Role of sulphate ions on the formation of calcareous deposits on steel in artificial seawater; the formation of Green Rust compounds during cathodic protection

Electrochimica Acta 54 (2009) 3580–3588

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Role of sulphate ions on the formation of calcareous deposits on steel in artificial seawater; the formation of Green Rust compounds during cathodic protection Ch. Barchiche a , C. Deslouis b , O. Gil c , S. Joiret b , Ph. Refait a,∗ , B. Tribollet b a

Laboratoire d’Etude des Matériaux en Milieux Agressifs, EA 3167, Université de La Rochelle, Bât. Marie Curie, Avenue Michel Crépeau, F-17042 La Rochelle Cedex 01, France Laboratoire des Interfaces et Systèmes Electrochimiques, UPR 15 - CNRS, Université Pierre et Marie Curie T22, 4 Place Jussieu, F-75252 Paris Cedex 05, France c Equipe de Recherche en Physico-Chimie et Biotechnologie, Université de Caen, Campus 2, Science 2, Boulevard du Maréchal Juin, 14032 Caen Cedex, France b

a r t i c l e

i n f o

Article history: Received 15 July 2008 Received in revised form 8 January 2009 Accepted 11 January 2009 Available online 19 January 2009 Keywords: Calcareous deposit Steel Impedance Cathodic protection Artificial seawater

a b s t r a c t Cathodic protection of steel in seawater could be optimized by taking into account the calcareous deposits forming on the steel surface. The aim of this work was to study the influence of sulphate ions on the kinetics and mechanisms of formation of these deposits. The experiments were performed at 20 ◦ C, with an applied potential of −1.0 V/saturated calomel electrode (SCE), in artificial seawater-like solutions with various SO4 2− concentrations. The deposition was monitored by chronoamperometry and electrochemical impedance spectroscopy (EIS). Micro-Raman analyses were performed in situ in a specific electrochemical cell to identify the solids forming on the steel surface. It could be demonstrated that sulphate ions had an important effect on the formation of both Ca(II)- and Mg(II)-containing phases. In the solution enriched with sulphate ions, the deposition of CaCO3 was almost totally inhibited. Experiments performed in Ca2+ free solutions demonstrated that the Mg-based deposit was, in contrast, favoured by the increase of the sulphate concentration. In situ Raman spectra of the solid forming at the early stages of the cathodic protection proved to be characteristic of Green Rusts (GRs). This compound was favoured by the presence of Ca2+ and/or Mg2+ cations, and is more likely a GR-like M(II)-Fe(III) hydroxysulphate, with M = Fe, Mg and Ca. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Cathodic protection is widely used to protect immersed steel structures from corrosion. In the range of potentials applied, the reduction of dissolved oxygen occurs onto the steel surface, and is accompanied for the more cathodic potentials by hydrogen evolution. Both reactions produce hydroxyl ions OH− , inducing the formation of a Mg(II)-based precipitate provided that the interfacial pH is sufficiently high [1]. These reactions also lead to changes in inorganic carbonic equilibrium at the metallic interface, favour CO3 2− over HCO3 − and allow for CaCO3 precipitation. The formation on the metallic surface of the so-called calcareous deposits promotes a physical barrier against oxygen diffusion and thus decreases the current density, or the sacrificial anode consumption, needed to keep efficient protection [2,3]. Chloride, sulphate and hydrogenocarbonate are the main anionic species in seawater. The chloride to sulphate molar ratio is about 19 and the sulphate to carbonate species molar ratio is about 12. It was demonstrated that sulphate ions participated directly to

∗ Corresponding author. Tel.: +33 5 46 45 82 27; fax: +33 5 46 45 72 72. E-mail address: [email protected] (Ph. Refait). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.01.023

the corrosion process of iron in seawater by promoting the formation of an intermediate Fe(II–III) hydroxysulphate of the Green Rust (GR) type [4,5]. GRs are mixed valence Fe(II–III) hydroxysalts forming as precursors of Fe(III) oxyhydroxides. They appear during the early stages of the corrosion process, before the usual “brown” rust. They are characterised by a crystal structure that consists of the stacking of Fe(OH)2 -like layers carrying a positive charge as due to the presence of Fe(III) and interlayers constituted of anions and water molecules. Several GRs are known, e.g. GR(Cl− ), GR(CO3 2− ), GR(C2 O4 2− ), GR(SO4 2− ), etc. This layered structure is characterised by a strong affinity for divalent anions [6,7], which explains why the variety obtained in seawater is the sulphated GR(SO4 2− ) even though the Cl− /SO4 2− molar ratio of seawater is 19. The chemical composition of GR(SO4 2− ) is FeII 4 FeIII 2 (OH)12 SO4 ·8H2 O [8], sometimes developed as [FeII 4 FeIII 2 (OH)12 ]2+ [SO4 ·8H2 O]2− to remind that the crystal structure is built on positive hydroxide layers alternating with negative interlayers. As another consequence of its relative abundance in seawater, sulphate could have a strong influence on the composition, structure and property of the calcareous deposits. But, although a noticeable number of works has already been devoted to the calcareous deposition, only a few of them indicated that sulphate anions did play a role [9,10]. The solubility of CaSO4 and

Ch. Barchiche et al. / Electrochimica Acta 54 (2009) 3580–3588

MgSO4 is higher than that of the corresponding carbonates and these compounds are not likely to form on the steel substrate. However, the adsorption of sulphate ions on the substrate or on the crystals of CaCO3 and Mg(OH)2 could modify both nucleation and growth processes of the deposited minerals, favour one of the allotropic forms of CaCO3 or even lead to specific compounds. The aim of the study was to confirm and precise the effects of sulphate ions previously described [9,10], to clarify the interactions of sulphate with (i) the deposition of aragonite (CaCO3 ) and (ii) the deposition of the Mg(II)-containing phase. Chronoamperometry and electrochemical impedance spectroscopy (EIS) were then coupled to monitor the formation of the deposits. The solids forming on the steel surface at the early stages of the deposition were analysed in situ by micro-Raman spectroscopy whereas the deposits finally obtained were analysed ex situ by scanning electron microscopy (SEM). 2. Materials and methods Experiments were performed in an artificial solution containing the main species present in natural seawater, as described by the ASTM D1141 standard [11]. The composition was then NaCl: 0.42 mol L−1 ; Na2 SO4 : 2.88 × 10−2 mol L−1 ; CaCl2 ·2H2 O: 1.05 × 10−2 mol L−1 ; MgCl2 ·6H2 O: 5.46 × 10−2 mol L−1 ; KCl: 9.32 × 10−3 mol L−1 ; NaHCO3 : 2.79 × 10−3 mol L−1 . Solutions with various sulphate concentrations, from 0 to 8.64 × 10−2 mol L−1 (that is three times as much as the sulphate concentration of artificial seawater), were prepared by varying the Na2 SO4 concentration. The pH of the solutions was set at 8.2 by addition of sodium hydroxide. The composition of the solutions considered in this study is given in Table 1. An electrochemical cell with a double wall in glass for the control of temperature was used. Its large volume (500 mL) restricted the variations of species concentrations during the formation of deposits. The working electrodes were rotating disks obtained as the cross-section of cylindrical rods (2 cm2 area). The electrode material was carbon steel, with approximate composition in weight % of: 98.2% Fe, 0.12% C, 0.21% Si, 0.64% Mn, 0.016% P, 0.13% S, 0.12% Cr, 0.02% Mo, 0.11% Ni and 0.45% Cu. The steel surfaces were polished with silicon carbide (particle size 25 ␮m), rinsed thoroughly with Milli-Q water and carefully dried. Potentials were measured vs. the saturated calomel electrode (SCE, +242 mV/SHE). A large platinum grid was used as counter electrode. All studies were performed at 20 ◦ C, with a rotation speed of the electrode of 600 rpm and an applied potential of −1.0 V/SCE, except for one specific experiment, performed at −1.1 V/SCE. Under these conditions, the main component of the deposit is CaCO3 under the form of aragonite [12]. A Mg-containing compound, that could be detected only by in situ electrochemical methods, is present as a rather unstable porous layer [12], presumably the precursor

3581

of the Mg(OH)2 (brucite) layer that is obtained at more cathodic potentials. It forms during the early stages of the polarisation and the order-of-magnitude of its thickness was calculated as 10−8 m [13]. In order to study the specific interactions of SO4 2− with this Mg-containing phase, CaCl2 ·2H2 O-free artificial seawater was used to avoid the co-deposition of aragonite. Additional experiments were also performed in MgCl2 ·6H2 O-free solutions and in solutions enriched with Mg(II) (see Table 1 for the composition of these solutions). Electrochemical experiments were driven under potentiostatic conditions using SOLARTRON 1287 interface associated with a SOLARTRON Frequency Analyser 1260 controlled by a microcomputer. The blocked area ratio of the electrode was determined from EIS measurements by calculating the capacitance values in the high frequency domain, CHF , as done in previous studies [15–17]. The EIS investigations were performed at cathodic polarisation potential over the frequency range 50–1 kHz with a variation of ±30 mV of the ac applied perturbation potential corresponding to linear conditions. CHF was determined from the linear part of the Nyquist diagram that is between 1 and 2.81 kHz, where 10 measurements were performed. Micro-Raman spectra were obtained with a LABRAM spectrometer (Jobin Yvon). The red line at 632.8 nm of a He–Ne laser with a power fixed at 9.7 mW was used. A confocal microscope with a hole fixed at 200 ␮m and an 80 ULWD objective with a lateral resolution of 1.2 ␮m2 was used to accurately select the crystal to be analysed. Raman analyses were performed under various conditions: (i) the electrolyte was not stirred, the electrode was static and the analysis was performed in situ during the cathodic protection in a specific electrochemical cell. (ii) A pump was used to create a circulation of the electrolyte inside the electrochemical cell. The electrolyte was then stirred and the Raman analysis could also be performed in situ during cathodic protection. (iii) Cathodic protection was performed on a rotating electrode (600 rpm). The Raman analysis was in this case performed ex situ immediately after stopping the polarisation. Of course, in this last case the steel surface was exposed a few minutes (just before and during analyses) to the atmosphere. The polarisation of the steel electrodes was maintained for at least 48 h. The experiments were stopped once the current was stabilised at a negligible value, or after 120 h (5 days). The steel electrodes were rinsed thoroughly with distilled water so as to avoid the formation of NaCl crystals that otherwise would mask an important part of the electrodeposited minerals. The deposits obtained were characterised afterwards by SEM. SEM pictures were obtained in the secondary electron mode on a JEOL 5410 Low Vacuum and coupled microanalyses were performed using an energy dispersive X-ray spectrometer (EDS) Oxford Link ISIS 300, without specific calibration. ZAF corrections were applied to quantitative measurements. The beam voltage was set at 20 kV for both observations and chemical analyses.

Table 1 Composition of the solutions used. The concentrations are given in mol L−1 . Salt

Artificial seawatera

Solutions with various SO4 2− concentrations

Ca-free solutions with various SO4 2− concentrations

NaCl Na2 SO4

0.42 2.88 × 10−2

0.42 0 1.44 × 10−2 5.76 × 10−2 8.64 × 10−2

CaCl2 MgCl2 KCl NaHCO3

1.05 × 10−2 5.46 × 10−2 9.32 × 10−3 2.79 × 10−3

1.05 × 10−2 5.46 × 10−2 9.32 × 10−3 2.79 × 10−3

0.42 0 1.44 × 10−2 2.88 × 10−2 5.76 × 10−2 8.64 × 10−2 0 5.46 × 10−2 9.32 × 10−3 2.79 × 10−3

a

Derived from ASTM D1141 standard.

Solution enriched in Mg(II) 0.42 2.88 × 10−2

1.05 × 10−2 10.92 × 10−2 9.32 × 10−3 2.79 × 10−3

Mg-free and Mg- and Ca-free solutions 0.42 2.88 × 10−2

1.05 × 10−2 or 0 0 9.32 × 10−3 2.79 × 10−3

3582

Ch. Barchiche et al. / Electrochimica Acta 54 (2009) 3580–3588

Fig. 1. Influence of SO4 2− concentration on the chronoamperometric curves for calcareous deposition at −1.0 V/SCE and 600 rpm.

3. Results 3.1. Deposition in artificial seawater-like solutions with various sulphate concentrations The chronoamperometric curves are presented in Fig. 1. The current is normalized by I0 , the current value at t = 0, in order to facilitate comparison between different experiments, even if I0 proved to be independent of the sulphate concentration. First, it must be noted that in most cases the cathodic current slightly increases during the first hours of polarisation so that the ratio I/I0 reaches a maximum larger than 1 after ∼3 h. This initial increase of the current density was not observed during calcareous deposition on gold electrodes in similar conditions [15] but was already mentioned in the case of steel electrodes [13]. It may result of an increase of the active area, due for instance to the progressive dissolution of the remnants of the native oxide film formed while the steel electrode was exposed to air. Secondly, it can be seen that SO4 2− ions do influence calcareous deposition. The larger the sulphate concentration, the slower the deposition and the larger the residual current at longer times. For instance, the value I = 0.2I0 is reached after 15 h without sulphate and after 30 h for [SO4 2− ] = 8.64 × 10−2 mol L−1 . I/I0 tends toward a very small value (near zero) for small sulphate concentrations (<2.88 × 10−2 mol L−1 ), but is about 0.1 for [SO4 2− ] = 5.76 × 10−2 mol L−1 and 0.15 for [SO4 2− ] = 8.64 × 10−2 mol L−1 after 60 h. So, SO4 2− ions seem to hinder the formation of aragonite, the main component of the deposits in the experimental conditions considered here [12]. The results obtained by EIS are presented in Fig. 2. It must be reminded that CHF is equal to the double layer capacitance Cdl over a surface free from deposit at the beginning of calcareous deposition. When the deposit grows up, the measured capacitance is the sum of the capacitance Cf , over the area covered by the deposit and the double layer capacitance Cdl on the deposit-free surface of the electrode, by considering the equivalent circuit already proposed for an organic coating [14]. As for the same area, Cf is much smaller than Cdl (at least three orders of magnitude), for moderate blockage ratios of the electrode, one can assume that the resulting parallel combination of those two capacitances is equivalent to Cdl which is proportional to the free electrode surface. Determination of the capacitance values in the high frequency domain therefore follows closely the free metallic area when calcareous deposit forms [15–17].

The CHF vs. time curves displayed in Fig. 2a confirm the role of sulphate. When [SO4 2− ] is smaller or equal to 2.88 × 10−2 mol L−1 , the curves are quite similar and CHF reaches a very small value indicating a quasi-total surface coverage. In contrast, when [SO4 2− ] is larger than 2.88 × 10−2 mol L−1 , the CHF vs. time curves are considerably modified and CHF does not tend towards zero. After 70 h, it is for instance equal to 8 ␮F/cm2 for [SO4 2− ] = 8.64 × 10−2 mol L−1 , which corresponds to a surface coverage of ∼75%. The evolution of CHF with time differs from that of the cathodic current, since in contrast to CHF , the current is not proportional to the active area. This was confirmed by plotting CHF vs. I/I0 (Fig. 2b). It can be seen that, whatever the sulphate concentration, CHF varies more rapidly than I/I0 during the first stages of the process (that is for high I/I0 ). For the largest sulphate concentration (8.64 × 10−2 mol L−1 ), after this rapid initial decrease CHF stabilises at an intermediate value (about 18 ␮F/cm2 ) while I/I0 keeps on decreasing from 0.9 to 0.2. This behaviour is typical of the porous Mg-rich layer forming together with aragonite [10,12]: first, CHF decreases rapidly at the initial times as the layer forms and covers part of the surface. Then, CHF decreases much more slowly as the porous layer growths essentially in the direction perpendicular to the surface, leaving the uncovered surface proportion approximately constant. CHF is then independent of the layer thickness. In contrast, the cathodic current depends on the layer thickness since O2 diffusion occurs through the pores and decreases as the length of the pores increases. Finally, at the latest stage of the process (low I/I0 ), CHF decreases more rapidly than I/I0 . At this point, the cathodic current is mainly limited by the diffusion of O2 along the pores and

Fig. 2. Influence of SO4 2− concentration on the (a) CHF vs. time and (b) CHF vs. I/I0 curves for calcareous deposition at −1.0 V/SCE and 600 rpm.

Ch. Barchiche et al. / Electrochimica Acta 54 (2009) 3580–3588

becomes more or less independent of slight variations of the active area, whereas CHF remains proportional to the active area. In contrast, for [SO4 2− ] ≤ 2.88 × 10−2 mol L−1 , after the initial decrease of CHF (high I/I0 ), both I/I0 and CHF decrease regularly to negligible values. This means that the deposit grows mainly along directions parallel to the steel surface. So, growth of the deposit leads to the covering and blocking of the surface and CHF and I/I0 are both decreasing. This behaviour corresponds to the formation of a deposit mainly composed of CaCO3 [12,15]. Finally, for [SO4 2− ] = 5.76 × 10−2 mol L−1 , it can be observed an evolution intermediate between that of the 3D porous layer effect of the Mg-rich film and the pure 2D blocking behaviour of the aragonite deposit. SEM observations of the deposits obtained after 120 h of polarisation in solutions with various sulphate concentrations are presented in Fig. 3. They are consistent with the electrochemical analyses. The morphology of the crystals visible on each micrograph is typical of aragonite [10,12]. Actually, calcite crystals are cubic and vaterite forms hemispheric spherulites [18]. Brucite appears as a homogeneous layer, the individual crystals being too small to be distinguished at the magnification used [12]. The aragonite layer obtained at [SO4 2− ] = 1.44 × 10−2 mol L−1 covers completely the steel surface, in agreement with the fact that both I/I0 and CHF tend to zero. Some uncovered areas appear in the deposit obtained for 2.88 × 10−2 mol L−1 . But the drastic modifications are observed for [SO4 2− ] ≥ 5.76 × 10−2 mol L−1 . At 5.76 × 10−2 mol L−1 , less than half of the steel surface is covered by aragonite. At 8.64 × 10−2 mol L−1 , only a few small-sized aragonite clusters are seen. First, this confirms that SO4 2− ions hinder the formation of aragonite. However, the cathodic current flowing through the same electrode decreased to less than 20% of its initial value I0 (Fig. 1). Since aragonite is no more the main component of the deposit, and since the electrochemical behaviour becomes characteristic of the formation of the Mg-rich porous layer described previously [10,12], it must be

3583

assumed that the decrease of the current (and of CHF ) is due to the formation of a similar Mg-rich film. It must be reminded that this film could only be detected by in situ electrochemical techniques [1,12], as it is more likely removed during the preparation of the samples for ex situ analyses, when the surface is thoroughly rinsed with distilled water. Therefore, sulphate ions do not hinder the growth of a Mgcontaining film. However, they may have modified it or even favoured its growth. Actually, since aragonite is (almost) not obtained with the largest sulphate concentration, it is likely that this film grows more easily. Its thickness may be much more important than the 10−8 m of the film formed during the early stages in seawater [13]. This may explain the value of ∼0.15 I0 reached by the residual current at the end of the experiment. It must be reminded that the formation of the Mg-containing film is favoured by the decrease of the potential E applied to the electrode that is by the increase of the interfacial pH. In experimental conditions similar to those used here, brucite Mg(OH)2 was detected together with aragonite for E = −1.2 V/SCE and was the only component of the mineral layer for E = −1.3 V/SCE [12]. Mg compounds could not be detected, i.e. only aragonite was seen, for −0.9 V/SCE ≤ E ≤ −1.1 V/SCE [12]. However, in situ electrochemical monitoring indicates that Mg-containing porous film is forming, at least at the beginning of the process, even for E = −1.0 V/SCE. The formation of this film also explains why I/I0 can decrease down to ∼0.15 while aragonite does not form, as observed above with the largest sulphate concentration. The presence of this film under the deposit of aragonite is demonstrated by the SEM observations displayed in Fig. 4. These images relate to an experiment performed at E = −1.1 V/SCE. As seen in Fig. 4a, a fragment of the aragonite layer has been expelled after the drying of the sample. Under this fragment, a mineral layer is visible on the steel surface. It is not homogeneous and thicker zones appear here and there in an otherwise rather thin film. The Ca and Mg mappings of the sur-

Fig. 3. SEM photographs of the deposits obtained after 120 h of polarisation at −1.0 V/SCE and 600 rpm on a steel substrate at various SO4 2− concentrations: (a) 1.44 × 10−2 mol L−1 ; (b) 2.88 × 10−2 mol L−1 (artificial seawater); (c) 5.76 × 10−2 mol L−1 ; (d) 8.64 × 10−2 mol L−1 .

3584

Ch. Barchiche et al. / Electrochimica Acta 54 (2009) 3580–3588

Fig. 5. Chronoamperometric curves in Ca2+ -free seawater-like solutions. Rotation speed of the electrode: 600 rpm, applied potential: −1.0 V/SCE.

Fig. 4. SEM observations of a deposit obtained at −1.1 V/SCE and 600 rpm. (a) SEM photograph showing a zone where a fragment of the aragonite layer has been expelled at the end of the rinsing and drying procedure. (b) Ca and Mg mappings of the same area obtained using the K␣ radiation of the elements. More abundant is the element and lighter is the image.

face confirm that this film is made of Mg-based compound. These results confirm definitively that the partial “3D porous layer effect” observed in the −0.9 to −1.1 V/SCE potential range is due to the formation of Mg-containing film. So, the discrepancy between electrochemical data (Figs. 1 and 2) and SEM observation (Fig. 3) is due to the fact that results obtained in situ are compared to results obtained ex situ: it is not possible to obtain a complete accordance between in situ electrochemical results and ex situ SEM observations. This illustrates that in some cases in situ methods can provide unique information. 3.2. Deposition in Ca2+ -free seawater-like solutions with various sulphate concentrations In order to investigate whether SO4 2− ions directly hinder the formation of aragonite or favour the Mg(II)-containing component, the formation of the deposits in Ca2+ -free seawater-like solutions was studied similarly. Note that, as it was the case with the solutions containing Ca2+ , I0 proved to be independent of the sulphate concentration. The chronoamperometric curves plotted in Fig. 5 show that the cathodic current decreases more rapidly and to a smaller residual value as the sulphate concentration increases. This indicates that SO4 2− ions favour the formation of the Mg(II)-containing layer. In contrast, the variations of CHF vs. time, presented in Fig. 6, are not modified significantly by the variations of the sulphate concentration. In any case, CHF decreases rapidly during the first 15 h before to stabilise. But this discrepancy between the evolutions with time of I/I0 and CHF is a characteristic of the Mg(II)-containing porous layer. The value of CHF at 70 h depends slightly on the sulphate concentration, which means that the active area is almost independent of the sulphate concentration. The decrease of the residual cathodic current induced by an increase of the sulphate concentration must be interpreted as the consequence of an increase of the

thickness of the Mg(II)-containing layer. Therefore, SO4 2− ions do favour the growth of the Mg-rich layer in the direction perpendicular to the steel surface. It must be noticed that the residual value reached by the cathodic current at the end of the experiments for [SO4 2− ] ≥ 2.88 × 10−2 mol L−1 is about 0.2 I0 . A similar value was measured in the presence of Ca2+ with the largest sulphate concentration (Fig. 1) in agreement with the fact that in this solution, only a few crystals of aragonite were observed (Fig. 3d). Ex situ SEM observations after rinsing thoroughly the electrode surface (not represented) did not reveal the presence of a mineral layer. It was then attempted to rinse with a very special care the surface of the electrode, in order to try to wash out the seawaterlike solution without removing totally the Mg-based layer. The results proved somehow satisfying as remnants of this layer could be clearly observed. SEM observations and EDS analyses of the surface of a steel electrode polarised 120 h at −1.0 V/SCE in the Ca2+ free solution with [SO4 2− ] = 2.88 × 10−2 mol L−1 are presented in Fig. 7. The photograph Fig. 7a is a large view showing particles scattered over the steel surface. Large particles (length between 20 and 60 ␮m) are seen together with smaller ones (length ∼5 ␮m) more

Fig. 6. CHF vs. time curves in Ca2+ -free seawater-like solutions. Rotation speed of the electrode: 600 rpm, applied potential: −1.0 V/SCE.

Ch. Barchiche et al. / Electrochimica Acta 54 (2009) 3580–3588

3585

Fig. 7. SEM photographs of the deposits obtained after 120 h of polarisation at −1.0 V/SCE and 600 rpm on a steel substrate in Ca2+ -free seawater-like solution with [SO4 2− ] = 2.88 × 10−2 mol L−1 : (a) large view and (b) focus on a particle. Corresponding EDS spectra obtained by focusing on: (c) an apparently uncovered area and (d) the particle. Rinsing of the electrode prior to the analyses was made as carefully as possible.

homogeneously spread over the surface. The second photograph (Fig. 7b) was obtained by focusing on one of these particles. The particle has no definite shape, which means that the individual crystals are much smaller than the size of the particle (∼26 × 13 ␮m). It looks like a fragment of the brucite layer forming at lower potentials (E ≤ −1.2 V/SCE) [12]. EDS analyses were performed on the (apparently) uncovered steel surface (Fig. 7c) and on particles. Note that the volume analyzed via EDS can only be roughly estimated, in particular because it depends on the analyzed materials. The order of magnitude of each of its three dimensions (x, y, z) is a micrometer. The spectrum corresponding to the apparently uncovered surface (Fig. 7c) reveals only iron and possible traces of magnesium that could be due to a very thin film. The spectrum obtained by focusing on a particle (Fig. 7d) shows unambiguously the characteristic peak of magnesium. The intense peak of iron indicates that the width of the particle is much smaller than 1 ␮m. The small peak corresponding to S suggests that the electrodeposited Mg(II) compound also contains sulphate ions. 3.3. In situ Raman spectroscopy study of the early stages of calcareous deposition The phases forming during the first stages of the calcareous deposition were analysed by micro-Raman spectroscopy. In situ analyses with or without stirring and ex situ analyses led to the same results and only the kinetics of the phenomena varied with the experimental conditions. For this reason only the results obtained in situ in stirred conditions are presented. Typical Raman spectra are displayed in Fig. 8. Spectrum Fig. 8a illustrates what could be found on the steel surface after the polishing procedure and after an exposure time to the atmosphere similar to that required for the preparation of the electrode before electrochemical measurements. Only one vibration band is visible at 670 cm−1 . It corresponds to magnetite [19]. Such a spectrum could only be obtained once, implying that the surface was almost totally free of corrosion products. Spectra Fig. 8b and c were obtained after 30 and 90 min of polarisation in artificial seawater ([SO4 2− ] = 2.88 × 10−2 mol L−1 ). Analyses performed on various spots of the surface after 30 min led to spectra where vibration bands at 435, 505 and 980 cm−1 are seen. These bands correspond neither to any of the CaCO3 variety nor to Mg(OH)2 nor to

Fig. 8. In situ micro-Raman spectrum of a corrosion product formed on steel before immersion in seawater (a) and of the solids forming during polarisation of steel after 30 min (b) and after 90 min (c) in artificial seawater. Applied potential: −1.0 V/SCE, stirred electrolyte.

MgCO3 . In fact, the sharp peak at 980 cm−1 correspond to the sulphate ions in solution [20], whereas the bands at 430 and 510 cm−1 are typical of GR compounds [4,21–24]. It is only after 90 min of polarisation that aragonite crystals could be seen on the steel surface. The corresponding Raman spectrum is then mainly composed of the vibration bands of aragonite at 701, 704 and 1084 cm−1 [25]. The bands of GR(SO4 2− ) and SO4 2− ions in solution are still visible. It must be noted that GR compounds could not be detected before the immersion of the electrodes into artificial seawater. The corrosion product (rarely) identified on the steel electrode after

3586

Ch. Barchiche et al. / Electrochimica Acta 54 (2009) 3580–3588

the polishing procedure and prior to the immersion was magnetite (Fig. 8a). This demonstrates that the GR compound detected in situ via Raman spectroscopy has formed in seawater during the early stages of the cathodic protection. Actually, seawater is a good electrolyte to form GRs, and it was recently demonstrated that GR(SO4 2− ) was present in any case on steel coupons immersed for 6–12 months in various harbour sites [5]. The formation of this corrosion product on the surface of cathodically protected steel was not expected. Looking at the equilibrium conditions between GR(SO4 2− ) and solution, it is possible to give an estimate of the dissolved Fe(II) species concentration required for the precipitation of the GR. These conditions are given by [26] 6 Fe2+ + 12 H2 O + SO4 2− ↔ FeII 4 FeIII 2 (OH)12 SO4 + 12 H+ + 2 e− (1)

leading to :

E SHE = 1.81 − 0.1773 log [Fe2+ ] − 0.0296 log [SO4 2− ] − 0.3546 pH

(2)

The potential applied to the electrode is E = −1.0 V/SCE = −0.758 V/SHE, the sulphate concentration is 2.88 × 10−2 mol L−1 . Assuming that the interfacial pH is ∼10, and taking the activities equal to the concentrations, its is computed that the dissolved Fe(II) species concentration corresponding to the equilibrium conditions is around [Fe2+ ] ∼ 10−5 mol L−1 . But the principle of cathodic protection is to keep the dissolved Fe species concentration lower than 10−6 mol L−1 . For instance, in seawater at E = −1.0 V/SCE, [Fe2+ ] should be around 10−8 mol L−1 . However, other divalent cations often substitute for Fe2+ in GR compounds, and for instance the mineral fougerite, structurally similar to GRs, contains a significant amount of Mg(II) [27,28]. For this reason, experiments were performed in solutions enriched with Mg(II), free of Mg(II) and free of Mg(II) and Ca(II) (see Table 1 for the composition of the solutions). The first result (not represented) is that the GR could not be detected without Ca(II) and Mg(II). In the solution enriched with Mg(II), the number of GR crystals visible through the optical microscope was significantly larger than in seawater, confirming that Mg(II) favours the formation of the GR. Raman spectrum Fig. 9a was obtained after 15 min of polarisation. It is very similar to the spectrum Fig. 8b obtained in seawater after 25 min of polarisation. The same area was analysed 40 min later that is after 55 min of polarisation. The corresponding Raman spectrum Fig. 9b is similar to the previous one, but the bands of the GR are more intense. This is evident if one compares the intensity of the GR peaks with that of the sharp peak due to SO4 2− ions. This demonstrates that GR (SO4 2− ) is still forming at this stage of the cathodic protection. Finally, experiments were performed in Mg(II)-free solutions. It is well known that Mg2+ ions inhibit the formation of calcite, which is the reason why calcareous deposits in seawater are made of aragonite. So, in Mg(II)-free solutions, calcite is formed. It is already detected after 200 s of cathodic polarisation, as it can be seen in spectrum Fig. 9c. But together with the very intense sharp lines of calcite, the two vibration bands of GR(SO4 2− ) are still visible. 4. Discussion In the experimental conditions considered here, sulphate ions clearly promote the formation and growth of the Mg-rich porous layer. This conclusion contradicts the results reported by Lin and Dexter [9] who observed that an addition of sulphate hindered the formation of the Mg(II)-containing deposit on steel. However, their experimental conditions were different. The temperature and

Fig. 9. In situ micro-Raman spectra of the solids forming during polarisation of steel in the electrolyte enriched with Mg(II) after 15 min (a) and after 55 min (b), and in the Mg(II)-free electrolyte after 200 s (c). Applied potential: −1.0 V/SCE, stirred electrolyte.

applied potential were slightly larger (25 ◦ C and −0.9 V/SCE) and the rotation speed of the electrode was considerably smaller (50 rpm). As it was clearly illustrated previously [10], varying various parameters can lead to cross-effects that may induce apparently opposite behaviours, especially the potential which may have a strong influence on the interfacial pH. The Mg(II)-compound deposited on steel could be like the structure described by Shirasaki [29], who obtained hydrated magnesium hydroxide Mg(OH)2 ·mH2 O (1 < m < 2). The crystal struc-

Ch. Barchiche et al. / Electrochimica Acta 54 (2009) 3580–3588

ture consists of brucite-type layers with interlayers of water molecules sandwiched in between. Adsorption of SO4 2− ions on those Mg(OH)2 layers may then lead to the formation of a hydroxide sulphate, that is a compound consisting of Mg(OH)2 layers alternating with (MgSO4 )·nH2 O layers. Such basic sulphates are actually present in nature in submarine geothermal systems [30–32]. Then, magnesium hydroxide sulphate hydrates, xMg(OH)2 ·MgSO4 ·nH2 O would form preferentially in the presence of sulphate and hydrated magnesium hydroxide would then form only in the absence of sulphate. The increase of the sulphate concentration would favour the growth and stability of hydroxide sulphate hydrates. This was confirmed by the EDS analysis of the remnants of the Mg layer observed ex situ by SEM: it revealed the presence of S in the electrodeposited Mg-based compound. Raman analyses performed during the early stages of the process did not allow us to identify this compound. In contrast, Raman spectroscopy detected a GR-like compound that was not present on the steel surface before immersion, and was still forming during the application of cathodic protection. Two hypotheses can be forwarded. (i) The Mg layer is too thin at the early stages of the cathodic protection to be detected by Raman spectroscopy. This hypothesis is supported by previous work [13]. (ii) The Raman bands of the Mg layer are masked by those of the GR. For instance, the main band of Mg(OH)2 is found at 443 cm−1 [33,34] and can be masked by the broad vibration band of GRs found at 430 cm−1 . The formation of the GR compound proved to be favoured by the presence of Mg(II) and could not be observed in the absence of both Mg(II) and Ca(II). This suggests that Mg2+ and probably Ca2+ are incorporated into the GR structure. In fact, the Fe(OH)2 -like layers of GRs are structurally similar to the layers of Mg(OH)2 . Mg(II)–Fe(III) hydroxysalts, such as pyroaurite the Mg(II)–Fe(III) hydroxycarbonate, are isomorphous to GRs, the Fe(II)–Fe(III) hydroxysalts [35,36]. Fougerite, a GR-like mineral recently discovered in hydromorphic soils [37,38], proved to be a Mg(II)–Fe(II)–Fe(III) compound [27,28]. Laboratory experiments also confirmed that Mg(II) could easily substitute for Fe(II) in GR compounds [27,39]. The formation of such mixed Mg–Fe compounds would occur when the conditions are inappropriate for the formation of a Fe–GR. It is clearly the case at the interface of a steel electrode under cathodic protection, where the Fe2+ concentration is kept low. Note that the GR-like compound could not be detected at the end of the polarisation (120 h), indicating that its formation takes place only at the early stages of the process (∼2 h). It quickly becomes a very minor component of the mineral electrodeposited layer, and is thus almost impossible to detect. Finally, it was demonstrated that sulphate ions hindered the formation of aragonite. It may be an indirect consequence of the beneficial effect of sulphate on the formation of the Mg-based layer. 5. Conclusions The combination of electrochemical and micro-Raman spectroscopy analyses resulted in new information concerning the role of sulphate during calcareous deposition on a steel electrode under cathodic protection in seawater (at −1.0 V/SCE and 20 ◦ C). First of all, sulphates do influence the phenomenon by hindering CaCO3 precipitation. Secondly, they promote the formation of the Mg-containing porous layer. In seawater, the Mg-containing film forming on cathodically protected steel would then be Mg basic sulphate, xMg(OH)2 ·MgSO4 ·nH2 O. It can be forwarded that the beneficial effect of sulphate ions on the Mg-based layer may indirectly hinder the formation of aragonite. In situ Raman spectroscopy revealed that a GR compound was forming during the early stages of the cathodic protection of steel

3587

in seawater. As a matter of fact, the Fe(II–III) hydroxysulphate GR(SO4 2− ) is a corrosion product of steel in seawater [4,5]. Its precipitation on cathodically protected steel results from the slight dissolution of iron occurring even under cathodic protection and the reduction of O2 that favours the formation of hydroxysalts GR via OH− production. But, like the GR mineral fougerite discovered in hydromorphic soils [27,28,37,38], this GR-like compound forms in conditions where the dissolved Fe species concentration is not sufficient for its precipitation. Mg(II) and Ca(II) dissolved species proved to favour its formation and it can be assumed that, like the mineral fougerite, this compound is a Fe(II)–Mg(II)–Fe(III) or even a Fe(II)–Mg(II)–Ca(II)–Fe(III) hydroxysalt. The formation on steel of similar compounds during the early stages of cathodic protection should then occur in other media than seawater, and for instance in soils, where the groundwater contains divalent and trivalent metal cations that can promote layered double hydroxysalts of the GR type. Acknowledgement This work, part of the PhD thesis of C. Barchiche, was supported financially by the Conseil Général of Charente Maritime (17). References [1] C. Deslouis, D. Festy, O. Gil, V. Maillot, S. Touzain, B. Tribollet, Electrochim. Acta 45 (2000) 1837. [2] R.U. Lee, J.R. Ambrose, Corrosion 44 (1988) 887. [3] S. Rossi, P.L. Bonora, R. Pasinetti, L. Benedetti, M. Draghetti, E. Sacco, Corrosion 54 (1998) 1018. [4] Ph. Refait, J.B. Memet, C. Bon, R. Sabot, J.M.R. Genin, Corros. Sci. 45 (2003) 833. [5] S. Pineau, R. Sabot, L. Quillet, M. Jeannin, Ch. Caplat, I. Dupont-Morral, Ph. Refait, Corros. Sci. 50 (2008) 1099. [6] S. Miyata, Clays Clay Miner. 31 (1983) 305. [7] Ph. Refait, S.H. Drissi, J. Pytkiewicz, J.-M.R. Génin, Corros. Sci. 39 (1997) 1699. [8] L. Simon, M. Franc¸ois, Ph. Refait, G. Renaudin, M. Lelaurain, J.-M.R. Génin, Solid State Sci. 5 (2003) 327. [9] S.H. Lin, S.C. Dexter, Corrosion 44 (1988) 615. [10] Ch. Barchiche, C. Deslouis, O. Gil, Ph. Refait, B. Tribollet, Electrochim. Acta 49 (2004) 2833. [11] Standard ASTM D1141, American Society for Testing and Materials, Philadelphia PA, US, vol. 11.02, 1999. [12] Ch. Barchiche, C. Deslouis, D. Festy, O. Gil, Ph. Refait, S. Touzain, B. Tribollet, Electrochim. Acta 48 (2003) 1645. [13] K.E. Mantel, W.H. Hartt, T.-Y. Chen, Paper no. 374, presented at Corrosion/90, National Association of Corrosion Engineers, Houston, USA (1990). [14] L. Beaunier, I. Epelboin, J.C. Lestrade, H. Takenouti, Surf. Technol. 4 (1976) 237. [15] C. Deslouis, D. Festy, O. Gil, G. Rius, S. Touzain, B. Tribollet, Electrochim. Acta 43 (1998) 1891. [16] O. Devos, C. Gabrielli, B. Tribollet, Electrochim. Acta 51 (2006) 1413. [17] C. Deslouis, A. Doncescu, D. Festy, O. Gil, V. Maillot, S. Touzain, B. Tribollet, Mater. Sci. Forum 289–292 (1998) 1163. [18] C. Gabrielli, G. Maurin, G. Poindessous, R. Rosset, J. Cryst. Growth 200 (1999) 236. [19] D.L.A. de Faria, S. Venâncio Silva, M.T. de Oliveira, J. Raman Spectrosc. 28 (1997) 873. [20] M.M. Tlili, M. Ben Amor, C. Gabrielli, S. Joiret, G. Maurin, P. Rousseau, J. Electrochem. Soc. 150 (2003) C485. [21] N. Boucherit, A. Hugot Le Goff, S. Joiret, Corros. Sci. 32 (1991) 479. [22] Ph. Refait, A. Charton, J-M.R. Genin, Eur. J. Solid State Inorg. Chem. 33 (1998) 655. [23] S. Simard, M. Odziemkowski, D.E. Irish, L. Brossard, H. Ménard, J. Appl. Electrochem. 31 (2001) 913. [24] L. Legrand, G. Sagon, S. Lecomte, A. Chaussé, R. Messina, Corros. Sci. 43 (2001) 1739. [25] C. Gabrielli, R. Jaouhari, S. Joiret, G. Maurin, J. Raman Spectrosc. 31 (2000) 497. [26] Ph. Refait, A. Géhin, M. Abdelmoula, J.-M.R. Génin, Corros. Sci. 45 (2003) 656. [27] Ph. Refait, M. Abdelmoula, F. Trolard, J.-M.R. Génin, J.-J. Ehrhardt, G. Bourrié, Am. Miner. 86 (2001) 731. [28] F. Trolard, G. Bourrié, M. Abdelmoula, Ph. Refait, F. Feder, Clays Clay Miner. 55 (2007) 323. [29] T. Shirasaki, Denki Kagaku 29 (1961) 656. [30] J.L. Bischoff, W.E. Seyfried, Am. J. Sci. 278 (1978) 838. [31] D.R. Janecky, W.E. Seyfried, Am. J. Sci. 283 (1983) 831. [32] R.M. Haymon, M. Kastner, Am. Miner. 71 (1986) 819. [33] P. Dawson, C.D. Hadfield, G.R. Wilkinson, J. Phys. Chem. Solids 34 (1973) 1217. [34] P. Dawson, J. Raman Spectrosc. 1 (1973) 359.

3588

Ch. Barchiche et al. / Electrochimica Acta 54 (2009) 3580–3588

[35] P.P. Stampfl, Corros. Sci. 9 (1969) 185. [36] Ph. Refait, M. Abdelmoula, J.-M.R. Génin, Corros. Sci. 40 (1998) 1547. [37] F. Trolard, M. Abdelmoula, G. Bourrié, B. Humbert, J.-M.R. Génin, Comput. Rend. Acad. Sci. Paris, Série IIa 323 (1996) 1015.

[38] F. Trolard, J.-M.R. Génin, M. Abdelmoula, G. Bourrié, B. Humbert, A.J. Herbillon, Geochim. Cosmochim. Acta 61 (1997) 1107. [39] Ph. Refait, S. Drissi, M. Abdelmoula, J.-M.R. Génin, Hyperfine Interact. 139–140 (2002) 651.