Elsevier

Corrosion Science

Volume 50, Issue 1, January 2008, Pages 62-69
Corrosion Science

New insight in the behaviour of Co–H2O system at 25–150 °C, based on revised Pourbaix diagrams

https://doi.org/10.1016/j.corsci.2007.07.002Get rights and content

Abstract

Revised Pourbaix diagrams (potential/pH diagrams) for the Co–H2O system are presented at 25–150 °C using a new set of standard thermodynamic data in accordance with the recommended CODATA key values.

Introduction

The search for more reliable and precise thermodynamic standard functions: free formation enthalpy (ΔGf), formation enthalpy (ΔHf), entropy (S) and specific heat (Cp), than those selected by Pourbaix in 1963 in his capital atlas [1], may modify significantly E-pH diagrams and the metallic oxide and hydroxide solubility established at 25 °C by this author. In the case of the Co–H2O system, the revision of the whole set of thermodynamic data is highly stimulated by the numerous uses of cobalt compounds and, in particular, their oxides.

Among the applications of cobalt compounds, we can outline the uses of cobalt-oxide coatings, as catalysts [2] and electrocatalysts [3], [4], insertion compounds for electrochromic devices [5] and batteries [6], semi-conductors [7]. Another example is given by LiCoO2, which has been identified recently as a potential protective coating for the state-of-the-art molten carbonate fuel cell (MCFC) cathode, LixNi1−xO [8], [9], [10], [11]. LiCoO2 can be produced by lithiation of Co3O4 or CoOOH thin films elaborated by a potentiostatically-controlled deposition technique. Their formation proceeds from the oxidation of dissolved Co(OH)2. These oxides have been formed in conditions which are in contradiction with the predictions of the original Pourbaix diagram [1], showing the necessity of upgrading these well-known data. It is also important to establish a set of E-pH diagrams at high temperature in order to predict the corrosion behaviour of cobalt and cobalt alloys in aqueous media. It is worthy to note that revised Pourbaix diagrams have been presented recently in the literature for a certain number of metals, including iron [12], [13], nickel [12], [14], chromium [12], [15], zinc [16], [17], [18] and copper [19] for example. Krupta and Serne [20] have recently presented a review on the geochemical behaviour of cobalt, and have published E-pH diagrams for cobalt with [Co]tot = 10−12 and 10−14.8 mol L−1 with the presence of dissolved chloride, nitrate, sulfate and carbonate, but only at 25 °C. Another important field for cobalt–water system thermodynamic analysis is radiochemistry with, for instance, the purification of pressured-water from the primary reactor coolant which requires a good insight in the speciation of cobalt species and, in particular, dissolved 60Co dissolved into reactor water [21], [22] (60Co is a strong γ-emitter with a half live, t1/2, of 5.27 years).

This study will analyse, on the basis of recent thermodynamic sources, the behaviour of Co–H2O system in a large temperature range: 25–150 °C and two Co(II) concentrations: 10−6 and 10−2 mol kg−1 (molal). It will include a selection of the standard thermodynamic data (25 °C and 1 bar) concerning the compounds belonging to the Co–H2O system and, from this set of selected data with an internal coherence, the establishment of Pourbaix diagrams as a function of the temperature. It should be précised that the unit mol kg−1 was selected because of its non ambiguous signification with respect to the temperature variation range 25–150 °C (in contrast to mol L−1).

The search of new thermodynamic data is often a delicate task due to the dispersion of the experimental values given in the literature, their precision, the methodology used, the purity and the origin (natural or synthetic) of the compounds analysed and the thermodynamic functions of the key chemical substances (H+, H2(g), O2(g), H2O(l)…) used as auxiliary values in the calculations.

A recall of the principal thermodynamic aspects useful for the comprehension of this work is presented in annex 1.

In the present work, the chemical symbol is followed by (g) for the gaseous state, by (l) and (s) for liquid and solid states, respectively. In the case of soluble ionic species, the charges are indicated by z+ and z− and sign for non-charged dissolved species. We have chosen Co(OH)n2-n for cobaltous hydroxyl monomers, in agreement with Baes and Mesmer [23], but distinct from the nomenclature used by Pourbaix [1] and Shock et al. [24]. The difference between these nomenclatures is one or two water molecules (CoO+H2O(l)=Co(OH)2,HCoO2-+H2O(l)=Co(OH)3- and CoO22-+2H2O=Co(OH)42-).

Section snippets

Cobalt properties

Cobalt, one of the three elements of the iron family (group VIIIB), exists in nature in a sulphide or arsenosulphide form, but also as oxide and carbonate. It is a white-silver metal, ductile and inalterable in air at room temperature. It crystallises at temperatures lower than 450 °C in the α-hexagonal system. Its compounds correspond to oxidation degrees +II, +III. Degree IV only exists in few cases. CoO2(s), a very strong oxidising agent, is unstable in presence of water.

In aqueous solutions,

Selection of thermodynamic data for the Co–H2O system

In order to obtain the E-pH equilibrium diagram of the Co–H2O system at 25 °C, Pourbaix [1] used the Gibbs free energy (ΔGf) published by Latimer in 1952 [26] for Co(s), Co(OH)2(s), Co(OH)3(s), Co3+ and Co2+, by Besson in 1947 [27] for Co3O4(s) and by Gayer and Garret in 1950 [28] for HCoO2- (or Co(OH)3-) (Table 1). In 1976, Baes and Mesmer [23] have recommended a set of stability constants for the mononuclear and polynuclear hydroxide complexes of Co2+, as the solubility product of Co(OH)

Elaboration of Pourbaix diagrams for the Co–H2O system at 25–150 °C

Our selection of standard thermodynamic functions ΔHf, S and Cp have been collected, in the form of a specific database, in the HSC CHEMISTRY® for WINDOWS. This software, which uses the STABCAL algorithms [35], permits, on the basis of the registered data, the calculations of the thermodynamic functions corresponding to the chemical and electrochemical reactions related to the Co–H2O system and the establishment of Pourbaix diagrams at different temperatures. The diagrams obtained are

Comparison of diagrams E-pH of the Co–H2O system at 25 °C

The potential-pH diagrams obtained at 25 °C with [Co]tot = 10−6 molal, with the data used by Pourbaix [1] and those of our selection (Table 1), without taking into account CoOOH, are plotted in Fig. 1. The diagram depicted in Fig. 1A is similar to that published by Pourbaix [1] for the cobalt–water system.

Comparison of Fig. 1A and B (without CoOOH(s)) shows that with our thermodynamic data set:

  • the stability domain of Co3O4(s) increases considerably with respect to Co(OH)2(s) and Co(OH)3(s);

  • the

Conclusion

The revised Pourbaix diagrams of the Co–H2O system presented in this work are based on a selection of new thermodynamic data, which is in accordance with CODATA recommendations. The E-pH diagrams, established between 25 and 150 °C, allowed to estimate the evolution of the stability domains of the dissolved and solid species involved in the Co–H2O system with the temperature and the concentration of dissolved cobalt. These diagrams allow to evaluate the composition of Co(II) solutions and, in

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