Electrochemical study of lepidocrocite reduction and redox cycling for the mechanistic modelling of atmospheric corrosion

H. Antonya, L. Maréchalb, L. Legranda*, A. Chausséa, S. Perrinb, P. Dillmannc

a Laboratoire Analyse et Environnement, UMR 8587 CNRS-Université d’Evry-CEA, Université d’Evry Val d’Essonne, rue du Père Jarland, 91025, EVRY, France Tél : 33(0)169477705 – Fax : 33(0)169477655 – [email protected] b Laboratoire d'Etude de la Corrosion Aqueuse DEN/DPC/SCCME, CEA Saclay 91191 Gif Sur Yvette, France c LRC CEA DSM01.27 : Laboratoire Pierre Süe, CEA/CNRS, DSM/DRECAM/SCM, CEA Saclay 91191 Gif Sur Yvette, France et Institut de Recherche sur les Archéomatériaux, UMR5060 CNRS

Abstract- During the long term storage of low alloy steel containers, the container walls may be exposed to cyclic wet and dry periods and will suffer from indoor atmospheric corrosion at room temperature. In order to predict the damage due to this corrosion for very long period (more than 100 years), a mechanistic modelling has been proposed by considering the phenomena occurring during the three stages of a wet dry cycle: the wetting stage, the wet period and the drying stage. For this modelling, the reduction by iron of one of the phase composing the rust layer, lepidocrocite (g-FeOOH), plays a significant role on the corrosion processes. It was therefore of importance to gain a confirmation of the galvanic coupling between lepidocrocite and iron, as well as a better description of the electrochemical reduction of this ferric oxihydroxide and its possible redox cycling. The electrochemical reduction and redox cycling of lepidocrocite were investigated at ambient temperature in neutral or mildly alkaline solutions containing chloride, sulphate or bicarbonate anions. The working electrodes were either a thin lepidocrocite film electrodeposited on inert gold substrate or a composite electrode made by compacting a graphite / lepidocrocite powder mixture into a platinum grid. Electrochemical measurements were coupled to in-situ electrochemical quartz crystal microbalance (EQCM) and micro- Raman spectroscopy. Ex-situ SEM and FTIR were also used. The reduction of lepidocrocite occurs in any of the electrolytes considered here. Dissolution phenomena during the reduction of thin films are revealed by EQCM measurements. A fraction of the reduced product remains on the film, as adsorbed species or as a precipitate, which may be re-oxidised during a further anodic transient. The efficiency of the reduction process (FeIII to FeII) is lower for composite electrode than for thin film, mainly due to the easier saturation by FeII of the aqueous solution in the pores in the former case. The comparison of g-FeOOH reduction potentials and iron corrosion potentials let us state that the galvanic coupling is possible. The redox cycling of g-FeOOH was also demonstrated, but the stabilised coulombic charges were rather low, less than 10%. Finally, some electrochemical studies were performed with powders extracted from corrosion layers of ancient steel materials which have been exposed to indoor atmospheric corrosion.

Keywords : lepidocrocite, electrochemical reduction, indoor atmospheric corrosion, redox cycling.

1 1. Introduction

Nuclear waste could be packaged in low alloy steel containers for interim storage which could last more than 100 years. In the storage conditions, the condensation on the containers walls can conduct to atmospheric corrosion caused by a succession of wet and dry periods. Atmospheric corrosion is an electrochemical process needing aqueous conditions for its occurrence. An aqueous layer, which acts as an electrolyte, is formed in indoor conditions by water condensation. The time of wetness, which defines the duration of the electrochemical processes, is strongly dependent on many parameters which include the relative humidity of the atmosphere at a given temperature. The relative humidity and temperature variations lead to cyclic wet and dry periods, called wet-dry cycles. As far as iron or low alloy steels are concerned, atmospheric corrosion can be summarized by the stoichiometric equilibrium: 4Fe +3O2+2H2O = 4FeOOH (1) The atmospheric corrosion behaviour of iron-based materials has generally been well predicted by the way of the well-known bilogarithmic laws, for periods of a few decades [1]. For longer time periods - one to a few centuries - which could be the case for some nuclear waste containers in very long-term storage, predictions are rare and probably more uncertain. The main reason of this lack of reliability is the complex mechanisms involving in the rust layer during atmospheric corrosion cycles and the subsequent modifications of the protective properties of the rust scale. In order to obtain a relevant and robust prediction of the damage due to atmospheric corrosion, a mechanistic modelling has been established in the frame of the COCON research program [2]. This modelling describes the processes occurring in the rust layer during a wet dry cycle (see next part). A significant assumption of the model concerns the rust layer reactivity. It is supposed that one of the constituent of the rust layer, lepidocrocite can be reduced during the cycle. As a consequence, the experimental study of the lepidocrocite electrochemical reduction is a keypoint in order to confirm the model. In this paper, electrochemical studies on the lepidocrocite behaviour will be performed on one hand on synthesized powders and, on the other hand, on the rust layer of a 200 years old archaeological artefact.

2. Mechanistic modelling

2.1. Rust layer characterization In order to better understand the phenomena occurring in the rust layer during atmospheric corrosion, it is useful to have a good description of the rust layers properties. For this reason, characterizations of rust layers, from archaeological artefacts gathered in ancient monuments and from synthetic samples rusted in climatic chamber, were performed. A corpus of eleven archaeological iron artefacts exposed several centuries to indoor atmospheric corrosion has been collected. The morphology, composition and structures of rust layers have been studied by Mercury porosimetry, BET measurements, SEM, OM, EPMA and µXRD [3]. These results bring very important information to understand the rust layer morphology. The most relevant parameters for atmospheric corrosion modelling are show on Figure 1. In a first step, two phases are taken into account for the rust layer model, the lepidocrocite (g-FeOOH) and the goethite (a-FeOOH). The ratio between the two phases (a/g) is called protective ability index because the goethite is supposed to be a protective constituent whereas the lepidocrocite is supposed to participate to the corrosion processes [4].

2 a b

g-FeOOH a-FeOOH Fe3O4

d Electrolyte

Rust layer L L L Iron Metal Interfaces a-FeOOH Porosity e g-FeOOH Porosity Tortuosity t Þ a /g Protective Specific area Sp Ability Index

Figure 1: The rust layer. (a) SEM micrograph of a 150 years old rust layer [3]. (b) Schematic representation of the rust layer used for modelling. L and d are the rust layer and electrolyte thickness.

2.2. Presentation of the mechanistic modelling The mechanistic modelling of a wet dry cycle is based on the works of Evans [5] and Stratmann [6, 7]. As show on the Figure 2, which represents the rate of oxygen and iron consumption during a typical wet dry cycle, the cycle can be divided in three stages.

Wetting Wet Stage Drying

a ----- Iron consumption Dissolved O2 consumption

O2 + FeII ® FeIII FeIII + Fe ® FeII

CR O2 + Fe ® FeII

d0 b d

0 Time Figure 2: The Wet-Dry cycle. At the back the schematic variations of electrolyte thickness (d) with time. At the top the corresponding variations of Iron (dotted line) and Oxygen (solid line) consumption rates (CR) (from [6]).

During the first stage (wetting), the anodic dissolution of iron is mainly balanced by the reduction of lepidocrocite. This reduction is considered to be the rate-limiting step of the corrosion. This corrosion rate depends among many parameters on the morphology and the

3 composition of the rust layer. During the second stage of the cycle, the wet period, the reduction of dissolved oxygen on the lepidocrocite, previously reduced, is controlling the mechanism. The amount of oxidized metal depends on the quantity of reduced lepidocrocite and also on the oxygen diffusion in the electrolyte and in the rust layer. At the end of the cycle, the blocking of the anodic sites is considered to describe the extinction of electrochemical corrosion during the drying. The reoxidation of the rust layer, conducting to the beginning of a new cycle, is not examined for the moment in the modelling. The mechanisms of the three stages of a cycle are presented on the Figure 3.

Wetting

O (gas) Por Rust Wet stage 2 0

+ 1 monolayer Electrolyte d ~100 mm H O2

g-Fe.OH.OH g-FeOOH d 2+ Fe 2e- some monolayers Fe k - Fe Rust layer H2O+O2 +4e¾¾®4OH L Oxidation :Fe ® Fe2+ + 2e- d+L Reduction :g-FeOOH + H+ + e- ® g -Fe.OH.OH Oxidation : Fe ® Fe2+ + 2e- Reduction : O + 4e- + 2H O ® 4OH- Oxidation: Fe ® Fe 2+ + 2e- 2 2 - - Reduction: O2 + 4e + 2H2O ® 4OH 2+ - Precipitation: Fe +2 OH = Fe(OH)2,s

Fe(OH)2 O 2 OH- Ø-FeOOH Drying Fe2+ e- Fe(OH)2 Ø-Fe.OH.OH

Figure 3 : Representation of the mechanisms occurring during a wet dry cycle.

This short description of the modelling show the relevant role of the lepidocrocite on the corrosion processes. As a consequence, a group of experiments has been achieved in order to study the lepidocrocite reduction and the galvanic coupling between lepidocrocite and iron.

3. Experimental and samples

3.1. Lepidocrocite synthesis 3.1.1. Chemicals Na2SO4, 10H2O (ACS Reagent, Aldrich), NaHCO3 (ACS Reagent, Aldrich), NaCl (RP Normapur Prolabo), FeCl2, 4H2O (99%, Aldrich), HCl (1M, Titrinorm, Prolabo), NaOH (30% solution, RP Normapur, Prolabo), 1-methyl-imidazole (99%, Aldrich, pH-buffering agent with pKa ~ 7.2) and 1,4-piperazinediethanesulfonic acid, disodium salt – PIPES - (Aldrich, pH-buffering agent with pKa ~ 6.8) and graphite (Fluka) were used without further purification. The solutions were prepared with 18 MW cm nano-pure water and deaerated with Argon (U, Air Liquide).

3.1.2. Synthesis of lepidocrocite powder. Lepidocrocite (g-FeOOH) powder was synthesised at 25°C following a procedure drawn from Schwertmann and Cornell [8]. 100 mL of 18 MW cm nano-pure water were introduced into a 200 mL glass beaker equipped with a stirrer, a combined pH electrode and a burette

4 containing 1 M NaOH solution. Then, 4 g of FeCl2, 4H2O were added and the mixture was left in contact with air under stirring (500 rpm). NaOH (about 30 mL) was continuously added during the synthesis in order to maintain the pH within the 6.7-6.8 range. After about 3 hours, the completion of the oxidation reaction was obtained, as revealed by the orange colour of the suspension. Filtration was done and the solid was dried at ambient temperature.

3.1.3. Electrochemical synthesis of lepidocrocite thin film Lepidocrocite thin films were synthesised at the surface of gold substrate by potentiostatic oxidation (E = 0V) of soluble Fe(II) in 0.4 M NaCl / 0.02 M Met-Imidazole / 0.01 M FeCl2 solution at pH 7.5 and T ~22°C under argon bubbling. The main features of electrolysis and electrodeposits are only summarised here; complementary information can be found in our previous paper [9]. The current density was in the 0.05 to 0.15 mA cm-2 range; it decreased with oxidation time duration, consistently with the progressive surface coverage by ferric oxyhydroxide particles. The coulombic charge values were between several tenths to several hundreds mC cm-2. A mass gain was observed along the oxidation experiment. A Dm-Q linear relationship was obtained and the formation of partially hydrated lepidocrocite, FeOOH, n H2O with n ~1, was assumed from the value of Dm-Q plot slope. FTIR, XRD and SEM measurements showed the formation of pure lepidocrocite thin films with a tendency to (0 0 1) preferentially-oriented growth. The mass of lepidocrocite deposit and coulombic charge involved during the synthesis will thereafter be expressed as mo and Qo, respectively.

3.1.4. Electrodes Polished gold disc (Goodfellow, 99.95%, laminated, 0.785 cm2) or 5 MHz Au/Ti polished quartz crystal (Maxtek Inc., 1.37 cm2) were used as a working electrode for the electrochemical studies of lepidocrocite thin films. The working electrode used for the electrochemical studies of lepidocrocite powder consisted of a Pt grid with a geometric area of 1 cm2 on which the cathodic material, a mixture of lepidocrocite (20% wt) and graphite, was pressed (mass: 15-20 mg, pressure: 5 tons/cm2). Au wire and home-made Ag/AgCl (0.1M NaCl) electrode were employed as the counter and reference electrodes, respectively. All potentials are referred to SHE.

3.2. Apparatus Electrochemical measurements were conducted with an EG&G PAR 273A potentiostat/galvanostat. Electrochemical quartz crystal microbalance (EQCM) measurements were performed with a Maxtek Inc. RQCM plating monitor coupled to an AUTOLAB PGSTAT30 potentiostat/galvanostat (Eco Chemie). IR spectra of thin films were recorded on a Bruker IFS28 FTIR spectrometer, using infrared reflection-absorption spectroscopy (IRRAS). A reflection-absorption tool (Grazeby Specac) with variable incidence angle allowed us to analyse the film directly on the disc surface without scraping procedure. The FTIR spectrum of lepidocrocite powder was taken using transmission mode and KBr pellet. XRD patterns were obtained using a Bruker D8 diffractometer with a Cu anticathode in q – 2q geometry.

3.3. Archaeological sample The archaeological specimen used for this study is a 200 years old rod coming from the framework of one of the towers of the Marly viaduct. More details on this sample can be found in [3]. Quantitative powder XRD measurements were performed on rust powder scraped from the artefact (Philips diffractometer using a Co anticathode) [10]. The rust is constituted by 4 different phases: maghemite (?-Fe2O3) and or magnetite (Fe3O4), goethite (a- FeOOH) and lepidocrocite (?-FeOOH).

5 4. Results and discussion

4.1. Electrochemical reduction of lepidocrocite The reduction behaviour of lepidocrocite thin film was studied in 0.1 M NaCl / 0.05 M PIPES -2 solution at pH 7.5 and T = 25°C by applying a cathodic current density Ic = 12.7 µA cm , to the electrode; the recorded E-t and Dm-t transients are converted to E-Q and Dm-Q transients (Figure 4 (a) and (b)). The Dm-Q transients obtained for the deposition of lepidocrocite (c) thin film is also reported in Figure 4. The potentiometric curve displays a potential decrease followed by a plateau around –0.2 V; both processes are related to the reduction of g-FeOOH thin film since they are not observed on the blank curve of Au electrode. The end of this reduction process is revealed by the potential drop. At the beginning of reduction, Dm remains close to 0. Then, Dm gradually decreases and a quasi-linear variation is reached from which a negative slope value of 69 ± 2 g/F can be determined. Finally, Dm becomes constant again as the potential drop occurs. The slope value is lower than those expected from the one-electron II reduction reaction of solid g-FeOOH or solid hydrated g-FeOOH, n H2O (d) into soluble Fe species, 89 g/F or 115 g/F, respectively. FTIR and XRD analysis of thin film after electrochemical reduction only reveal the presence of lepidocrocite. From figure 4, the coulombic charge involved during the reduction (Qr), the masses of reduced and dissolved lepidocrocite (mr,d), of reduced and not dissolved lepidocrocite (mr,nd) and of not reduced and not dissolved lepidocrocite (mnr,nd) can be computed. A part of lepidocrocite thin film, 49%, is mr,d Q not affected by the reduction process. The ratio, 25%, is lower than r ratio, 51%, mo Qo indicating that a part of reduced lepidocrocite remains at the electrode surface, as a solid phase or as adsorbed FeII species. Further in-situ measurements will aim at providing information about this reduced compound.

The galvanostatic reduction of lepidocrocite/graphite composite electrode was also investigated. The applied current was –100 µA. The potential value at the plateau, - 0.45 V, and the value of the coulombic charge involved in the reduction process, 0.18 F with respect to one mole Fe, are lower than those recorded for thin film. This may be due to the higher discharge regime or to the low pore volume of electrolyte within the composite electrode (saturation by FeII and precipitation of ferrous solid phase).

Lepidocrocite can be electrochemically reduced into soluble FeII species and/or solid FeII phase remaining at the electrode. The discharge depth should strongly depends on the structural and morphological features of lepidocrocite samples (thin layer, porosity, mixing with other compounds, conductivity, …). The comparison of lepidocrocite reduction potential value and iron corrosion potential value in a given electrolyte indicates that the galvanic coupling of iron/ g-FeOOH is possible. The reaction considered during the first stage of wet/dry cycle, FeIII + Fe ® FeII, is therefore validated in the case where FeIII is lepidocrocite. It is to be noted that the reduction of goethite/graphite composite electrode studied under the same conditions as above, occurs at potential value negative to that of lepidocrocite and involves lower coulombic charge (-0.55 V and 0.12 F with respect to one mole Fe).

6 g-FeOOH g-FeOOH 0.1 deposition réduction 300 0 (b) m r,d

-2 -0.1 (c) E / V 200 m r,nd m SHE o -0.2 (a) m / µg cm D -0.3 100 Q (d) m o Q nr,nd r -0.4

0 -0.5 0 100 200 300 400 500 Q / mC cm-2

Figure 4 : (a) E-Q and (b) Dm-Q transients recorded for the reduction of g-FeOOH thin film -2 in 0.1 M NaCl / 0.05 M PIPES solution at pH 7.5 and T = 25°C, Ic = 12.7 µA cm . (c) Dm-Q transient recorded for the deposition of g-FeOOH thin film (see text for conditions). (d) Theoretical reduction Dm-Q transient calculated from (c).

4.2. Redox cycling of lepidocrocite Galvanostatic redox cycles were performed with lepidocrocite/graphite composite electrode at ±100 µA within the – 0.53 V and + 0.73 V potential window (Figure 5). The highest Qred value is recorded for the first cycle. Beyond the first cycle, stabilised Qred and Qox are obtained, around 0.06 and 0.04 F/mole Fe (6% and 4%), respectively. The fact that Qred and II Qox are not equal may be explained by the loss of reduced product as soluble Fe species (mr,d) occurring during each reduction cycle. The part of material that can be re-oxidised may therefore correspond to reduced and not dissolved lepidocrocite (mr,nd). The galvanostatic cycling (± 10 µA within the – 0.53 V and + 0.73 V potential window) of lepidocrocite thin film was also studied. A rapid deterioration of the cycling efficiency is observed; Qred and Qox recorded at the 4th cycle are 10% and 7%, respectively. To state, the redox cycling of lepidocrocite in a corrosion layer may reasonably be considered for several tenth (or more) of cycles, but the quantity of material should be less than 10%.

7 0.8

Qred 0.1 0.4 Qox

0 0.05 Q (F/mo le Fe) E / V vs ENH

-0.4

0 0 0.1 0.2 0.3 0.4 1 3 5 7 9 11 13 15 Q (F/mole Fe) Cycle number

Figure 5 : (a) Galvanostatic cycling of g-FeOOH/graphite composite electrode in 0.1 M NaCl / 0.05 M PIPES solution at pH 7.5 and T = 25°C, I = ±100 µA. (b) Qred and Qox as a function of cycle number.

4.3. Electrochemical reduction of archaeological artefacts rust Figure 6 reports the reduction E-Q curves at –100µA of blank graphite electrode (a), synthetic a-FeOOH/graphite (b), synthetic g-FeOOH/graphite (c) and archaeological artefact rust (MAR01)/graphite (d), composite electrodes.

0

-0,1

-0,2

-0,3

-0,4ESH E / V -0,5 (c) (d) -0,6 (b)

-0,7 (a) -0,8 0 0,002 0,004 0,006 0,008 0,01 normalized Q / F.g-1 Figure 6 : Galvanostatic reduction of (a)blank graphite electrode and (b) synthetic a- FeOOH/graphite, (c) synthetic g-FeOOH/graphite, (d) archaeological artefact rust (MAR01)/graphite composite electrodes in 0.1 M NaCl / 0.05 M PIPES solution at pH 7.5 and T = 25°C, I = -100 µA.

The rapid drop of potential observed in curve (a) indicates that graphite does not give any reduction response. The reduction capability of ferric products can be estimated through the coulombic charge involved until a potential value about – 0.62 V is reached. The corresponding values are (b) 0.5 10-3, (c) 3.9 10-3 and (d) 1.9 10-3 F.g-1 (values corrected from the blank time value). By assuming that (i) the Q values (b) and (c) are related to 100% a- FeOOH and 100% g-FeOOH, respectively, (ii) the rust layer only contains only a- and g-

8 FeOOH, the composition of MAR01 rust may be computed from electrochemical reduction data, a-FeOOH 59% and g-FeOOH 41%. Considering the age of the sample, this ratio is relatively is in relative good agreement with precedent measurement made on several archaeological samples [3].

Finally, electrochemical experiments have also been achieved on the archaeological artefact rust layer in order to confirm that lepidocrocite (g-FeOOH) could also be reduced on such natural and more complex layer. An electrode has been conceived as described on figure 7(a) by protecting the metal surfaces, which has been created during the cut up of the artefact original sample. This protection has been made by using a non-conducting resin and allows to have only rust in contact with the electrolyte during the experiments.

-350

-400

-450

-500

-550 E / V vs ENH

-600

-650

-700 0 0,5 1 1,5 2 Q (C) (a) (b) Figure 7: (a) Archaeological artefacts rust layer electrode used during the electrochemical experiments. (b) Two hundred years old archaeological artefact rust layer reduction in 0.5 M Na2SO4 electrolyte. T = 20°C. I=-50 mA.

The potential variation in function of the charge is presented on the figure 7(b) and showed a similar aspect to the reduction of synthetic g-FeOOH/graphite and archaeological artefact rust (MAR01)/graphite electrodes with a small but significant threshold indicating the end of the layer reduction after about 5 hours of reduction. As goethite has been experimentally shown to be a very less reducible phase than lepidocrocite, it appears clearly that the amount of lepidocrocite contained in the rust layer can be reduced during the wetting stage described in the atmospheric corrosion modelling.

5. Conclusion

Synthethic powder of lepidocrocite and rust layer of archaeological samples were studied by electrochemical methods in order to confirm the hypothesis of lepidocrocite reduction possibility during the first stage of the atmospheric corrosion wet/dry cycle. Experiments performed on synthetic lepidocrocite powder and thin film enable to conclude that lepidocrocite is a reducible phase and the galvanic coupling between lepidocrocite reduction and iron oxidation is possible. All lepidocrocite is not reduced and a part of the reduced lepidocrocite is dissolved in the electrolyte. The reduced lepidocrocite not dissolved can be re-oxidised. With the electrochemical experiments set up on archaeological artefact, two main conclusions can also be underlined. Lepidocrocite in an archaeological artefact sample can also be reduced even on natural rust layer. By using reduction experiments of pure components (goethite and

9 lepidocrocite), an estimation of the phases contents of an archaeological artefact rust layer can be obtained.

Finally this study shows clearly that lepidocrocite can be reduced during the atmospheric corrosion of iron. This point is a fundamental advance for the understanding and modelling of atmospheric corrosion of iron on very long term periods. One of the outlooks of this work is to try to characterize the structure of the phase formed during the lepidocrocite reduction.

References

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