Marc Metian

Mohamed M Eltayeb

Laëtitia Hédouin

Thomas Lacoue-Labarthe

Jean-Louis Teyssié

Chantal Mugnier

Paco Bustamante

Michel Warnau
email: warnaumichel@yahoo.com

Laetitia Hédouin

Jean- Louis Teyssié

Metal and metalloid bioaccumulation in the Pacific blue shrimp Litopenaeus stylirostris (Stimpson) from New Caledonia: laboratory and field studies

Keywords: crustacean, metals, biokinetics, radiotracers, moulting, contamination

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

Introduction

In many countries, shrimp is one of the most consumed seafood and wild-caught stocks are heavily fished (e.g., [START_REF] Cascorbi | Wild-caught warmwater shrimp (Infraorder Penaeus-the Penaeid shrimps). Seafood Watch, Seafood Report[END_REF]. While most species show no clear signs of overfishing, shrimp are believed to be exploited at or near their maximum sustainable yield [START_REF] Cascorbi | Wild-caught warmwater shrimp (Infraorder Penaeus-the Penaeid shrimps). Seafood Watch, Seafood Report[END_REF]. Alternatively, shrimp culture has been adopted and consolidated as one of the largest profitable aquaculture activities all over the world (it has e.g. the most important commodity traded in value terms; [START_REF] Fao | The state of world fisheries and aquaculture[END_REF]. Since 1997, the aquaculture shrimp production shows a strong expansion with an increase of 165% (viz., a 15% annual growth, [START_REF] Fao | The state of world fisheries and aquaculture[END_REF].

Among the shrimp species that are cultured, the Pacific blue shrimp Litopenaeus stylirostris, which is originally from the Pacific coast of Latin America, is well reared in the South Pacific region, especially in New Caledonia. This Overseas French Territory is the first world aquaculture producer of L. stylirostris with 2278 tonnes produced in 2006 [START_REF] Fao | ProdSTAT: livestock (primary and processed), FAOSTAT, FAO[END_REF], of which more than the half is exported. Indeed, in 2005, 73% of the production was exported to France, Japan and Australia, and represented the second major export of the country [START_REF] Gfa | Groupement des Fermes Aquacoles (GFA)[END_REF].

New Caledonia is also known for another natural wealth: its nickel resources. Indeed, the largest resources of Ni laterites in the world (20-25%) are found in the soils of New Caledonia, which have been largely exploited for more than a century [START_REF] Dalvi | The past and the future of nickel laterites[END_REF].

Nowadays, mining activities constitute the major economical resource of the Territory, which is the third world largest producer of nickel. Obviously, open-cast mining exploitations result in a considerably increased discharge of metals into the lagoon waters either directly or indirectly (via e.g. deforestation and enhanced soil erosion). This input threatens and damages the coastal ecosystems, local biodiversity, and quality of local seafood [START_REF] Labrosse | New Caledonia in[END_REF][START_REF] Hédouin | Nickel bioaccumulation in bivalves from the New Caledonia lagoon: Seawater and food exposure[END_REF][START_REF] Labonne | Otolith chemical signature and growth of Chaetodon speculum in coastal areas of New Caledonia[END_REF]Metian et al., 2008a;[START_REF] Chouvelon | Hg concentrations and related risk assessment in coral reef crustaceans, molluscs and fish from New Caledonia[END_REF].

Mining-originating metal contamination is due essentially to Ni and the main mining byproducts occurring at elevated concentrations in Ni ores, such as Co, Cr and Mn (Metian et al., 2008a,b).

Metal-enriched soils are obviously also a potential non negligible contamination source for shrimp ponds which are located near the coast [START_REF] Mugnier | Métaux et crevettes d'élevage en Nouvelle Calédonie. Travail préliminaire d'analyse de la teneur en métaux chez les crevettes et le sédiment[END_REF]. However, to the best of our knowledge, there are few studies available in the open literature that have investigated metal levels in the blue shrimp L. stylirostris (see e.g., Paez-Osuna and Ruiz-Fernandez, 1995a;[START_REF] Frías-Espericueta | Metal content of the Gulf of California blue shrimp Litopenaeus stylirostris (Stimpson)[END_REF]. Among them, only one considered the New Caledonian blue shrimp with a focus on the risk linked to its consumption [START_REF] Chouvelon | Hg concentrations and related risk assessment in coral reef crustaceans, molluscs and fish from New Caledonia[END_REF].

Therefore the aim of this study was to investigate the bioaccumulation and tissue distribution of a range of elements in the shrimp L. stylirostris, using both laboratory experiments under controlled conditions and in situ investigations. For this purpose, the bioaccumulation kinetics and body distribution of five selected radiotracers ( 110m Ag, 109 Cd, 57 Co, 51 Cr, and 65 Zn) were determined in the blue shrimp experimentally exposed to the metal radiotracers via seawater and their food. In the field study, eight metals (Ag, Cd, Co, Cr, Cu, Mn, Ni, and Zn) and one metalloid (As) were analysed in the tissues of L. stylirostris from an aquaculture farm to assess their levels and the human risk related to shrimp consumption.

Materials and Methods

Radiotracer experiments in the laboratory.

Origin and acclimation of organisms

Exposure via seawater

A 24-d radiotracer exposure was carried out to determine the uptake kinetics of dissolved metals in L. stylirostris. Physico-chemical parameters of the seawater (salinity, temperature and pH) were checked twice daily all along the experiment duration. The selected metals (Ag, Cd, Co, Cr, Zn) were introduced into the experimental microcosm as high specific activity radiotracers purchased from Amersham, UK ( 109 Cd as CdCl 2 in 0.1M HCl, T 1/2 = 426.6 d and 51 Cr as Na 2 CrO 4 in saline solution, T 1/2 = 27.7 d), CERCA, France ( 110m Ag as AgNO 3 in 1M HNO 3 , T 1/2 = 249.8 d and 57 Co as CoCl 2 in 0.1M HCl, T 1/2 = 271.8 d), and Isotope Product Lab., USA ( 65 Zn as ZnCl 2 in 0.5M HCl, T 1/2 = 243.9 d). Seawater was spiked with a low nominal activity of each selected radiotracer: 0.5 kBq l -1 ( 110m Ag, 57 Co, 65 Zn), 1 kBq l -1 ( 51 Cr) and 1.5 kBq l -1 ( 109 Cd). In term of stable metal concentration, these additions corresponded to Ag (19 pmol l -1 ), Cd (0.7 pmol l -1 ), Co (0.5 pmol l -1 ), Cr (2.9 pmol l -1 ) and Zn (92 pmol l -1 ), i.e. activities that are 1 to 3 orders of magnitude lower than the background concentrations of these metals in open seas [START_REF] Bruland | Trace elements in seawater[END_REF]. No change in pH was detectable in the aquarium (close circuit) after tracer addition. Seawater and spikes were renewed daily for 5 d, then every second day in order to keep exposure activities constant [START_REF] Hédouin | Allometric relationships in the bioconcentration of heavy metals by the edible tropical clam Gafrarium tumidum[END_REF]. Activity of the metal tracers in seawater was checked daily, before and after each seawater renewal, to determine time-integrated activities (Warnau et al., 1996a;Rodriguez y Baena et al., 2006a). For the entire experimental time course, the time-integrated tracer activities in seawater were 0.46 kBq 110m Ag l -1 , 1.56 kBq 109 Cd l -1 , 0.56 kBq 57 Co l -1 , 1.27 kBq 51 Cr l -1 , and 0.46 kBq 65 Zn l -1 . Shrimp were fed briefly (30 min) before each seawater and spike renewal in clean seawater so as to avoid any possible ingestion of radiotracer through the food. Shrimp were individually and periodically γ-counted alive over the 24-d exposure period to determine the radiotracer uptake biokinetics [START_REF] Warnau | Radiotracer techniques: a unique tool in marine ecotoxicological studies[END_REF].

At the end of the exposure period, the 20 blue shrimp were carefully transferred to another 70-l aquarium supplied with natural flowing seawater (open circuit; water renewal: 40 l h -1 ). They were fed daily and were periodically γ-counted to follow the radiotracer depuration kinetics over 43 d. After 4 and 43 d, 3 individuals were dissected: the abdominal exoskeleton, the abdominal muscle, the cephalothorax, the hepatopancreas and the intestine were separated, weighed (wet wt) and radioanalysed to determine the radiotracer body distribution.

When an ecdysis event occurred during the experiment, the exuvia was immediately removed from the PVC container, carefully rinsed with clean filtered seawater, weighed, and γ-counted.

Exposure via the food

Preparation of the radiolabelled food was carried out by exposing for 3 weeks commercial mussels (Mytilus edulis) via natural seawater spiked with the radiotracers: 0.5 kBq l -1 ( 110m Ag, 57 Co, 65 Zn), 1 kBq l -1 ( 51 Cr) and 1.5 kBq l -1 ( 109 Cd) [START_REF] Boudjenoun | Lead bioaccumulation in the Mediterranean mussel Mytilus galloprovincialis[END_REF].

The 20 shrimp placed in the 70-l glass aquarium (open circuit; water renewal: 40 l h -1 )

were each provided with one radiolabelled mussel that was introduced in each PVC container.

Shrimp were allowed feeding on the mussel for 3 h (pulse-chase feeding method; see e.g. Warnau et al., 1996a;Metian et al., 2008c). In parallel, a group of 3 additional shrimp was placed in the same aquarium in 3 additional PVC containers and fed uncontaminated mussels. These individuals were used as a control for any possible radiotracer recycling from seawater due to radiotracer leaching from the contaminated food or, later on, from shrimp depuration. At the end of the feeding period, each shrimp (including the control ones) was whole-body γcounted alive. From then onwards, organisms were regularly radioanalysed to follow the radiotracer depuration kinetics over 39 d. After 39 d of depuration, 3 individuals were dissected to determine the body distribution of the radiotracers. As for seawater experiment, when moulting events did occur, exuviae were immediately removed to be radioanalysed.

Radioanalysis

Radioactivity was measured using a high-resolution γ-spectrometer system composed of three Germanium -N or P type-detectors (EGNC 33-195-R, Eurysis ® ) connected to a multi-channel analyzer and a computer equipped with a spectra analysis software (Interwinner ® 6). The radioactivity was determined by comparison with standards of known activity and of appropriate geometry. Measurements were corrected for counting efficiency and physical radioactive decay. Counting time was adjusted to obtain propagated counting errors less than 5% (Rodriguez y Baena et al., 2006b). Counting times usually ranged from 10 to 20 min for the radioanalysis of living organisms.

Metals in organisms from the field

Trace element analysis

Twelve L. stylirostris were collected in the ponds of the "Station d'Aquaculture Ifremer de Saint-Vincent" in March 2005. Each individual was rinsed thrice with seawater to remove sediment and other particles, weighed (5.3 ± 1.2 g wet wt) and then dissected to separate exoskeleton (abdominal exoskeleton including pleiopods and tail), muscle (abdominal muscle), cephalothorax, hepatopancreas, and intestine. All tissue samples were dried at 60°C until reaching a constant weight, ground to powder and digested according to the method described in Hédouin et al. (2008). Elements were then analysed using a Varian ® Vista-Pro ICP-OES (As, Cr, Cu, Mn, Ni, and Zn) or a Varian ® ICP-MS Ultra Mass 700 (Ag, Cd and Co). Measurements of Certified Reference Materials (dogfish liver DOLT-3, NRCC, and lobster hepatopancreas TORT-2, NRCC) were in good agreement with the certified values given and indicated the following recoveries (in %): 103 (Ag), 98 (As), 103 (Cd), 112 (Co), 79 (Cr), 95 (Cu), 84 (Mn), 113 (Ni), ands 106 (Zn). The detection limits (µg g -1 dry wt) were 10.1 (As), 0.8 (Cr), 0.5 (Cu), 0.04 (Mn), 1.1 (Ni) and 0.7 (Zn) for ICP-OES and 0.1 (Ag), 0.3 (Cd) and 0.03 (Co) for ICP-MS. Mean element concentrations are given on a dry weight basis (µg g -1 dry wt).

Preliminary risk assessment for consumers

From the element levels measured in the shrimp muscles, a recommended maximum number of shrimp to be consumed was assessed on the basis of the Provisional Maximum Tolerable Daily Intake (PMTDI) and Provisional Tolerable Weekly Intake (PTWI) given by [START_REF] Who | Environmental Health Criteria 18: Arsenic[END_REF] and by the Joint Expert Committee on Food Additives (JECFA, 2006(JECFA, , 2008)).

The computation method is described in Metian et al (2008a) and [START_REF] Chouvelon | Hg concentrations and related risk assessment in coral reef crustaceans, molluscs and fish from New Caledonia[END_REF] and was carried out for As, Cd, Cu and Zn (for which a PMTDI or PTWI does exist), taking into account abdominal muscle, abdominal muscle + intestine or abdominal muscle + intestine + hepatopancreas (i.e., the three ways shrimp are eaten). PMTDI for Cu and Zn are 500 and 1,000 µg kg -1 human body weight d -1 , respectively and PTWI for As and Cd are 15 and 7 µg kg -1 human body weight wk -1 , respectively.

Data treatment

Whole-body uptake and depuration kinetics of the seawater and feeding exposure experiments were fitted using the kinetic models, nonlinear regression routines and iterative adjustment (Statistica® 6), and statistical methods as described by Warnau et al. (1996a,b) and [START_REF] Metian | Interspecific comparison of Cd bioaccumulation in European Pectinidae (Chlamys varia and Pecten maximus)[END_REF].

Statistical analyses of the field data regarding metal and metalloid concentrations in the shrimp were performed by one-way ANOVA, followed by the multiple comparison test of Tukey [START_REF] Zar | Biostatistical analysis[END_REF].

The level of significance for statistical analyses was always set at α = 0.05.

Results

Radiotracers experiment:

Exposure via seawater

The bioaccumulation kinetics of 57 Co, 51 Cr, and 65 Zn in whole-body L. stylirostris were best described by a first-order saturation model whereas bioaccumulation of 110m Ag and 109 Cd was best fitted by a linear model (Fig. 1, Table 1). The values estimated for the uptake rate constant, k u , allowed ranking the radiotracers according to the following order of initial uptake efficiency: 65 Zn > 110m Ag > 57 Co > 109 Cd > 51 Cr (Table 1) whereas the concentration factors observed at the end of the exposure period, CF 24d , ranked the elements according to the following order: 110m Ag > 65 Zn > 109 Cd > 57 Co > 51 Cr.

When non-contaminating conditions were restored, the depuration kinetics of the 5 radiotracers were followed for 43 d. The whole-body depuration of 110m Ag was best fitted by a single-component exponential model, whereas depuration kinetics of the 4 other radiotracers were best described by a double component exponential model (Fig. 1, Table 1). Estimated biological half-lives indicate that in terms of metal retention strength, the elements examined are ranked according to the following order: 109 Cd > 51 Cr > 65 Zn ≈ 110m Ag ≈ 57 Co.

Exposure via the food

During the food experiment, frequent monitoring of the control group of shrimp demonstrated that no significant secondary contamination from radiotracer recycling via seawater did occur over the entire observation period. Whole-body depuration kinetics of 110m Ag and 57 Co ingested with food (i.e. radiolabelled mussels) were best described by a single-component exponential model whereas depuration of ingested 109 Cd, 51 Cr, and 65 Zn were best fitted by a double-component exponential model (Fig. 2, Table 1). Assimilation efficiency (AE; i.e. A 0l ) of the radiotracers ingested with food was generally elevated (AE > 38%) whereas the biological half-life of the radiotracers was generally short, especially for 110m Ag, 109 Cd and 57 Co (T b½ < 11 d) (Table 1).

Organotropism

During the depuration phase of both seawater and feeding experiments, dissections were carried out in order to determine the metal distribution among selected body compartments of L. stylirostris at 4 and 43 d of depuration following the 24-d seawater exposure and at 39 d following the single radiolabelled feeding (Table 2). Because of the fast physical decay of 51 Cr, this radiotracer was only detected at the first dissection time (4 th d of seawater depuration). 110m Ag was always mainly distributed in the hepatopancreas (> 50%) whereas 65 Zn was mainly occurring in both the abdominal muscle and hepatopancreas.

Regarding 109 Cd, 57 Co and 51 Cr, the body distribution at the 4 th d of seawater depuration clearly showed a concurrent role of hepatopancreas and cephalothorax in element storage.

Later on in the seawater depuration period, as well as at the end of the post-feeding depuration period, the cephalothorax became the predominant body compartment for 109 Cd (≥ 66%) whereas 57 Co was mainly distributed in the cephalothorax and the exoskeleton.

Interestingly, body distributions at the end of seawater and feeding depuration period were found to be not significantly different from each other (Table 2).

In term of radiotracer activity concentration (Table 3), three compartments were found to display the highest values: hepatopancreas, intestine, and cephalothorax. The activity concentrations generally decreased significantly (up to one order of magnitude) in hepatopancreas and cephalothorax during the depuration after seawater exposure. In contrast, the activity concentrations in the intestine remained unchanged ( 109 Cd) or increased significantly ( 110m Ag and 57 Co) during the depuration phase (Table 3).

Element release during ecdysis

No moulting events were observed during the metal exposure periods. In contrast, a total of 23 individuals moulted at 6 different times during the depuration phases (8 individuals in the seawater experiment and 15 in the feeding experiment). Except for 110m Ag, metal proportions released with the exuviae were lower in the feeding experiment than in the seawater experiment. During seawater depuration, exuvia-associated 51 Cr and 65 Zn proportion tended to be somewhat higher during the first 2 weeks; the repartition of the other elements remained similar during the depuration phase. Considering all moulting events together, the proportion (average and range) of metal associated with the exuviae was: 0.4 ± 0.4% (0.1 -1.3%) for 110m Ag, 2.4 ± 1.5% (0.7 -4.5%) for 109 Cd, 33 ± 15% (17 -52%) for 57 Co, 25 ± 16% (0.4 -45%) for 51 Cr, 10 ± 6% (1.5 -19%) for 65 Zn during the seawater depuration phase (n = 8) and 0.7 ± 0.8% (0.1 -2.4%) for 110m Ag, 0.8 ± 0.7% (0.2 -2.6%) for 109 Cd, 6.9 ± 3.6% (1.9 -14%) for 57 Co, 13.1 ± 5.4% (8 -28%) for 51 Cr, 1.3 ± 0.7% (0.2 -2.9%) for 65 Zn during the feeding depuration phase (n = 15).

Field Study

Metal and metalloid analysis

Arsenic and metal concentrations measured in the different body compartments of the shrimp collected from the Saint-Vincent aquaculture farm are given in Table 4. For all elements, hepatopancreas and/or intestine displayed the highest concentrations.

In terms of body burden distribution, the elements were distributed is a quite variable way among body compartments (Table 5). The only exception was Ag that was mainly (98%) contained in the hepatopancreas. The abdominal muscle displayed the highest load of As and Zn (40 and 43%). Cd was mainly (93%) occurring in cephalothorax and hepatopancreas.

Regarding Co, Cu and Mn, the major part of these metals (74 -88%) was localized in the exoskeleton and cephalothorax, whereas Cr and Ni were mainly distributed (82 -87%) in muscle and exoskeleton.

Preliminary risk assessment

The concentrations measured in the edible part (Table 4) allowed computing the number of shrimp that a human must eat to reach the PMTDI and PTWI of the elements for which such a parameter is provided, i.e. As, Cd, Cu and Zn [START_REF] Who | Environmental Health Criteria 18: Arsenic[END_REF]JECFA, 2006JECFA, , 2008)). Computations were realised with different sets of data corresponding to several combinations of body compartments in order to take into account all the parts that can be eaten according to the care with which the animal is decorticated (i.e., abdominal muscle alone and abdominal muscle + intestine) and the taste of the consumer (abdominal muscle + hepatopancreas and abdominal muscle + intestine + hepatopancreas).

Calculations indicated that the number of shrimp to be eaten to reach a contamination risk was always high (several hundreds to several thousands; data not shown). When considering abdominal muscle alone, the worst scenario was found to be As: a 50-kg person should eat 195 shrimp a week (i.e. 3.84 µg of total As) to reach the maximum recommended weekly intake. If abdominal muscle + intestine + hepatopancreas are considered, the same person should eat a slightly lower amount of shrimp per week (viz., 184 wk -1 ).

Discussion

Metal releases resulting from the nickel mining industry and related activities are an environmental conservation issue of concern in New Caledonia [START_REF] Labrosse | New Caledonia in[END_REF][START_REF] Labonne | Otolith chemical signature and growth of Chaetodon speculum in coastal areas of New Caledonia[END_REF]Metian et al. 2008a).

Surprisingly, few information are available in the open literature regarding metal contamination in seafood from New Caledonia (see e.g., [START_REF] Labonne | Otolith chemical signature and growth of Chaetodon speculum in coastal areas of New Caledonia[END_REF]Metian et al. 2008a;[START_REF] Chouvelon | Hg concentrations and related risk assessment in coral reef crustaceans, molluscs and fish from New Caledonia[END_REF]. In particular, besides the recent study of [START_REF] Chouvelon | Hg concentrations and related risk assessment in coral reef crustaceans, molluscs and fish from New Caledonia[END_REF] very little has been published on the blue shrimp L. stylirostris, for which this country is the first world aquaculture producer [START_REF] Fao | ProdSTAT: livestock (primary and processed), FAOSTAT, FAO[END_REF]. Moreover, to the best of our knowledge, studies that have investigated metals in L. stylirostris from other regions of the world were devoted to the measurement of metal concentrations in the main edible part (i.e. the abdominal muscle) (e.g. Paez-Osuna and Ruiz-Fernandez, 1995a;[START_REF] Frías-Espericueta | Metal content of the Gulf of California blue shrimp Litopenaeus stylirostris (Stimpson)[END_REF]. Hence, nothing is known regarding the behaviour of this shrimp towards metals occurring in different compartments of its environment (dissolved vs particulate metals). This information is however of great interest in the context of a cultivated organisms, as it could allow controlling -at least partly-the metal intake in the shrimp by regulating environmental parameters (water vs added food quality).

Our work was carried out using both laboratory and field approaches. Laboratory experiments were designed to better understand the metal bioaccumulation capacity of the shrimp exposed via either the dissolved phase or its food. The field study was then carried out in order to establish a baseline for metal and metalloid levels in L. stylirostris organs and to assess the risk related to its consumption for humans.

Laboratory experiments have shown that L. stylirostris readily takes up Ag, Cd, Co, Cr and Zn when dissolved in seawater. This was especially true for Ag and Cd which displayed linear uptake kinetics over the 24-d observation period, indicating that these two metals were still far from reaching steady-state equilibrium and can reach, over the longer term, concentration factors (CF) much higher than those observed here (80 and 20 for Ag and Cd, respectively). The experiments also revealed that Ag, Cr and Zn were retained in shrimp tissues with biological half-lives (Tb ½ ) ≤ 35 d, whereas Cd and Co were retained much more strongly (Tb ½ : 796 and 152 d, respectively).

Following a single-feeding exposure, Ag, Cd, and Co were efficiently assimilated from food (AE > 77%) compared to Cr and Zn (AE < 54%). However, the Tb ½ of these elements did not exceed 32 d. To the best of our knowledge, trophic transfer of metals is not documented for L. stylirostris. However data on Cd and Zn have been published for other shrimp, such as Penaeus indicus [START_REF] Nunez-Nogueira | Assimilation efficiency of zinc and cadmium in the decapod crustacean Penaeus indicus[END_REF] and Palaemon spp [START_REF] Wallace | Cadmium resistance in an oligochaete and its effect on cadmium trophic transfer to an omnivorous shrimp[END_REF]Rainbow et al., 2006a,b). AEs estimated in L. stylirostris for Cd and Zn indicate that their values fall within those observed in the aforementioned shrimp species (i.e. 20 to 83%). In addition, as it has been reported for P. indicus and Palaemon spp, our observations indicate that Zn is more efficiently assimilated by L. stylirostris than Cd.

Regarding the other metals, Ag is the only element whose trophic transfer has been previously investigated in another shrimp species (Rainbow et al., 2006a,b). The latter studies reported AE for Ag in Palaemon spp ranging between 33 and 50%. Conversely, here we found an estimated A 0l value of 100% for Ag. This could mean that L. stylirostris would be able to assimilate the totality of Ag ingested with food (i.e., AE = A 0l = 100%). Such extreme assimilation efficiency has already been documented in some organisms (e.g., [START_REF] Rainbow | Trace metal concentrations in aquatic invertebrates: why and so what?[END_REF][START_REF] Luoma | Why is metal bioaccumulation so variable? Biodynamics as a unifying concept[END_REF], such as in the sea star Asterias rubens [START_REF] Temara | Experimental cadmium contamination of Asterias rubens (Echinodermata)[END_REF]Warnau et al., 1999;[START_REF] Danis | Contaminant levels in sediment and asteroids (Asterias rubens, Echinoderm) from the Belgian coast and Scheldt estuary: polychlorinated biphenyls and metals[END_REF]. In the sea star, the almost 100% AE for Co is related to the particular feeding mode of the organism (Warnau et al., 1999): A. rubens has an extraoral digestion and ingests/swallows its prey as a pre-digested material, which makes internal assimilation extremely efficient (e.g., [START_REF] Jangoux | Digestive systems: Asteroidea[END_REF]. In the case of L. stylirostris, however, such particular feeding mode is lacking [START_REF] Dall | The biology of Crustacea, internal anatomy and physiological regulation[END_REF]. In addition, observed T b1/2 was quite short (11 d) and the remaining activities measured in the body compartments 39 d after the feeding were very low (see Table 3). This suggests that the ingested Ag would remain in the digestive tract from where it could then be progressively eliminated along with the faeces rather than being entirely assimilated. Hence, the single exponential compartment that describes the depuration of dietary Ag in the Pacific blue shrimp would represent a shortlived compartment rather than a long-lived one. These observations regarding dietary Ag also suggest that the food pathway would be a minor route of contribution to the global Ag bioaccumulation in L. stylirostris. This could also explain why Ag concentrations measured in the shrimp muscles from the field were so low (see Table 4).

In invertebrates, and in crustaceans in particular, the digestive gland or hepatopancreas is well known to play an important role in metal accumulation and detoxification (e.g., [START_REF] Gibson | The decapod hepatopancreas[END_REF][START_REF] Dall | The biology of Crustacea, internal anatomy and physiological regulation[END_REF][START_REF] Rainbow | Phylogeny of trace metal accumulation in crustaceans[END_REF][START_REF] Bianchini | Short-term silver accumulation in tissues of three marine invertebrates: shrimp Penaeus duorarum, sea hare Aplysia californica, and sea urchin Diadema antillarum[END_REF].

Concentrations of Cd, Cu, Mn and Zn that we measured in the hepatopancreas of L. stylirostris from New Caledonia were approximately twice those reported for the same species from the Gulf of California [START_REF] Ruelas-Inzunza | Distribution and concentration of trace metals in tissues of three penaeid shrimp species from Altata-Ensenada del Pabellon Lagoon (S.E. Gulf of California)[END_REF]. Regarding Ag, As, Cd, and Co, we measured similar concentrations as those reported in the hepatopancreas of the black tiger shrimp Penaeus monodon from contaminated areas from South Vietnam [START_REF] Tu | Regional variations in trace element concentrations in tissues of black tiger shrimp Penaeus monodon (Decapoda: Penaeidae) from South Vietnam[END_REF]. To the best of our knowledge, the concentrations in Cd, Cr, Mn, and Ni that were found in the hepatopancreas of the New Caledonian L. stylirostris are among the highest reported for shrimp and even crustaceans (e.g., [START_REF] Eisler | Trace metal concentrations in marine organisms[END_REF][START_REF] Méndez | Mineral concentration in tissues during ovarian development of the white shrimp Penaeus vannamei (Decapoda: Penaeidae)[END_REF][START_REF] Tu | Regional variations in trace element concentrations in tissues of black tiger shrimp Penaeus monodon (Decapoda: Penaeidae) from South Vietnam[END_REF].

In contrast to hepatopancreas, the intestine is not generally considered as a major bioaccumulator tissue for metals, although some authors have reported that the hindgut may play a significant role in the elimination or sequestration of several xenobiotics, including metals such as Ag, Al, Cd, Fe and Zn (e.g., Buchanan et al., 1984;[START_REF] Berthet | Bioaccumulation, toxicity and physico-chemical speciation of silver in bivalve molluscs: ecotoxicological and health consequences[END_REF][START_REF] Bendell-Young | Comparison of metal concentrations in the fore and hindguts of the crayfish Cambarus bartoni and Orconectes virilis and implications regarding metal absorption efficiencies[END_REF][START_REF] Warnau | In vitro and in vivo investigations of the excretory function of the rectal caeca in the asteroid Asterias rubens (Echinodermata)[END_REF]. In agreement, our results have shown that when L. stylirostris is exposed to metals and then placed in clean conditions, the concentrations of 3 elements significantly increased (Ag and Co) or remained unchanged (Cd) in the shrimp intestine over the time. Overall, our dataset supports the assumption of a physiological strategy that tends to result in higher metal concentrations in the digestive organs (i.e. intestine and hepatopancreas) possibly through detoxification and/or storage mechanisms.

Following seawater or food exposures, Cd, Co and Cr were mainly associated with the cephalothorax and Zn was mainly associated with both the cephalothorax and abdominal muscle. Metal and metalloid analyses carried out on the farmed Pacific blue shrimp confirmed that the cephalothorax contained a significant fraction of As, Cd, Co, Cu, Mn (28 -57%; see Table 5). This body compartment is composed of many sub-compartments that include, among other, gills and antennal glands, which are well known to play a significant role in metal bioaccumulation and depuration in decapods (e.g. [START_REF] Arruda-Freire | Fine structure of the gills of the fresh-water shrimp Macrobrachium olfersii (Decapoda): effect of acclimation to high salinity medium and evidence for involvement of the lamellar septum in ion uptake[END_REF][START_REF] Paez-Osuna | Concentration and distribution of heavy metals in tissues of wild and farmed shrimp Penaeus vannamei from the Northwest Coast of Mexico[END_REF][START_REF] Rainbow | Phylogeny of trace metal accumulation in crustaceans[END_REF]. Obviously, cephalothorax is also composed to a large extent of exoskeleton. In this respect, our radiotracer experiments have shown that a significant fraction of Co, Cr and Zn can be found associated with the exoskeleton (see Table 2) and then be eliminated from the organism during moulting (ecdysis). Such a process has already been observed in other crustaceans and is reported as a detoxification/metal excretion mechanism (e.g., [START_REF] Keteles | The contribution of ecdysis to the fate of copper, zinc, and cadmium in grass shrimp, Palaemontes pugio Holthius[END_REF]Rodriguez y Baena et al. 2006a[START_REF] Fao | ProdSTAT: livestock (primary and processed), FAOSTAT, FAO[END_REF]. A significant metal fraction associated with the exoskeleton was also observed for Co, Cu, Cr, Mn, Ni and Zn (32 -53%; see Table 5) in the Pacific blue shrimp from the farm of St Vincent. The related distributions are in good agreement with the results reported by [START_REF] Keteles | The contribution of ecdysis to the fate of copper, zinc, and cadmium in grass shrimp, Palaemontes pugio Holthius[END_REF]. This observation was somewhat expected as these cations are known to be able to either substitute Ca in the cuticular CaCO 3 (i.e., Mn; [START_REF] Tu | Regional variations in trace element concentrations in tissues of black tiger shrimp Penaeus monodon (Decapoda: Penaeidae) from South Vietnam[END_REF] or adsorb and bind strongly to the cuticle constituents (e.g., Rodriguez y Baena, 2006a[START_REF] Fao | ProdSTAT: livestock (primary and processed), FAOSTAT, FAO[END_REF]. However, it was quite surprising to note that both experimental and field observations indicated that Ag and, secondarily, Cd were quite poorly associated with the shrimp cuticle. Indeed, Ag is well known to be a particlereactive element [START_REF] Sañudo-Wilhelmy | Anthropogenic silver in the southern California bight: a new tracer of sewage in coastal waters[END_REF]Metian and Warnau, 2008) and would thus be expected to bind in significant proportion onto the cuticle.

Our field and laboratory observations indicated that a significant proportion (20 -45%) of As, Co, Cr, Cu, Ni and Zn was associated with the abdominal muscle of L. stylirostris. In terms of element concentrations measured in shrimp from the field, the New-Caledonia Pacific blue shrimp displayed metal and metalloid levels falling generally within the range of concentrations reported for the same, or closely related species, from other Pacific areas (Table 6). This was also true for elements which occur in high levels in New Caledonian laterites (Co, Cr, Ni and Mn). This suggests that 1) these elements are not particularly bioavailable to the shrimp when the organisms are in their aquaculture habitat (Metian et al., 2008a) and/or 2) the New Caledonian blue shrimp could have developed physiological strategies to regulate its body concentrations of these metals [START_REF] White | Regulation of zinc concentration by Palaemon elegans (Crustacea: Decapoda): zinc flux and effects of temperature, zinc concentration and moulting[END_REF].

The element concentrations measured in the abdominal muscle of L. stylirostris were used to carry out a preliminary risk assessment for the human consumer in a similar way as those done by [START_REF] Frías-Espericueta | Metal content of the Gulf of California blue shrimp Litopenaeus stylirostris (Stimpson)[END_REF] for the Pacific blue shrimp from the Gulf of California and by Metian et al. (2008a) for the scallop Comptopallium radula from New Caledonia. To do so, metal and metalloid concentrations measured in the shrimp abdominal muscle (alone or combined with other organs or tissues that can be eaten along) were confronted to maximum thresholds recommended by international organisations (FAO, WHO). Computations indicated that for any element considered consumption of shrimp did not present any significant risk. Indeed, in the "worse" situation (related to arsenic), the number of shrimp to be eaten in a single week by a 50-kg adult was > 180 to reach the WHO maximum recommended weekly intake.

Conclusion

The present laboratory and field study showed that the 9 metalloid and metals considered are mainly bioaccumulated, detoxified and/or stored in the digestive system (hepatopancreas and intestine) of L. stylirostris and are not highly bioaccumulated in its abominal muscle (i.e. the main edible part). Laboratory experiments have also shown that metals bioaccumulated by the shrimp (either via water or via the food) were relatively poorly retained in its tissues and that moulting can contribute substantially to metal depuration (mainly for Co, Cr, and, to a lesser extent, Zn). With regards to the relatively low element concentrations measured in the abdominal muscle, no significant seafood safety problem could be identified for the elements examined. This is a particularly important observation in the context of the metalliferous environment of New Caledonia [START_REF] Ambatsian | High metal inputs to closed seas: the New Caledonia lagoon[END_REF][START_REF] Mugnier | Métaux et crevettes d'élevage en Nouvelle Calédonie. Travail préliminaire d'analyse de la teneur en métaux chez les crevettes et le sédiment[END_REF], as it suggests that metals typically found in New Caledonia soils are poorly bioavailable to the farmed Pacific blue shrimp.

Table 1. Litopenaeus stylirostris. Parameters (mean ± ASE, n = 20) of the whole-body uptake and/or depuration kinetics of 5 metal radiotracers in the Pacific blue shrimp exposed to the radiotracers via seawater (A) and via the food (B). Uptake parameters: CF ss : concentration factor at steady state; k u : uptake rate constant (d -1 ); L and E: linear [CF t = k u t] and exponential [CF t = CF ss (1-e -k e t )] model, respectively. Depuration parameters: A 0s and A 0l : activity (%) lost according to the short-and the long-lived exponential component, respectively; T b½ : biological half-life (d) [T b½ = ln 2 / k e ]; O and T: one-compartment [At = A 0 e -k e t ] and two-compartment [At = A 0s e -k es t + A 0l e -k el t ] exponential model, respectively. ASE: asymptotic standard error; r 2 : determination coefficient of kinetics.

A

  batch of approximately 100 Litopenaeus stylirostris (Stimpson, 1874) was obtained from the Ifremer experimental farm (Station d'Aquaculture Ifremer de Saint-Vincent, New Caledonia) in January 2002 and shipped to MEL premises in Monaco. Prior to experimentation shrimp were acclimated for 2 months to laboratory conditions (open circuit 400-l aquarium; 0.45-µm filtered water renewal: 40 l h -1 ; salinity: 36 p.s.u.; temperature: 25 ± 0.5°C; pH: 8.0 ± 0.1; light/dark cycle: 12h/12h) simulating the conditions prevailing in pond waters where shrimp are reared. During acclimation, shrimp were fed daily pre-cooked mussels. Recorded mortality was lower than 3%. Forty penaeids of similar size and weight (7.2 ± 1.1 g wet wt) were then collected and divided in two groups of 20 individuals. Each group was placed in a 70-l glass aquarium (0.45-µm filtered water; constant aeration; salinity: 36 p.s.u.; temperature: 25 ± 0.5°C; pH: 8.0 ± 0.1; light/dark cycle: 12h/12h; open vs. close circuit as indicated below) in which the experiments were conducted (see below). In order to avoid cannibalism (e.g. during moulting) and to facilitate individual recognition, during the whole duration of the experiments, each shrimp was kept individually in a cylindrical PVC container (160 mm height × 80 mm diameter) covered above and below with 300-µm mesh size net (to allow for free water circulation).

  Figure 1. Litopenaeus stylirostris. Uptake and depuration kinetics of 5 dissolved radiotracers ( 110m Ag, 109 Cd, 57 Co, 51 Cr and 65 Zn) during a 24-d exposure (Concentration factor; mean ±

Figure 2 .

 2 Figure 2. Litopenaeus stylirostris. Depuration kinetics (Remaining activity -%-; mean ± SD; n = 20) of 5 radiotracers ( 110m Ag, 109 Cd, 57 Co, 51 Cr and 65 Zn) after a single feeding on

Table 2 .

 2 Litopenaeus stylirostris. Radiotracer distribution (%; mean ± SD, n = 3) among the tissues and organs of the Pacific blue shrimp after 4 and 43 d of depuration following seawater exposure and after 39 d following radiolabelled food ingestion.

	Uptake

Table 3 .

 3 Litopenaeus stylirostris. Activity concentration (Bq g -1 wet wt; mean ± SD, n = 3) of radiotracers in the body compartments of the Pacific blue shrimp during the depuration period following seawater and food exposures.

		Weight (g)	110m Ag	109 Cd	57 Co	51 Cr	65 Zn
	Seawater experiment		4 d of depuration
	cephalothorax	2.61 ± 0.48 6.31 ± 1.27	56.8 ± 7.2	2.22 ± 0.16 3.76 ± 1.34	20.2 ± 2.9
	exoskeleton	0.71 ± 0.21	< 0.01	18.2 ± 2.9	1.57 ± 1.01 2.71 ± 0.89	13.5 ± 8.9
	intestine	0.04 ± 0.02 1.83 ± 0.86 88.8 ± 52.7 2.77 ± 1.41 20.2 ± 10.3 24.9 ± 20.1
	hepatopancreas	0.25 ± 0.05	276 ± 99	468 ± 177	35.5 ± 24.5 23.6 ± 11.3 39.5 ± 23.6
	muscle	2.79 ± 0.92	< 0.01	4.14 ± 1.73 0.21 ± 0.02 0.82 ± 0.02	11.9 ± 2.8
				43 d of depuration
	cephalothorax	2.75 ± 0.17 0.31 ± 0.04 31.3 ± 3.72 0.27 ± 0.03	< d.l.	6.52 ± 0.74
	exoskeleton	1.17 ± 0.58	< 0.01	9.11 ± 1.41 0.54 ± 0.31	< d.l.	3.39 ± 0.68
	intestine	0.02 ± 0.01 12.7 ± 5.55 86.3 ± 41.1 8.79 ± 3.40	< d.l.	9.05 ± 7.83
	hepatopancreas	0.33 ± 0.08 17.0 ± 9.85	38.2 ± 9.2	0.62 ± 0.25	< d.l.	6.82 ± 0.96
	muscle	3.47 ± 0.25	< 0.01	2.18 ± 0.43 0.11 ± 0.05	< d.l.	5.75± 0.71
	Feeding experiment		39 d of depuration
	cephalothorax	3.74 ± 0.31 0.06 ± 0.02	18.4 ± 9.2	0.09 ± 0.02	< d.l.	11.4 ± 1.4
	exoskeleton	1.32 ± 0.11	< 0.01	1.44 ± 0.77 0.14 ± 0.04	< d.l.	6.73 ± 0.59
	intestine	0.02 ± 0.01 4.28 ± 2.88 52.9 ± 32.6 8.02 ± 3.86	< d.l.	8.40 ± 2.21
	hepatopancreas	0.39 ± 0.12 1.97 ± 0.95 74.0 ± 55.7 0.38 ± 0.05	< d.l.	14.7 ± 1.1
	muscle	4.51 ± 0.62	< 0.01	0.41 ± 0.29 0.05 ± 0.01	< d.l.	9.91 ± 0.83

Table 4 .

 4 Litopenaeus stylirostris. Element concentrations (µg g -1 dry wt; mean ± SD, n = 12) in tissues and organs of the Pacific blue shrimp collected in the aquaculture farm of Saint-Vincent, SW New Caledonia.

		Ag	As	Cd	Co	Cr	Cu	Mn	Ni	Zn
	Cephalothorax	< 0.01	8.6 ± 2.1 0.24 ± 0.11 0.27 ± 0.02 0.9 ± 0.6	129 ± 16	21 ± 5	1.5 ± 0.5	74 ± 6
	Exoskeleton	< 0.01	7.3 ± 0.9 0.02 ± 0.01 0.20 ± 0.06 3.7 ± 3.3	158 ± 19	18 ± 8	4.6 ± 3.7	81 ± 14
	Hepatopancreas	1.8 ± 1.3	22 ± 12	6.5 ± 3.8	3.1 ± 1.7	2.5 ± 2.8 1097 ± 726 64 ± 41	16 ± 22	269 ± 158
	Intestine	0.04 ± 0.04 18 ± 18	0.5 ± 0.4	1.2 ± 1.6	12 ± 17	118 ± 118	50 ± 36	21 ± 14	158 ± 92
	Muscle	< 0.01	6.4 ± 1.1	< 0.01	0.03 ± 0.02 1.8 ± 2.0	40 ± 10	1.8 ± 1.9	1.7 ± 1.4

Acknowledgements

Authors thank the Ifremer Station of St Vincent, New Caledonia for providing the organisms, O. Cotret (IAEA-MEL) for skilful technical assistance and A.G.J. Tacon (Aquatic Farms Ltd, Hawaii) for critical reading of a previous version of the manuscript. MW is an Honorary Senior Research Associate of the National Fund for Scientific Research (NFSR, Belgium) and holds a 2008 Invited Expert position at LIENSs (CNRS-Université de La Rochelle), supported by the Conseil Régional de Poitou-Charente. This work was supported by the IAEA, the French PNEC Programme (Chantier "Nouvelle-Calédonie"), IRD and the French GIP Seine-Aval (PhD grant to MM). The IAEA is grateful for the support provided to its Marine Environment Laboratories by the Government of the Principality of Monaco.


Jean Geringer
email: geringer@emse.fr

Bernard Normand
email: bernard.normand@insa-lyon.fr

Catherine Alemany-Dumont
email: catherine.alemany-dumont@insa-lyon.fr

Robert Diemiaszonek
email: robert.diemiaszonek@insa-lyon.fr

Catherine Alemany

Assessing the tribocorrosionnext

Keywords: Tribocorrosion, Cu, Al, EIS

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

Tribocorrosion is defined as materials degradation of solids in contact resulting from chemical, electrochemical and mechanical processes. These surface interactions occur during rolling and/or sliding contact between two materials in a corrosive medium such as an aqueous solution [START_REF] Landolt | Electrochemical methods in tribocorrosion: a critical appraisal[END_REF]. Thus, in mechanical devices, decreased performance decreasing and environment contamination occur. Tribocorrosion affects numerous industrial fields such as nuclear industry [START_REF] Kaczorowski | Wear problems in nuclear industry[END_REF], food manufacturing or medical implants [START_REF] Yan | Tribocorrosion in implants-assessing high carbon and low carbon Co-Cr-Mo alloys by in situ electrochemical measurements[END_REF]. The electric and electronic fields are also increasingly concerned by tribocorrosion. The investigated materials, in this work, are copper and aluminium that are related with these fields.

Since the 70s, tribocorrosion processes have been increasingly investigated. Different methods (including design of new tests) are developed to understand, to predict and to model the impact of the mechanical solicitation on the electrochemical properties of materials [START_REF] Landolt | Electrochemical and materials aspects of tribocorrosion systems[END_REF].

As a result, tribocorrosion can be analyzed using classical electrochemical techniques: open circuit potential evolution according to normal load or frequency [START_REF] Celis | Tribo-corrosion of materials: Interplay between chemical, electrochemical, and mechanical reactivity of surfaces[END_REF]; potentiostatic measurements for quantitative analysis of wear [START_REF] Geringer | Wear analysis of materials used as orthopaedic implants[END_REF]; current and potential monitoring under open circuit conditions, i.e. electrochemical noise measurements [START_REF] Berradja | An electrochemical noise study of tribocorrosion processes of AISI 304 L in Cland SO 4 2media[END_REF]. Electrochemical Impedance Spectroscopy (noted EIS) was also described by several authors as an interesting tool to investigate mechanisms occurring on both material surfaces in contact [START_REF] Ponthiaux | Utilisation du bruit électrochimique pour déterminer la surface dépassivée par frottement, cas d'un acier Z2CND 17-13 en milieu chloruré (NaCl 3 %)[END_REF][START_REF] Ponthiaux | Electrochemical techniques for studying triocorrosion processes[END_REF][START_REF] Macdonald | Relections on the history of electrochemical impedance spectroscopy[END_REF].

Interfacial reactivity can be modelled by an association of basic electrical components such as resistance, capacitance or inductance [START_REF] Gabrielli | Identification of electrochemical processes by frequency response analysis[END_REF]. In this study EIS experiments were performed to obtain these physical values.

A model tribosystem based on Al-Cu has been chosen for this study because it highlights galvanic coupling due to the high electrochemical potential gap in the considered electrolyte [START_REF] Pourbaix | Atlas d'équilibres électrochimiques[END_REF]. Cu effect on the Al corrosion resistance is widely studied in material science [START_REF] Keddam | Exfoliation corrosion of Al alloys examined by electrode impedance[END_REF][START_REF] Guillaumin | Localized corrosion of 2024 T351 Al alloy in chloride media[END_REF]. Recently galvanic corrosion between Cu and Al was particularly studied by local Electrochemical Impedance Spectroscopy (LEIS) [START_REF] Jorcin | Etude du comportement en corrosion d'alliages modeles Al/cuivre à l'aide de la spectroscopie d'impedance electrochimique locale[END_REF]. In contrast, the electrochemical behaviour of the Cu/Al contact under sliding has been much less considered in acidic solution using EIS technique. Acid solution was used for promoting the dissolution of metal and for enhancing galvanic coupling.

In this paper, the electrochemical behaviour of Cu and Al was studied separately to characterize the Potential of Zero Charge (noted PZC). This physical characteristic informs about attraction or repulsion phenomena (which play a role in the friction evolution) between two surfaces in contact [START_REF] Geringer | Wear analysis of materials used as orthopaedic implants[END_REF]. The static contact was investigated to estimate the galvanic coupling. Finally, the electrochemical behaviour of dynamic contact, i.e. under sliding conditions was characterized. In addition, SEM and EDS were used to understand the wear mechanisms.

Experimental

Materials

The chemical composition of Al alloy (A 2017 [START_REF] Vargel | [END_REF]) was measured by spark emission spectroscopy on a SEO JY 50 E equipment (Table 1).

[Table 1]

Cu composition was measured by electrolysis and confirmed by atomic absorption spectroscopy. Because minor metallic elements such as Cr, Ni, Ag, Fe are lower than 0.002 % weight percent, Cu purity was established to 99.80 %.

Tribocorrosion device

A wear corrosion apparatus was developed in MATEIS laboratory, for pure sliding, from reciprocating displacement, in a pin/disc configuration, figure1.

[Figure 1]

Displacement amplitude of ± 8.6 mm at a frequency of 1 Hz was applied. Electrical contacts, between device and samples, were eliminated by insulation of samples and elements of the tribosystem.

Al alloy pin is presented schematically on figure 1, with an area of 0.28 cm -2 . The upper part was embedded in poly (methyl methacrylate) for electrical insulation. The lower part was previously covered by cataphoretic epoxy-amine base paint.

The paint protects the lateral surface from corrosion in an acid solution and defines the size of reactive surface. The contact diameter was equal to 6 mm. In addition, to obtain a planar contact, a polishing step of the pin was carried out on the friction device with grit paper of grading 180 to 2500.

The Cu samples were discs with a diameter of 25 mm (area of 19.63 cm 2 ) and a thickness of 1 mm machined in industrial production. A surface roughness of Ra = 0.10 ± 0.05 µm was obtained after a polishing procedure including grit paper 180 to 1200 and finally a paraffin solution and alumina particles to obtain mirror surface.

Normal load equal to 6.7 N was applied by dint of calibrated rod fixing the contact diameter and the temperature was maintained to 23 ± 2 °C.

Electrochemical measurement

Acid solution was chosen to maintain an active state of materials constituting the tribosystem [START_REF] Pourbaix | Atlas d'équilibres électrochimiques[END_REF]. The electrolyte was an aerated solution of 0.5 mol.L -1 Na 2 SO 4 having a pH of 1.7 obtained by a H 2 SO 4 buffer.

Electrochemical measurements, including free corrosion potential (E corr ) as a function of time and electrochemical impedance measurements, were carried out. K 2 SO 4 solution, was used and the ionic junction was located very close to the surface of the working electrode. The potential difference against the standard hydrogen electrode (SHE) was measured about 0.615 V at 25°C. EIS spectra were performed in the frequency range from 10 kHz to 15 mHz at 10 points per Hertz decade with an AC voltage amplitude of 10 mV. Data were validated by Kramers-Koenig (K-K) transformation [START_REF] Macdonald | Characterizing electrochemical systems in the frequency domain[END_REF][START_REF] Orazem | Systematic approach toward error structure identification for impedance spectroscopy[END_REF]. Cu discs and Al pins were studied separately as a working electrode before EIS diagram was performed during friction. Exposed areas of Cu and Al samples were 19.63 cm² and 0.20 cm² respectively. Graphite counter electrode was used with a diameter of 5 mm and a length of 70 mm. Impedance experiments were conducted using Princeton Applied Research (Parstat 2263) equipment. Impedance monitoring was performed at Ecorr and applied potential.

Scanning Electron Microscope

Cu and Al samples were observed with a Scanning Electron Microscope (SEM) JEOL 6400 coupled with an EDS probe. A gold skin covered the worn surfaces, after friction tests, to highlight transferred particles between two materials in contact. These particles were oxide.

They require a gold coating to obtain the best images and to avoid charge accumulation.

Results and discussion

Data analysis

Here, the selected pH is leading to a dissolution of Al and Cu. Figure 2 illustrates experimental EIS response recorded for Al alloy immersed during 3 hours in acid solution.

Spectrum exhibits two time constants. At high frequencies, the depressed capacitive loop corresponds to the interfacial capacitance with a native oxide layer on the alloy surface [START_REF] Keddam | Exfoliation corrosion of Al alloys examined by electrode impedance[END_REF].

At low frequencies, the inductive loop is associated with the relaxation processes related to the weakening of the protective efficiency of the oxides layer due to dissolution [START_REF] Keddam | Exfoliation corrosion of Al alloys examined by electrode impedance[END_REF]. Outliers recorded at very low frequencies can be attributed to the non-stability of the Al corrosion processes. Their elimination was improved from Dygas et al. approach [START_REF] Dygas | Variance of errors and elimination of outliers in the least squares análisis of impedance spectra[END_REF].

[Figure 2]

The Nyquist diagram of Cu is represented in figure 3. The diagram is characterized by one depressed capacitive loop with the center under the abscissa axis. Because the pH of the test electrolyte induces an active dissolution of Cu, the reaction mechanism proposed for the larger electrode can be interpreted as the charge transfer resistance (R ct ) in parallel with the double layer capacity (C dl ).

[Figure 3]

This approach aims at getting the electrical parameters that are consistent with the materials reactive surface from EIS measurements. Their contributions to the total impedance are well interpreted by analyzing the system thanks to an equivalent circuit.

To be useful, the elements of an equivalent circuit should always have a physical meaning in the electrochemical system. In this work, the elements were chosen following these two conditions:

• numerous electrical models are proposed for Al alloy sample in different medium including numerous associations of resistance, capacitance and inductance [START_REF] Snogan | Characterisation of sealed anodic films on 7050 T74 and 2214 T6 Al alloys[END_REF][START_REF] Nigam | Electrochemical impedance spectroscopy and cyclic voltammetry study of carbon-alloyed iron aluminides in sulphuric acid[END_REF][START_REF] Wit | Electrochemical impedance spectroscopy as a tool to obtain mechanistic information on the passive behaviour of Al[END_REF][START_REF] Harrington | Stability and electrochemical impedance of mechanisms with a single adsorbed species[END_REF].

Because this study is dedicated to compare the electrochemical behaviour of each sample separately with one of both samples coupled electrically during friction, the equivalent electrical circuit has to be chosen both for each material and for the tribosystem constituted by both materials in contact.

• Reciprocating test lead to a surface activation that could induce some wrong EIS results, especially for low frequencies, in accord with corrosion phenomena. Thus, to get around this point, one considers only high frequencies.

By the way, a simple equivalent circuit of a resistor and a capacitor in parallel is sufficient to extract parameters from diagrams of the impedance, at high frequencies. From Nyquist diagrams, the high frequency domain corresponds to the charges transfer. Indeed, one might suggest that friction will exhibit this phenomenon. So, the inductive behaviour of Al alloy sample has been filtered.

[Figure 4]

The selected electrical circuit was a series association R e (CPE hf , R hf ) presented in figure 5. R e is the electrolyte resistance. R hf is a resistance that is associated in parallel with the CPE hf .

CPE hf is the Constant Phase Element (hf: high frequencies) used in order to include the divergence from pure capacitive behaviour. The value of CPE hf is as

1 () CPE n hf Z Qj ω = (eq. 1)
Q hf is the numerical value of the admittance ( )

1/ CPE Z at ω = 1 rad.s -1 (S.s n .cm -2 ), j 2 = -1 the
imaginary unit, ω is the angular frequency (rad.s -1 ) and n is the CPE exponent (0 ≤ n ≤ 1)

[Figure 5]
Then electrical values, resistances and capacitances, calculated from CPE, were compared between Al alloy and Cu samples, between the static contact (Al-Cu) and sliding contact (Al/Cu). These results allow calculating the capacitance according to Brug's relation [START_REF] Brug | The analysis of electrode impedances complicated by the presence of a constant phase element[END_REF]:

1- () n n hf hf hf hf CQ Q R = (eq. 2)

Impedance diagrams for Al alloy and Cu, separately

Al alloy sample

During experiments, the Ecorr changes were lower than 10 mV, which is consistent with the small amplitude of the perturbation voltage for EIS measurements.

The values of the electrical components were calculated for the same Al sample and different time of storage in the acid solution, they are shown in Table 2.

[Table 2]

Table 2 exhibits no drastic evolution of the capacitance, the resistance, and n. Thus, it is worth noting that, after 4.5 hours, the electrical behaviour of Al does not suffer changes.

Cu sample

During experiments, the Ecorr of the Cu sample did not suffer significant changes as the Al sample. The capacitance increases according to the exposure time, in acid solution and R hf decreases as summarized in Table 3. The CPE exponent values show that the double layer is less and less homogeneous, probably due to the modification of roughness during dissolution.

Moreover, due to the Cu dissolution, sulfate diffusion could occur in order to balance charges.

As the potential can be considered as constant, charges number increases according to the exposure time.

[Table 3]

Because pH is 1.7, Cu dissolution can be described following two steps [START_REF] Ma | Impedance spectroscopic study of corrosion inhibition of Cu by surfactants in the acidic solutions[END_REF]. The first fast step corresponds to the Cu dissolution from Cu (s) to Cu(I). Cu(I) ads is generally considered as an adsorbed species at the Cu surface and does not diffuse into the bulk solution. The second slower step corresponds to oxidation of Cu(I) in Cu(II). Consequently, one might suppose that higher value of R hf for Cu than R hf value obtained with Al can be related to a barrier effect of salt of Cu(II) formation decreasing the kinetic rate of Cu dissolution. In addition, the mass transport has a low influence on dissolution of Cu.

Potential at the Potential of Zero Charge (PZC) for Al alloy and Cu

Sliding phenomena partially depend on electrostatic state of material surface in contact. The potential of zero charge (PZC) plays a major role in electrostatic processes. Different PZC values for Cu immersed in different solution are proposed in literature i.e. Bockris and Reddy's report [26]. EIS offers a good method to determine the PZC of materials in solution which is considered like the minimum of the capacitance according to the applied potential [START_REF] Ma | Impedance spectroscopic study of corrosion inhibition of Cu by surfactants in the acidic solutions[END_REF]. If the capacitance is minimum, the thickness of the double layer is maximum and the charges equilibrium is reached to the zero charge state.

A series of impedance spectra for Cu and Al electrodes has been recorded on sulphuric acid solution in a large range of potential (between -0.47 and + 0.37 V vs. E corr ). Double layer capacitances evaluated from electrical equivalent circuit described previously, are plotted against applied potential in Figure 6. Capacitances at Open Circuit Potential (OCP) are also reported on the figure 6.

[Figure 6]

Figure 6 shows that C hf of Al sample is approximately constant. The C hf , calculated at open circuit potential (Ecorr), is four times higher than the one calculated at applied potential.

Although the potential value is the same, the previous cathodic polarization modifies the Al alloy surface during applied potential tests. The curve does not exhibit a minimum of capacitance.

For the Cu sample, C hf is the lowest at Ecorr. The minimum corresponding potential is more negative than the Ecorr. At high frequencies, a capacitive loop from the relaxation of the double-layer capacitance is observed. The faradic process is slow compared to double layer relaxation (phenomena can not be coupled) and specific ions are not adsorbed. However, this last hypothesis could be questioned. Indeed, the capacitance of Cu, at open circuit potential (OCP) potential is not rigorously constant and the salts on the surface could participate in this evolution [START_REF] Ma | Impedance spectroscopic study of corrosion inhibition of Cu by surfactants in the acidic solutions[END_REF]. The same behaviour could occur at applied potential, close to Ecorr. However, the evolution of the Cu C hf shows a minimum that is relevant to the PZC value.

In this work, the PZC value of Cu sample is estimated to be -0.47 ± 0.04 V versus MSE that is in accordance with Ma et al. [START_REF] Ma | Impedance spectroscopic study of corrosion inhibition of Cu by surfactants in the acidic solutions[END_REF] who found a value of -0.56 V in sulphuric acid, 0.5 mol.L -1 .

However, experimental protocol does not allow to find the PZC of Al alloy. Thus, one might suggest that the PZC of Al alloy is close to the OCP around -1.13 V vs MSE [26]. Further investigations have to be carried out with only one sample per value of applied potential.

Figure 7 shows evolution of OCP of the contact between Al and Cu before, during and after sliding.

[Figure 7]

The OCP value is between one of PZC of Cu and Al. Thus, positive charges on Al alloy surface are opposite to negative ones on Cu surface. If the electrostatic attractions are considered, the friction coefficient would be higher in these conditions meaning that EIS measurements contribute to revealing the synergistic effect in tribocorrosion. However, no friction coefficient can be measured with this equipment. Additional works will be carried out in order to correlate friction coefficient evolution at different applied potential. Related to this electrostatic effect between surfaces in contact, this problem was already highlighted in the case of 316L stainless steel / poly(methyl)methacrylate contact [START_REF] Geringer | Wear analysis of materials used as orthopaedic implants[END_REF].

Impedance diagrams for static and dynamic contacts

Static contact between Cu and Al

Table 4 shows the capacitance, the resistance and χ 2 , defined as

( ) 2 2
,, log log

i experimental i calculated i ZZ χ =-∑
, for the static contact between Cu and Al alloy, at high frequencies. The capacitance can be considered constant as well as the resistance.

[Table 4]

Table 5 presents capacitances and resistances of: -Al electrode before contact; -Cu electrode before contact; -Al/Cu static contact: association of Al alloy and Cu in static contact. Table 5 presents normalized results (table 5a) and no normalized ones (table 5b) according to areas of each surface.

[Table 5]

On Table 5 a), the capacitance calculated for the static contact was close to the addition of the capacitances of Al alloy and Cu. Moreover, the resistance, R hf , is close to (1/R hf (Al)) + (1/R hf (Cu)). As a result, the Al alloy and Cu contact is consistent with a parallel association for normalized electrical parameters. On Table 5 b), the results are reported without normalization. The sum of capacitances that are consistent with a parallel association of capacitances is not equal to the capacitance of the static contact. The parallel association of resistances is not consistent with the resistance of the static contact. The no-normalized R hf , respectively C hf , of the static contact is 20 times lower, respectively 10 times higher, than that of normalized value. Due to the decrease of the R hf , one might suggest that the dissolution is triggered by the galvanic coupling. The potential difference between both materials is significant. Al and Cu samples play roles of the anode and the cathode, respectively. Cathodic reaction (oxygen reduction) occurs on Cu surface and anodic dissolution on Al alloy. The entire Cu surface does not play the role of cathode. Anodic reaction should occur on the Cu worn zone, due to galvanic coupling, during friction. Around the friction zone, one might suggest the local potential should decrease from anodic to cathodic values far from the contact zone.

Dynamic contact made of Cu and Al alloy and post contact

During friction, numerous wear phenomena occur: transfer, oxidation, mechanical degradation as ploughing, wear, etc. The worn area represents 4 % of the entire Cu area. The particular behaviour of the worn track area could not be clearly isolated. As the entire Cu surface was selected, worn track can be neglected for resistance and capacitance consideration. This work could be considered as the first approach.

Steps 1, 2, and 3

Steps 1 and 2 were analyzed in previous paragraphs. Step 3 indicates that the C hf value decreases when the friction starts. Thus, C hf increases during friction. The R hf increases at the beginning of friction and during friction. The R hf .C hf time constant is the same, at the beginning of friction, as the one at the end of static contact. Thus, during friction, the time constant increases according to the time. At the beginning of friction and during friction, the convection occurs due to Al alloy pin displacement in the solution. The active surface is then disturbed. During the friction, the Cu surface becomes active. After 10 hours of friction, the time constant is higher than that of Al or Cu samples. This increase could be related to the dissolution involved by friction. This electrochemical behaviour is reproducible, two series of experiments were carried out and the same values were obtained. S is the double layer area, e is the thickness, ε and ε 0 are the permittivity of the adsorbed water and of vacuum, respectively. R hf .S = ρ.e (eq. 4)

ρ is the resistivity that varies to the inverse of the ions concentration. increases, either ρ or ε increases. The friction triggers the dissolution; indeed, the ions concentrations should increase. Thus, the resistivity should decrease contrary to the increase of the product R hf .C hf . Therefore, the relative permittivity should increase. The permittivity of the water molecules could account for this evolution. Indeed, according to the orientation of the adsorbed water molecules, the relative permittivity should be changed [START_REF] Landolt | Corrosion et chimie de surfaces des métaux[END_REF]. Because data seem to prove the increase of ε, the wear mechanisms could modify the charges orientation on materials surface. The EIS results, with taking care attention about the stationary, seem to be consistent with the modification of the double layer. One might suggest that the permittivity is the global physical value that changes. At the ions scale, the corrosion triggers metal ions dissolution. The charge balance involves the anions displacement close to the double layer. In next steps for modelling, the permittivity could be described as a function of ion concentration and/or friction parameters taking into account the permittivity evolution, especially the imaginary part for the dissipative phenomena.

Step 4

Step 4 concerns the Al-Cu contact after friction. 5 hours after the end of rubbing, the values of the electrical components are close to those of step 3, i.e. during friction. One can conclude that the electrochemical behaviour becomes similar to the one during friction. The dissolution continues being active, because of the enhancement of the galvanic coupling between Cu and Al alloy, after removing the passive layers during friction. Finally, one might suggest that the quality of the rebuilding passive layer is not sufficient for protecting the materials in contact.

Step 5

Step 5 shows that the resistances of both samples are weaker than those during friction. Thus, the capacitances values increase and the values exhibit discrepancy.

The Al resistance, step 5, is approximately equal to Al resistance, step 1. The Cu resistance of step 5 is approximately 100 times lower than that of step 1. The Al and Cu capacitances of step 5 are the same as that of step 1, and 100 times higher than that of step 1, respectively.

Finally, one might conclude that the time constant is almost the same for each sample, before contact and after friction. However, The R hf C hf of Al increases according to the time after friction.

The decrease of Cu resistance, after friction, could be related to the decrease of the transfer charges resistance on the surface. The increase of the Cu capacitance could be related to the worn zone. The Cu surface increases due to friction zone, in eq. 3. The discrepancy of Cu capacitances (resistances and time constants) values is higher than that of Al sample capacitances. The friction zone on Cu has a significant effect on the electrochemical behaviour. The time constant of Cu sample, after friction, indicates a huge discrepancy.

Moreover, due to Al particles, see part 3.4, the galvanic coupling could reinforce the corrosion of Cu. The worn zone is the location of a drastic dissolution; the oxides film is not as efficient as the one outside the worn zone. The contribution of the third body could play a significant role, the interpretation is, nowadays, not obvious. Further investigations will be carried out to better understand the third body contribution. The impedance diagrams of the static contact exhibit that Cu and Al in contact were interpreted by EIS as a parallel association of impedance, with normalized data. The results are consistent with delocalized reactions. About the dynamic contact, EIS informs about an ions concentration decreasing in the double layer. Moreover, the relative permittivity of adsorbed water molecules and ions could account for increasing the time constant.

Wear track analyses

One can conclude that EIS is an interesting tool in order to follow degradations of surfaces submitted to friction. EIS related to tribology could be, in next years, a useful tool for following material behaviour submitted to friction in a solution.

Additional works are in progress in order to improve numerous results. The surface ratio on the Cu plate, between a worn surface and an exposed one, will change in order to understand and to follow, by EIS, the location of the cathodic reaction and the reaction rate. Further tests will be carried out with a friction device that will allow monitoring tangential load and the dissipated energy for relating the mechanical parameters to the electrochemistry. Moreover, EIS could be an interesting method to complete previous investigations, such as frettingcorrosion tests. Table 1: A 2017 chemical composition (weight %).

Tables

Figure captions

Figure 4

 4 Figure 4 exhibits the Nyquist diagram of a Cu disc and an Al pin during friction. It

Figures 8 a

 8 Figures 8 a), b), and c) show the capacitances, the resistances, and the time constants for

Figure 9

 9 Figure9shows the qualitative EDX analyses and SEM observations of wear track on Cu

Figure

  Figure9apresents EDX spectrum in the worn zone. It can be compared with figure9b that

Figure 10

 10 Figure10shows qualitative EDX analyses and SEM observations of wear track on Al pin.

Fig. 1 :

 1 Fig. 1: a) Experimental device of tribocorrosion; b) Al pin, design and insulating.

Fig. 2 :

 2 Fig. 2: Al EIS diagrams, Nyquist plot representation, raw impedance data, after immersion in Na 2 SO 4 (0.5 mol.L -1 ) buffered to pH 1.7 by H 2 SO 4 for 3 hours.

Fig. 3 :

 3 Fig.3: Cu EIS diagrams, Nyquist plot representation, raw impedance data, after immersion in Na 2 SO 4 (0.5 mol.L -1 ) buffered to pH 1.7 by H 2 SO 4 for 5 hours.

Fig. 4 :Fig. 5 :

 45 Fig. 4: Al & Cu EIS diagrams, raw impedance data, after 45 hours of immersion Fig. 5: Equivalent electrical circuit for high frequencies; R e : electrolyte resistance; CPE hf : constant phase element, R hf : transfer resistance.

Fig. 6 :

 6 Fig. 6: Capacitances evolution for Al and Cu electrodes, according to applied potential; Mercurous Sulphate Electrode (MSE); Open Circuit Potential (OCP).

Fig. 7 :

 7 Fig. 7: Ecorr during friction between Al pin and Cu plate; solution: Na 2 SO 4 0.5 mol.L -1 and sulphuric acid, pH = 1.7.

Fig. 8 :

 8 Fig. 8: a) Capacitances (C hf ) log scale, b) resistances (R hf ) log scale and c) time constants (C hf .R hf ) linear scale during 5 steps: (1)-before contact, Al and Cu separately, (2)-static contact between Al and Cu, (3)-dynamic contact between Al and Cu, (4)-static contact between Al and Cu after friction), (5)-after friction and separated samples of Al and Cu.

Fig. 9 :

 9 Fig. 9: a) EDX spectrum of Cu sample on the center of the worn zone, after step 5; b) EDX spectrum of Cu sample on the outer worn zone; c) SEM image on the center of the worn zone, Cu sample; d) SEM observation.

Fig. 10 :

 10 Fig. 10: a) EDX spectrum of Cu transferred on Al electrode, after step 5; b) EDX spectrum of Al sample on a worn zone; c) SEM image of Al sample, deposit of Cu on Al; d) SEM image of Al sample on the center of the worn zone.

Fig. 1 Fig

 1 Fig.1

Fig. 5 Fig. 6 - 1

 561 Fig. 5

Table 2 :

 2 EIS results calculated from Al sample immersed in acid solution.

Table 3 :

 3 EIS results calculated from Cu sample immersed in acid solution.

Table 4 :

 4 EIS results calculated from static contact between Cu and Al.

Table 5 :

 5 EIS results from: -Al electrode; Cu electrode; Al-Cu static contact ; a) results are reported according to an area normalization of samples, Al area of 0.28 cm2, Cu area of 19.64 cm2; b) results are reported without normalization.

Table 1 :

 1 A 2017 chemical composition (weight %).

	Elements	Cu Mn Mg	Si	Fe	Zn	Cr	Pb	Ti	Sn	Ni	Al
	Weight % 4.00 0.71 0.66 0.66 0.44 0.17 < 0.05 < 0.05 0.03 < 0.02 < 0.01 bal

Table 2 :

 2 EIS results calculated from Al sample immersed in acid solution.

	Duration	C hf	Err (C hf )	n	Err (n)	R hf	Err (R hf )	χ 2
	hour	µF	µF			ohm	ohm
	1.5	12.8	4.8	0.88	0.05	6110	1360	1.0.10 -3
	3	12.6	4.2	0.89	0.07	5950	1165	1.7.10 -3
	4.5	12.8	4	0.90	0.07	5575	1315	1.6.10 -3

Table 3 :

 3 EIS results calculated from Cu sample immersed in acid solution.

	Duration	C hf	Err (C hf )	n	Err (n)	R hf	Err (R hf )	χ 2
	hour	µF	µF			ohm	ohm
	3	98	4	0.85	0.04	1590	42	4.2.10 -3
	5	118	4	0.84	0.12	997	25	3.8.10 -3
	6	171	8	0.81	0.01	615	15	4.6.10 -3
	6.5	192	8	0.81	0.01	549	13	4.2.10 -3
	7.5	200	8	0.81	0.01	519	13	4.0.10 -3

Table 4 :

 4 EIS results calculated from static contact between Cu and Al.

	Duration	C hf	Err (C hf )	R hf	Err (R hf )	χ 2
	hour	µF	µF	ohm	ohm
	23	1577	122	52	1	2.0.10 -3
	25	1573	110	50	1	1.7.10 -3
	27	1656	20	52	1	1.6.10 -3
	29	1658	20	53	2	1.7.10 -3
	31	2060	169	45	1	2.2.10 -3

Table 5 :

 5 EIS results from: -Al electrode; Cu electrode; Al-Cu static contact ; a) results are reported according to an area normalization of samples, Al area of 0.28 cm 2 , Cu area of 19.64 cm 2 .

	b) results are reported without normalization
	a)	C hf	Err (C hf )	R hf	Err (R hf )
		µF.cm -2	µF.cm -2	ohm.cm 2	ohm.cm 2
	Al	64	22	1176	256
	Cu	8	1	16.78.10 3	0.43.10 3
	Al-Cu static contact	87	5	989	26
	C hf (Al) + C hf (Cu)	72	23
	(1/R hf (Al) + 1/R hf (Cu)) -1			1099	141
	b)	C hf	Err (C hf )	R hf	Err (R hf )
		µF	µF	ohm	ohm
	Al	13	4	5878	1280
	Cu	156	6	854	22
	Al-Cu static contact	1705	88	50	1
	C hf (Al) + C hf (Cu)	169	7
	(1/R hf (Al) + 1/R hf (Cu)) -1			746	22

Acknowledgements

The authors are grateful to Robert Di-Folco for his significant technical contribution, to Dr. Kyungmok Kim and to Dr. Nicolas Mary for readings and helpful discussions.


Imran Rafiq Quadri

Yassin Elhillali
email: yassin.elhillali@univ-valenciennes.fr

Samy Meftali

Jean-Luc Dekeyser

Model based design flow for implementing an Anti-Collision Radar detection system

I. INTRODUCTION

In order to ensure and increase the safety and reliability of transport systems, these systems are becoming more and more intelligent. They integrate more and more sensors and communication systems. Each of these functionalities can be implemented on a System-on-Chip (SoC). These functionalities are carried out by massive computations. As the number of integrated functionalities increase in the transport systems, the design and implementation complexity also increases at a tremendous rate. Implementation of these functionalities can be carried out either via FPGAs or DSP (digital signal processors) platforms. FPGAs are considered the ideal choice as they accelerate the computations by executing the algorithms in a parallel manner.

Actually the initial steps in existing design flows are the development of the complex algorithms; and then their manual implementation, which are daunting tasks. Change in the scale of the algorithm requires starting the implementation over from scratch which results in increase design time. Normally these steps are carried out by teams of different domains which could result in compatibility issues.

We propose a high abstraction level design methodology for implementation of these algorithms in a graphical manner. The advantages offered by our approach aim to reduce time to make up or time to market. Changes in the nature of the algorithm can be easily carried out due to their graphical nature and the code can be generated automatically rapidly. Afterwards the implementation can be carried out on the target FPGA platforms.

Model Driven Engineering (MDE) is an emerging domain and can be seen as a High Level Design Flow in order to resolve the issues related to SoC co-design. MDE enables system level (application/architecture) modeling at a high specification level allowing several abstraction stages (i.e. IRs). Thus a system can be viewed globally or from a specific point of view of the system, allowing to separate the system model into parts according to relations between system concepts defined at different abstraction stages. This Separation of Views (SoV) allows a designer to focus on a domain aspect related to an abstraction stage thus permitting a transition from solution space to problem space. Using a graphical modeling language i.e. UML (Unified Modeling Language) for system description increases the system comprehensibility. This allows designers to provide high-level descriptions of the system that easily illustrate the internal concepts (task/data parallelism, data dependencies and hierarchy). These specifications can be reused, modified or extended due to their graphical nature. Finally MDE's model transformations allow to generate executable models (or executable code) from high level models bridging the gap between these models and execution platforms.

Gaspard [START_REF] Dekeyser | GASPARD SoC Framework[END_REF], [START_REF] Gamatié | A model driven design framework for high performance embedded systems[END_REF] as shown in Figure .1 is an MDE-based SoC Co-design framework dedicated to parallel hardware and software. It is based on the UML MARTE profile [START_REF] Omg | Modeling and analysis of real-time and embedded systems (MARTE)[END_REF] that allows to model real-time and embedded systems; and allows to move from high level MARTE specifications to different execution platforms such as RTL synthesis in FPGAs [START_REF] Quadri | A model driven design flow for fpgas supporting partial reconfiguration[END_REF]. Gaspard exploits the inherent parallelism included in repetitive constructions of hardware elements or regular constructions such as application loops. The applications targeted by Gaspard also focus on a specific application domain, that of intensive data-parallel applications.

Figure.1: A global view of the Gaspard framework

In this paper we present the modeling and final implementation of a key integral part of an anti-collision radar detection system. This part is based on delay estimation using a correlation algorithm. This part is modeled at an high abstraction level using the MARTE profile in the Gaspard framework. Afterwards using the model transformations present in our design flow, we have generated the necessary RTL level code for synthesis on a target FPGA using commercial tools. An important point to observe is that the final code generation from the high level models usually is carried out in a few seconds resulting in a huge save in the overall system conception development time.

II. CASE STUDY: DELAY ESTIMATION CORRELATION

MODULE

Correlation algorithms are among the type of digital processing largely employed in DSP (digital signal processing) based systems. They offer a large applicability range such as linear phase and stability. A correlation algorithm normally takes some input data values and compute an output which is then multiplied by a set of coefficients. Afterwards the result of this multiplication is added together to produce the final output. While a software implementation can be utilized for implementing this functionality, the correlation functionality will be sequentially executed. Where as a hardware implementation allows the correlation functions to be executed in a parallel manner and thus increases the processing speed.

A. Algorithm

We propose to study a case where our radar uses a PRBS (Pseudorandom binary sequence) of length of 127 chips. In order to produce a computation result, the algorithm requires 64 multiplications between the 127 elements of the reference code and the last 127 received samples. The result of this multiplication produces 64 data elements. The sum of these 64 data elements produces the final result. This result can be sent as input to other parts of the Anti-Collision radar detection system.

The mathematical expression of the DECM is described as:

C cy ( j) = 1 N N-1 ∑ i=0 c(i) • y(i + j) (1)
Where c(i) represents reference code, y(i + j) the received signal and N the length of the referenced code. In this case stydy, the normalization 1 127 does not improves the detection quality itself, we consider the following expression:

C cy ( j) = 126 ∑ i=0 c(i) • y(i + j) (2)
Figure .2 shows the global overview of our design flow. The figure only shows the top hierarchical level of our correlation application. Once the application has been completely modeled using the UML MARTE profile, the model transformations allow to move from high level models to lower detailed models which add detailed information and concepts related to RTL semantics. Once the final low level RTL model is generated, the model to text transformations can be invoked which allow to convert the models into user specified Hardware description language. In our case, we generated the VHDL source code for the modeled application, however, Verilog code can be easily generated as well. The automatic generation of the code allows to convert the application into a hardware functionality, i.e., a hardware accelerator for final FPGA implementation.

The application contains temporal as well as spatial dimensions which can be easily expressed in our design flow. Similarly, task parallelism and data parallelism can be specified at the high abstraction levels, and the generated HDL code expresses the parallelism specified at the modeling level. The generated HDL code has been synthesized and simulated on Xilinx Virtex II-Pro XC2VP30 as well as Stratix II 2S180 FPGAs in order to validate our design methodology.

1) High level view: Figure .3 shows the top level of our modeled DCEM module. The instance trm of the component TimeRepeatedMultiplication determines the multiplications while instance trat of component TimeRepeatedAddition-Tree determines the sum.

The instance trdg of component TimeRepeatedDataGen produces the data values for the received signal while the instance trcg of component TimeRepeatedCoeffGen produces the reference code. For the received signal, as the port out TRDG of trdg has an infinite flow of data; it has a shape specification of {*}. As only the 4 MSB (most significant bits) of the input signal are retained, we utilize the type INTEGER RANGE -8 TO 7 for this signal. The reference code is initially composed of 127 samples, and in order to standardize this input port with a power of 2; we add a 128th element in the reference code. The value of this element is neutral and does not effect the final computation result. The reference code can have different values: from a range of -1 to 1 where 0 allows to encode the added element. Therefore type of the port out TRCG of trcg is INTEGER RANGE -1 TO 1 with a shape of {128,*}. The choice to model the reference code in the form of a flow in time (on the basis of * in its dimension) permits to modify the code during execution of the algorithm. Figure .3 shows the top level view of the filter application.

The output out TRAT port of the instance trat also indicates infinite flow as illustrated by its dimension {*}. However, the algorithm allows us to specify that the maximum value of the output (type of the output port of trat) will be between -4096 and 4095; hence the primitive type associated is INTEGER RANGE -4096 TO 4095. In order to standardize the system output, another component could be created between trat and trdc to convert the output into INTEGER. This step has not been taken in this case study. This value is then sent out to the radar. The input port inAdditionTree of this tree has a dimension of {128} while the output port outAdditionTree is a scalar: shape of {1}.

The addition computation has been decomposed in a tree, with each stage of this tree carrying out partial additions. The dimensions of the ports between each stage in this pipeline of tasks reduce by a factor of 2 (128 → 64 . . . 2 → 1). Figure .6 represents the first stage, which realizes a partial addition of sixty four elements on an input port and produces thirty two elements on its output port. The computation task Addition is repeated thirty two times and is elementary in nature.

In conclusion, we have presented a novel design methodology to model complex intensive data-parallel applications. The modeling is carried out using the UML graphical language and the MARTE standard. Afterwards, automatic code generation can be carried out via MDE tools and technologies. Finally the code can be synthesized and implemented on a target FPGA.

B. Simulation Results

Figure. 2 :

 2 Figure.2: An overviw of our complete design flow

2 ) 3 )

 23 Modeling of the Multiplication step: The modeling of the modeling step is shown in Figure.4 by means of two components. The component TimeRepeatedMultiplication expresses the repetition in time while RepeatedMultiplication expresses the repetition in space. At the level of TimeRepeat-edMultiplication, the port inDataM of the instance rm is composed of an array of 128 data elements and is constructed by a sliding window in time of length 127. This sliding window is expressed via the Tiler connected to port inDataTRM which expresses the data dependency between the two ports. The component RepeatedMultiplication realizes 64 multiplications between the data on the ports inDataM and inCoeffM by means of the repeated instantiations of the elementary component Multiplication. Modeling of the Addition step: Figure.5 represents the component which realizes the addition of 128 data elements.

Figure. 7

 7 Figure.7 and Figure.8 shows the simulation results of our DCEM.

Figure. 4 :

 4 Figure.4: Modeling of the Multiplication stage