A G Guézennec
email: a.guezennec@brgm.fr

M Hanke

A Chmielarz

C Joulian

Y Ménard

P D'hugues

BIO-HYDROMETALLURGY: AN ALTERNATIVE TO PYROMETALLURGY FOR COPPER RECOVERY IN A POLYMINERAL CONCENTRATE

Keywords: Bioleaching, copper, low grade ore, electrowinning

The copper concentrate used in this study is produced by flotation of a black shale organic rich ore. This resource is a carbonate-rich, polymineral concentrate. In the last 5-6 years, ore characteristics changed and the concentrate grades degraded. This study evaluated bioleaching of the concentrate in stirred tank reactors as an alternative to the pyrometallurgical technology that is currently applied, and which may no longer be feasible in future given the concentrate chemistry.

The first part of the study focused on testing non-traditional operating conditions during continuous bioleaching experiments: high solids concentration (> 20% solids), reduced agitation and aeration rates. For the same residence time, bioleaching performance at high solids loading (25%) were the same as those reached during previous bioleaching experiments at 15% solids. No mixing or microbial issues were encountered. The bacterial consortium used in the experiments demonstrated a high copper tolerance.

The second part of the study was devoted to technology development for metal recovery from copper bearing solutions obtained during bioleaching experiments. Due to the high copper concentration of the leachate, solvent extraction prior to electrowinning was not necessary. After iron removal, high quality copper cathodes (morphology and composition) were obtained from the PLS by electrowinning.

INTRODUCTION

Biohydrometallurgy is a proven technology already industrially applied all over the world for the exploitation of refractory gold ores, low-grade copper ores and in one case for cobaltiferous pyrite [START_REF] Arrascue | Biooxidation of arsenopyrite concentrate using BIOX® process: Industrial experience in Tamboraque, Peru[END_REF][START_REF] Van Aswegen | The BIOX process for the treatment of refractory gold concentrates[END_REF][START_REF] Morin | Bioleaching of a cobalt-containing pyrite in stirred reactors: a case study from laboratory scale to industrial application[END_REF][START_REF] Dreisinger | Copper leaching from primary sulfides: Options for biological and chemical extraction of copper[END_REF][START_REF] Watling | The bioleaching of sulphide minerals with emphasis on copper sulphides -A review[END_REF]. However, it remains a niche application which can compete with other pyrometallurgial or hydrometallurgical technologies only for specific resources. Cost can be one advantage of biohydrometallurgical processing and is also one of the most important drivers for the industry. Another advantage is the low environmental impact, sulphur dioxide and arsenic emissions to air occurring during smelting being completely avoided [START_REF] Brierley | Present and future commercial applications of biohydrometallurgy[END_REF]; [START_REF] Rawlings | Biomineralization of metal-containing ores and concentrates[END_REF]).

The copper concentrate subject of this study is produced by flotation from a black shale organic rich ore. The main copper-bearing minerals are: chalcocite, bornite, chalcopyrite and covellite. In the last few years, an increase of organic carbon and As contents as well as a decrease of Cu content in ore were observed, resulting in a decrease of the concentrate quality. There was a need to look for new methods of concentrate treatment. Bioleaching efficiency for this type of ore was previously demonstrated in the frame of the Bioshale FP6 European project [START_REF] D'hugues | Bioshale FP6 European project: Exploiting black shale ores using biotechnologies[END_REF]. However, some improvements were still needed to be achieved in order to meet process economic viability [START_REF] Spolaore | Bioleaching of an organic-rich polymetallic concentrate using stirred-tank technology[END_REF]. In bio-hydrometallurgy, the most important investment costs generally come from the bioleaching and electrowinning unit operations. When focusing on bioleaching itself, capital costs largely depend on global tank volume and agitators. Our study aims at improving the profitability of the bioleaching of the copper concentrate in stirred tank reactors with the following objectives:

 increasing revenues by improving chalcopyrite dissolution and thus bioleaching operation efficiency;  reducing capital costs by reducing global bioleaching tank volume, which means decreasing residence time in the bioleaching unit and increasing the feed solids content from 15% to 25%;  reducing capital and operating costs by performing electrowinning without solvent extraction step.

The first part of the experimental study consisted in testing non-traditional operating conditions during continuous piloting bioleaching experiments: high solids concentration (> 20% solids), reduced agitation and aeration rates. In the second part of the study, electrowinning experiments were conducted on copper bearing solutions obtained during the bioleaching experiments cited above. Current efficiency and energy consumption were monitored. The influence of iron and chloride removal from the bioleaching solution on copper recovery was also tested.

MATERIALS AND METHODS

Characterization of the sulphidic materials

The copper concentrate used in the study is produced from a black shale ore. The main characteristics of the sample are as follows: Cu 14.6%, Ag 900 mg/kg, Fe 7.5%, sulphur as sulphide 15.9%, inorganic C 1.9% and organic C 8.2%, particle size (80% cumulative passing) 60 µm. Copper sulphides in the concentrate mainly consist of bornite (Cu 5 FeS 4 ), chalcocite (Cu 2 S), chalcopyrite (CuFeS 2 ) and covellite (CuS).

Bacterial culture and nutrients

The tests were run using a mixed bacterial consortium from the Bioshale project. Bioshale-BRGM bacterial consortium has already been fully described [START_REF] Spolaore | Bioleaching of an organic-rich polymetallic concentrate using stirred-tank technology[END_REF]. The predominant organisms in the culture are affiliated to the genera Leptospirillum, Acidithiobacillus and Sulfobacillus.

Nutritive medium

The nutrient medium is derived from the "0Km" medium [START_REF] Battaglia | Dissolution of cobaltiferrous pyrite by Thiobacillusferrooxidans and Thiobacillusthiooxidans -factors influencing bacterial leaching efficiency[END_REF]. Following the Bioshale project, it was demonstrated that a dilution of the 0Km medium by a factor 3 did not affect bioleaching performance. For economic reasons, the 0Km/3 medium is used in this study. Its standard composition is the following: (NH 4 ) 2 SO 4 , 1.23 g.L -1 ; H 3 PO 4 , 0.27 g.L -1 ; MgSO 4 •7H 2 O, 0.17 g.L -1 ; KOH, 0.16 g.L -1 .

Pregnant leach solution

The initial solution used in the electrolytic copper recovery experiments was sampled during bioleaching of the copper concentrate. This copper rich solution is called PLS1 and its main characteristics are the following: Cu 2+ , 44.91 g.L -1 ; Fe 3+ , 5.5 g.L -1 ; Fe 2+ , 0.27 g.L -1 ; Cl -, 0.46 g.L -1 ; pH, 1.78.

Laboratory-scale continuous bioleaching operation

Apparatus

The continuous bioleaching operation was carried out in a laboratory-scale unit equipped with three stirred reactors, one of 50 L (R1) followed by two of 20 L operating capacity (R2, R3). The tanks are all made from 316L stainless-steel and have a height/diameter ratio equal to 1. The reactors are arranged in cascade so that the pulp flows from one tank to the next one by overflowing. CO 2 -enriched air (1%) is injected beneath the turbine at the bottom of the tank. The top of the reactors is connected to a condenser system to prevent excessive evaporation. The same mixing system (BROGIM® -BRGM/MRM) is mounted in all tanks on a rotating shaft.

Operating conditions

The feed was made up of a high density pulp (50% wt.wt -1 in water), a concentrated nutritive medium flow and a H 2 SO 4 (20% v.v -1 ) flow. The three feed flows were pumped separately into the first tank to obtain the desired solids ratio for the feed pulp (20% or 25%) and a pH below 1.8.

Temperature was maintained constant at 42°C by circulating cold water through an internal stainless-steel coil for cooling and by an external electric ribbon for heating. When necessary, the pH was regulated in the second tank of the unit (R2) between 1.5 and 1.7 by adding H 2 SO 4 (20% v v -1 ) in the pulp. pH regulation was not necessary in the last tank. The main operating conditions in the pilot are presented in Table 1. Three configurations were tested during these experiments:

-Configuration 1 (solid concentration of the feed pulp: 20%) corresponds to the first attempt to test the feasibility of a bioleaching operation at high solid content. This configuration was tested during 24 days. -In Configuration 2 (solid concentration of the feed pulp: 20%, reduced agitation/aeration rates in R1), the objective was to reduce redox potential (< 420 mV Ag/AgCl) by controlling oxygen transfer rate in order to improve chalcopyrite dissolution. This configuration was tested after Configuration 1, for a period of 16 days.

-Configuration 3 (solid concentration of the feed pulp: 25%, reduced agitation/aeration rates in R1, R2 and R3) combines both objectives of Configurations 1 and 2, i.e. increasing solid content of the feeding pulp and decreasing redox potential. This configuration was tested after Configuration 2, for a period of 27 days. The residence time was reduced from 5.1 days in Configuration 1 to 4.6 days in Configurations 2 and 3.

Analytical techniques

Redox potential (vs. Ag/AgCl), pH and dissolved oxygen concentration (Mettler Toledo probe InPro 6850i) were measured directly in the pulp. Copper and total iron concentrations were measured by atomic absorption spectroscopy (Varian SpectrAA-300) in the supernatant fraction from 0.45 µm filtered culture samples. The oxygen concentration in the inlet and outlet gas of each reactor was measured using a paramagnetic analyzer (ADC -MGA 3000). The gas balance was used to calculate the oxygen uptake rate (OUR).

Laboratory-scale electrowinning tests

Apparatus

Copper removal by electrowinning was tested in an experimental stand which consists of a main electrolyte tank, electrolytic cell and spent electrolyte tank with stirrer. Thus the electrolyte flows through a circuit. The characteristics of the cell are as follows:

cell tank volume: 2 dm 3 electrode set-up: 2 anodes -PbAg, 1 cathode -316 L stainless steel cathode operating dimensions (cm): 8.2 x 10.2 x 0.2 cathode operating surface: P = 167.28 cm 2 anodes operating dimensions (cm): 7.5 x 10.0 x 0.25 anode spacing: 90 mm

Operating conditions

Before elctrowinning trials, several pre-treatment processes were tested on the copper rich bioleaching solution:

-Hydrolytic Fe(III) removal using calcium carbonate: 8.5 dm 3 of bioleaching solution was heated up to the temperature of 90 -95°C. 115 g of CaCO 3 was then proportioned with continuous mechanical stirring for 1 hour. The precipitated suspended matter was filtered on Buchner funnel and the cake was washed with water. The solution obtained is called PLS1-IR1. -Fe(III) removal by jarosite precipitation: bioleaching solution was brought to pH of 1.7, and then maintained in the state of boiling under reflux condenser for 20 h. The process was conducted in 5 steps where 1.5 dm 3 of the bioleaching solution was treated. To accelerate precipitation of iron in a form of jarosite, crystal nuclei were added to the solution. After cooling the solution down, the precipitated residue was filtered (easy filtration) on Buchner funnel and the cake was washed with water. The solution obtained is called PLS1-IR2. -Fe(III) removal by jarosite precipitation combined to Cl -removal: Fe(III) was removed from the bioleaching solution as described above. The excess of chloride ions was removed from the solution by weighed amount of Ag 2 SO 4 added in a form of aqueous solution. The precipitated silver chloride was filtered and the excess of silver ions was then cemented on metallic copper. The solution obtained is called PLS1-IR2-CR.

Continuous copper removal by electrowinning was then tested using the bioleaching solution without pretreatment (PLS1) and the three solutions obtained after pre-treatment (PLS1-IR1, PLS1-IR2, PLS1-IR2-CR). The tests were conducted in the conditions of maintaining the Cu 2+ content in the solution at a constant level of about 30 g/dm 3 . Electrolyte flow was equal to 33.3 cm 3 /min which means that volume of the electrolytic cell was exchanged after one hour. Inhibitors (glue and thiourea) were continuously proportioned to the spill tank. The temperature of the electrolyte was maintained at 55°C. The cathode current density was 200 A/m 2 .

Analytical techniques

Current intensity, bath voltage, current efficiency and electrical energy consumption were continuously registered during the tests. Copper content of the electrolyte was measured by means of iodometric copper titration. The composition (Cu, Fe, Ag, Pb, Zn, As contents) of the copper cathodes was determined by means of inductively coupled plasma optical emission spectroscopy (ICP-OES).

RESULTS AND DISCUSSION

Bioleaching experiments

Results obtained with configuration 1 (solid concentration of the feed pulp: 20%) After 15 days of continuous operation, all physico-chemical parameters (pH, Eh, Cu and Fe concentrations) were stable in the first reactor of the unit (R1): the reactions reached a steady-state. R2 was also close to a steadystate but in R3, redox potential and iron concentration were decreasing constantly and the pH was gradually increasing. It reached values around 2.2. It was assumed that bacterial activity in this reactor was affected by the high pH, limiting iron oxidation rate and causing Eh and iron concentration decrease. At this pH, iron precipitation also occurred. As the pH was already high in R2 (between 1.8 and 1.9), it was decided to start pH regulation in this reactor. It would help to decrease the pH in R3 and stabilize bacterial culture and bioleaching performances. Sulphuric acid (20% v v -1 ) addition started in R2 to obtain a pH between 1.5 and 1.7. This was successful and after 24 days, the whole unit reached a steady-state. Table 2 presents the values of pH, redox potential, dissolved oxygen concentration, OUR and copper concentration in the liquid phase reached at steady-state in the reactors R1, R2, R3. With this first set of operating conditions, in R1, copper dissolution rate was 355 mg.L -1 .h -1 and copper recovery was 48%. After 5.2 days of residence time, global copper dissolution was 60% (Figure 1). Almost no copper dissolution occurred in the last reactor.

Figure 1 -Influence of operating conditions on copper dissolution vs. residence time during continuous bioleaching (X, configuration 1; , configuration 2; , configuration 3; , Bioshale)

Results obtained with configuration 2 (solid concentration of the feed pulp: 20%, reduced agitation/aeration rates in R1)

During the Bioshale project, the limitation of copper recovery had been demonstrated to be linked to incomplete dissolution of chalcopyrite. To improve chalcopyrite dissolution, many authors recommend working at reduced redox potential (< 420 mV) to limit passivation of the mineral surface. The reduction of redox potential is obtained by controlling the oxygen transfer rate [START_REF] Córdoba | Leaching of chalcopyrite with ferric ion. Part I: General aspects[END_REF][START_REF] Pinches | A method of operating a bioleach process with control of redox potential[END_REF][START_REF] Third | Control of the redox potential by oxygen limitation improves bacterial leaching of chalcopyrite[END_REF][START_REF] Tshilombo | The influence of applied potentials and temperature on the electrochemical response of chalcopyrite during bacterial leaching[END_REF]. In this study, the limitation of redox potential was applied in the first reactor. As bacterial growth mostly takes place in this reactor, a particular attention was taken to provide sufficient oxygen in order to maintain the necessary growth rate avoiding a washout of the unit. Agitation and aeration rates were gradually decreased to their lower acceptable limit to obtain a good homogeneity of the pulp (340 rpm, 100 L.h -1 , Table 1). A slight decrease of dissolved oxygen concentration was observed and OUR was cut in half. This was not sufficient to allow a redox potential decrease (Table 2). Redox potential increased in R2 and R3 but OUR in these reactors did not change significantly.

Copper dissolution was positively impacted by these changes of operating parameters: in R1, copper dissolution rate reached 445 mg.L -1 .h -1 , copper concentration was 24 g.L -1 and copper recovery was 68%. When considering the whole operation (R1, R2, R3, R4), global copper dissolution did not improve in the same proportions. After 4.7 days residence time, final copper concentration in R4 was 28.4 g.L -1 corresponding to 73% recovery. No significant copper dissolution was again observed in R4.

Results obtained with configuration 3 (solid concentration of the feed pulp: 25%, reduced agitation/aeration rates in R1, R2, R3) R1 performances at 20% solids being satisfactory, the feed solid content was increased to 25%. Similarly, as the agitation and aeration rate limitation improved copper dissolution in R1, the same modification was applied to R2 and R3. After 10 days of continuous running with these new operating parameters, a steady-state was reached. A decrease of pH in R1 was observed. Dissolved oxygen concentration also decreased and OUR increased in proportion to the solids content (Table 2). Copper recovery in R1 was identical to the one at 20% solids (67%, Figure 1). Copper dissolution rate was in consequence improved from 445 to 538 mg.L -1 .h -1 .

In R2 and R3, the decrease in agitation and aeration rates caused a limitation in oxygen transfer: dissolved oxygen concentration dropped under 1 mg/L and OUR significantly decreased. Redox potential also decreased but its value remained above 420 mV (Table 2). In R3, an increase in pH was observed. Copper recovery in these two reactors was positively impacted by the reduction of agitation and aeration rates. It was 81% in R2 and 86% in R3 (Figure 1). The copper concentration reached very high values all along the continuous unit.

The results obtained in this last part of the bioleaching continuous experiment can be compared to those obtained in the Bioshale project on the same resource. In the latter, 92% copper recovery in 6.6 days residence time was the best performance reached at 15% solids. In this study, the best result was 86% copper recovery at 25% solids after 4.7 days residence time. In Figure 1, it appears that both processes reached the same recovery after 4.7 days of residence time. So, it should be assumed that at 25% solids, if residence time was extended, recovery would at least reach 92%.

Bioleaching consortium

The bioleaching consortium at the end of the 50L batch culture just prior the start-up of the continuous operation was the following: Sulfobacillus (Sb.) benefaciens 6%, Sb. thermosulfidooxidans 10%, Leptospirillum (L.) ferriphilum 64% and Acidithiobacillus (At.) caldus 20%. All species of the Bioshale-BRGM consortium were still present.

When the unit was run in continuous mode, the diversity of the population in R1 decreased gradually. After 21 days, Sb. thermosulfidooxidans was not detected anymore and after 46 days, At. caldus was also under the CE-SSCP fingerprinting technique detection level (Figure 2). The final population was only composed of two organisms, Sb. benefaciens and L. ferriphilum, which is even less diverse than what is usually observed in continuous operations [START_REF] Okibe | Enumeration and characterization of acidophilic microorganisms isolated from a pilot plant stirred-tank bioleaching operation[END_REF][START_REF] Norris | Acidophile diversity in mineral sulphide oxidation[END_REF]. Nevertheless, all the functions needed in a bioleaching consortium were still present: iron and sulphur oxidation and autotrophic and heterotrophic growth. The fact that Sb. benefaciens developed over Sb. thermosulfidooxidans could be due to the faster growth rate of Sb. benefaciens in autotrophic condition. Sb. thermosulfidooxidans is more likely to washout in a stirred tank continuous operation [START_REF] Johnson | Sulfobacillusbenefaciens sp. Nov., an acidophilic facultative anaerobic Firmicute isolated from mineral bioleaching operations[END_REF]. The population evolution was similar in the two other reactors (R2 and R3, data not shown).

Electrowinning experiments

Efficiency of the Fe(III) removal processes

The results of Fe(III) removal experiments (Table 3) show that 96% of Fe(III) is removed from PLS1 solution by hydrolysis. There is, however, significant loss of copper in the residue which most probably results from difficulties in filtration of the precipitated iron residue. Jarosite precipitation removes only 70% of the iron from the solution, but the precipitate is much easier to filter than the precipitate obtained with the hydrolysis method and the loss of copper in the residue is much lower. The results of the continuous electrowinning experiments performed on PLS1 (see Table 4) show that copper removal is efficient and copper cathodes are produced in a compact metallic form. However, the current efficiency is quite low during the operation (in the range 61 -77%), which entails high energy consumption. Fe(III) removal from the PLS1-IR1 solution radically increases current efficiency (above 91%) and decreases electrical energy consumption (to the level of 2000 kWh/Mg Cu). Copper deposits obtained from electrowinning of PLS1, PLS1-IR1 and PLS1-IR2 are characterized by smooth metallic surface and presence of loosely bounded bushy accretions mainly located on the cathode rims whereas the cathode obtained from PLS1-IR2-CR exhibits a significant decrease of dendritic accretions. These results confirm that chloride removal improves the quality of copper cathodes. The qualities of the copper deposits obtained were very good and average impurities level in the cathodes was as follows [ppm]: Ag, 24.24; Pb, 1.74; Fe, 2.15; Zn, 3.04; As, 0.37.

CONCLUSIONS

Non-traditional operating conditions were tested on the bioleaching of a carbonate-rich, multi-element (Cu, Ag) and multi-mineral concentrate. Electrowinning of the bioleaching solution for downstream copper recovery was also examined.

The bioleaching performances obtained were compared to the ones reported in the Bioshale project on the same resource. In the latter, 92% copper recovery in 6.6 days residence time was the best performance reached at 15% solids. In this study, the best result was 86% copper recovery at 25% solids after 4.7 days residence time. It appears that both processes reached the same recovery rate after 4.7 days of residence time. So, it should be assumed that at 25% solids, if residence time was extended, recovery would at least reach 92%. The feasibility of the copper concentrate bioleaching at high pulp density (25%) was also demonstrated. No mixing or microbial issues were encountered. The Bioshale-BRGM consortium has shown a rare copper tolerance: copper content increased up to more than 40g/L without any negative effect on the bacterial community. Combining high solid content and reduced residence time, this bioleaching process would need 2.3 less tank volume than the "Bioshale bioleaching process" to reach a copper recovery of 86%, which would significantly reduce capital costs.

Electrowinning experiments performed using solution coming from bioleaching tests show that copper removal can be achieved directly by electrowinning without any solvent extraction step. Preliminary removal of Fe(III) from the bioleaching solution significantly increases current efficiency of electrowinning operations. Iron can be successfully removed by precipitation as jarosite, which demonstrated good filtration properties at low copper loss in the residue. An improvement of the structure of copper cathodes can be achieved by decreasing chloride ion content in the bioleaching solution from the level of about 0.5 g.dm -3 to 0.01 g.dm -3 . Chloride content reduction should be done in a separate process of concentrate washing before bioleaching. These results offer new perspectives for the design of an alternative process to smelting.

Figure 2 -

 2 Figure 2 -Evolution of the bacterial community structure determined from CE-SSCP fingerprints in the first reactor of the continuous unit. Grey bars Sb. benefaciens; black and white bars Sb. thermosulfidooxidans; white bars L. ferriphilum; black bars At. caldus.

Table 1 -

 1 Main operating conditions of the continuous bioleaching unit. Comparison with best operating conditions applied during the Bioshale project.

	Configuration	Solid concentration	Tank	Residence time (days)	Agitation rate (rpm)	Aeration rate (L.h -1 )
			R1	2.5	450	800
	1	20%	R2	1.3	420	400
			R3	1.3	420	400
			R1	2.4	340	100
	2	20%	R2	1.1	420	400
			R3	1.1	420	400
			R1	2.4	340	100
	3	25%	R2	1.1	300	50
			R3	1.1	300	50
			R1	2.1	390	500
	Bioshale	15%	R2	2.2	250	100
			R3	2.3	250	100

Table 2 -

 2 pH, redox potential, dissolved oxygen concentration and OUR values reached at steady-state in the three reactors of the continuous bioleaching unit. Comparison between the three different sets of operating parameters.

	Configuration		1	2	3
		R1	1.6	1.8	1.3
	pH	R2	1.5	1.5	1.6
		R3	1.7	1.6	1.9
		R1	600	592	601
	Eh	R2	611	642	472
		R3	616	639	480
		R1	5.6	3.2	1.9
	dissolved O 2 (mg.L -1 )	R2	5.6	5.1	0.6
		R3	5.4	5.3	0.5
	OUR (mg.L -1 .h -1 )	R1 R2	193 202	86 163	115 101
		R3	147	126	79
		R1	21.0	24.4	31.7
	Cu (mg.L -1 )	R2	25.6	25.7	37.2
		R3	26.8	27.6	40.6

Table 3 -

 3 Results of the Fe(III) removal tests

		Initial solution	Mass of		Final solution		Removal to residue
	Fe(III) removal	(g.dm -3 )	precipitated		(g.dm -3 )		(%)
	tests	Cu 2+	Fe	residue (g)	pH	Cu 2+	Fe	Cu 2+	Fe
	Hydrolytic Fe(III)
	removal using	44,91	5,76	161,8	3,0	43,08	0,23	4,07	96,0
	calcium carbonate
	Fe(III) removal by
	jarosite	44,91	5,76	51.9	1,24	43,58	1,67	2,93	71,0
	precipitation
	Results of the electrowinning experiments

Table 4 -

 4 Results of the continuous electrowinning tests

	Initial solution	Cu 2+ content in solution (g.dm -3 ) initial final	Mass of copper cathode (g)	Average current (A)	Cell voltage (V) stand. average deviat.	Current efficiency (%)	Energy consumption (kWh/t Cu)
	PLS1	44.9	29.9	133.8	3.341	2.21	0.06	70	2653
	PLS1-IR1	43.1	30.5	106.3	3.345	2.26	0.09	96	1828
	PLS1-IR2	43.6	28.8	97.6	3.347	2.22	0.08	91	2055
	PLS1-IR2-CR	41.3		90.8	2.348	2.18	0.07	91	2012

ACKNOWLEDGEMENTS

The research leading to these results has received funding from the European Community's Seventh Framework Programme ([FP7/2007-2013] [FP7/2007-2011]) under grant agreement n° 228559. This publication reflects only the author's view, exempting the Community from any liability.


Andreea Dogaru

Céline Pichon

Rodica Ababei

Dmitri Mitcov

Catalin Maxim

Loïc Toupet

Corine Mathonière

Rodolphe Clérac
email: clerac@crpp-bordeaux.cnrs.fr

Marius Andruh
email: mandruh@yahoo.com

New bidimensional honeycomb CoII-FeIII and brick wall FeII-CoIII cyanido-bridged coordination polymers: Synthesis, crystal structures and magnetic properties

Keywords: Heterometallic complexes, Cyanido-bridged complexes, Iron complexes, Cobalt complexes, Magnetic properties. New bidimensional cyanido-bridged heterometallic coordination polymers,

come

Introduction

The high interest in the chemistry of heterometallic cyanido-bridged complexes was triggered by the synthesis of the first molecular magnets with high critical temperatures, which belong to the Prussian Blues family [1]. Another milestone in this field is due to Ohba, Okawa et al. [2], who showed that the hexacyanido-complexes can be used as metalloligands not only towards bare (actually hydrated) metal ions, but also towards complex cations with one or more coordination sites blocked by ancillary ligands. Following this strategy, a plethora of cyanido-bridged complexes with interesting magnetic properties have been described [START_REF] Dunbar | Several excellent reviews on heterometallic complexes constructed using hexacyanido metalloligands have been[END_REF]. The denticity of the ancillary ligands has a strong influence on the nature of the polymetallic cyanido-bridged complexes resulted from the self-assembly processes (oligonuclear species versus coordination polymers with various dimensionalities and network topologies). A rich variety of organic molecules can be used to block several coordination sites of the assembling cations with various denticities (ranging from monodentate to pentadentate), geometries (e.g. linear, tripodal or macrocyclic) and charge (neutral or often anionic). Let us recall briefly the case of macrocyclic blocking ligands. They are usually coordinated into the equatorial plane of the metal ions, the axial positions being occupied by weakly bonded groups (usually solvent molecules), which can be easily replaced by cyanido bridges. Generally, the self-assembly process involving trans-[M(mac)(solv) 2 ] 2+ (where mac stands for macrocyclic ligand) and [M' (CN)

6 ] 3-ions yields 2-D [M 3 M' 2 ] networks. Within a layer, each [M(mac)] 2+ moiety is linked to two [M'(CN) 6 ] 3-ions in
trans positions, and each [M'(CN) 6 ] 3-ion is surrounded by three [M(mac)] 2+ units. If the [M'(CN) 6 ] 3-tecton employs three facial cyanido groups as bridges, layers with a stair-shaped honeycomb architecture are formed [4]. On the other hand, flat brick wall-like layers are assembled when the three cyanido bridges are disposed in meridional positions [5].

Two interesting macrocyclic ligands, L 1 and L 2 , both potentially pentadentate, result from the condensation reaction of 2,6-diacetylpyridine with 3,4-dioxaoctane-1,8-diamine, and triethylenetetramine, respectively (Scheme 1). These two ligands favour a pentagonal bipyramidal geometry around metal ions. Another important property of these ligands arised from the coordination geometry imposed to the metal ions. They generate very robust Co II and Mn II complexes with high preference for octahedral coordination and prevent their oxidation to Co III and Mn III , respectively. Moreover, it has been shown that the geometry of these ligands can enhance the local magnetic anisotropy of high spin Fe II complexes [6]. Interestingly, the selfassembly process between [M(L 1/2 )(solv) 2 ] 2+ and [M'(CN) 6 ] 3-ions do not always lead to 2-D coordination polymers. For example, by reacting [Mn(L 2 )(H 2 O) 2 ]Cl 2 with K 3 [Fe(CN) 6 ], we obtained a linear pentanuclear [Mn II 3 Fe III 2 ] complex [7], while reactions between [10]. The synthesis of 2 was performed in a glove box operating under argon using degassed solvent by freeze-pump-thaw technique prior to use.

[Mn(L 2 )(H 2 O)

Physical measurements

The IR spectra were collected in the 4000 -400 cm -1 range on a Bruker Tensor 37 spectrometer for 1 (KBr pellets) and on a Thermal Scientific Nicolet 6700 spectrometer equipped with a Smart iTR diamond window for 2. Magnetic measurements were carried out with a Quantum Design MPMS-XL SQUID magnetometer equipped with a 7 T magnet. Measurements were performed on crystals of 1 (7.9 mg) and 2 (13.6 mg) covered and thus restrained in a minimum of their frozen mother liquor within a sealed straw to prevent desolvation of the solid. No evaporation of the mother liquor was observed during the measurements. The mass of the sample was determined after the measurements and subsequent mother liquor evaporation. These measurements were collected in the temperature range 1.8-280 K and in the field range -7 to 7 T.

An M vs. H measurement was performed at 100 K to confirm the absence of ferromagnetic impurities. Ac magnetic susceptibility data were collected in zero dc field in the temperature range 1.8-10 K, under an ac field of 3 Oe, oscillating at frequencies in the range 1-1000 Hz. The magnetic susceptibility data were corrected for the sample holder and the diamagnetic contributions.

Synthesis

Synthesis of complex 1

[{Co(L 1 )}

X-ray data collection and crystal structure refinement

The crystals are losing quickly their interstitial solvent molecules once filtered from their mother solution, resulting in a loss of crystallinity. For this reason, the crystals were mounted on a fiber CrysAlis RED programs from Oxford Diffraction [START_REF]CrysAlis CCD and CrysAlis RED[END_REF]. The structure of 1 was solved with the SHELX-97 crystallographic software package [START_REF] Sheldrick | SHELXL-97; Program for Crystal Structure Refinement[END_REF], while 2 was solved using SIR-97 [START_REF] Altomare | [END_REF], and refined by a full-matrix least squares method on F 2 with SHELXL-97. The non-H atoms were refined with anisotropic displacement parameters. Hydrogen atoms were located at calculated positions using suitable riding models except the ones on methanol and water molecules that were not introduced, but are taken into account in the compound formula. The structure of 2

presents a disorder rather difficult on one of the two macrocyclic complex contained in the unit cell. Also, it was necessary to use numerous restraints and set isotropic four nitrogen atoms. C29 and N14 from a cyanide group coordinated to Co(1) are disordered on two positions and were refined with relative occupations of 0.642 : 0.358. A summary of the crystallographic data and the structure refinement for crystals 1 -2 are given in Table 2.

Results and Discussion

Syntheses and general description

The two compounds, 2), have been obtained in a similar way, by slow diffusion of a solution containing the assembling cation, [M(L 1/2 )] 2+ , into the solution of the corresponding hexacyanometallate. The characteristic bands of the cyanido group are observed by infrared spectroscopy for the two compounds as follows: 2145 and 2110 cm -1 for 1, and 2122 cm -1 for 2.

[{Co(L 1 )} 3 {Fe(CN) 6 } 2 ]•6CH 3 OH•6H 2 O (1) and [{Fe(L 2 )} 3 {Co(CN) 6 } 2 ]•2CH 3 OH•13H 2 O (

Crystal structure of complex 1

The crystallographic investigation of the two compounds reveals an assembly of layers with different topologies driven by the relative position of the three bridging groups from the hexacyanidometallo-ligands (facial for {Fe(CN) 6 } in 1 and meridional for {Co(CN) 6 } in 2). The asymmetric unit for 1, presented in Figure 1, reveals two crystallographically independent heptacoordinated cobalt ions, both with a pentagonal bipyramidal stereochemistry. Their equatorial plane is occupied by the macrocyclic L 1 ligand, with metal-donor atom distances varying between 2.318(3) and 2.117(3) Å for Co1 and between 2.257(2) and 2.127( 4

Crystal structure of complex 2

The asymmetric unit for compound 2, shown in Figure 3, contains two crystallographically independent heptacoordinated iron(II) ions with a pentagonal bipyramidal geometry. The L 2 ligand coordinated to Fe1 is disordered over two positions with relative occupancy of 0.642 : 0.358. The Fe -N distances within the equatorial plane vary between 2.076(11) and 2.355( 12) Å (for Fe1), and between 2.180(6) and 2.290( 5) Å (for Fe2). Each {Co(CN) 6 } building-block interacts with three iron ions through meridional cyanido groups, resulting in a 2-D coordination polymer with a distorted brick wall topology (Figure 4). The distortion is due to the deviation from the linearity of the Co-CN-Fe bridges: Fe1 -N12 -C25 = 147.4( 5), Fe1 -N13 -C26 = 153.8( 5) and Fe2 -N9 -C24 = 164.9(4)°. The distances between the metal ions bridged by cyanido groups are:

Co1•••Fe1 = 5.018, Co1•••Fe2 = 5.086, Co1•••Fe1 i = 4
.947 Å (i = 0.5-x, -0.5+y, 0.5-z). Each mesh of the network contains six cobalt ions (each one shared between three meshes) and six iron ions (each one shared between two meshes). Similarly to the case of compound 1, the stacking of the layers, parallel to the ab plane, generates channels, which are filled by the solvent molecules.

Selected bond distances and angles for the two compounds are gathered in Table 1.

Magnetic properties

The magnetic properties of the two compounds have been investigated. The magnetic susceptibility shown as a T vs. T plot for compound 1 is represented in Figure 5. The room temperature value of the T product, 10.8 cm 3 mol -1 K ( defined as M/H per Co 3 Fe 2 unit), corresponds to the value expected for three high spin Co II and two low spin Fe III non-interacting magnetic centers (10.8 cm 3 mol -1 K with g Co = g Fe = 2.6), having in mind that both high spin Co II and low spin Fe III ions are characterized by a significant orbital contribution to their magnetic moment. By decreasing the temperature, T product increases slowly, then more abruptly until reaching 71 cm 3 mol -1 K at 2.7 K, indicating dominant ferromagnetic interactions between the Co II and Fe III ions through the cyanido bridges. The exchange interaction between high spin Co II and low spin Fe III ions within cyanido-bridged complexes is frequently ferromagnetic [15], and in fewer cases it was also found to be antiferromagnetic [16]. Below 2.7 K, T decreases due to a saturation associated to the long-range magnetic order (vide infra). The field dependence of the magnetization recorded between 1.8 and 8 K is shown Figure 6. The magnetization reaches 10.7

µ B at 1.85 K under 7 T. Below 3 K, the magnetization increases very fast at low field, indicating the occurrence of a magnetic order. This magnetic order appears to be ferromagnetic, as no inflection point (that could be related to an antiferromagnetic interactions and thus antiferromagnetic order) is seen on the M vs. H plot. Moreover, the ferromagnetic order is also in agreement with the increase of the χT vs T data discussed above. This magnetic order is further confirmed by the ac measurements displaying (i) out-of-phase susceptibility χ'' different to zero below 3 K and (ii) no significant frequency dependence of the in-phase χ' and χ'' components, as seen on the data shown in Figure 7. The absence of an inflection point on the M vs H curve, even at low fields, and the out-of-phase signal below 3 K strongly suggest that the interlayer interaction is ferromagnetic.

The temperature dependence of the χT product for compound 2 at 1000 Oe is presented in Figure 8. The magnetic contribution for 2 is due solely to the high spin Fe II ions, and the room temperature value of 10.9 cm 3 mol -1 K corresponds exactly to the expected values for three S = 2

Fe II ions per formula unit with g Fe = 2.2. The χT product remains constant down to ca 40 K, then is decreases abruptly, because of the quite strong ZFS effect associated to the iron(II) ion within this stereochemistry [6]. Intralayer antiferromagnetic interactions between the Fe II nodes, if any, are very weak because of the quite long Fe•••Fe distances (7.699 and 7.514 Å). The presence of the magnetic anisotropy is confirmed by the non-superposition of the M vs. H/T curves (Figure 8). Consequently, the magnetic data of 2 have been fitted using the following Hamiltonian: Table 1. Selected bond lengths (Å ) and angles (°) in compounds 1 and 2.

i = 1-x, y, 0.5z i = -x, y, 0.5-z ii = 1.5-x, -0.5+y, 0.5-z ii = 0.5-x, -0.5+y iii = 1. 5-x, 0.5+y, 0.5-z

  loop and immediately placed under the nitrogen flux to prevent desolvatation. X-ray diffraction measurements for crystal 1 were performed at 173 K on a STOE IPDS II diffractometer operating with Mo K (λ = 0.71073 Å) X-ray tube with graphite monochromator. A four-circle Oxford Diffraction Xcalibur 3 diffractometer with graphite-monochromated Mo-K radiation ( = 0.71073 Å) with a 2D Saphire 3 CCD detector coupled with an Oxford CryoSystem nitrogenflow cryostat was used for the data collection at 100(2) K of 2. The structures were solved by direct methods and refined by full-matrix least squares techniques based on F 2 . The unit cell parameters and the data integration of 2 were respectively obtained with the CrysAlis CCD and

  ) Å for Co2.The Co -O bond lengths are slightly longer than the Co -N ones (Co1 -O1 = 2-N10 = 2.127(4) Å). The apical positions are occupied by the nitrogen atoms arising from the cyanido bridges (Co1 -N3 = 2.088(3), Co1 -N4 ii = 2.101(3) Å with ii = 1.5-x, -0.5+y, 0.5-z; Co2 -N8 = 2.117(3) Å). The Fe -C distances (1.945(3) -1.952(3) Å) fall in the normal range for cyanido complexes of iron(III)[7],[9]. The {Fe(CN) 6 } building block involves three facial cyanido groups as bridges. The three Fe-CN-Co bridges deviate significantly from linearity (Co1 -N3 -C7 = 166.1(3), Co2 -N8 -C6= 150.0(3), Co1 iii -N4 -C2 = 158.6(3)° with iii 0.5-x, 0.5+y, 0.5-z). Each {Fe(CN) 6 } unit is connected to three cobalt ions, and each cobalt center is connected to two hexacyanido-iron(III) units, resulting into a distorted honeycomb topology (Figure2a). The intralayer Co -Fe distances are: Fe1•••Co1= 5.125, Fe1•••Co2 = 4.964, Fe1 ii -Co1 = 5.083 Å ( ii 1.5-x, -0.5+y, 0.5-z). The layers are parallel to the ab plane. Each mesh of the network contains six cobalt atoms (shared between two meshes) and six iron atoms (shared between three meshes). The deviation from the regular honeycomb topology is caused by the non-linearity of the Co-NC-Fe bridges. The stacking of the layers generates channels, which run along the crystallographic c axis and host the solvent molecules (Figure2b).

Figure 1 .

 1 Figure 1. Perspective view of the asymmetric unit of compound 1 (i = 1-x, y, 0.5-z).

Figure 2 .

 2 Figure 2. (a) View along the crystallographic c axis of the coordination polymer 1. (b) Packing diagram showing the formation of the channels containing the solvent molecules. The macrocyclic ligands coordinated to the cobalt ions (pink) have been removed for the sake of clarity.

Figure 3 .

 3 Figure 3. Perspective view of the asymmetric unit of compound 2.

Figure 4 .

 4 Figure 4. View along the crystallographic c axis of the coordination polymer 2 showing only the major component of the disordered macrocyclic complex for the sake of clarity.

Figure 5 .

 5 Figure 5. Temperature dependence of the χT product at 1000 Oe (with defined as the molar magnetic susceptibility equal to M/H) for compound 1; Inset: enlarged view of the main figure emphasizing the high temperature behaviour of the χT product.

Figure 6 .

 6 Figure 6. Field dependence of the magnetization for 1, recorded at different temperatures.

Figure 7 .

 7 Figure 7. Temperature dependence below 10 K of the in-phase (χ', top) and out-of-phase (χ", bottom) components of the ac susceptibility at different frequencies between 1 and 1000 Hz with a 3 Oe ac-field in zero dc-field for compound 2. Solid lines are guides for eyes.

Figure 8 . 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7

 8234567 Figure 8. Temperature dependence of the χT product at 1000 Oe (with defined as the molar magnetic susceptibility equal to M/H) for compound 2; Inset: M vs. H plots at different temperatures below 8 K. The solid lines are representing the best simulation of the experimental data as described in the text.

Figure 8

 8 Figure 8

3 {Fe(CN) 6 } 2 ]•6CH 3 OH•6H 2

  .5 cm diameter tube and layered with 1 mL CH 3 OH/H 2 O 1:1. A solution of [Fe(L 2 )(H 2 O) 2 ]Cl 2 (26 mg, 0.05 mmol) dissolved in CH 3 OH (2 mL) was layered very carefully.

	After 1 day, big dark blue crystals appeared at the interface, which were collected and dried with
	diethylether. Yield: 15 mg (55% based on Fe for C 59 H 95 Co 2 Fe 3 N 27 O 13 ). Anal. calcd for
	C 57 H 95 Co 2 Fe 3 N 27 O 13 (2-2MeOH, 1651.95 g/mol): C 41.44; H 5.80; N 22.89%. Found C 41.6; H
	5.0; N 23.1%. The crystals are quickly losing some solvent molecules as seen by the fast loss of
	crystallinity during the crystal mounting. Methanol molecules are expected to leave easily
	compared to water ones so the elemental analysis was calculated for
	[{Fe(L 2 )} 3 {Co(CN) 6 } 2 ]•13H 2 O. IR data (ν /cm -1 ): 3283 (w, NH), 2920 (w), 2862 (w), 2122 (s,
	conj. CN), 1639 (s), 1589 (m), 1461 (w), 1426 (m), 1378 (m), 1351 (m), 1289 (w), 1267 (m),
	1198 (m), 1180 (w), 1125 (w), 1109 (m), 1049 (m), 961 (w), 914 (w), 887 (w), 813 (s), 742 (m),
	728 (m), 651 (m), 624 (w), 617 (w), 608 (w).
	Yield 20
	mg (42%, based on Co for C 57 H 75 Co 3 Fe 2 N 21 O 12 ). IR data (cm -1 ): IR data (ν /cm -1 ): 3349 (w,
	NH), 2927 (w), 2881 (w), 2124, 2110 (m, conj. CN), 1647 (m), 1588 (m), 1423 (m), 1369 (m),
	1354 (m), 1270 (w), 1242 (m), 1203 (m), 1123 (w), 1097 (w), 1080 (m), 1045 (m), 957 (w), 896
	(w), 836(w), 816 (s), 743 (m), 728 (m), 652 (m), 583 (w), 617 (w), 572 (w).
	2.3.2. Synthesis of complex 2

O(1). K 3 [Fe(CN) 6 ] (23 mg, 0.07 mmol) in H 2 O (3mL) was introduced in a 1.5 cm diameter tube and layered with 10 mL CH 3 OH/CH 3 CN 1:1 . A methanolic solution (3mL) containing [Co(L 1 )(H 2 O) 2 ]Cl 2 (46 mg, 0.1 mmol) was then layered. After 2-3 days, green crystals were formed at the interface, collected and dried with diethylether. [{Fe(L 2 )} 3 {Co(CN) 6 } 2 ]•2CH 3 OH•13H 2 O(2). K 3 [Co(CN) 6 ] (16 mg, 0.05 mmol) in H 2 O (2 mL) was introduced in a 1

Acknowledgements.

We are grateful to the UEFISCDI (project PN-II-ID-JRP-RO-FR-2011-2-0034), University of Bordeaux, the CNRS, the Aquitaine Région, the IUF (Institut Universitaire de France) and the ANR (ANR-12-IS07-0004, CREMM project) for financial support.

The theoretical susceptibility calculated from this Hamiltonian fits quite well and simultaneously the T vs. T and M vs H/T data of 2 (solid lines in Figure 8) leading to D/k B = -5.5 K and g = 2.21 (5).

The magnetic properties of 1 and 2 are strongly affected by the nature of the hexacyanidometallate bridging unit, either paramagnetic (S Fe(III) = ½) for compound 1 or diamagnetic (S Co(III) = 0), resulting in a ferromagnetically ordered phase below 3 K for 1 or a paramagnetic state even at low temperatures for 2.

Conclusions

In summary, we described two new cyanido-bridged 2-D coordination polymers constructed from [M(CN) 6 ] 3-spacers and pentagonal bipyramidal complex cations acting as structuring nodes. The two compounds show honeycomb versus brick-wall network topologies observed for the first time for these types of building blocks. These two examples illustrate also the robustness of the precursors, obtained using the pentadentate macrocyclic ligands L 1 and L 12) N9-Fe2-N7 = 94.98(18) N9-Fe2-N8 i =86.36(18) N9-Fe2-N8 = 89.14(18) N9-Fe2-N7 i =86.89(19) Fe1-N12-C25 = 147.4(5) Fe1-N13-C25 = 153.8(5) Fe2-N9-C24 = 164.9(4)

GRAPHICAL ABSTRACT

New bidimensional cyanido-bridged heterometallic coordination polymers, [Co II 3 Fe III 2 ] and [Fe II 3 Co III 2 ], have been assembled following the building-block approach. The cobalt(II)-iron(III) derivative shows a ferromagnetic order below 3 K.

SYNOPSIS (Text)

New bidimensional cyanido-bridged heterometallic coordination polymers, [Co II 3 Fe III 2 ] and [Fe II 3 Co III 2 ], have been assembled following the building-block approach. The cobalt(II)-iron(III) derivative shows a ferromagnetic order below 3 K.


Alix Vancostenoble

Claude Duret -Thual

Cédric Bosch

David Delafosse

Stress Corrosion Cracking of ferrito-pearlitic steel in aqueous environment containing dissolved CO 2

Keywords: Stress corrosion cracking, spheroidized pearlitic steel, hydrogen embrittlement

A confined aqueous environment is defined by a very low water-volume to exposed steel-area ratio. In such media containing dissolved CO 2, siderite is formed and acts as a protective film. An addition of applied stress and/or environmental fluctuation can disturb the balance between the steel and this protective film, causing the fracture of the latter and leading to Stress Corrosion Cracking (SCC). The material studied is a cold drawn and rolled high strength steel composed of ferrite and spheroidized pearlite and has a strong microstructural anisotropy due to the specific cold work process. To investigate its susceptibility to SCC, Slow Strain Rate Tests (SSRT) were carried out on smooth and notched specimens allowing to separate crack initiation and crack propagation. The environment is an aqueous chloride solution saturated in CO 2 at pH around 6. Tests were performed under open circuit potential (OCP) and at cathodic potential. Under OCP, localized anodic dissolution in shear bands is responsible for crack initiation. Under cathodic potential, crack initiation is delayed due to the absence of critical defect on the surface. Both fractographic analyses and results obtained on notched specimens showed that hydrogen plays an important role in crack propagation.

INTRODUCTION

Cold-drawn and cold-rolled ferrito-pearlitic steels are widely used for their high yield strength while a maintaining sufficient ductility for use as structural materials. However, like any carbon steel, they can suffer from uniform corrosion in aqueous solution. In de-aerated solution containing dissolved CO 2 , the formation and precipitation of siderite (FeCO 3 ) as corrosion product depends on the temperature and the supersaturation level of Fe 2+ . [START_REF] Dugstad | The importance of FeCO3 supersaturation on the CO2 corrosion of carbon steels[END_REF] These conditions can be met in a confined environment where the water-volume to steel-area ratio is very low and the content of ferrous ions is very high. As a consequence, the precipitated siderite at the surface acts as a protective film and reduces the corrosion rate. [START_REF] Van Hunnik | The formation of protective FeCO3 corrosion product layers in CO2 corrosion[END_REF][START_REF] Crolet | Role of conductive corrosion products on the protectiveness of corrosion layers[END_REF] Applying stresses and/or environmental fluctuations destabilize the protective layer, causing its fracture and leading to Stress Corrosion Cracking (SCC) damage. Moreover, the cold working process of pearlitic steel produces a strongly anisotropic microstructure that could influence the SCC susceptibility and the mode of fracture. In many SCC phenomena, both anodic dissolution and hydrogen effects are involved and interact with the localized crack-tip plasticity. Experimentally, the respective roles of these processes are not easy to separate and require a methodology based on the mechanisms of corrosion-deformation interactions. An approach involves the use of micronotched specimens subjected to dynamic deformation like Slow Strain Rate Tests (SSRT) to trigger the mechanisms at the notch tip and monitor them during the test. The control of the electrochemical and mechanical conditions allows exploring the key factors influencing damage. [START_REF] Delafosse | The roles of crack-tip plasticity, anodic dissolution and hydrogen in SCC of mild and C-Mn steels[END_REF] , [START_REF] Bosch | Effects of strain and trapping on hydrogen-induced cracking in high strength low alloy steels[END_REF] This methodology is applied to a high strength steel in a chloride solution saturated in CO 2 . The mechanisms of SCC are investigated using slow strain rate tests on smooth and micro-notched specimens under different electrochemical conditions. The specific roles of anodic dissolution and hydrogen effects are particularly considered during the crack initiation and crack propagation.

EXPERIMENTAL PROCEDURE

The material studied was a cold-rolled and cold-drawn ferrito-pearlitic steel containing 0.3% of carbon. The steel was annealed after cold-working. As a consequence, the microstructure was constituted of spheroidized pearlite and recrystallized ferrite. The largest carbides are located along the boundaries between adjacent pearlite colonies and between these colonies and ferrite grains. The anisotropy of the microstructure produced by the cold work process is preserved despite the thermal treatment. Thus, it presents pearlite colonies elongated along the rolling direction (RD) (figure 1a) while in the transverse direction (TD), the microstructure is equiaxed (figure 1b). SCC behavior was studied using Slow Strain Rate Tests (SSRT) on smooth and micro-notched specimens. Notched specimens were used in order to separate the specific roles of the anodic dissolution and hydrogen effect in the crack initiation and propagation mechanisms. The µ-notch provides a localized plastic zone that favors the corrosion-deformation interactions. [START_REF] Delafosse | The roles of crack-tip plasticity, anodic dissolution and hydrogen in SCC of mild and C-Mn steels[END_REF] , 5 Specimens were sampled along the rolling direction of the steel. The gage length and the cross-section were respectively 12 mm and 2.5x4 mm². Surfaces of specimen were polished with emery paper to a 1500 grit finish. The micro-notch was machined at mid-height of the gage length with a diamond wire saw to avoid any local strain hardening (figure 2). Smooth and notched specimens were tested respectively at a strain rate of 10 - 7 s -1 and 10 -6 s -1 .

Figure 2: micro-notched specimen

The SCC tests were conducted in a corrosion cell with three electrodes, using a counter electrode in platinum and a saturated calomel electrode (SCE) connected to the cell with an external salt-bridge for the reference potential. Two electrochemical conditions were explored, at free potential in open circuit (OCP) and at cathodic potential (E c = -1200 mV/SCE). Table 1 summarizes the composition of the solutions used and the potentials applied or measured. At OCP, the solution was confined by adding carbon steel coupons to saturate the solution with metallic cations. The ratio of water-volume to steel-area exposed to the solution, which measures the confinement of the solution, is equal to 0.3 mL/cm 2 . In these conditions, the pH of the seawater solution saturated with CO 2 was 6.

Table 1 Environmental conditions applied during SSRT

At OCP (E 0 = -670 mV/SCE) Under cathodic polarization (E c = -1200 mV/SCE) Synthetic seawater saturated in CO 2 confined solution (V/S=0,3 mL/cm 2 ) oxygen concentration < 5 ppb Synthetic seawater saturated in CO 2 oxygen concentration < 20 ppb

An additional SSRT with a potential jump was performed. The test was started at OCP then the potential was shifted to E c = -1200 mV/SCE at 5% elongation. Fractographic analyses were performed on all the specimens after failure using a scanning electron microscope with field emission gun (SEM-FEG). Microstructures were observed on cross-sections etched with a 5% nital solution. The effect of environmental conditions was evaluated through the ratio of elongation-to-failure obtained in the test environment to that in air.

RESULTS

SSRTs on smooth specimens

Figure 3 shows stress vs elongation curves obtained on smooth specimens at free and cathodic potentials, compared with the reference test in air. Elongation-to-failure obtained for the two tests in aqueous environment is considerably reduced compared to the test in air. However, one notes a quick stress decrease after the Ultimate Tensile Stress (UTS) for at OCP, while at Ec some amount of stable necking is observed before the final fracture. In both cases, high plastic strains have to be applied in aqueous environment to trigger environmentally-assisted fracture.

Figure 3: Stress-elongation curves on smooth specimens

Fractographic observations on the reference specimen tested in air reveal a fracture surface constituted of ductile tearing and longitudinal delaminations (figure 4a). These delaminations results from brittle crack propagations in the rolling direction (which is also the tensile direction). The longitudinal delaminations in the center are surrounded by a region of ductile tearing characterized by the presence of dimples (figure 4b). Delaminations occur both along boundaries between adjacent pearlite colonies where the largest carbides are located, and along ferrite/pearlite colony interfaces (figure 5). Under cathodic polarization (CP), no main delamination was observed at mild-thickness in the specimen (figure 6). Conversely, numerous small longitudinal delaminations were observed, which initiated from the lateral edges of the sample and propagated towards the center. The initiation sites seem to be shear bands which emerge at the surface. The others parts of the fracture surface show ductile shearings and are very similar to those observed in air. At OCP, these small longitudinal delaminations located at the lateral edges of the smooth specimen are also observed (figure 7a). The difference with previous results is that at free potential, initiation sites in shear bands are also sites of preferential dissolution (figure7b). This can promote earlier crack initiation compared to tests at cathodic potential. Figure 8 presents a cross section of the specimen close to the fracture surface showing dissolved shear bands which act as critical defects for crack initiation. The very thin crack tip to less than 20 µm from the surface suggests that two different mechanisms operate for crack initiation and crack propagation. The former obviously involves anodic dissolution, while hydrogen may play a role in the latter.

Figure 9 : Stress-elongation curves on micro-notched specimens

Fractographic investigations on the micro-notched specimen tested in air reveals a ductile fracture mode by shearing and some small longitudinal delaminations. Under cathodic polarization, the fracture surface shows different features: a first brittle zone fractured by a pseudo-cleavage mode perpendicular to the loading direction (figure 10b); a second zone, just below the previous one, results from numerous short longitudinal delaminations and small ductile tearing (figure 10c); and a third region of shear ductile fracture, which caused the final fracture.

Observation on cross-sections close to the fracture surface has revealed several micro-voids which nucleated at cementite/ferrite interfaces. The comparison between specimens tested in air and under CP, shows that the number and the size of these micro-voids are higher under CP than in air as illustrated in figure 11. At OCP, the fracture surface is marked by the anodic dissolution (figure 12a). Close to the notch tip, it is characterized by numerous longitudinal delaminations and small ductile tearings (figure 12b). A region of final fracture by ductile shearing is still present. The SSRT which started at OCP before shifting the potential to a cathodic value at 5% elongation gave the lowest elongation to rupture. The failure occurred very quickly after the change in potential.

The fracture surface is shown on figure 13. The region nearest to the notch tip is similar to that observed at OCP (figure 13a). It is characterized by numerous longitudinal delaminations and small ductile tearing with traces of anodic dissolution. A second zone following the previous shows a brittle region with a cleavage-like appearance, similar to that observed under CP (figure 13b). Finally, the third one is the final fracture surface showing the ductile shearing.

DISCUSSION

Tensile curves on smooth specimens have shown that environment affects the ductility only at high plastic strains, typically after the UTS when the mechanical damage process begins to operate. This process involves the formation and coalescence of microvoids at ferrite/cementite interfaces. The preferential sites of void nucleation are the largest carbides of cementite located on the boundaries of adjacent pearlite colonies. Ultimately, growth and coalescence of microvoids lead to the longitudinal delaminations because of both the anisotropic microstructure (the larger carbides are aligned with the rolling direction which is also the tensile direction) and the triaxiality of stresses which increases with the beginning of the necking. This process is illustrated in figure 11a where the hydrostatic stress is increased at the notch tip.

Under CP, hydrogen is introduced into the material by electrolytic charging and is trapped at the metallurgical sites (precipitate/ferrite interfaces, grain boundaries, dislocations …) weakening the steel by different possible mechanisms. The microscopic events leading to failure seem to be unchanged compared to the reference test in air on smooth specimens. These features are consistent with those described in literature. [START_REF] Cialone | The role of hydrogen in the ductile fracture of plain carbon steels[END_REF][START_REF] Enos | A Critical-Strain Criterion for Hydrogen Embrittlement of Cold-Drawn. Ultrafine Pearlitic Steel[END_REF][START_REF] Oriani | Hydrogen-enhanced nucleation of microcavities in AISI 1045 steel[END_REF] Indeed, several authors have studied the effect of hydrogen on spheroidized pearlitic steels but the mechanism of hydrogen embrittlement is still not clear. Cialone explained that more microvoids could be formed for a given macroscopic elongation, leading to failure at lower macroscopic elongation with the increase of hydrogen concentration in the neck. [START_REF] Cialone | The role of hydrogen in the ductile fracture of plain carbon steels[END_REF] Garber showed that hydrogen does not affect the nucleation or the growth of microvoids but accelerates the coalescence process. [START_REF] Garber | Hydrogen assisted ductile fracture of spheroidized carbon steels[END_REF] It is well established that hydrogen diffuses towards the locations of maximum triaxial-stress in the material, due to the lowering of the chemical potential. The high local concentration of hydrogen present within the necking zone, where the triaxial-stress state is maximized, could assist the growth and coalescence of microvoids. This explains both the increasing number of longitudinal small delaminations observed on the fracture surface and the microvoids size observed on metallographic sections.

The results obtained on micro-notched specimens under CP have shown brittle facets corresponding to cleavage-like cracking. This suggests that localized plasticity is necessary to observe a significant hydrogen effect. A critical hydrogen concentration is achieved in the location of the maximum triaxial stress state causing early crack propagation. In this case, microvoids could act as nuclei for cleavage facets.

At OCP, fractographic analyses suggest that anodic dissolution plays a key role in the crack initiation. The emergence of shear bands at the surface produces preferential sites of dissolution caused by a slip-dissolution type of process. However, this process does not seem to be involved in crack propagation. The slip dissolution model postulates that dynamic plastic strain at the crack tip can break a protective film, allowing a burst of metal dissolution prior to repassivation; repetition of this sequence extends the crack. [START_REF] Ford | Corrosion sous contrainte. Phénoménologie et mécanismes[END_REF] Nevertheless at OCP, no trace of dissolution has been observed at the crack tip as illustrated in figure 8. Therefore, another mechanism must be responsible for the crack propagation. Considering the cathodic discharge of hydrogen, accompanied by the anodic dissolution in this de-aerated environment, the ingress of hydrogen into the steel and its effects might be involved in the mechanism of embrittlement. On micro-notched specimens, the localized anodic dissolution process is sufficient to initiate a crack. Indeed, the fractographic surface is highly marked by anodic dissolution close to the notch tip driving by the high local strain. Just after this zone, the fracture surface is characterized by numerous longitudinal small delaminations, and this cannot be explained only by a dissolution process. The characteristic features of this region are similar to those observed under CP, which suggests that hydrogen plays a role in the crack propagation even at OCP. Hydrogen produced by electrochemical reaction at the crack tip surface, is absorbed in the material and diffused ahead of the crack tip where there is a high concentration of stress and local plastic strain. A critical local concentration of hydrogen could be reached and cause early crack initiation at the critical defect produced by dissolution.

The SSRT started at OCP and then switched to cathodic potential supports these results. At OCP, the crack is initiated by anodic dissolution, dissolution rate being accelerated and localized at the notch tip. The crack has a sharper tip than the artificial micro-notch machined on the specimen. The sharpness has an influence on the local plasticity at the crack tip, which consequently leads to a higher hydrogen concentration. For the same hydrogen concentration more stress is necessary to trigger crack propagation at a blunt crack tip. [START_REF] Troiano | The role of hydrogen and other interstitials in the mechanical behavior of metals[END_REF] After the stage at OCP, a high plastic strain region is localized at the crack tip. In this local plastic area the hydrogen concentration increased when the potential was shifted to a negative value and caused the cleavage-like cracking at lower strain and stress than that obtained with the notched specimen subjected to SSRT under CP.

CONCLUSIONS

The mechanisms of stress corrosion cracking of a cold drawn and rolled high strength steel in a de-aerated chloride solution saturated in CO 2 have been explored using a methodology based on the corrosiondeformation interactions. [START_REF] Delafosse | The roles of crack-tip plasticity, anodic dissolution and hydrogen in SCC of mild and C-Mn steels[END_REF] At free potential, crack initiation occurs on sites of preferential dissolution, which are all sites of strain localization, and in particular, shear bands emerging at the surface. The localized dissolution on these sites creates critical surface defects from which cracks are initiated. Hydrogen does not take part in the crack initiation process at OCP. A brittle cleavage-like cracking occurs when a critical content of hydrogen and a specific stress are reached ahead of the crack tip. At cathodic potential, crack initiation from the micro-notch is delayed compared to that produced by anodic dissolution.

The sensitivity of the cold drawn and rolled high strength steel in de-aerated chloride solution to SCC in CO 2 is thus dependent on the presence and the geometry of surface defects. The preferential dissolution in shear bands promotes the formation of severe surface defects which increase the susceptibility to SCC.

Figure 1 :

 1 Figure 1: Microstructure of the steel: (a) rolling direction, (b) transverse direction.

Preprint

  Author Copy -Vancostenoble, A., C. Duret-Thual, C. Bosch, and D. Delafosse. 2014. "Stress Corrosion Cracking of Cold Drawn Ferrito-Pearlitic Steels in Confined Aqueous Solutions Containing Dissolved CO2." In NACE Corrosion Conference 2014, 51314-4321-SG. Houston, Tx: NACE International. http://www.nace.org/cstm/Store/Product.aspx?id=762f2a5c-a5ba-e311-a396-0050569a007e.

Figure 4 :Figure 5 :

 45 Figure 4: (a) Fracture surface of reference specimen tested in air, and (b) magnification of tearing and dimples in a ductile region,

Figure 6 :

 6 Figure 6: (a) Fracture surface of specimen tested under CP, and (b) magnification on the numerous longitudinal small delaminations

Figure 7 :

 7 Figure 7: (a) Fracture surface of specimen tested at OCP, and (b) preferential dissolution of shear bands on the lateral surface.

Figure 8 :

 8 Figure 8: Cross-section of specimen tested at OCP. Notice that the crack tip very thin and the dissolved shear band acts as a critical defect for crack initiation.

Preprint

  Author Copy -Vancostenoble, A., C. Duret-Thual, C. Bosch, and D. Delafosse. 2014. "Stress Corrosion Cracking of Cold Drawn Ferrito-Pearlitic Steels in Confined Aqueous Solutions Containing Dissolved CO2." In NACE Corrosion Conference 2014, 51314-4321-SG. Houston, Tx: NACE International. http://www.nace.org/cstm/Store/Product.aspx?id=762f2a5c-a5ba-e311-a396-0050569a007e.

Figure 10 :

 10 Figure 10: (a) Fracture surface of specimen tested under CP, (b) magnification of the first brittle zone ahead of the notch tip, (c) second zone with numerous small delaminations and ductile tearing.

Figure 11 :

 11 Figure 11: Cross-section of specimens observed in the rolling direction and close to the fracture surface, (a) tested in air, and (b) under CP.

Figure 12 :

 12 Figure 12: (a) Magnification of the fracture surface of specimen tested at OCP close to the notch tip, and (b) Fracture zone resulting from numerous longitudinal small delaminations and small ductile tearing.

Figure 13 :

 13 Figure 13: Fracture surface of the specimen tested with a jump of potential from OCP to CP. (a) Magnification close to the notch tip, and (b) further from the first, corresponding to mode of fracture under CP.

Preprint Author Copy -Vancostenoble

  , A., C. Duret-Thual, C. Bosch, and D. Delafosse. 2014. "Stress Corrosion Cracking of Cold Drawn Ferrito-Pearlitic Steels in Confined Aqueous Solutions Containing Dissolved CO2." In NACE Corrosion Conference 2014, 51314-4321-SG. Houston, Tx: NACE International. http://www.nace.org/cstm/Store/Product.aspx?id=762f2a5c-a5ba-e311-a396-0050569a007e.

  Preprint Author Copy -Vancostenoble, A., C. Duret-Thual, C. Bosch, and D. Delafosse. 2014. "Stress Corrosion Cracking of Cold Drawn Ferrito-Pearlitic Steels in Confined Aqueous Solutions Containing Dissolved CO2." In NACE Corrosion Conference 2014, 51314-4321-SG. Houston, Tx: NACE International. http://www.nace.org/cstm/Store/Product.aspx?id=762f2a5c-a5ba-e311-a396-0050569a007e.

  Preprint Author Copy -Vancostenoble, A., C. Duret-Thual, C. Bosch, and D. Delafosse. 2014. "Stress Corrosion Cracking of Cold Drawn Ferrito-Pearlitic Steels in Confined Aqueous Solutions Containing Dissolved CO2." In NACE Corrosion Conference 2014, 51314-4321-SG. Houston, Tx: NACE International. http://www.nace.org/cstm/Store/Product.aspx?id=762f2a5c-a5ba-e311-a396-0050569a007e.