from the middle of the 80's. Examples of such structures are presented in the following, considering the three types of natural hazards for illustrating the variety in designs and functions. The cases are described from the available literature. Then, the advantages in using geosynthetics for reinforcing the passive protection structures are mentioned. Some specific difficulties with the designing such structures are also discussed.

ROCKFALL PROTECTION

This application is the oldest and most widespread one among passive protection structures. Reinforced embankments and bunds may be used to arrest rock blocks with kinetic energies ranging from 1 to 50 000 kJ, as well as rockslides. A ditch is most often associated to the embankment to collect the fallen material. In some rare cases, structures aim at deviating the rock block. Two out of the many available references describing real rockfall protection embankments are discussed as follows.

Varzo, Italy. The need for protecting the Simplon road against 15 000 kJ in kinetic energy rockfall motivated the construction of a huge embankment close to Varzo, Italy [START_REF] Rimoldi | Impressive Reinforced Soil Structures in Italy[END_REF]. The embankment is 20 m in height and 600 m long (Figure 1). The choice of a reinforced embankment with 70° steep faces was imposed by the available space and the steepness of the mountain slope. The reinforcement layer consisted a heavy duty woven polypropylene geogrids, with a 3 mm mesh and a 340 kN/m tensile strength. Many other technical consideration were included in the design of the embankment. Italy [START_REF] Rimoldi | Impressive Reinforced Soil Structures in Italy[END_REF].

Figure 2. Rockfall protection embankment along the Trans-Canada highway. General view and cross-section (adapted from [START_REF] Simons | Design and construction of a high energy rockfall embankment on the Trans-Canada Highway[END_REF].

Hells gate rapids, B. C., Canada. In order to protect a portion of the Trans-Canada Highway, a 8m high and 96 m long rockfall protection embankment was built using a double sided reinforced soil structure (Simmons et al., 2010). The facings consist of gabions (Figure 2). The same hexagonal wire mesh as that used for the gabions is used as reinforcement tail. The structure is designed to withstand a maximum rockfall impact energy of 10,000kJ maximum and it accounts for the limited space constraint. [START_REF] Coulon | Les merlons renforcés de protection[END_REF].

SNOW AVALANCHE PROTECTION

The control of snow avalanche run out has motivated the construction of impressive reinforced structures. Geosynthetics allow building four types of passive structures forming an obstacle on avalanche paths. First, guiding walls prevent from excessive lateral expansion of snow avalanches towards settlements. Second, deflecting dams aim at deviating avalanches towards areas with no elements at risk. Third, mounds -or cone, splitters, wedge-may be used to break down the avalanche velocity. Last, dams may be used to contain the snow avalanche. These structures are rather massive, with an elevation defined based on the avalanche characteristics and are either placed normal or transversally to the avalanche propagation direction.

Similarly as for rockfall protection embankments, geosynthetics allow building structures with a greater height and steeper slope while keeping the structure total volume acceptable and with a steeper face opposed to the avalanche.

Neskaupstaður, Iceland. Since 2002, the village of Neskaupstaður, Iceland, is protected by 13 breaking mounds, 10 m in height, placed upstream a catching dam 400 m in length and 17 m in height (see Figure 3). These structures are reinforced with strips, with a well graded rock as fill, and the face is 76° steep and made from galvanized steel facing. The strips consist of closely packed high tenacity polyethylene terephthalate (PET) fibers encased in a low density polyethylene (LDPE) sheath [START_REF] Halifax | Design and construction of a reinforced soil avalanche barrier at Neskaupstaður[END_REF].

Thereafter, nine large facilities with different designs have been built in Iceland, providing a rather exhaustive experience for the use of geosynthetics in snow avalanche protective structures [START_REF] Indrioason | Experience and evaluation of reinforced soil systems in catching dams in Iceland 1998-2017[END_REF].

Belle-Plagne, France. In order to protect the ski resort, a snow avalanche deflecting dam was built on a 50° steep slope up hill the resort (Figure 4). This wall consists of two superimposed reinforced structures, with maximum heights of 8.5 and 4.3 m respectively. Reinforcement consists of geogrids made from polyester. The local slope material was used as fill which fine content induced constraints for the construction phase.

DEBRIS FLOWS PROTECTION

The use of reinforced structures for building debris flows protection structures is limited up to now. Only a few cases of such applications of geosynthetics are reported in the literature.

Miseregne, Italy. In order to protect the village of Miseregne, Val d'Aoste, Italy, a debris flow catchment basin was created by building a reinforced earth wall 4.39 m in height, 6.27 and 2 m in width at the top and base respectively (Figure 5) [START_REF] Di Pietro | Remedial works against debris flows after the 2000 floods in Valle d'Aosta[END_REF]. The 65° inclined faces were vegetated. The wall was built perpendicular to the incoming flow, with a large opening to let the water through. In addition, the wall was built on a gabion foundation to insure progressive drainage of the stored debris flow material. Similar structures were also used for deflecting debris flows [START_REF] Di Pietro | Remedial works against debris flows after the 2000 floods in Valle d'Aosta[END_REF].

Whistler, B.C., Canada. In view of protecting Whistler against damaging effects of large debris flows, a ground reinforced structure (GRS) was used as abutment for a debris flow barrier (Figure 6). It has two function; first, to serve as support for the steel arch forming the barrier and, second, to contain the debris flow material [START_REF] Strouth | Geosynthetic reinforced soil walls for debris barrier in Whistler[END_REF]. This abutment is in particular designed to withstand the load transferred by the steel archs and that result from the interception of the debris flow by the barrier. A high -strength woven polypropylene geotextile was used as reinforcement. The fill is a well graded crushed granular material. The GRS is protected against erosion and impact by a 1-m thick facing made of cobbles that is grouted and contained by a welded wire covered with shotcrete.

ADVANTAGES OVER OTHER SOLUTIONS

Various benefits results from the use of geosynthetics in these applications, in particular by comparison with structures made of compacted earth or concrete.

First, geosynthetics allow building tall structures, up to 25m in height in some cases. In such applications, the structure height is indeed a key input parameter for the structure design as it relates to its capacity in containing the avalanche, rockslide or debris flow volume or in intercepting bouncing rock blocks.

Second, the face of reinforced structure exposed to the natural event may be given a high inclination, up to 90°. This prevents from any spring board effect, associated with low inclinations and resulting in structure passing over by the moving volume to intercept.

Geosynthetics improve the structure internal stability with respect to the natural-event-induced loading. This was clearly demonstrated for rockfall protection [START_REF] Blovsky | Model tests on protective barriers against rockfall[END_REF][START_REF] Peila | Ground reinforced embankments for rockfall protection: design and evaluation of full scale tests[END_REF]. For this specific application, geosynthetics in fact distribute the loads along the structure longitudinal axis, from the impacted area.

As a corollary, these structures may be given a high height-to-width ratio, for meeting space constraints associated with some specific contexts (Simmons et al., 2010) or for limiting the structure mass in case the structure is built on a close-to-instability natural slope [START_REF] Grimod | Reinforced soil rockfall embankment to absorb high energy impacts along the regional road SR 507 Aymavilles-Cogne, Aosta (Italy)[END_REF]. Also, slender structures allows saving fill material [START_REF] Mannsbart | Geosynthetic reinforced protection structures in mountainous regionsexamples of safe and cost-effective alternatives to conventional structures[END_REF].

Reinforced structures can conveniently be equipped with specific facings. The aim may be to meet aesthetic constraints (vegetated facing even if steep, rocky appearance…) (Bygness 2007, di Pietri and[START_REF] Di Pietro | Remedial works against debris flows after the 2000 floods in Valle d'Aosta[END_REF]. Also, specific facings may improve the structure impact strength, for example creating a cushion or sacrificial layer made from crushable material, recycled tires… (Simmons et al., 2010;[START_REF] Lambert | Design of rockfall protection embankments: A review[END_REF][START_REF] Strouth | Geosynthetic reinforced soil walls for debris barrier in Whistler[END_REF]. In addition, facings prevent reinforcement layers from ageing by atmospheric agents.

Last and not least, structures reinforces with geosynthetics generally constitutes, all in all, costeffective alternative to conventional material and structures [START_REF] Mannsbart | Geosynthetic reinforced protection structures in mountainous regionsexamples of safe and cost-effective alternatives to conventional structures[END_REF][START_REF] Strouth | Geosynthetic reinforced soil walls for debris barrier in Whistler[END_REF].

SPECIFIC DESIGN ISSUES

Almost all reinforced structures mitigating gravity driven natural hazards consist of massive structures in elevation with respect to the soil. As such, most of the design principles detailed in the various recommendations and standards for mechanically stabilized earth (MSE) wall apply. This in particular concerns the structure global stability, overturning, base sliding, and bearing capacity. Nevertheless, the design of these protective structures requires also addressing some very specific facets. Some of these are detailed in the following.

Loading related issues. Rockfall, snow avalanches and debris flows exert very specific loadings on the lateral faces of passive protection structures. This loading is either localized in the case of rockfall and distributed, but not uniform, in the case of snow avalanches and debris flows. The typical loading orientation is along the horizontal axis in the latter cases and is inclined by an angle ranging from 0 to 50° in the former. Last, and not least, the hazard induced loadings vary with time and last between a few tens of milliseconds in the former case to a few seconds in the latter ones. These loadings are thus very specific and confront designers with uncommon issues relating to the actual whole structure response with time.

The first issue relates to the loading area and orientation. Figure 7 illustrates the case where the upper part of a reinforced breaking mound slipped along a geosynthetic layer due to an excessive lateral loading by an avalanche. To some extent, a similar observation can be made from Figure 8 where the structure face opposite to the impact by a projectile evidences differential sliding from one layer to the other. Both these observations relate to the fact that the loading is applied on the faces of the structure with an orientation close to that of reinforcement layers. Limited work to improve the design of the structure with respect to such lateral loading has been done up to now (Blovsky, 20004;[START_REF] Yang | Lateral bearing capacity and failure mode of geosynthetic-reinforced soil barriers subject to lateral loadings[END_REF][START_REF] Cuomo | Deformation mechanisms of deformable geosynthetics-reinforced barriers (DGRB) impacted by debris avalanches[END_REF][START_REF] Korini | The influence of geosynthetics design on the behavior of reinforced soil embankments subjected to rockfall impacts[END_REF]. Figure 8. Rockfall protection embankment after impact evidencing relative displacement between soil layers associated with horizontal reinforcement (from [START_REF] Peila | Ground reinforced embankments for rockfall protection: design and evaluation of full scale tests[END_REF].

The definition of the proper loading to consider for the design is also an issue. Some recommendations exists for computing the loading applied on a structure by a rock block, a snow avalanche or a debris flow based on its velocity and mass (or unit mass) in particular (e.g. [START_REF] Barbolini | The design of avalanche protection dams Recent practical and theoretical developments[END_REF]ONR, 2013;GEO, 2015). Nevertheless, structure design approaches consisting in considering a pseudo-static loading that is equivalent to this loading may not be relevant. This is particularly true for rockfall protection structures due to the very short impact duration. The difficulty concerns the equivalent loading to consider, the maximum value being not necessarily relevant for a loading which amplitude strongly varies with time. In addition, the response of the whole structure to such rapid loadings depends on mechanisms that are not accounted for by pseudo-static approaches [START_REF] Lambert | Design of rockfall protection embankments: A review[END_REF]. In this context, employing numerical tools to model the actual response of these structures seems to be more satisfactory. Numerical modeling could be useful to extend design capabilities if enough data from full-scale tests and performance monitoring, report and analysis is available. Time and very specific skills are required for the numerical models to be reliable.

Site specific constraints. Gravity driven hazards passive protection structures are often built in environments characterized by steep and potentially unstable slopes, space constraints or poor quality of the onsite soil. The on-site soils often contains medium to large granular materials, with angular shapes. Soil characteristics at the toe of mountain slopes may be extremely variable with space. Their fine content and water content may be high.

As for the global stability issue, the problem relates to the mass of structures built on steep slopes. This issue mainly concerns rockfall protection embankments built on slopes and the solution consists in building a structure with a high height-to-width ratio [START_REF] Rimoldi | Impressive Reinforced Soil Structures in Italy[END_REF]. For the same reason, the ditch that is generally associated to the embankment may be suppressed [START_REF] Grimod | Reinforced soil rockfall embankment to absorb high energy impacts along the regional road SR 507 Aymavilles-Cogne, Aosta (Italy)[END_REF].

The on-site soil characteristics may pose various problems during the design and construction phases. General requirements for soil backfill characteristics when in contact with geosynthetics may not be met. The grain size distribution may require on-site sorting to eliminate large quantities of granular materials that are not suitable to reinforcement layers due to their size and angular shape [START_REF] Rimoldi | Impressive Reinforced Soil Structures in Italy[END_REF]. This process requires space, which may constitute a constraint in narrow for example.

Site specific testing of onsite materials should include shear properties, enabling the design of required reinforcement strengths and length. In addition, high water content and high fine content may pose a problem during compaction, for its feasibility, and for the structure stability. The poor quality of the onsite soil may also pose a foundation problem, in terms of bearing capacity, requiring geotechnical investigations.

Last, the interaction of the structure with other natural events occurring on slopes should be considered. For example, long embankments may constitute an obstacle to precipitation run-off with dramatic consequences in some cases [START_REF] Jaecklin | Innovative design for repairing Gondo mudslide by 20 m high geogrid wall[END_REF]. Specific and efficient drainage systems are thus necessary uphill the structure. In the same spirit, openings through the structure may be required to let torrential flow through, if any.

CONCLUSION

The use of geosynthetics for building passive protective structures against rockfall, debris flows and snow avalanches now benefits from a significant experience. The most frequent application concerns rockfall protection embankments. The review of the available literature reveals that geosynthetics allows tackling with a wide variety of constraints, in particular associated with the site characteristics and the function of the structure. Passive protective structures reinforced with geosynthetic appear extremely versatile, and present technical and cost advantages over conventional solutions. However, their design and building requires accounting for some constraints that are specific to the sites where that are built, as well as for the specific loading resulting from the interaction with debris flows, snow avalanches and rockfall.

Figure 1 .

 1 Figure 1. Huge rockfall protection embankment protecting the Simplon road, in Varzo,Italy[START_REF] Rimoldi | Impressive Reinforced Soil Structures in Italy[END_REF].

Figure 3 .

 3 Figure 3. Avalanche control facility in Neskaupstaður, Iceland.

Figure 4 .

 4 Figure 4. Snow avalanche deflecting dam in Belle-Plagne, France[START_REF] Coulon | Les merlons renforcés de protection[END_REF].

Figure 5 .

 5 Figure 5. Cross section of a debris flows catchment wall (adapted from Di Pietro and Tinti, 2008). DEBRIS FLOWS PROTECTION

Figure 6 .

 6 Figure 6. Debris flow barrier which abutment is reinforced with geosynthetics. General view and cross-section (Strouth et al., 2012).

Figure 7 .

 7 Figure 7. Avalanche breaking mound after completion (left) and after partial destruction (right). Location: Taconnaz, France. (J.M. Tacnet, INRAE).