C Berne

E Andrieu

J Reby

J-M Sobrino

C Blanc

Corrosion behavior of α,β'-Brass in NaNO 3 Solutions

The corrosion behavior of an α,β'-brass CuZn40Pb2 (CW617N) was studied in solutions with basic pHs and various nitrate ion concentrations.

Corrosion at the open circuit potential was observed to proceed by the galvanic coupling of α and β' phases, leading to a surface dezincification of the β' phase and the evolution of the passive layer. When the alloy was polarized in the anodic domain, a pseudo-passive or a passive stage followed by a passive breakdown was observed.

10.

Introduction

In various corrosive environments, α,β'-brasses, used in components in gas distribution networks, are believed to be susceptible to simultaneous dissolution (Cu, Zn) and/or selective dissolution (Zn, dezincification) processes. These phenomena are expected to play a major role during the first stages (incubation and initiation) of stress corrosion cracking (SCC) damage, leading to rupture, that can be observed on network components in service. In this framework, it is of interest to develop a good knowledge of α,β'-brasses corrosion mechanisms. However, abundant literature can be found for α-brass (in nitrite solutions, for example: (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)) but not for α,β'-brasses (11) (12) (13) (14). Therefore, the present study aimed to contribute to a better understanding of the corrosion processes of α,β'-brass. The target of this study was to test the influence of the environment (pH, composition) on the corrosion behavior using electrochemical techniques. The specimens investigated in the present work were composed of α,β'-brass CuZn40Pb2 (CW617N) removed from stamping rods that were used to manufacture gas transfer valves. This specific alloy, CW617N, used principally for its good machinability and resistance to corrosion, is principally studied for the development of water taps (15) (16) (17) (18). In the present work, corrosion tests were performed in NaNO 3 solution, selected on the basis of a literature review on α-brass (19), although in literature the solutions that are most often used are ammonia (14) (20) (21), sulfuric acid (22) (23) (24), and nitrite solutions (9). Results obtained over a wide range of conditions (pH, nitrate ion concentrations) were analyzed by comparing open circuit potential (OCP) measurements, anodic polarization curves, and surface characterizations (Optical Microscope, Scanning Electron Microscope and X-Ray photoelectron spectrometry).

Experimental

Material

The samples of α,β'-brass CuZn40Pb2 were removed from casting rods. Their chemical composition (Zn 38.35, Al 0.008, Ni 0.051, Fe 0.205, Mn 0.004, Sn 0.150, Pb 1.875, Si 0.001, Cu 59.35, wt. %) was similar to that from the standard composition CW617N. Their metallurgical state was that of the heart of a 65mm diameter stamping rod (Figure 1). The α phase (64 at% Cu, 36 at% Zn) was included in an enveloping β' phase (55at% Cu, 45at% Zn). A transverse section of the rod was characterized by equiaxed grains (α and β' phase) with an average size of 24 µm in diameter for the α phase and 20µm for the β' phase. In the longitudinal sections of the rod, a texturation of the β' grains was observed: their size varied from 20µm to 120µm compared to 20µm in the transverse direction. Moreover, the α grains distribution in the longitudinal section was heterogeneous: α grains had preferentially germinated at the β' grain boundaries, leading to a specific chaining of β' and α phases. Open Circuit Potential (OCP) measurements, anodic potentiokinetic polarization experiments and corrosion tests at constant potential were performed with a standard device composed of a potentiostat connected to a reference electrode (Saturated Calomel Electrode), a counter electrode (platinum) and a working electrode composed of an α,β'brass sample. The surface exposed to the electrolyte corresponded to the plane perpendicular to the extrusion direction. For the OCP measurements and the polarization tests, the surface area was equal to approximatively 4 cm². The sample surface was polished with 4000 grit SiC paper before the experiments. Tests were performed in a beaker with 300 mL of the solution that was open to the air and moderately stirred with a magnetic stirrer. During the tests, the beakers were placed in a thermostatically controlled water-bath maintained at 23.5±2 °C. OCP values were measured for 15 minutes upon immersion of the working electrode, and then the anodic polarization curves were plotted from the last OCP value to a potential sufficient to observe the passivity breakdown at a scan rate of 0.070 mV/s.

Complementary tests

Surface observations and analyses after the corrosion tests were performed through optical microscopy (OM) (Olympus 2000) and X-ray photoelectron spectroscopy (XPS K alpha ThermoScientific, and monochromatic excitation source A1Kα 1486.6eV). Semiquantification and XPS peak deconvolutions were made after selections of the peak Binding Energy (BE) or the Kinetic Energy in common database.

Results

Corrosion behavior of α,β'-brass CuZn40Pb2 at OCP

The corrosion behavior of α,β'-brass CuZn40Pb2 during immersion in different NaNO 3 solutions is presented as OCP vs. time curves. These curves were plotted during 15 minutes upon immersion of the samples in the electrolyte for various chemical conditions (different pH values and nitrate ion concentrations) (Figure 2).

Whatever the pH and the nitrate ion concentration (Figure 2 a and b), the OCP first increased rapidly (except for pH 13, Figure 2 a) and then tended to stabilize, which could be associated to passivation/pseudo passivation of the exposed surface. The results showed that the pH (Figure 2 a) significantly influenced the OCP measurements. The evolution rates of the OCP from one curve to another were quite different: when the pH increased, the OCP evolution slowed down during the first 400s, which was markedly visible for pH 13. Furthermore, the OCP value was stabilized at -0.24 V/SCE for a pH of 13 whereas for a pH equal to 12 and 11, the OCP reached a value of approximately -0.17 V/SCE after 15 minutes in solution but that was not totally stabilized, as confirmed by longer measurements. The results were probably correlated to a surface evolution that was strongly dependent on pH. Figure 2 b shows the influence of the nitrate ion concentration on OCP values for a pH 11 solution. The effect of the nitrate ion concentration was less significant than that of the pH. Globally, all curves presented the same shape with a rapid increase of the OCP in the first part of the immersion. The OCP value reached after 15 minutes in solution increased with the nitrate ion concentration; however longer tests showed that the OCP values were slightly dependent on the nitrate ion concentration. The preliminary results presented in Figure 2, characterizing surface evolution through potential evolution, were completed further by OM observations (Figure 3) and XPS analyses (Figure 4 and Figure 5). Because the pH was found to be the major parameter on OCP values, attention was paid to surface evolution depending on the pH of the electrolyte in a 0.5M NaNO 3 solution. Comparison of OM observations before immersion (Figure 3 These evolutions were assumed to be mainly due to the formation of a passive/pseudo-passive layer and its evolution as well as a dezincification process of the sample surface, in particular of the β' phase. All solutions contained 0.5M NaNO 3 . On each figure, XPS analyses were performed for a non-corroded sample, a sample after a 15 minute immersion at the OCP, and samples after the polarization until the potentials before and after the passivity breakdown.

Results showed that, for the non-corroded sample, the intensity of Cu 2p 3/2 (Figure 5 a andb) and Zn 2p 3/2 (Figure 5 c andd) peaks was strong. Deconvolution of these peaks, as well as that of Auger Cu peak (Figure 5 e and f), showed that the surface was covered by a passive film mainly composed of Cu 2 O and ZnO for a lower part. However, the signals of both metallic Zn and Cu were strong showing that this passive film was very thin. After a 15 minutes immersion at OCP in the pH 11 solution, the general spectrum (Figure 4) evidenced a decrease of Zn 2p 3/2 peak. Figure 5 c confirmed that the Zn 2p 3/2 peak was significantly reduced and its intensity was even lower for the pH 12 solution (Figure 5 d) compared to pH 11. This was confirmed by XPS semi-quantitative analysis of the samples (Figure 6). The results thus corroborated the dezincification hypothesis and showed that the dezincification process was increased when the pH increased. This was correlated with an evolution of the composition of the surface with, after immersion at The anodic behavior of the material, evaluated through anodic polarization curves plotted after 15 minutes at OCP, is shown in Figure 7. Globally, all curves presented a similar shape with a first breakdown potential corresponding to the corrosion potential, E corr , then a passivity or pseudo-passivity plateau followed by a second breakdown potential, E bp , and finally a strong increase of the current densities corresponding to the passivity/pseudo passivity breakdown. Figure 7a shows the influence of the pH on the current-potential curves. The shift of the OCP towards more cathodic values when the pH value increased, as shown in Figure 2 a, was clearly evidenced on the polarization curves with the shift of E corr . Furthermore, a significant influence of the pH was observed on the current densities and on the values of the second breakdown potential, E bp . At pH 11, a passivity plateau was observed that was characterized by low current densities OCP in pH 11, a passive layer mainly composed of Cu 2 O with a very low amount of ZnO; at pH 12, the passive layer seemed to be thicker and mainly composed of Cu 2 O, Zn being significantly depleted at the surface. The differences in the chemical composition of the surface layer depending on the pH should explain the OCP results (Figure 2 a). The slowing down of the OCP evolution rate and the decrease of the OCP values when the pH increased should be related to a higher dezincification process leading to a more porous structure of the brass surface. At pH 11, during the passivity stage, OM observations (Figure 8a) and XPS analyses (Figure 5) did not show any significant evolution of the surface on the passivity plateau compared to the surface analyzed after OCP. The formation of a Cu 2 O layer was revealed (Figure 5 a and e), with Cu in metallic form, demonstrating that the passive layer should be thin. After the breakdown potential, OM observations (Figure 8 b) showed that the β' phase started to dissolve locally while the α phase appeared to remain stable. XPS analyses (Figure 5) showed no significant differences concerning the major elements, i.e., Cu and Zn, compared to the results on the passivity plateau. This was probably because c d b a approximately 10 -6 A.cm -2 before E bp . For a pH 12 solution, an intermediate breakdown potential, E bi , was evidenced: E corr was first followed by a passivity plateau at higher current densities than for pH 11, i.e., approximately 5.10 -6 A.cm -2 ; then, after E bi , a pseudo-passivity plateau characterized by current densities approximately 6.10 -5 A.cm -2 was observed before the breakdown potential E bp . At pH 13, only the pseudo-passivity plateau was observed before E bp with current density values similar to those values for pH 12. Concerning the breakdown of the passivity or the pseudo passivity, observations showed that an increase in the pH shifted the breakdown potential, E bp , towards more anodic potentials. Similarly to the OCP results (Figure 2 b), the effect of nitrate ion concentration on the current-potential curves was studied for a pH 11 solution (Figure 7b). The results showed that a variation in nitrate ion concentration did not show a significant effect on the current-potential curves. It had no effect on the current densities of the passivity stage; however, a decrease in the nitrate ion concentration led to a small shift of the second breakdown potential, E bp , towards more anodic potentials Characterization of the α,β' brass CuZn40Pb2 anodic behavior was completed by OM surface observations (Figure 8) and XPS chemical analyses (Figure 5) after interrupted polarization tests performed in solutions at pH 11 and 12. Selected arrest potentials for analyses were chosen at points before and after the passivity/pseudo-passivity breakdown, i.e .20mV/SCE and 50 mV/SCE at pH 11 and 50 mV/SCE and 140mV/SCE at pH 12.

the breakdown of the passivity corresponded to local corrosion phenomena on the β' phase. Quantification of the Cu and Zn amounts corroborated the results (Figure 6). The dezincification process occurred during immersion at OCP; it was not significantly promoted by an anodic polarization in a pH 11 solution for a short time polarization. At pH 12, before the end of the pseudo-passivity stage, OM observations (Figure 8c) showed an inversion in color between the α and the β' phases. A new surface layer seemed to be formed on the α phase. XPS analyses (Figure 5 b, d andf) showed the formation of a layer mainly composed of Cu 2 O, CuO and Cu(OH) 2 . After the breakdown potential, OM observations showed that the β' phase was considerably dissolved (Figure 8 d) while a layer remained on the α phase. XPS analyses (Figure 5 b, d andf) revealed some significant changes in the chemical composition of the surface layer. CuO and Cu(OH) 2 signals were not observed while Cu 2 O was still detected. Concerning Zn, Figure 5d showed that ZnO participated in the formation of the thick surface layer observed during the pseudo-passivity stage. However, after the pseudo-passivity breakdown, the Zn signal was significantly reduced. Figure 6 clearly evidenced this surface evolution. As previously stated, the immersion of the α,β' brass at the OCP in a pH 12 solution led to a dezincification phenomenon stronger than at pH 11. When the brass was polarized at potentials corresponding with the pseudo-passivity, a thick surface layer formed, leading to an additional dezincification of the brass in the bulk and an incorporation of Zn as ZnO in this surface layer. Therefore, the Cu/Zn ratio measured by XPS was lower than that after the OCP. When the sample was polarized at a higher potential, after the pseudopassivity breakdown, this thick surface layer was removed and its chemical composition strongly evolved. Dissolution phenomena on the β' phase continued and a strong dezincification was observed, leading to a very high Cu/Zn ratio.

Discussion

Immersion of the α,β'-brass CuZn40Pb2 in basic solutions of sodium nitrate (pH 11, 12 and 13) at the OCP led to a spontaneous surface dezincification. This occurred simultaneously with the formation of a passive/pseudo-passive oxide layer that was identified by XPS analyses. The results showed that this layer was mainly composed of copper oxide. At the microstructural scale, OM observations indicated that the α phase was relatively stable in the nitrate solutions compared to the β' phase, for which the chemical composition strongly evolved due to complex dissolution mechanisms. The galvanic coupling between the two phases probably acted according to the mechanism proposed by Assouli et al. (25). Small electrochemical cells could exist at the microstructural scale with the α phase, which contains more copper than the β' phase, being more noble than the β' phase and acting as the cathode while the β' phase was the anode. Therefore, passivation could occur in the α phase with the formation of a copper oxide layer. Concerning the β' phase, an initial dezincification process should be necessary to achieve a surface that was more noble and susceptible to passivation, leading to the formation of a copper-rich oxide layer.

Influence of pH. OCP values were found to decrease with an increase in pH, which was in agreement with previous studies on α-brass (19). This could be attributed to the formation of a poorly efficient passive layer as the pH increased. It was assumed that the passive film formed at pH 12 should evolve during the immersion in the nitrate solution and, for example, the participation of Cu(OH) 2 in the oxide layer should make it more porous and therefore less protective. Concerning the selective dissolution of the Zn present in the β' phase, the Pourbaix diagram for Cu and Zn (pure metals) provides data on the stability of the species under different pH conditions. This approach is quite simplistic; the metals are studied separately in a very simple aqueous solution. Nevertheless, these diagrams show that Zn stability decreases when the pH increases. Zn(OH) 2 is the most stable species until pH 10. Beyond that, HZnO 2 -and ZnO 2 2-are the thermodynamically stable species, which could explain the corrosion that the β' phase undergoes in solution over the range of pH that was tested. The level of the β' phase dezincification as a function of pH may be associated to the propensity of the Zn dissolution in this phase. Dezincification was faster as the pH increased, but, for immersion at OCP, dezincification did not affect the material in the bulk in agreement with the stabilization of the OCP. This could be due to the formation of a copper-rich surface layer which is stable in this pH range. Indeed, the range of pH corresponded to the stability of Cu, Cu 2 O and CuO (for higher potentials) according to Pourbaix's diagram of pure copper. These data could be easily extended to brass to explain the formation of the Cu 2 O passive layer in this range of pH for the α phase and for the β' phase after the initial stage of surface dezincification. Concerning the behavior of the brass under anodic polarization, the pH effect was strong. At low pH (11), a passivity stage was observed whereas at higher pH (12 or 13), only a pseudo-passivity phenomenon was observed. With increasing the pH value, the passivity layer formed should be considered as less efficient. The results could be explained by the species detected by the XPS analyses at pH 12, i.e., Cu(OH) 2 /CuO. Cu(OH) 2 was probably less efficient in achieving a passivity of the brass. However, the question of efficiency of the passive layer is complex since, as shown by the current-potential curves, the passivity/pseudo-passivity breakdown potentials increased with the pH, which could be due to the stability/thickness of the passive layers formed as a function of pH.

Influence of nitrate ion concentrations. Variation in the nitrate ion concentration led to small differences in the OCP values for tests of 15 minutes, but longer experiments showed that the OCP values were quite similar over the range of nitrate ion concentrations tested. Nitrate ion concentration affects the O 2 availability in solution, which could explain why, depending upon the nitrate ion concentration, the rate of the OCP evolution versus time was different and longer experiments were necessary to obtain an OCP stabilization. Concerning the corrosion behavior of the brass under an anodic polarization, the nitrate ion concentration was found to significantly influence the passivity breakdown. Additional experiments showed that, in adjusted pH solutions without NaNO 3 , the passivity breakdown was not observed at pH 11 and pH 12, whereas the current densities during the passivity stages were similar to those measured in the presence of nitrate ions. As stated by Fernandez et al. (19), for α-brass, NO 3 -could certainly play a role in the corrosion mechanism observed after the passivity breakdown.

Conclusions

Results for the α,β'-brass CuZn40Pb2 in NaNO 3 solutions are summarized in the following points.

-Preferential Zn dissolution, promoted by the galvanic coupling between the α and the β' phases was observed on the β' surface at the open circuit potential (OCP). The kinetics depended essentially on the pH.

-The anodic behavior investigated by the current-potential curves was characterized by a passivity/pseudo-passivity stage followed by a passivity breakdown for which NO 3 participation was a key parameter. A copper-rich passive layer was formed in basic solutions of NaNO 3 but the properties of the passive layer depended significantly on the pH of the solution and on the corresponding oxides that were formed. This work was performed in the framework of the CETIMAT. The CIRIMAT and the CETIM collaborate for certain aspects of their research activities; this collaboration is performed in a joint laboratory, called CETIMAT.

Figure 1 .

 1 Figure 1. Optical microscope observations of the α,β'-brass CuZn40Pb2 (CW617N)

Figure 2 .

 2 Figure 2. OCP vs. time of the α,β' brass CuZn40Pb2 (a) at various pH, in a 0.5 M NaNO 3 solution (b) at various NaNO 3 concentrations, pH 11.

Figure 3 .

 3 Figure 3. Optical microscope observations of the α,β' brass CuZn40Pb2 sample surface (a) before immersion (b) after 15 minutes at OCP in a 0.5 M NaNO 3 solution at pH 11 (c) after 15 minutes at OCP in a 0.5 M NaNO 3 solution at pH 12.

  a) and after 15 minutes at OCP in a 0.5 M NaNO 3 solution at pH 11 (Figure 3 b) showed an orange discoloring of the β' phase after immersion. This discoloring became more pronounced after 15 minutes at OCP in a 0.5 M NaNO 3 solution at pH 12 (Figure 3 c).

Figure 4 .

 4 Figure 4. General XPS spectra and identification of the characteristic peaks on α,β' brass CuZn40Pb2. Spectra are plotted for a non-corroded sample and for a sample after a 15 minute immersion at OCP in a pH 11 solution (0.5M NaNO 3 ).

Figure 5 .

 5 Figure 5. XPS analyses of α,β'-brass CuZn40Pb2. Focus on (a) and (b) Cu2p3/2 peaks, (c) and (d) Zn2p3/2 peaks, and (e) and (f) Cu LMN peaks. Figures (a), (c) and (e) correspond to a pH 11 solution. Figures (b), (d) and (f) correspond to a pH 12 solution.All solutions contained 0.5M NaNO 3 . On each figure, XPS analyses were performed for a non-corroded sample, a sample after a 15 minute immersion at the OCP, and samples after the polarization until the potentials before and after the passivity breakdown.

Figure 6 .

 6 Figure 6. XPS semi-quantification for the characterization of the dezincification phenomenon of α,β' brass CuZn40Pb2 during immersion at the OCP and anodic polarization in a 0.5M NaNO 3 solution (a) at pH 11 (b) at pH 12. The Cu/Zn ratio of a non-corroded sample (reference) is reported for comparison.

Figure 7 .

 7 Figure 7. Current density vs. potential curves for the α,β' brass CuZn40Pb2 (a) at various pHs, 0.5 M NaNO 3 (b) at various nitrate ion concentrations, pH 11

Figure 8 .

 8 Figure 8. Optical microscope observations of the α,β' brass CuZn40Pb2 after interrupted anodic polarization tests in a 0.5 M NaNO 3 solution (a) before breakdown potential (20 mV.SCE), pH 11 (b) after breakdown potential (50 mV.SCE), pH 11 (c) before breakdown potential (50 mV.SCE), pH 12 (d) after breakdown potential (140 mV.SCE), pH 12.

Acknowledgments

The authors would like to express their sincere thanks to J. Esvan for the XPS analyses performed in the CIRIMAT laboratory.


C Berne

E Andrieu

J Reby

J-M Sobrino

C Blanc

Corrosion susceptibility of an α,β'-brass CuZn40Pb2 (CW617N), used for gas transfer devices, was investigated through accelerated corrosion tests at a constant anodic potential in NaNO 3 solution with basic pHs. The anodic dissolution behavior of the α,β'-brass was characterized by a two stages-mechanism, with each stage including both dezincification and simultaneous dissolution phenomena of the β' phase but with different kinetics at each stage. Compressive or tensile stresses applied on brass specimens during the accelerated tests were observed largely to influence the anodic dissolution kinetics. Stresses were assumed to open or close the pores present in the dezincified β' phase, which promoted or slowed the dezincification mechanism, including with the Zn diffusion into the solution trapped inside of the pores. 10.1149/06427.0021ecst Development of Representative Tests to Quantify the Susceptibility to Stress Corrosion Cracking of α,β'-Brass Used for Gas Transfer Devices

Introduction

Gaz transfer valves composed of α,β'-brasses and used in a various range of environmental conditions were analyzed after failure generated in service. Throughcracking phenomena were observed attributed to SCC damage coupled with simultaneous dissolution (Cu, Zn) and/or selective dissolution (Zn, dezincification) processes. Dissolution phenomena are expected to play a major role during the first stages (incubation and initiation) of stress corrosion cracking (SCC) damage, leading to rupture, that can be observed on the network components in service. Failure analyses of some components showed a strong dissolution phenomenon through all the depth of the material, and a dissolution assisted by the stress until the rupture was expected. On balance, the dissolution phenomena were identified as the elementary corrosion mechanism to study in order to develop representative accelerated tests to evaluate the susceptibility to corrosion and SCC of α,β'-brasses and to ensure a long term durability of brass components. In literature, the corrosion behavior of α-brass was largely studied (in nitrite solutions, for example: (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)) but not the corrosion behavior of α,β'brasses (11) (12) (13) (14). The dissolution mechanisms proposed in the literature for metallic alloys (15) are divided into two categories: selective and non-selective. The nonselective mechanism refers to a complete dissolution of metallurgical phases of the alloys. Regarding brasses, selective mechanisms correspond to dezincification, and here two main mechanisms have also been distinguished. The first corresponds to a preferential dissolution of Zn controlled by Zn diffusion in the Cu-rich phase that has been formed. The second corresponds to a simultaneous dissolution of Zn and Cu, which turns into a preferential dissolution with re-deposition of Cu in solution (16). The present study aimed to contribute to a better understanding of the dissolution processes of α,β'-brass under a constant anodic potential in a NaNO 3 solution in the framework of the development of accelerated SCC tests. The approach consisted in testing the influence of the environment (pH, potential) and mechanical stress on the dissolution kinetics. In the present work, α,β'-brass CuZn40Pb2 (CW617N) was studied; samples were removed from stamping rods that were used to manufacture gas transfer valves. CW617N is used principally for its good machinability and resistance to corrosion, and particularly for water taps manufacture (17) (18) (19) (20). NaNO 3 solutions were selected to perform accelerated corrosion tests on the basis of preliminary results and a literature review on α-brass (21) although the solutions that are most often used are ammonia (14) (22) (23), sulfuric acid (15) (24) (25), and nitrite solutions (9). The discussion section focuses on the identification of dissolution mechanisms and key parameters controlling the dissolution kinetics. The influence of compressive or tensile stresses on the dissolution mechanisms was discussed to understand the SCC incubation and initiation stages (i.e., crack initiations) better.

Experimental

Material

The heart of a 65mm diameter stamping rod of the α,β'-brass CuZn40Pb2 (CW617N; Zn 38.35, Al 0.008, Ni 0.051, Fe 0.205, Mn 0.004, Sn 0.150, Pb 1.875, Si 0.001, Cu 59.35, wt. %) was used to sample the material studied in the present work. The α phase (64 at% Cu, 36 at% Zn) was included in an enveloping β' phase (55at% Cu, 45at% Zn), which is in agreement with the solidification processes expected for CuZn40Pb2 alloys (Figure 1). Equiaxed grains (α and β' phase) with an average size of 24 µm in diameter for the α phase and 20µm for the β' phase were observed in the transverse section of the rod, whereas, in the longitudinal sections of the rod, a texturation of the β' grains (their size varied from 20µm to 120µm in the longitudinal direction compared to 20µm in the transverse direction) was detected. Moreover, a preferential germination of α grains at the β' grain boundaries led to an heterogenous distribution of α and β' in this section of the material.

Corrosion tests A 0.5M NaNO 3 solution (AnalaR NORMAPUR® ACS, ISO, Reag. Ph. Eur. analytical reagent) with adjusted pH (11 or 12) by the addition of NaOH (AnalaR NORMAPUR® ACS, ISO, Reag. Ph. Eur. analytical reagent) was prepared for all corrosion tests. A first set of corrosion tests at constant anodic potential (20 mV/SCE or 50 mV/SCE) without mechanical stress was performed with a standard device composed of a potentiostat connected to a reference electrode (Saturated Calomel Electrode), a counter electrode (platinum) and a working electrode composed of an α,β'-brass sample. The surface exposed to the electrolyte corresponded to the plane perpendicular to the extrusion direction. In reference conditions, a surface area of 0.2 cm 2 (ratio surface/volume = 6.7. 10 -4 cm 2 /mL) was selected by using a varnish. The sample surface was polished with 4000 grit SiC paper before the experiment. Tests were performed in a beaker with 300 mL of the solution that was open to the air and that was moderately stirred with a magnetic stirrer. During the tests, the beakers were placed in a thermostatically controlled water-bath maintained at 23.5±2 °C. A second set of experiments was performed to study the influence of a mechanical stress on the corrosion behavior of the α,β'-brass CuZn40Pb2 using a bending device developed in the laboratory (Figure 2), allowing for the specimen to be completely immersed in 500 mL of the electrolyte solution. Geometry of the tested specimen is also presented in Figure 2. The stress was perpendicular to the extrusion direction. Either surface under tensile or compressive stress was exposed to the electrolyte using a protective varnish on the non-exposed surface. In each case, the surface exposed to the electrolyte corresponded to a zone in a rectangular shape (10 x 3.5 mm 2 ; ratio surface/volume = 7. 10 -4 cm 2 /mL) in the center of the specimen between the two interior support rollers. An initial maximal stress of 140 MPa was applied to achieve 70% of the yield strength at 0.2% of strain, which was previously determined by traction tests at 10 - 3 s -1 and found to be 205 MPa. The corrosion tests were then performed at constant mechanical stress. Displacements during the tests were followed with a sensor (Keyence-GT2-H12K). During the tests, the potential was applied using the same standard device For corrosion tests with or without mechanical stress, the potential was applied after 15 minutes of immersion at the open circuit potential (OCP). The duration of the corrosion tests was between 2 and 48h.

Complementary tests

Cross-sections of the corroded material were prepared after corrosion tests at constant potential to follow the corrosion propagation. They were observed using an optical microscope (OM, NIKON) to measure the dissolution depths. More detailed observations were also performed by using a scanning electron microscope (SEM, LEO 435VP) coupled with an Energy-dispersive X-ray spectrometer (EDS) for chemical analyses. The evolution of the corrosive solution was followed by a pH-meter and X-ray diffraction (XRD) experiments were performed on solid corrosion products removed and dried from the solution after corrosion tests: 2-θ scans were performed from 20° to 70° using Cu K-α radiation and SEIFERT-3000TT equipment.

Results

Figure 3 presents the typical damage caused by the corrosion tests at a constant potential in the conditions called reference conditions (0.5M NaNO 3 solution, at pH 11, without mechanical stress and under 50mV/SCE; surface/volume ratio equal to a 0.2cm² surface per 300 mL of solution).

-In the first stage (t< 18h) the dezincification front merged with the simultaneous dissolution front. OM observations in the cross-sections showed the presence of a porous structure that consisted of a non-corroded α phase only on the outer layer of the sample but possessed no discoloring of the β' phase, i.e., no visible signs of a Curich phase, under this outer layer.

-In the second stage (t> 18h) the dezincification front preceded the dissolution front. At this stage, the porous structure corresponding to the simultaneous dissolution of the β' phase was still observed in the outer layer but an inner layer was observed. This inner layer was composed of non-corroded α grains and a porous copper-rich phase replacing the β' phase, which was pink-colored. These observations clearly show that, during the second stage, the complete dissolution of the β' phase was not stopped but continued to proceed with a time lag compared to the dezincification.

Effects of potential and pH. To identify the effect of the two parameters on the dissolution kinetics, the results obtained with different experimental conditions were compared with those obtained for the reference conditions. The results are presented in Figure 4.

-At a lower potential than that of the reference conditions, i.e., 20 mV/SCE (Figure 4 a), results showed a first stage where both dissolution depths were very low. After a critical time, a significant evolution of the simultaneous dissolution front and the dezincification front was observed. From this time, the kinetics of the dezincification process became very quickly higher than that of the simultaneous dissolution. Then both The damage area evidenced an elliptical shape corresponding to the β' phase dissolution. The dissolution starts at the edges of the exposed zone that was separated from the protected zone by a varnish (scheme in Figure 3 c). Due to the shape of the damage area (dissolution depth constant in the center of the exposed area), the quantification of the corrosion damage, i.e., dissolution measurements, was performed in the center of the corroded zone. Two types of dissolution were identified. The first type was a complete dissolution of the β' phase, corresponding essentially to a simultaneous dissolution of Cu and Zn present in this phase. β' complete dissolution led to a porous layer composed of non-corroded α grains only. The second type of dissolution was a selective dissolution of Zn contained in the β', called dezincification. This second process was easily identified by OM with discoloring of the β' phase which became pink-colored but remained around the non-corroded α phase. Chemical analyses by EDX were performed through the sections of the corroded specimens (Figure 3 b). The discolored β' phase was identified as a Cu-rich phase, totally depleted in Zn. No gradient of the Zn content was observed through the thickness of the dezincified layer. To quantify the dissolution damage, two parameters were identified and recorded versus the duration of the corrosion tests: the depth of the simultaneous dissolution front, i.e., the average distance between the initial surface of the material and the first β' grain not completely dissolved, and the depth of the dezincification front, i.e., the average distance between the initial surface of the material and the first intact β' grain. This method was chosen to evaluate the kinetics of dissolution because the depth of dissolution is expected to be one of the decisive factors for the lifetime of brass component. Figure 4 shows the dissolution depth vs. time curves: each point corresponds to an independent corrosion test at a constant potential where both dissolution depths were measured. The results for the reference conditions were characterized by a two stages-mechanism separated by a critical time equal to 18 hours: Effect of a mechanical stress. Figure 5 shows the effects of a mechanical stress on the dissolution kinetics, i.e., simultaneous dissolution and dezincification. New curves of dissolution depths vs time for the reference conditions were plotted with samples placed on the bending device without applying a mechanical stress to eliminate the effects of the bending device (stirring, position of the exposed surface) and to have the same surface/volume ratio with and without a mechanical stress. This explained the differences kinetics of dissolution were lower than those for a 50 mV/SCE potential but the global shape of depth vs time curves were similar to that of the higher potential.

-At higher pH than that of the reference condition, i.e. pH 12, results showed also a first stage where the kinetics of both dissolution processes were similar but were more critical for pH 11 than pH 12. In the first few hours, the kinetics of dissolution were very low at pH 12. After a critical time equal for both pHs, the second stage initiated with the dezincification front preceding the dissolution front; during this stage, the kinetics of both dissolution processes became higher at pH 12 compared to pH 11. Effect of chemical composition of the electrolyte versus time. The evolution of the corrosive solution during corrosion tests at a constant potential is expected to be a key factor on the dissolution kinetics, more precisely the solution in contact with the exposed surface, which can be different depending on the local stirring. In a first approach, the evolution of the bulk solution was studied. A blue coloring of the bulk due to the corrosion products was observed after a few hours of corrosion test. This evolution should correspond to a significant change in the aggressiveness of the environment that should modify the critical time corresponding to the transition from the first stage (similar kinetics for both dissolution processes) to the second stage (dezincification faster than in the reference curves in comparison to those plotted in Figure 4. Comparison of the curves plotted in Figure 5, without stress and with stress, allowed to identify the isolated effect of a compressive or tensile stress. As previously described, all curves evidenced two stages with a first stage corresponding to similar kinetics for both simultaneous dissolution and dezincification and, after a critical time, a second stage where dezincification occurred faster than the simultaneous dissolution. Concerning the effect of mechanical stress, major results were that:

(i) there was no effect of a tensile mechanical stress on the first stage and on the critical time compared to samples without a mechanical stress. However, under tensile mechanical stress, the dezincification kinetics were significantly affected during the second stage. Dezincification process under tensile stress was faster than in the reference conditions without mechanical stress while the simultaneous dissolution process was not significantly affected during the entire corrosion test (Figure 5 a) (ii) under compressive mechanical stress, the simultaneous dissolution and dezincification processes were similar over long periods of time compared to the results without mechanical stress (Figure 5 b). The depths of dezincification were never significantly deeper than the depths of complete dissolution.

Previous results evidenced that stirring significantly influenced the results. Indeed, dissolution kinetics for the reference conditions where quite affected by the use of a bending device and the position of the exposed surface with regard to the magnetic stirrer at the bottom of beaker which modified the local stirring. This result suggested that, during the corrosion tests, a significant evolution of the chemical composition of the electrolyte should occur depending on the surface/solution volume ratio and on the solution stirring. To evaluate this effect, additional experiments were performed.

Discussion

Anodic dissolution of the β' phase was observed during corrosion tests in the reference solution (a 0.5 M NaNO 3 solution, pH 11) at a constant potential of 50 mV/SCE. The first simultaneous dissolution). This finding meant that the critical time should be correlated with a chemical composition of the electrolyte.

Blue corrosion products in the solution were removed, dried and analyzed by XRD (Figure 6 a). Cu 2 (NO 3 )(OH) 3 , called Rouaite, was identified and also Cu(NO 3 ) 2 . The quantity of corrosion products accumulated on the surface that was exposed to the electrolyte and in the beaker seemed to be dependent on the pH of the solution, with more corrosion products at pH 12 compared to experiments at pH 11. Moreover pH vs. time data points that were measured for an experiment at the reference conditions (Figure 6 b) showed a decrease of the pH value over time, which was mainly related to the formation of the corrosion products. These results suggested a link between the evolution of the aggressiveness of the solution with the pH and the Cu concentration in solution.

To confirm the influence of the evolution of the chemical composition of the corrosive solution on the dissolution kinetics, dissolution kinetics were determined for two additional values of the ratio of the surface exposed to the electrolyte to the volume of the solution (S/V) generating different rates of corrosion product accumulation (Figure 7). Experiments were performed at a constant volume (300 mL in a beaker, without the bending device) with 0.02cm² and 4cm² of the surface exposed (respectively S/V=6.7.10 - 5 cm²/mL and 1.3.10 -2 cm²/mL) compared to the ratio used for previous experiments i.e., 6.7.10 -4 cm²/mL with 0.2cm² of surface exposed. The results clearly showed that an increase in the S/V value shifted the critical time to a lower value, i.e., led to a more rapid differentiation of the dezincification and simultaneous dissolution fronts (Figure 7 b). However, mainly for long tests (> 40h), the dezincification depths were smaller for the large S/V value while the simultaneous dissolution depths were quite similar whatever the ratio (Figure 7a).

stage corresponded with a simultaneous dissolution of the β' phase, essentially the Zn and Cu content, through the following possible reactions [1][2][3][4][5][6].

Zn (s) + 2HO -Zn(OH) 2 (s) + 2e -(precipitation white color) [ During the second stage, the dissolution rate of Zn was faster than that of Cu [1-3] [4-6] in the β' phase. The values of the solution pH, the applied potential, the nature of the mechanical stress were identified in significantly influencing the kinetics of the two dissolution processes of the β' phase, i.e., simultaneous dissolution and dezincification, throughout the material. In the following part, their influence on the dissolution kinetics will be discussed, beginning with the initiation step and then followed by the propagation step. The initiation step was considered as the period before the dissolution goes in depth and the propagation step was the period where the dissolution was quantified by the OM observations in the cross-sections. Therefore, the initiation step corresponds to the period during which the dissolution was contained to the extreme surface. When the dissolution starts to be quantifiable with depth measurements by OM in cross section, it corresponds to the beginning of the propagation step.

Initiation step

Influence of pH. The nature of the Cu and Zn corrosion products that were observed depended upon the pH according to the different potential-pH diagrams; depending on the corrosion products present on the surface sample, it was assumed that the kinetics of dissolution could be affected. Taking into account this data, the low dissolution kinetics observed at pH 12 during the first hours can be explained. At this stage for a pH of 12, a larger amount of the blue corrosion products was observed in the solution compared to pH 11. The corrosion products were predominantly identified as Rouaite. They seemed to be sufficiently adhesive on the material to generate the pseudopassivity stage and to slow down the dissolution processes during a relatively short time period (12 h at pH 12). Figure 6 b is in agreement with the expected reaction [1][2][3][4][5][6]. It was assumed that, when a sufficient drop in the pH value was achieved at the surface, a part of the hydroxide (Rouaite) that was formed dissolved and the pseudo-passivity stage was over, leading to an increase of the dissolution kinetics.

Influence of potential. It was also shown that, for a low potential, i.e., 20 mV/SCE, the dissolution kinetics were very low in the first stage defined on the dissolution kinetics versus time curves (Figure 4 a). This could be expressed as the existence of an incubation time; after this incubation time, a polarization of the brass at 20 mV/SCE led to an active dissolution process. This could be explained by a time necessary to achieve the breakdown of the material passivity. As shown by Fernandez et al. (21), when an intensity-potential curve is plotted, a higher scan rate moves the breakdown potential to more anodic values and vice versa. Further experiments should be performed to determine if a pseudo breakdown potential -much lower -exists as identified by Propagation step Parameters identified as relevant in explaining the dissolution kinetics during the propagation step are discussed below: the chemical composition of the solution was expected to be the key parameter.

Influence of pH. The active dissolution of the α,β'-brass, with a rate increasing with the depth of dissolution as observed in Figure 4 should be explained by the global decrease of the pH during the corrosion test (Figure 6 b). Indeed, a decrease in the pH could increasingly reduce the precipitation of the copper hydroxide, which should limit dissolution and also facilitate a mechanism of local acidification as proposed by Fernandez et al. (21).

Influence of corrosion product concentration. During the corrosion test, corrosion products accumulation is not negligible according to the precipitation observed and characterized by XRD (Figure 6 a). In particular, the Cu concentration increase was expected to control at least partially the dissolution kinetics. Indeed, previous studies in the literature (26) demonstrated the major role of the copper concentration in solution on the transition between the simultaneous and preferential (dezincification) dissolution of the Cu/Zn elements.

The first hypothesis states that copper in solution accelerates the dezincification process through a reaction [7]. This corresponds to a two stages-dezincification process: a simultaneous Cu/Zn dissolution followed by a second stage with Cu re-deposition from solution.

2Cu + + Zn (s) 2Cu (s) + Zn 2+ [7] However, this re-deposition corresponds to the reduction of copper on the sample surface. Under anodic polarization, this reaction is quite unlikely, as postulated by Pchelnikov et al. (16). Moreover, the analysis of the Cu-rich phase due to the β' dezincification showed a Sn content and distribution similar to that of the non-corroded β' phase (Figure 3 b). Therefore, it seems irrelevant to assume that the β' phase was totally dissolved. This mechanism, including a total dissolution of the β' phase, can be excluded.

The other hypothesis corresponds to a second dezincification mechanism controlled by Zn diffusion. This hypothesis is compatible with the chemical composition of the Curich phase, and, in particular, with its Sn content. In this mechanism, Zn is preferentially dissolved; dezincification goes on through the material due to Zn diffusion through the porous Cu-rich phase that has been formed. However, no gradient of Zn was observed throughout the Cu-rich phase (Figure 3 b), which is usually observed in the case of Zn diffusion-controlled mechanisms. This observation might put in doubt this hypothesis.

Fernandez et al. (21) for α-brass. Under this pseudo breakdown potential, the dissolution processes could not be propagated in depth.

Influence of a mechanical stress. This parameter did not show a significant impact on the initial stage of the dissolution process.

On balance, the two mechanisms proposed do not fit perfectly with the usual observations found in literature. However, the Cu concentration in solution should be considered as a relevant parameter, for example as a solubility limit achievement for Cu, which should explain the transition between the simultaneous dissolution process and the dezincification process. This transition was identified for each of the experiments performed using a different S/V ratio which corresponded with different Cu and Zn concentrations in the bulk. The relationship between the dissolution kinetics and the S/V ratio should be easily correlated to a corrosion product accumulation rate in particular Cu accumulation near the corroded surface. A critical Cu content in the bulk of 6-8 mg.cm - 2 .L -1 was determined. When this critical Cu concentration in solution was reached, the equilibrium between the two dissolution mechanisms was modified, leading to a faster Zn dissolution compared to the Cu dissolution and therefore promoting the Zn dezincification compared to the simultaneous dissolution. As a remainder, it is worth noticing that the Cu concentration at the bottom of the corrosion defects that were formed should be significantly different than that measured in the bulk; this concentration should be the one to consider, And so, it should be assumed that the deeper the corrosion defects, the shorter the time necessary to reach the critical Cu concentration. However, in first approach, taking into account the concentration in the bulk seemed to be relevant.

Concerning the Zn dezincification process throughout the material, no gradient of Zn through the Cu-rich phase that was formed was noticed. Regarding the high Zn proportion in the β' phase, the defect density of the β' phase should be considered as strong after dezincification, and much more than for the α phase of α-brass. According to the literature, it was demonstrated that dezincification induced a tensile stress in the dezincification layer of brass (23) (27) (28). The assumption here is that, for the β'-phase in α-β' brass, the stress level could be high enough to induce the coalescence of the defects and then create porosity, more specifically, open porosity. OM observations were in agreement with this statement. For α-brass with the Zn content being lower, this phenomenon could not occur. This should explain the changes in the mechanism of dezincification between the α-β' brass and the α-brass. A rapid dissolution of Zn through the pores of the Cu-rich phase, i.e., through the liquid phase trapped inside the pores and not through the solid Cu-rich phase, could explain this phenomenon. Assuming the above, the Cu concentration had to be considered as a key parameter in both dissolution processes. It had been previously noticed that the evolution of the chemical composition of the solution could be largely affected by local stirring (see for example the problems of the accumulation of the corrosion products from the sample surface when the bending device was used). In the framework of the development of an accelerated SCC test, the geometry of the experimental device correlated to a local stirring that was identified as one of the key parameters.

Influence of a mechanical stress. A significant effect of the mechanical stress on the dissolution kinetics (free of bending device effect) was demonstrated (Figure 5). On the one hand, a compressive mechanical stress promoted the simultaneous dissolution to the detriment of dezincification. When a favorable environment was achieved to promote dezincification, the porosity of the Cu-rich phase that was formed was closed by the effects of the compressive mechanical stress. If the dezincification mechanism was controlled by the diffusion of Zn 2+ in the liquid phase, the decrease in the dezincification is in agreement with the results. On the other hand, a mechanical tensile stress accelerated the dezincification process. When the critical Cu-value was reached that promoted the dezincification kinetics, the porosity of the Cu-rich phase that was formed was opened due to the tensile stress. Therefore, if the dezincification mechanism was controlled by the diffusion of Zn 2+ in liquid phase, the dezincification process would be accelerated, as was observed.

Conclusions

Results presented in this study, concerning the α,β'-brass CuZn40Pb2 in NaNO 3 solutions are summarized below.

-The typical corrosion damage observed after corrosion tests, in a 0.5 M NaNO 3 solution at pH 11 at a constant anodic potential of 50mV/SCE, was the dissolution of the β' phase.

-For a lower potential (20mV/SCE), an incubation time, before dissolution occurred through the material, was observed.

-At higher pH ( 13), a pseudo-passivity stage could be achieved because of the accumulation of hydroxide copper.

-The dissolution process through the material was described with two propagation stages: an initial stage corresponding to the simultaneous dissolution of copper and zinc, followed by a second stage called dezincification where the Zn dissolution was faster than that of copper.

-The process of dezincification proposed was based on Zn diffusion through the pores of the Cu-rich phase that was formed. These pores were assumed to be due to the β' phase dezincification and the stress induced.

-The porosity of the Cu-rich phase was significantly affected by an external mechanical stress which consequently affected the kinetics of the β' phase dezincification and/or its simultaneous dissolution.

-The bending device built for the experiments allowed to study the dissolution kinetics of the α,β'-brass. The conditions of the accelerated tests allowed to obtain relevant results. This work was performed in the framework of the CETIMAT. The CIRIMAT and the CETIM collaborate for certain aspects of their research activities; this collaboration is performed in a joint laboratory, called CETIMAT.

Figure 1 .

 1 Figure 1. Optical microscope observations of the α,β'-brass CuZn40Pb2 (CW617N)

Figure 2 .

 2 Figure 2. Bending device used to determine the influence of a mechanical stress on the open circuit potential values, anodic polarization curves, and corrosion test results at a constant potential and geometry of the specimen.

Figure 3 .

 3 Figure 3. Analysis of a cross-section of the α,β'-brass CuZn40Pb2 sample after a 48h corrosion test at the reference set, i.e., 0.5 M NaNO 3 solution at pH 11 under anodic polarization at 50 mV/SCE by (a) optical microscope and (b) EDX analysis. (c) Scheme of the corrosion damage.

Figure 4 .

 4 Figure 4. Influence of (a) applied potential and (b) pH on the dissolution depths (i.e., simultaneous dissolution and dezincification) of the α,β' brass CuZn40Pb2. In each graph, the effect of the specific parameter is studied in comparison with the results obtained for the reference conditions, i.e., experiments in a 0.5 M NaNO 3 solution, a pH adjusted to 11, and an applied potential equal to 50 mV/SCE.

Figure 5 .

 5 Figure 5. Influence of (a) tensile mechanical stress and (b) compressive mechanical stress on the depth of the dissolution (i.e., complete dissolution and dezincification) of the α,β' brass CuZn40Pb2. In each graph, the effect of the mechanical stress is studied in comparison with the results obtained for the reference conditions, i.e., experiments in a 0.5 M NaNO 3 solution, a pH adjusted to 11, and an applied potential equal to 50 mV/SCE without mechanical stress.

Figure 6 .

 6 Figure 6. (a) XRD analysis of the blue corrosion products (b) pH vs. time during a corrosion test at the reference conditions, i.e., at 50 mV/SCE in a 0.5 M NaNO 3 solution at pH 11, performed on α,β' brass CuZn40Pb2.

Figure 7 .

 7 Figure 7. (a) Dissolution kinetics of the α,β' brass CuZn40Pb2 in a 0.5M NaNO 3 solution, pH 11 for different surface/volume ratios (b) critical time versus the surface/volume ratio. All of the measurements were performed for three values of the surface/volume ratio. The ratio 6.7.10 -4 cm²/mL corresponded to a surface exposed to 0.2 cm 2 , i.e., that of the previous experiments (Figure 4 a).

Acknowledgments

The authors would like to express their sincere thanks to J-C. Salabura for the design of the bending device utilized in the study and R.Mainguy for providing the technical support to establish the experimental device.


Observing institutional experiments in citizens involvment in public spaces projects

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

Should alternatives become models?

Observing institutional experiments in citizens involvement in public spaces projects Hello I'm Margaux, I'm a landscape architect beginning my third year of PhD, in the architecture school of Nantes in France. I apologize, I'm going to read my text cause my English is not so good, but I hope at least understandable. I also want to say I'm still in the heart of my research, so my presentation will focus more on fieldwork than on theory, and bringing more questions than answers. My PhD research is studying the interaction between experimentation and institutionalization dynamics within citizens involvement in public spaces development projects, in two European cities : Nantes and Brussels. These projects use generally speaking alternative approaches, previously mainly initiated by civil society, and now seized by public stakeholders and progressively turning into real public policies. These approaches are not really new but are today reused in a totally different context where the participation of the citizens has become a generalized idea and even an injunction. What is new is their position and role in the urban transformation and I'm asking if they produce a transformative impact on urban fabric or not. I do not have the time to go in details but I would like to introduce a little on each case. In Brussels: This is the Tour and Taxis site that is the former disused customs of Belgium, on the canal side, between the municipality of Brussels and the municipality of Molenbeek, I'm sure you heard about Molenbeek. Regional authorities wanted to develop a green spaces sequence from the Northwest to the historical center, but this hope was promptly in danger, as the biggest and central lot has been sold by the state to a private owner in 2000. Bruxelles Environnement, which is the administration in charge of green spaces development and management, decided to initiate two experiments on the plots they still control the property. Parckdesign was a temporary festival, of which remain a few devices now managed by an association of inhabitants: a greenhouse; a chicken coop; some beehives; a vegetable garden; etc. Allée du Kaai is a temporary occupation of former industrial sheds, set on a plot that is supposed to be transformed into a public park. The idea is to occupy the site for 4 years to initiate a social dynamic, and also to experiment uses in parallel of the programming and conception period of the future park. The place is running quite well, there is a strong appropriation, local associations are managing the site and leading various activities each week. In Nantes: This is the Île de Nantes; it is an island on the river Loire, in the center of the city. It is a well--know urban project in France since 20 years, on a former industrial area, and already quite well developed. The project is conducted by the SAMOA, which is a SPL société publique d'aménagement. Very briefly, a SPL is a society whose capitals are owned by public actors, and mandated for specific distric development on a given period. The SAMOA had previously some disappointing citizens participation experiences that they judged as too events oriented, not producing substantial impacts on the planning practice and attracting an insufficiently diverse population. Consequently they initiate a vast program of citizen's involvement. As of now, I'm currently following a specific project, just beginning. The idea is that the inhabitants become the sponsors of the future development, and also to reach the precarious people, quite numerous in the neighborhood. Are these cases alternatives and alternatives to what? I did the exercise of taking the way you tried to define what made alternatives alternative in the call for paper and confront it to my cases, and finally combine the results in three topics: one about the socio--economical aspects, one about the governance questions, and one about the urban fabric issues.

1/ SOCIO--ECONOMICAL ASPECTS My first answer is schizophrenia. In these popular neighborhoods, public authorities are trying to keep the disadvantaged people living there, and, at the same time, are trying to attract other social groups, in the name of the social mix, which became in urban politics in France an undisputed watchword. But it seems to be a declared issue more than a real issue, the projects are targeting disadvantaged groups, but the resources are not going along with the rhetoric. Furthermore, the way public authorities mobilize social categories is quite caricatural, often polarizing on one side a real popular inhabitant, who is supposed to be the target through a condescending speech, and on the other side a middle class invasive, creative and active inhabitant towards whom they develop a suspicion discourse. They fantasize a kind of inhabitants who should be "only" inhabitant and really popular. This seek is fore sure vain, but what is interesting it that it has brought them to focus interests and efforts on some specific disadvantaged populations, such as the youngsters, the black community or the homeless people, in a sort of positive discrimination approach which is quite unusual, at least in France. 2/ GOVERNANCE At first sight, we can easily say that yes, urban society and disadvantaged groups are the driving force, but, the ways it is done is questioning.

--Bruxelles Environnement discourse is clear, they have more and more spaces and less and less funds to deal with. This lead them to adopt two strategies, experimented in the cases studies: on one side to develop parks which are rough and low cost; on the other side to encourage citizens to appropriate green spaces and take responsibility of its management, most of the times without a real or a sufficient funds delegation. In Brussels, public authorities tend to settle in a position of "support" more than driving, as if their role could become only for example giving or lending spaces. Most of people are happy with it as they reclaim less control and less bureaucratic inertia from the institutions. People want spaces; for public authorities is it a way of saving money, and it is lived by both actors as a kind of win--win strategy. However, the increase of public spaces self management by small communities can be seen as quite closed to a neoliberal vision of a city were self organization can play the game of social segregation and public funds reduction. --Compared to Belgium, France is characterized by a stronger municipal power, centralized and non--communitarian approaches, and civil society self--organization is less developed. This is strengthened in Nantes by a political unity and continuity since 20 years, so public authorities remain in a more controlling and driving force. The public developer want to mobilize inhabitants in his frame, to transform his way of doing his daily job, to improve his projects. We can say it is a kind of manipulation of citizens participation, but we can also say in a less negative way, they want to use the citizen's participation to produce tangible effects and changes, which is maybe good. Unlike to Brussels, they don't want to support self--directed projects, but rather want to imply civil society in the public projects. The issue is that it is in a frame that is so constrained, on financial, temporal and procedural aspects, that the marge is really weak for inhabitants to find an interest in it. 3/ URBAN FABRIC The term itself of neoliberal city is not so shared by the actors, so it's hard to say if they seek to redirect urban development away form the neoliberal model. But factually these projects do seek to establish political or "social" regulations:

--In Brussels, the projects have become ways for public authorities and local communities to get stronger in the power relationship with the private actor landowner of the main central T&T site. They were both fighting against him for years for not building the entire site in a market oriented strategy, for not ending with a gated community for rich people in a popular and immigrated background neighborhood, and for having there a park of a sufficient size with respects to the green spaces needs of the surrounding district. These negotiations were difficult and didn't give any results for a long time. The two experiments we study here are situated on both sides of the private plot. This was quite clever, Bruxelles Environnement refuse to admit publicly that is was a strategy but it really seems that it was. As most inhabitants are practicing both places, they get used to cross this open wasteland, despite the fences, despite the mud and despite the ongoing building works. As there is almost no other way to get from one side to another, the landowner let it open, as a temporary situation but now during for years. The possibility of re--enclose the site, was for a long time a threat the landowner used in the negotiations (to give an idea, it was something like "I will not open the park to the neighborhood if you don't let me do the offices square meters I want", etc). This routine of using this space that the inhabitants took, made that the landowner is today in a quasi impossibility to re--enclose the site. ----On another side the civil society also implemented by itself these experiments to gain a most important role in the game. Several associations and inhabitants created what they called "Plateforme T&T", which aim to be a force for bringing proposals and weigh in on the balance in front of the landowner. They positioned themself as a third actor in the negotiation process, trying to go further than the public authorities, and at the same time supporting their action. The role played in these dynamics by the two first small experiments we are talking about was really important. It has been physical places to meet and to organize, and also first experiences where people get observations and know--hows. The detour by the physical use of space allowed to return on the institutional struggle ground with a renewed power balance. --In Nantes, the project is not so advanced, so I cannot do so precise observations. Also, the context is different, as private actors are not so strong. The public developer is the main leader and so we kind of observe both opposite dynamics inside this only one institutional actor. To explain, the project is lead on a district that is said to be the last popular neighborhood in the city center, and the last neighborhood "untouched" by the urban project in the île de Nantes. To involve the inhabitants, and especially the precarious ones, is seen as a way to « regulate » the development which is finally soon arriving. They even say that the aim is to « strengthen» the neighborhood and his social dynamics, so it will be able to resist to the upcoming "surge" of the urban project. Urban project that they are also the leader. Here we see the schizophrenia and above all the limits of such an approach. The project seems to be only an attempt to limit damage, kind of a palliative regulation. OPEN QUESTIONS I will conclude finish with two open questions that are more methodological ones. The first one is the issue of studying tools or devices when doing research from and for the urban fabric world. Let's take the example of a so--called alternative which is temporary occupation: in Brussels, in 3 years it became a new recipe, meeting civil society needs (associations reclaiming spaces), public authorities needs (how to deal with waiting and empty spaces) and issues and complexities of the urban development in Brussels (how to better anticipate urban development and deal with long term planning and uncertainty). For some actors it implies social objectives, but for others it is only a pragmatic answer of space management. Temporary occupations projects blossomed, each time leaded by different institutional actors. Furthermore, several institutions in Brussels are actually trying to work together on a new public tool that provide possibility to set temporary occupations more easily and systematically. Broadly, we observe that it became a new dominant trend, most of the time catched up by public authorities or private stakeholders to valorize their plots. Besides of playing the game of property speculation, these projects are also accused to target a mainly middle class creative audience. At the same time "occupation" stays a tool for alternatives and social movements, all around the world. In Brussels, the occupation, even if it is only for a few years, produces real alternatives, in public spaces uses, in politicization processes, and in the ways protagonists self--organize. People experiment alternatives more in the "doing", through the daily running of the collectively managed space. Even if it will maybe not have a direct impact on the layout of the future public space, the occupied space have already been for 3 years a support for individual and collective emancipation and a resource for social groups and movements as diverse as illegal immigrants, climate activists, squatter networks, campaigns against wasting food, activities with psychiatric disorder people, etc. So, the question for me is, behind some "design" convergences is there always "content" convergences, or political project convergences? And if it's not, what is the sense of analyzing urban tools such as temporary occupation? What is the utility of criticize, praise or design some alternative devices if they can be used in any context and serving any political and social orientation? This is saying that devices are not good or bad on their on, are not inherently oriented for or against neoliberalism or any political or economical system but are always contingent and depend on the context in which they are set and on the way they are conducted. The last question concerns the action scale and the observation and analysis scale. As Max Rousseau wrote, there is two opposed trends, the francophone research urban fields, which promotes micro observation at the local scale; and the Anglophone urban studies, more characterized by macro analysis on urban capitalism changes. I'm clearly more situated in the first case, but this session was precisely for me the opportunity to confront to the second one, and maybe to participate to seek a third way. But I'm still and always asking myself if it is really judicious to observe these kinds of small spatial experiments. Indeed, these experiences stay marginal ones, small scale, small budget and small issues projects, not touching the heart of urban production. Furthermore, it seems sometimes that creating news spaces, even "better spaces" is not sufficient, and that alternatives like these ones should engage more in the institutional redesign. But, the survey show that it is even hard to talk about institution as an entity, as the projects are always supported by individuals, leaded by their personal engagements and convictions. We observe in all the cases coalitions around a common "cause", forms of alliances between institutional and non--institutional actors, and also forms of internal fights between institutional actors and their own hierarchy or the elected representatives they depend on. However, it most of the time get to a dead--end because issues were not really clearly discussed or not really shared. Is it really possible to change public action from the inside and if it does not want to? Or is it better to try to build independent alternatives, which are maybe condemned to stay experimental and not having any implementation on institutional routines? Thanks for your attention