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.