Sabrina Marcelin

Nadine Pébère
email: nadine.pebere@ensiacet.fr

C Passive

Synergistic effect between 8-hydroxyquinoline and benzotriazole for the corrosion protection of 2024 aluminium alloy: A local electrochemical impedance approach

A Aluminium B. EI S B.SEM C. Neutra! inhibition

Introduction

Among the numerous studies concerning the replacement of Cr(VI) for the corrosion protection of structural alloys, such as the 2XXX series, some compounds, such as rare earth salts (mostly cerium) [START_REF] Hinton | The inhibition of aluminum alloy corrosion by rare earth metal cations[END_REF][START_REF] Arnott | Cationic-film-forming inhibitors for the protection of the AA7075 aluminum alloy against corrosion in aqueous solution[END_REF][START_REF] Mansfeld | Corrosion protection of Al alloys and Al-based metal matrix composites by chemical passivation[END_REF][START_REF] Davenport | XANES investigation of the role of cerium compounds as corrosion inhibitors for aluminium[END_REF][START_REF] Aldykiewicz | The investigation of cerium as cathodic inhibitor for aluminum-copper alloys[END_REF][START_REF] Bethencourt | Lanthanide compounds as environmentally-friendly corrosion inhibitors of aluminum alloys: a review[END_REF][START_REF] Yasakau | Mechanism of corrosion inhibition of AA2024 by rare-earth compounds[END_REF][START_REF] Rosero-Navarro | Electrochemical techniques for practical evaluation of corrosion inhibitor effectiveness. Performance of cerium nitrate as corrosion inhibitor for AA2024T3 alloy[END_REF][START_REF] Paussa | Investigation of AA2024-T3 surfaces modified by cerium compounds: a localized approach[END_REF] or organic compounds, such as triazole, thiazole derivatives and more recently carbamate derivatives [START_REF] Casenave | An electrochemical impedance study of the corrosion inhibition of a 2024 aluminum alloy in neutral chloride solutions[END_REF][START_REF] Zheludkevich | Triazole and thiazole derivatives as corrosion inhibitors for AA2024 aluminium alloy[END_REF][START_REF] Lamaka | High effective organic corrosion inhibitors for 2024 aluminium alloy[END_REF][START_REF] Qafsaoui | Analysis of the inhibitive effect of BTAH on localized corrosion of Al 2024 from electrochemical noise measurements[END_REF][START_REF] Boisier | Corrosion inhibition of 2024 aluminium alloy by sodium decanoate[END_REF][START_REF] Williams | Inhibition of aluminium alloy AA2024-T3 pitting corrosion by copper complexing compounds[END_REF][START_REF] Qafsaoui | Coupling of electrochemical techniques to study copper corrosion inhibition in 0.5 mol L -1 NaCl by 1-pyrrolidine dithiocarbamate[END_REF], have presented interesting corrosion protection activity. lt was shown that this type of inhibitor acts on the copper-rich par ticles decreasing their cathodic activity. Despite their promising performance, these compounds do not achieve the same level of protection as chromates. To increase corrosion protection, the use of mixtures of inhibitors could be a good strategy. Recently, multifunctional inhibitors, offering bath anodic and cathodic inhi bition, were investigated: inorganic cations (rare earths) acting on the cathodic sites and organic anions, acting on the anodic sites [START_REF] Forsyth | Inhibition of corrosion on AA2024-T3 by new experimentally friendly rare earth organophosphate compounds[END_REF][START_REF] Ho | Cerium dibutylphosphate as a corrosion inhibitor for AA2024-T3 aluminum alloys[END_REF][START_REF] Garcia | Unravelling the corrosion inhibition mechanisms of bi-functional inhibitors by EIS and SEM-EDS[END_REF][START_REF] Catubig | The use of cerium and praseodymium mercaptoacetate as thiol-containing inhibitors for AA2024-T3[END_REF].

Among the organic compounds, 8-hydroxyquinoline (8-HQ) and benzotriazole (BTA) were studied as corrosion inhibitors in aque ous solution [START_REF] Casenave | An electrochemical impedance study of the corrosion inhibition of a 2024 aluminum alloy in neutral chloride solutions[END_REF][START_REF] Zheludkevich | Triazole and thiazole derivatives as corrosion inhibitors for AA2024 aluminium alloy[END_REF][START_REF] Lamaka | High effective organic corrosion inhibitors for 2024 aluminium alloy[END_REF] or encapsulated in reservoirs and added to organic or sol-gel coatings for the corrosion protection of AA2024 [START_REF] Yasakau | Influence of inhibitor addition on the corrosion protection performance of sol-gel coatings on AA2024[END_REF][START_REF] Kartsonakis | Multifunctional epoxy coatings combining a mixture of trapes and inhibitor loaded nanocontainers for corrosion protection of AA2024-T3[END_REF]. 8-HQ is known for its chelating properties on different metals [START_REF] Casenave | An electrochemical impedance study of the corrosion inhibition of a 2024 aluminum alloy in neutral chloride solutions[END_REF][START_REF] Zheludkevich | Triazole and thiazole derivatives as corrosion inhibitors for AA2024 aluminium alloy[END_REF][START_REF] Lamaka | High effective organic corrosion inhibitors for 2024 aluminium alloy[END_REF][START_REF] Garrigues | An investigation of the corrosion inhibition of pure aluminum in neutral and acid chloride solutions[END_REF][START_REF] Cicileo | Inhibitory action of 8-hydroxyquinoline on the copper corrosion process[END_REF] and BTA is well known as a corrosion inhibitor for copper [START_REF] Aramaki | Surface enhanced Raman scattering and impedance studies on the inhibition of copper corrosion in sulfate solutions by 5-substituted benzotriazoles[END_REF]. These compounds are generally studied separately.

The aim of the present work is to investigate the 8-HQ + BTA mix ture for the corrosion protection of2024 aluminium alloy and more particularly to obtain a clearer picture of the inhibitive effect of each compound. The final goal of this study is to develop a methodol ogy which could be used to evaluate and compare the efficiency of different compounds or inhibitive mixtures. The methodology chosen is based on the use of bath conventional and local electro chemical impedance spectroscopy (EIS). It is important to keep in mind that the degradation of the AA2024 is mainly due to local galvanic coupling between the intermetallic particles and the sur rounding matrix [START_REF] Liu | In-situ observation of intergranular stress corrosion cracking in AA2024-T3 under constant load conditions[END_REF][START_REF] Muller | Pitting potential of high purity binary aluminium alloys-I. Al-Cu alloys. Pitting and intergranular corrosion[END_REF][START_REF] Guillaumin | Localized corrosion of 2024 T351 aluminium alloy in chloride media[END_REF][START_REF] Zhang | Transitions between pitting and intergranular corrosion in AA2024[END_REF][START_REF] Blanc | Mechanistic studies of the corrosion of 2024 aluminum alloy in nitrate solutions[END_REF][START_REF] Schmutz | Corrosion study of AA2024-T3 by scanning Kelvin probe force microscopy and in situ atomic force microscopy scratching[END_REF][START_REF] Buchheit | Evidence for Cu ion formation by dissolution and dealloying the Al2CuMg intermetallic compound in rotating ring-disk collection experiments[END_REF][START_REF] Birbilis | Electrochemical characteristics of intermetallic phases in aluminum alloys: an experimental survey and discussion[END_REF]. To investigate the action of the inhibitor molecules on the particles, an Al/Cu model couple was used. ln a previous study, this model couple was designed to understand the corrosion phenomena associated with copper-rich intermetallics in aluminium alloys [START_REF] Jorcin | Galvanic coupling between pure copper and pure aluminum: experimental approach and mathematical model[END_REF] and was recently used to study the corrosion inhibition of AA2024 by sodium decanoate [START_REF] Boisier | Corrosion inhibition of 2024 aluminium alloy by sodium decanoate[END_REF].

Experimental procedure

The materials and the electrochemical techniques used are pre sented in this section. The AA2024 electrode surface was examined by optical and scanning electron microscopy after being immersed in the solution containing the inhibitors.

Materials

8-hydroxyquinoline (8-HQ) and benzotriazole (BTA) were analytical grade reagents (purity = 99%) from Alfa Aesar and Sigma-Aldrich respectively, and were used as received. The concentrations of 8-HQ (3 × 10 -3 M) or for BTA (10 -2 M) were chosen near their solubility limits in neutral solution. These concentrations allowed the highest efficiency of the compounds tested alone to be obtained.

For the conventional electrochemical measurements, the corrosive medium was prepared from deionised water by adding 0.1 M Na 2 SO 4 and a small concentration of chloride (0.05 M NaCl) (reagent grade). Sodium sulphate was chosen to increase the conductivity of the solution and for its low corrosiveness toward aluminium alloys. For the local impedance experiments (Al/Cu model couple), dilute solutions were used: 10 -3 M NaCl, 10 -2 M BTA, 3 × 10 -3 M 8-HQ or 10 -2 M BTA + 3 × 10 -3 M 8-HQ. The inhibitors were tested without NaCl in the deionised water to keep a solution with a low conductivity [START_REF] Jorcin | Galvanic coupling between pure copper and pure aluminum: experimental approach and mathematical model[END_REF]. The electrolytes were in contact with air at room temperature (20

• C ± 2 • C).
Aluminium alloy 2024 T351 was used for the investigations. The average chemical composition of the alloy is given in Table 1. Electrochemical experiments were carried out on an AA2024 T351 rod of 1 cm 2 cross-sectional area machined from a rolled plate (cylinder surface parallel to the plane of rolling). For comparison, some electrochemical tests were performed on pure aluminium (99.9999 wt.%), kindly provided by Praxair or on pure copper (99.9 wt.%) purchased from Alfa Aesar. The body of the rods (1 cm 2 cross-sectional area) was covered with a heat-shrinkable sheath, leaving only the tip of the rod in contact with the solution. The samples were abraded with successive SiC papers and diamond pastes (grade 1200-1 m), cleaned in ethanol in an ultrasonic bath and finally dried in warm air.

In order to study the interactions between the inhibitors and the matrix and/or the intermetallic particles, a simple system consisting of a pure aluminium/pure copper (Al/Cu) couple was used [START_REF] Jorcin | Galvanic coupling between pure copper and pure aluminum: experimental approach and mathematical model[END_REF]. The electrode was prepared as follows: a cylinder of pure aluminium (99.9999 wt.%) was drilled in its centre and a cylinder of pure copper (99.9 wt.%) was introduced by force into the hole (Fig. 1). The radii were 1 and 0.315 cm for the aluminium and copper bars, respectively. The assembly of the two materials gave a perfectly joined interface, avoiding crevice corrosion due to surface defects. The electrical resistance between Cu and Al was lower than 0.5 . The electrode was then embedded in an epoxy resin. Before immersion in the electrolyte, the Al/Cu disk electrode was prepared in the same way as the AA2024 or pure Al and Cu rods.

Electrochemical measurements

For the conventional experiments, a three-electrode cell was used with a platinum grid auxiliary electrode, a saturated calomel reference electrode (SCE) and the rod of AA2024 or the pure Al rod as rotating disk electrode. The rotation rate was fixed at 500 rpm. Polarisation curves and electrochemical impedance measurements were obtained using a Biologic VSP apparatus. The anodic and the cathodic parts were obtained independently from the corrosion potential at a potential sweep rate of 0.6 V/h. Impedance diagrams were obtained at the corrosion potential over a frequency range of 65 kHz to a few mHz with eight points per decade using a 20 mV peak-to-peak sinusoidal potential. The electrochemical results were obtained from at least three experiments to ensure reproducibility.

The corrosion behaviour of the model couple with and without inhibitor was studied by local electrochemical impedance spectroscopy (LEIS). The measurements were carried out with a Solartron 1287 Electrochemical Interface, a Solartron 1250 frequency response analyser and a Scanning Electrochemical Workstation Model 370 (Uniscan Instruments). This method used a five-electrode configuration. Details are provided elsewhere [START_REF] Jorcin | Delaminated areas beneath organic coating: a local electrochemical impedance approach[END_REF][START_REF] Frateur | Local electrochemical impedance spectroscopy: considerations about the cell geometry[END_REF][START_REF] Huang | Local electrochemical impedance spectroscopy: a review and some recent developments[END_REF][START_REF] Blanc | The origin of the complex character of the ohmic impedance[END_REF][START_REF] Jorcin | CPE analysis by local impedance spectroscopy[END_REF]. The probe (i.e., a bi-electrode allowing local current density measurement) was stepped across a selected area of the sample. The analysed part had an area of 14 mm × 14 mm and the step size was 400 m in the X and Y directions. Maps were obtained at a fixed frequency, chosen in the present case at 10 Hz, and admittance was plotted rather than impedance to improve the visualization of the result. Local impedance diagrams were recorded over a frequency range of 3 kHz-300 mHz with ten points per decade. The time to record all the local diagrams (from the centre to the edge) was lower than 60 min. The local impedance measurements were carried out in a low conductivity medium to optimize resolution. The measured conductivities of the different media are reported in Table 2. With the experimental set up used, only the normal component of the current was measured.

Surface characterization

The AA2024 surface was observed after 20 h of immersion in the solution (0.1 M Na 2 SO 4 + 0.05 M NaCl) containing the inhibitors by a scanning electron microscope with a Leo 435VP apparatus to obtain a better description of the corrosion morphology, particularly on the intermetallic particles. Observations of the Al/Cu interface of the model couple were also performed by optical microscopy with a Nikon Eclipse MA200 microscope after 20 h of immersion in the solution containing the inhibitors.

Results and discussion

Fig. 2 illustrates the variation of the free corrosion potential (E corr ) of the AA2024 in a 0.1 M Na 2 SO 4 + 0.05 M NaCl solution without inhibitor or in the presence of inhibitors: 8-HQ, BTA or a mixture of the two compounds. Without inhibitor, E corr stabilizes rapidly whereas in the presence of the inhibitors E corr increases with time The radii of the two cylinders were equal to 1 and 0.315 cm for aluminium and copper bars, respectively. The electrode was then embedded in an epoxy resin so that a disk electrode was obtained at the extremity.

Electrolyte conductivity with and without inhibitors in deionised water.

w -0.6

Ill -0. 7 > t: and then it stabilizes at approximatively 20 h. This immersion time corresponds to a stationary state and allows the compounds' efficiency to be compared. Thus, in the present study, ail the elec trochemical results and surface observations are shown after 20 h of immersion.

J -0.8 --e--e---0---o---e--e--e---0---o---� .. --• --•-- ..-- - � -• . .. ------•---•--. -. ---- ■', , •, K---X -- � --•----x ---* --x --x-- , ■ , ,X --X / ,

Polarisation curves

Fig. 3 shows the polarisation curves obtained for the M2024 in the solution containing 8-HQ or BTA or a mixture of the two compounds. The curve without inhibitor is also reported for com parison. ln the presence of BTA. the cathodic curve presents a region of mixed kinetic control of oxygen reduction. On the cathodic plateau, the current density is ten times lower than in the absence of inhibitor. ln the presence of 8-HQ, the length of the cathodic plateau is reduced and, below -0.85 V/SCE, a rapid raise of the cur -2.1 6.5 rent is observed. On the plateau, the current densities are lower than those measured in the presence ofBTA. ln the presence of the mixture of the two compounds. there is a significant drop of the cathodic current densities. Close to the corrosion potential (Ecorr ), within a potential range of 200 mV, the current densities remain low ( <1 µA cm-2 ) compared to the values measured in the absence of inhibitor (> 100 µAcm-2 ). ln the anodic range, near E corr , the polarisation curves obtained with the BTA or with the 8 -HQshow a slight decrease of the current densities. At more positive potentials (a round -0.5 V/SCE), an abrupt increase of the current densities due to the breakdown of the passive film and to the development of pits is observed (Epî t = -0.5 V/SCE). ln the presence of the mixture, Ecorr is shifted in the cathodic direction and an anodic current plateau can be seen. Then, around -0.5 V/SCE, an increase of the current is observed, as in the presence of 8-HQ or BTA. Under sponta neous corrosion conditions, pits will nucleate ifEcorr � Epî t • Without inhibitor, Ecorr = Eptt and in the presence of the inhibitive mixture, E co rr is shifted in the cathodic direction. Therefore, the probability that the development of pits occurs decreases.

Ex situ observations of the electrode surface were performed by SEM after 20 h of immersion in the electrolyte containing each inhibitor or the inhibitor mixture. SEM micrographs of the inter metallic particles are reported in Fig. 4. ln the presence of BTA, most of the particles are attacked (Fig. 4a andb ). A deep crevice was formed at the matrix/particle interface. The crevice is partially related to the dissolution of the Al matrix close to the interface [START_REF] Jorcin | Galvanic coupling between pure copper and pure aluminum: experimental approach and mathematical model[END_REF]. ln the presence of 8-HQ, mainly the A) i CuMg particles were attacked (Fig. 4c) and most of the others remained undamaged (Fig. 4d). ln the presence of the mixture, neither the observed par ticles (Fig. 4e andf) nor the matrix were corroded.

The results obtained from the polarisation curves were con firmed by the SEM observations and accounted for a synergistic effect between 8 -HQ and BTA for the corrosion protection of M2024. SEM micrographs showed that in the presence of BTA or 8 -HQ alone, the corrosion processes associated with the galvanic coupling between the particles and the matrix remained active 10-3 .......................... .._. ...................................... .._. ......................... _....

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•' , whereas in the presence of the 8-HQ + BTA mixture, galvanic cor rosion was strongly reduced.

lmpedance measurements

Global and local electrochemical impedance measurements were performed to elucidate the rote of each inhibitor on the corro sion behaviour of M2024. Global impedance diagrams were first obtained on pure Al with and without 8-HQ to separate the con tribution of the molecule from the impedance response obtained on M2024. Then, the diagrams were obtained on M2024 in the presence of 8-HQ or 8-HQ + BTA.

Global impedance measurements

The impedance diagrams obtained on pure aluminium after 20 h of immersion in the neutral solution with and without 8-HQ are shown in Fig. 5 in Bode coordinates. ln the absence of inhibitor, the diagram presents only one time constant which is associated to the response of the passive film and to oxygen reduction on the pas sive layer [START_REF] Jorcin | CPE analysis by local impedance spectroscopy[END_REF]. ln the presence of 8-HQ, the impedance modulus was little modified by comparison with the blank solution whereas two time constants are visible on the phase angle. The first time constant in the high frequency range was attributed to the forma tion ofa protective layer composed of insoluble aluminium chelates [START_REF] Garrigues | An investigation of the corrosion inhibition of pure aluminum in neutral and acid chloride solutions[END_REF] while the low frequency part of the diagram characterizes the aluminium oxide layer and oxygen reduction [START_REF] Boisier | Corrosion inhibition of 2024 aluminium alloy by sodium decanoate[END_REF]. ln the absence of inhibitor, an equivalent circuit composed of a ReF//CPEox ele ment can be proposed to describe the behaviour of pure Al in the electrolyte (Fig. Ga). The ReF resistance was added to account for the oxygen reduction reaction on the oxide layer. ln the solution containing 8-HQ, in order to take into account the presence of the two time constants, an additional Rf//CPEf circuit was introduced (Fig. 6b ). The two parts of the circuit were imbricated to follow the hypothesis that the presence of the aluminium chelates decreases the active surface in contact with the electrolyte. This equivalent circuit is similar to that often used to describe the behaviour of organic coatings [START_REF] Beaunier | Etude électrochimique, et par microscopie électronique à balayage, du fer recouvert de peinture[END_REF][START_REF] Bonnel | characterization of protective coating on aluminium alloys[END_REF]. The two circuits (Fig. 6) were chosen to combine the typical features of the impedance diagrams obtained with and without inhibitor and to account for: (i) the presence of the oxide film (anodic reaction); (ii) the film formed with the 8-HQ and (iii) the oxygen reduction reaction ( cathodic reaction). Parame ters a and Q, introduced into the equivalent circuits, are associated .. to a constant phase element ( CPE). They may result from the distri bution of the properties through the film [39.42). The experimental diagrams with and without 8-HQare perfectly fitted by the equiva lent circuits, as shown in Fig. 5 and the values of the parameters are reported in Table 3. The value of R 1 is relatively high ( 16 kfl cm 2 ) indicating the formation of a dense organic film on the pure Al sur face. Garrigues et al. have shown that 8-HQ is first adsorbed on the Al surface preventing the adsorption of chloride ions and the improvement of the corrosion resistance with increasing immer sion time was attributed to the slow formation of an aluminium chelate [START_REF] Garrigues | An investigation of the corrosion inhibition of pure aluminum in neutral and acid chloride solutions[END_REF]. The value of Qox is lower in the presence of8-HQ which tends to indicate that the oxide film is thinner in this case. The low values of (à and <2ox are indicative of the formation of thin films, either for the alumina film or for the dense layer of aluminium chelate. The value of RaF (Table 3) is lower in the presence of8-HQ, From the <2ox value, it was assumed that the aluminium oxide film thickness was thinner than that formed without 8-HQ due to the presence of the aluminium chelate which impeded the formation of the aluminium oxide. This hypothesis agreed with the polarisation curves obtained on pure aluminium with and without 8-HQ (not shown in the present manuscript). With 8-HQ, the cathodic cur rent densities were higher than that observed without inhibitor, which can explain the lower impedance value (Fig. 5). However, in the presence of8-HQ, the formation of the aluminium chelate pre vented the dissolution of the aluminium which led to lower anodic current densities. Thus, in the presence of8-HQ, the increase of the cathodic currents and the decrease of the anodic ones, by compar- ison with the curves obtained without 8-HQ, explained that, at the corrosion potential, the impedance modulus are close (Fig. 5). Fig. 7 presents the impedance spectra obtained on AA2024 in the solution containing 8-HQ and in the solution containing 8-HQ + BTA The diagrams have the same shape as those obtained on pure Al in the solution containing 8-HQ with the presence of two time constants (Fig. 5). lt can be observed that the high frequency part of the diagrams is superimposed for the two media whereas the impedance modulus in the low frequency range increased by a factor often when the BTA was added to the electrolytic solution. The diagrams are correctly fitted (Fig. 7) with the equivalent cir cuit proposed for pure Al in the solution containing 8-HQ(Fig. 6b) and the extracted parameters are reported in Table 4. lt can be seen that the values of the parameters associated to the organic film (af, (à and Rf) are relatively similar for the two solutions and the val ues are close to those obtained on pure Al with the 8-HQ(Table 3). This result indicates that the HF part of the impedance diagrams characterizes the action of 8-HQ and it can be concluded that 8-HQacts mainly on the Al matrix. The parameters associated to the oxide layer and to oxygen reduction (aox, Qo x and RsF ) are modi fied in the presence of BTA. The significant increase of RsF can be attributed to the specific action ofBTA on the Cu-rich intermetallic particles, impeding oxygen reduction. The inhibitive effect leads to a strong limitation of oxygen reduction and thus to an increase of the charge transfer resistance.

For comparison, the impedance diagram obtained in the absence of inhibitor is reported in Fig. 7. At the corrosion potential, the metal is not in the passive state as shown by the polarization curve (Fig. 3). The diagram is characterized by two time constants. The first one (around 1 Hz) is associated to the charge transfer process. mainly on the intermetallic particles, while the second, in the low frequency range, is not clearly defined and is due to the oxygen diffusion, particularly on the intermetallic partiel es. Comparison of the diagrams clearly shows the inhibitive action of the compounds. The synergistic effect was clearly seen on the impedance results ( Fig. 7 and Table 4). RsF is more than ten times higher with the mix ture of 8-HQ + BTA than with 8-HQ alone. Moreover, impedance diagrams obtained in the presence of BTA alone, at a concentra tion of 10-2 M (not reported here), showed that RsF was lower than 10 5 n cm 2 . The synergy can be explained by the two differ ent mechanisms by which BTA and 8-HQ inhibit corrosion of the 2024 aluminium alloy. From the global impedance measurements only an average response of the behaviour of the whole electrode surface was obtained. To identi fy the role of each inhibitor. par ticularly on galvanic corrosion at the matrix/intermetallic particles interface, LEIS measurements were performed on an Al/Cu mode( couple.

Local impedance measurements

Two mappings obtained at 10 Hz above the mode( couple in the solution without inhibitor and in the presence of the inhibitive mix ture are shown in Fig. 8. Without inhibitor ( Fig. Sa). admittance is higher (lower resistance) on the copper electrode. This is due to galvanic coupling: oxygen reduction occurs on copperwhereas alu minium is in the passive state [START_REF] Jorcin | Galvanic coupling between pure copper and pure aluminum: experimental approach and mathematical model[END_REF]. In the presence of the inhibitive mixture (Fig. Sb), it can be first noted that the admittance is sig nificantly lower than that obtained without inhibitor, showing the inhibitive efficiency of the compounds. It can be also seen that the admittance is lower on copper ( higher resistance ) confirming the inhibitive effect of BTA and the limitation of the oxygen reduction reaction on copper.

"" -• • ...--_ .• ; . .. _ 1 • -. . -i-• - � . . . .... •,i-• • -- ' --!--' � __ l _ Rr/('2cm 2 ) ±5% 25
Local impedance spectra were collected on both electrodes with the radial position of the bi-electrode as a parameter. The local diagrams were obtained in the solution containing BTA, 8-HQ or the inhibitive mixture. They are reported in Nyquist coordinates in Fig. 9a, b andc, respectively. Ail the diagrams are characterized by inductive loops in the high-frequency range, attributed to local ohmic impedance [START_REF] Blanc | The origin of the complex character of the ohmic impedance[END_REF] . In the presence of BTA, capacitive loops with high impedance values are seen on the Cu part indicating the inhibitive action of the molecule. The capacitive behaviour on Al is attributed to the presence of the passive layer on the aluminium surface. In the presence of 8-HQ, the local impedance diagrams on Cu present lower resistance values compared to those obtained with the BTAand conversely on the Al part, the diagrams are charac terized by a straight line with no curves, indicating high impedance values. With 8-HQ, the results underline the action of 8-HQ on Al but not specifically on Cu. In the presence of the inhibitive mixture, the local impedance diagrams are characterized by a capacitive behaviour both on Cu and on Al parts and the diagrams are almost superimposed. For each diagram obtained on Al or Cu, in the pres ence ofBTA. 8-HQand the mixture, the a and Qparameters were graphically extracted [START_REF] Jorcin | CPE analysis by local impedance spectroscopy[END_REF]. It can be underlined that the values are obtained in a relatively narrow frequency range and the accuracy on their determination is about 10%. Fig. 10 shows the variation of these parameters versus the radial position of the electrode.

In the presence of BTA (Fig. 10a) or in the presence of 8-HQ (Fig. 10b ), the Al/Cu interface is clearly visible whereas it is Jess marked in the presence of the inhibitive mixture (Fig. 10c). In Fig. 1 Oc, it can be seen that the presence of the two inhibitors "equalizes" the a and Qparameters on both sides (Cu and Al). This result can be analysed by comparison with Fig. 1 0 .9-�-�-�-�-�-�-3.510 • 7 0 .8

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.. . . r'

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. . , The Al/Cu interface was observed by optical microscopy after 20 h of immersion in the solution containing the inhibitors (Fig. 11). In the presence of BTA, a deep crevice was formed at the Al/Cu interface. In the presence of 8-HQ, the crevice was still visible and the copper surface presented a bright colour near the interface while, except in this zone, it appeared tarnished. With the inhibitive mixture, the crevice is no longer observed and the two parts of the model couple appear bright. The micrograph clearly shows that in the presence of 8-HQ + BTA the galvanic coupling between Al and Cu disappeared. This is in agreement with the SEM micrographs shown in Fig. 3(f) for the alloy. AA2024, even though the scale was not the same (4% of copper-rich intermetallic particles instead of 11% copper in the model couple), it can be assumed that the mechanisms are similar [START_REF] Jorcin | Galvanic coupling between pure copper and pure aluminum: experimental approach and mathematical model[END_REF].

The galvanic corrosion is due to the fact that the corrosion potentials of the two metals in the model couple are different (for the AA2024 the corrosion potential of the Al matrix is different from that of the intermetallic particles). Thus, to have a better insight into the modification of the corrosion potentials in the presence of the inhibitors, the corrosion potentials of pure Al and pure Cu in the presence of 8-HQ, of BTA and of the mixture were measured and compared to that of the Al/Cu model couple. The results are reported in Table 5. It can be seen that for pure Cu, E corr is shifted in the anodic direction in the presence of BTA. For pure Al, E corr is relatively similar with and without BTA, which agrees with the fact that 8-HQ acts only on Al. For the model couple, it is interesting to note that E corr is significantly shifted in the cathodic direction in the presence of the inhibitive mixture and its value is close to that of pure Al in the solution containing the two inhibitors. This result demonstrates that in the presence of the 8-HQ + BTA, the role of Cu becomes negligible due to the high efficiency of BTA which strongly limits oxygen reduction, and as a consequence the galvanic coupling disappears. In the presence of BTA or 8-HQ alone, the corrosion potentials of Cu and Al are different and galvanic coupling cannot be avoided as it observed in the micrographs (Fig. 11). Finally, for comparison, the corrosion potential of the AA2024 is reported in Table 5. Although the E corr values are different from those measured for the model couple (mainly due to the fact that the ratio Cu surface area/Al surface area was different in the model couple and for the alloy), a similar shift of E corr can be observed in the presence of the 8-HQ + BTA mixture by comparison with the E corr values measured in the presence of the compounds tested alone. Thus, in agreement with the other presented results, it can be assumed that the mechanisms are similar for the Al/Cu couple and for the AA2024 in the presence of the inhibitors.

Conclusions

The present study confirmed that 8-HQ and BTA are two effective corrosion inhibitors of AA2024. From both global and local electrochemical measurements, it was shown that the mixture of 8-HQ and BTA presented a synergistic effect for the corrosion protection of this alloy. It is explained by the two different mechanisms by which BTA and 8-HQ inhibit corrosion: the action of BTA is mainly limited to the copper-rich intermetallic particles; 8-HQ acts on the aluminium matrix due to its chelating properties. The more significant result is that, in the presence of both compounds, the galvanic coupling responsible for the corrosion process is strongly limited which is not the case when the inhibitors are used separately. This electrochemical result was confirmed by SEM observations.

Fig. 1 .

 1 Fig. 1. Schematic representation of the pure aluminium/pure copper model couple.The radii of the two cylinders were equal to 1 and 0.315 cm for aluminium and copper bars, respectively. The electrode was then embedded in an epoxy resin so that a disk electrode was obtained at the extremity.

9 O

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: 1 :

 1 1 0 -9 ........................................................................................................ ' ............................................... ........ -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 Potential vs SCE / V Fi g. 3. Polarization curves obtained on AA2024 after 20 h of immersion in a 0.1 M Na2S0• +0.05M NaCI solution: (0) without inhibitor; (■) with BTA; (e) with 8 -HQand ( x) with 8-HQ + BT A; electrode rotation rate: 500 rpm.

Fig. 4 .

 4 Fig. 4. SEM micrographs of M2024 surface after 20h of immersion in a 0.1 M Na25O4 +0.05 M NaCI solution containing: (a) and (b) BTA. (c) and (d) 8-HQand (e) and (f) 8-HQ + BTA; electrode rotation rate: 500 rpm.

  Fig. S. Electrochemical impedance diagrams obtained on pure aluminium after 20 h of immersion atf,orr in a 0.1 M Na25O4 +0.05 M NaCI solution: (O)without inhibitor and (e)with 8-HQ; electrode rotation rate: 500rpm.

Fitted

  parameters obtained for pure aluminium after 20 h of immersion in 0.1 M Na2SO4 + 0.05 M Na Cl solutions with or without 8

Fig. 6 .

 6 Fig. 6. Equivalent electrical circuits used to fit the impedance diagrams for: (a)pure Al in the solution without inhibitor and (b) pure Al and M2024 in the solution with 8-HQ and 8-HQ+ BTA; (R.: electrolyte resistance; Rr: resistance of the solution in the pores of the organic film; Qi and a1: parameters associated to the properties of the organic film; ReF: Resistance associated to the oxygen reduction reaction; Q., and a0,: parameters associated to the properties of the oxide film�

4 -

 4 ------... .. -... ... ',. , . ... 10 1 LL-""""""'i..........a1.,1.1.1,11......,.lollllll-;::-:.,:-;:--:;::: -�-��-10-3 10-2 10-1 1 o 0 10 1 10 2 10 3 10 4 10 5 .. ------0 1111&....u.a.l.lWL...u.a.l.lWL ........ .....i. ........ ......i....u......i....u......i....u......i....c.....i 10-3 10-2 10-1 10 ° 10 1 10 2 10 3 104 10 5 Frequency / Hz Fi g. 7. Electrochemical impedance diagrams obtained on AA2024 after 20 h of immersion at E,orr in a 0.1 M Na25O4 +0.05 M NaCI solution containing: (□) 8-HQ and (0) 8-HQ+BTA. The diagram obtained in the absence of inhibitor is reported in dotted line for comparison; electrode rotation rate: 500 rpm. Table4 Fitted parameters obtained for AA2024 after 20 h of immersion in 0.1 M Na2SO4 +0.05 M NaCI solution with 8-HQ and with 8-HQ + BTA mixture.

Fig. 8 .

 8 Fig. 8. Mappings obtained at l0Hz above the Al/Cu couple: (a) after 2 h of immersion in a 10-3 M NaCI solution and (b) after 20 h of immersion in deionised water(without NaCI) containing 8-HQ + BTA; static electrode.

6 Real part/ n cm 2 810 6 Fig. 9 .

 6269 Fig. 9. Local impedance spectra obtained for different radial positions above the Al/Cu couple after 20 h of immersion in aqueous solution ( deionised water without NaCI): (a) with BTA, (b) with 8-HQand (c) with 8-HQ+ BTA: starie electrode.

0

  ................................................................................................ ........ _9 ....................... T""T""T"",l'""T'""T""'T ............................. T""T ............

  '-'--'-o ................ 2 ........ '-'-4 ..................... 6 ................ 8'-'-........ 10 Radial position / mm Fi g. 10. (e)a and (0) Qparameters obtained from the local impedance spectra for different radial positions above the Al/Cu couple after 20 h of immersion in aqueous solution (deionised water without NaCI): (a) with BTA. (b) with 8-HQand (c) with 8-HQ+ BTA.(Fig.lOb)and with the mixture(Fig.10c). This obseivation supports the hypothesis that 8-HQ acts only on Al and BTA acts only on Cu. Thus, the action of BTA on Cu complemented the action of 8-HQ on AL To have a better analysis of the a and Q parameters and a better knowledge of the films formed on Cu and Al with BTA, with 8-HQor with the mixture, additional global and local measurements should be performed on pure Cu and pure AL

Fig. 11 .

 11 Fig. 11. Optical micrographs of the Al/Cu couple interface after 20 h of immersion in an aqueous solution (deionised water without NaCl): (a) with BTA, (b) with 8-HQ and (c) with 8-HQ + BTA; static electrode.

Table 1

 1 Chemical composition (wt.%) of 2024 T351 aluminium alloy.

	Cu	Mg	Mn	Si	Fe	Zn	Ti	Al
	4.50	1.44	0.60	0.06	0.13	0.02	0.03	Bal.

  1 ................................................................... ..._ ........ ...._ ...................... 1....1

	0	5	10 Immersion time / h 15	20

,,, .,,><., , ' X , / , -0. 9 ' -Fig. 2. Corrosion potential (f corr) of AA2024 as a function of time in a 0.1 M Na 2 S0 4 +0.05 M NaCI solution: (0) without inhibitor; (■)with BTA; (e)with 8-HQ and (x) with 8-HQ+ BTA; electrode rotation rate: 500rpm.

Table 5

 5 Corrosion potential values measured after 20 h of immersion.

		Ecorr/V vs SCE		Ecorr/V vs SCE
		(deionised water)	(0.1 M
					Na2SO4 + 0.05 M
					NaCl)
		Cu	Al	Al/Cu couple AA2024
	8-HQ (3 × 10 -3 M)	-0.05 -0.76 -0.47	-0.54
	BTA (10 -2 M)	0.09	-0.93 -0.64	-0.59
	8-HQ (3 × 10 -3 M) + BTA (10 -2 M) -0.05 -0.84 -0.81	-0.67

Acknowledgments

This work was carried out in the framework of the AERO 2 project with the financial support of the Agence Nationale de la Recherche (ANR-2011-RMNP-001 (2012-2015)). The authors gratefully acknowledge the partners of the project: Airbus Group Innovations (Dr. Sophie Senani and Martine Villatte), LCMCP (Dr. Lionel Nicole, Dr. Franc ¸ois Ribot and Alexandre Perrot) and Pylote SAS (Dr. Marie-Laure Desse and Dr. Loïc Marchin).


Nathalie Molines
email: nathalie.molines@cerma.archi.fr

Daniel Siret

Marjorie Musy

Dominique Groleau

Dominique Groleau Cerma

Benefits and limits of GIS for managing heterogeneous environmental data in sustainable urban design: example of the ADEQUA project

Keywords: GIS, Sustainable urban indicators, environmental analysis

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

INTRODUCTION

Cities have an important role to play as regards to sustainable development. Indeed, these territories are subjected to evolutions which may seriously compromise the economic, ecological and social balance. However, a sustainable approach of the urban development requires a reconfiguration of the traditional approaches based on single criterion. It thus directs planning towards an approach based on the joint and concerted analysis of heterogeneous parameters in which the environmental impacts and quality of life of the future residents play an important role. This new approach raises the question of the integration of the environmental data in the evaluation of the urban projects. It also results in wondering which can be the place of the Geographical Information Systems (GIS). Indeed, if the potential of these systems has been demonstrated in many territorial planning projects, there benefits in urban design at the district scale remain to be assessed. In this paper, we present a research that adresses the complexity of urban sustainability indicators as well as their integration into a GIS. This research project is called ADEQUA "Aménagement Durable d'un QUArtier" (Cherqui, Mora & al, 2004).

We will detail, in a first section, the question of the sustainability indicators. We will present then the results of the ADEQUA research project and will conclude this paper by the description of interests and limits of GIS in such problematics.

SUSTAINABILITY INDICATORS IN URBAN PLANNING

An indicator is a value derived from parameters, providing information on a phenomenon. It is a quantitative value which allows the characterization of an evolutionary situation, in order to evaluate and compare the different stages of this situation. Obvioulsy, an indicator presents a model of reality, not reality itself. Such a model needs to be informed by qualitative data and comments.

Needs for an approach based on multiple indicators

Formerly endorsed by the operational research, monocriterian approaches have shown their inadequacy for the planning problems [START_REF] Molines | Méthodes et outils pour la planification des grandes infrastructures linéaire et leur évaluation environnementale[END_REF]. Although these approaches do not postulate that "in the real world" only one indicator is concerned, they limit the decision-making support by presenting explicitly only one criterion (Bouyssou, 1993). The sustainable development is based upon the balance of environmental, social and economic constraints. It requires the integration of complex and multiple indicators which may be antagonist. The indicators, vector of communication, also increase the information and the awareness of each stakeholder such as city planner, prime contractor, social representatives or ordinary citizen. In this way, indicators take part in the mission of transparency, popularization and dialogue preached by sustainable planning approaches. Finally the characterization of an urban project by a set of indicators synthesizes its strong points and limits. It allows the decision makers to base their decision on actual points.

The Aalborg Charter (1994), the Lisbon Action Plan (1996), the European Sustainable Cities Report (1996) and the Communication titled "Sustainable urban development in the EU: a framework for action" (1998) successively pointed out the need for using indicators of sustainability of the urban systems, in order to assess the initial state, to observe and follow the progress achieved, or to work out the policies [START_REF] Dufrasnes | Note d'étape ADEQUA N°1 : Constitution d'un jeu d'indicateurs d'aménagement durable: 55p. European council of Town Planners, Vision for[END_REF]. However, by looking further into the assessment of a project, integration of multiple indicators of sustainability also involves a serious complexification of the analysis. On the one hand, this approach requires a preliminary reflexion on the indicators to be set up. On the other hand, it implies to work out new methodologies for creating and aggregating these indicators.

Several research projects have proposed various lists of indicators to achieve urban sustainable development. [START_REF] Dufrasnes | Note d'étape ADEQUA N°1 : Constitution d'un jeu d'indicateurs d'aménagement durable: 55p. European council of Town Planners, Vision for[END_REF] propose a state of the art of the national and international researches (e.g. European project E-Co-housing, HQE2R Project, Urban Development Strategy…). However few research projects tackle the problems of creation and aggregation of these indicators. The question of quantitative approach of sustainability in construction project at the district level has not been explored either.

To fill this gap, the ADEQUA Project addresses the issue of the sustainable urban development at the district level. It aims at providing a methodological guide of sustainable planning which will assist stakeholders during a planning project. It gives a detailed attention to the indicators that describe the environmental characteristics of public spaces and built envelopes.

Indicators of sustainability in urban design: the ADEQUA project

The sustainable development of a district requires the evaluation of differents kinds of objectives, themselves defined by various sets of indicators. These indicators are assessed using various methods such as solar simulations, thermal analysis, sonic simulations or observations... The quantification of these indicators may come from computer simulations, data bases or human experiment. Within the framework of the ADEQUA project, eight objectives were defined (Cherqui & al 2005, Figure 1).

Each objective is evaluated according to various indicators. The multiplicity of these indicators reflects the variety of the stated objectives. The indicators will be useful for many purposes such as measuring the consumed primary energy as well as thermal, acoustic or visual comfort of a site or a building, the volume of waste produced or the change in the ecological value of the site.

Within the ADEQUA project, a survey has been done to evaluate stakeholders' requirements in terms of data and indicators. The results of this survey highlight several characteristics of the urban planning projects. They confirm the complexity of the problem and outline the framework of a future operational GIS. The intervention of the partners varies according to the themes studied, the type of urban project as well as the step of the project developement.

Goals

Stakes Preserve the resources

Limit the impacts on the natural resources. Preserve the ecosystem Respect the fauna, the flora, the air, the water and the ground in place and minimize the risks for this environment. Improve environments' quality Improve environments' quality inside and outside buildings. Preserve health and manage the risks Offer conditions ensuring the health of the users and control both industrial and natural risks. Take into account culturage heritage Respect, preserve and develop the culturage heritage of a district.

Support local development

Encourage the local development of the district from economic, cultural, educational or social points of view.

Increase social life

Offer to the users of the district a pleasant framework of life by encouraging social cohesion. Develop the place of the district in the city Ensure the social and material connection between the district and the remainder of the city.

Figure 1: Goals and stakes

Social and operational approaches are combined with technical ones in the ADEQUA project. This project combines general solutions at the beginning of the urban design (analysis of the initial state, sketches of projects) with more particular approaches at the end of the design. The first steps are generally centered on the analysis of the site and on its relationships with the surrounding environment. The buildings are then materialized in the form of blocks and only their main characteristics are informed. The final steps involve the design of the buildings in their quasi final form. Indicators become then very precise.

The needs of data and analysis tools vary according to the type of urban project: operation of rehabilitation, creation of new districts, urban renewal for instance. Various indicators are required according to the different topics apprehended such as quality of life, environment, resources preservation, as well as the objects they are related to: block, building, frontage, level, roof... Some indicators characterize phenomena that are external to the site. Therefore, urban projects imply to work at multiple scales. Furthermore, some indicators like bioclimatic ones vary according to time and place. Last but not least, the various presentation and formats of data compexify the process of integration.

CONTRIBUTION AND LIMITS OF GIS FOR THE IMPLEMENTATION OF SUSTAINABLE INDICATORS

GIS at the various stages of the urban projects

Installation or rehabilitation district projects are generally subdivided in four stages:

-urban diagnosis and programming; -design and evaluation; -consultation-communication; -follow-up and observatory. GIS, with their capacities of management, seizure, analysis and visualization, supports stakeholders in each one of these stages.

Urban diagnosis -programming

The diagnosis is required to assess the site, its potentialities and constraints and its relations with the overall city. The site is studied on the environmental and the socio-economic levels. This study strongly varies according to the type of project such as the construction of a new district or the rehabilitation of old buildings. The first sketch of scenarios is carried out and analyzed. This phase is characterized by the use of overall indicators and the need for integrating concepts of flow and data coming from the rest of the city. In this phase of programming, the entities do not need a very accurate digitalization. It is particulary true for the "buildings" which are, at the beginning of the studies, simple blocks dedicated to multiple modifications. To carry out zoning tests, the data structure must be flexible and not very constraining. At this early stage of the project, the system does not need to be based on a rigorous conceptual model. This stage requires a flexible and multiscalar GIS which facilitates digitalization, analysis and modification of simplified scenarios. This tool must also allow the consultation of other data sources (other GIS, alphanumeric database, satellite images...).

Simple 2D spatial analysis can be performed such as the sum of builted area, the lenght of perdestrian raods or the number of bus stop at less of 500 meters of the building.

Design and evaluation

At the beginning of the design step, scenarios evolve quickly. The sketches carried out in the preceding phase are refined. Indicators coming from expert-softwares are integrated to assess the sustainability of sketches. Indicators have sometimes voluminal and temporal dimensions which are important to preserve. The 3D GIS potentialities serve this purpose.

The data base must be structured in order to ensure the permanence of the system, to facilitate the interoperability and to allow the automatic generation of entities. The evaluation process can begin with the system developed during the diagnosis step. However when the project is stabilized it is necessary to use a more rigorous data model. This will facilitate the integration of the indicators, which will allow possible connection with a non geographical DBMS and will authorize the automatic generation of 'under-entities' such as the frontage or the level for the entity "building". It also plays a part of supervisor centralized of the data. It reduces the redundancies and facilitates the updates [START_REF] Laurini | Les bases de données en géomatique[END_REF].

A Database model has been developed during the ADEQUA project (Figure 2). This model has been created to answer as best as possible stakeholders needs in terms of entity and indicators. This model is organized around a part "building" and a part "external spaces". The total entity is the overall site.

A building is described by its cadastral footprint, its frontages, its various levels, the walls of these levels (by distinguishing the external walls and the shared walls) and the possible characteristic elements affixed on the walls (Figure 3). In order to increase the environmental assessment, each part of the building corresponded to an entity which has its own indicators. For example, sunshine exposure or sound level indicators are attributed to the "frontage" entity and the energy consumption to the "level" entity (Musy, Siret & al, 2004). The buildings' part of the model has been explained more specifically elsewhere (Siret, Musy & al, 2004).

Entities are imbricates the ones with the others in a hierarchical way (district -> block -> parcel -> building...). On the one hand, this organisation allows to inform the whole of the territory and to perform spatial analysis. The number of entities is important besause this precision is necessary to the good repartition of the indicators. For example, the difference between "mineralized spaces" and "vegetalized spaces" can be used to inform more accurately vegetalized spaces (deciduous species proportion, sunshine factor...) in order to characterize some indicators. Difference between "roads" and "rails" is necessary to assess noise impact. On the other hand, this entity distinction avoids weighing down the database unnecessarily. Many analyses can be carried out. These analyses can overlay indicators coming from various softwares. For example we can calculate the total of uncomfortable areas in winter (aera wich receive less than 2 hours of sunshine and wich are very windy).

Consultation-communication

This step requires visualizations to facilitate the dialogue between stakeholder and the decisionmakers. These visualizations must emphasize the main characteristics of the project. They must underline its strength as well as its weakness. Speaking visualizations, illustrating a popularized and synthetic message, will be privileged. This step implies the use of technical indicators to take the heat out of the debates. However, if the use of overall indicators is privileged, it is necessary, to ensure the transparency of the project, to be able to reach original indicators (Figure 4).

Follow-up and observatory

This step can be done before and after comparisons and can be used to check the accuracy of the analysis as well as the recommendations emerging from the previous stages. This step requires recording the history of the project. With this stage, the indicators must be thematic and allencompassing. Considering the stakes defined at the beginning of the design, one or two agregated indicators may be sufficient.

Contributions and limits of GIS in a sustainable urban planning context

Contributions of GIS are very important. First, GIS give a geographical dimension to the project including a broader territory approach and a multiscalar approach. The spatial analyses tools of the GIS increase the level of understanding of the overall project. The ability of integrating indicators from different expert-softwares provide the material for a sustainable assessment of the project and authorizes an analysis of the area which takes into account the various facets of the problem.

Secondly, GIS ensure the perenniality of the information system by structuring the data and linking them with DBMS software.

By offering the support of the third dimension, essential dimension in the urban context, GIS increase the environmental analysis value. The third dimension enables 3D spatial analyses which are important in this context, for example for visibility analyses. It also provides visualizations adapted to the general public and to the urban field wich is dominatied by the role of volumes and verticality (in order to analyse, for example, the environmental impacts on the buildings frontages).

Finaly, GIS can also be considered as a federator tool of the indicators produced by expertsoftwares. They allow joining together all the capacities of analyses mobilized in the project. In the ADEQUA project, several expert software are monopolized to simulate indicators. For example, sunshine exposure comes from SOLENE, energy consumption from COMFIE and sound level from SOUNDPLAN. The GIS provide the data input necessary to some software (like footprint buildings and masks for SOLENE). GIS can be used to centralize, combine, amalyze and visualize the indicators produced by these software.

Beyond these potentialities GIS, in their current configuration, do not answer all the requirements of urban sustainable development project. Several limits may be pointed out:

-Weakness of the functionalities of 3D spatial analysis. It is very difficult to carry out analyses of impacts on the frontages of the buildings. However, these analyses are essential with the comprehension of urban environments (propagation and impact of the noise or wind). Currently these impacts assessment require over simplifications.

-Rigidity of the data base. The database structure cannot easily be transformed during the project. Additional information generally requires a complete modification of the database.

-Difficulties of producing alternatives comparisons. GIS can produce many indicators, but do not allow to synthesize these results in order to compare various scenarios between them . These comparisons must be done "manually" by the researchers or by using a multicriterion analysis software [START_REF] Molines | Méthodes et outils pour la planification des grandes infrastructures linéaire et leur évaluation environnementale[END_REF].

-Interfacing with the CAD and simulation softwares. Interchange formats between GIS and other tools are not really common. Additional developments are necessary to improve interoperability.

CONCLUSION

The assistance brought by GIS in sustainable urban planning projects is strong. In the one hand, GIS allow the management and the combination of heterogenous urban indicators resulting from observations or from physical simulations. In the other hand, GIS improve public participation and the decision making process. By producing an environmental information which is complete and apprehensive by everyone, GIS facilitate the dialogue between stakeholders. In this way, GIS play a capital role in the processes of communication and dialogue as regards to urban development [START_REF] Laurini | Information Systems for Urban Planning[END_REF].

However, new methods and techniques must be developed to better take into account the spatiotemporal character of the information which characterizes environmental analysis. New methods should also be developed to analyze 3D information more effectively. Our research center started to develop tools going in this direction [START_REF] Ramos | A3D GIS for managing building rehabilitation process[END_REF]. BIBLIOGRAPHY Bouyssou, D. 1993 "Décision Multicritère ou aide multicritère?", Newsletter of the European Working Group "Multicriteria Aid for Decisions", Series 2, 2, Spring, 1-2.

Cherqui F., Mora L., Wurtz E., Allard F., Method to assess sustainable design of a district, 21st International Conference Passive and Low Energy Architecture, Eindhoven, The Netherlands, 2004 p.813

Figure 2 :

 2 Figure 2: The data conceptual model

Figure 3 :

 3 Figure 3: Entities representing a typical building within the GIS data model

Figure 4 :

 4 Figure 4 : Example of synthetized map: areas of discomfort in external space (crossing of wind and solar indicators)