**4. Discussion**

In the considered experimental conditions, SO4<sup>2</sup>− and Cl− were the only anions available for the formation of LDH compounds. Consequently, the only Fe(II)-Fe(III) mixed valence compounds that could possibly form were GR(SO4<sup>2</sup>−), GR(Cl−), and Fe3O4. The traces of GR(CO3<sup>2</sup>−) detected in each case are due to CO2 and/or chemical impurities and the formation of this phase will not be further discussed. These experimental conditions were chosen so that in the absence of Mg(II) cations, the Fe(II)-Fe(III) SO4-LDH, i.e., GR(SO4<sup>2</sup>−), was obtained, only accompanied by traces of the Fe(II)-Fe(III) Cl-LDH, i.e., GR(Cl−). The aim was to reproduce the first stage of the corrosion process of carbon steel in seawater, which leads to GR(SO4<sup>2</sup>−) [1,2], via a precipitation reaction involving dissolved Fe(II) and Fe(III) species, OH- ions, and the main anionic species of seawater, i.e., Cl− and SO4<sup>2</sup><sup>−</sup>.

The first and more important effect of Mg(II) cations is to favor the formation of a Cl-LDH at the detriment of the SO4-LDH obtained with Fe(II) and Fe(III). This is clearly illustrated by the increase of the proportion of GR(Cl−) with the increase of the [MgII]/[FeII] substitution ratio and the formation of iowaite, the Mg(II)-Fe(III) Cl-LDH, when [FeII] = 0.

A Mg(II)-Fe(III) SO4-LDH could be obtained when Cl− ions were removed from the system. However, the solid phase identified in an aqueous suspension, structurally similar to GR(SO4<sup>2</sup>−), underwent a transformation upon drying, which led to a SO4-LDH structurally closer to GR(Cl−).

The main difference between the two GR structures is the organization of the interlayers, that involve two planes of anions and water molecules in GR(SO4<sup>2</sup>−) [23] and only one plane in GR(Cl−) [17] (and in GR(CO3<sup>2</sup>−), as well). Figure 9 displays a schematic representation of these structures. GR(Cl−) and GR(SO4<sup>2</sup>−) were initially called GR-1 and GR-2 [24] and a similar terminology can be retained to distinguish the structure of GR(Cl−) and GR(CO3<sup>2</sup>−) from that of GR(SO4<sup>2</sup>−). It must be noted that for the GR-1 rhombohedral R3m structure of GR(Cl−) [17], the stacking sequence is AcB i BaC i CbA i, where A, B, C are planes of OH− ions, a, b, c planes of Fe atoms, and i corresponds to the interlayers. In the case of the P3m1 trigonal structure of GR(SO4<sup>2</sup>−) [23], i.e., GR-2, the stacking sequence is AcB i AcB.

**Figure 9.** Schematic representations of the GR-1 and GR-2 structures, drawn according to the crystal structures of GR(Cl−) given in [17] and GR(SO4<sup>2</sup>−) given in [23].

The results obtained here show that Mg(II) cations favor the GR-1 structure. From a fundamental point of view, the cohesion of a LDH structure is due to (i) the water molecules of the interlayers that interact with the adjacent hydroxide layers and the intercalated anions via hydrogen bonds and (ii) the intercalated anions that interact with the hydroxide layers via electrostatic interactions and hydrogen bonds [21,25]. Changes in the hydroxide layers necessarily have an influence on the bonds linking these layers and the species (anions and water molecules) present in the interlayers. They have thus an influence on the cohesion of the LDH structure. The dependence between the cationic composition of the hydroxide layer and the structural stability has been studied and modelled in [21]. This study demonstrated how important the nature of cations for the stability of the crystal structure was.

The thorough analysis of the diffraction data showed that the *c* lattice parameter of GR(SO4<sup>2</sup>−) did not vary with the [MgII]/[FeII] substitution ratio. However, the distance between two planes of metal cations is higher for the Mg(II)-Fe(III) SO4-LDH, with 11.56 Å vs. 11.16 Å for GR(SO4<sup>2</sup>−) (Table 2). In contrast, the diffraction peaks of GR(Cl−) proved to be influenced by the [MgII]/[FeII] ratio. This suggests that the Mg(II) cations are not present, or only in a small amount, in the hydroxide layers of GR(SO4<sup>2</sup>−). Consequently, they would be preferentially incorporated in the GR(Cl−) structure or left in the solution. The small amount of Mg(II) possibly present in the hydroxide layers of GR(SO4<sup>2</sup>−) would explain the decrease of crystal/mean coherent domain size observed with the increasing Mg(II)/Fe(II) concentration ratio (Table 2).

An interesting first case is the [MgII]/[FeII] ratio of 1/3. With this Mg(II) amount, only a minor proportion of GR(Cl−) is present after 1 week of ageing, while 25% of Fe(II) is substituted by Mg(II). According to the initial amounts of reactants, the precipitation reaction could be written as:

$$\frac{9}{2}\text{Fe}^{2+} + \frac{3}{2}\text{Mg}^{2+} + 2\text{Fe}^{3+} + 12\text{OH}^- + \text{SO}\_4^{2-} + 8\text{H}\_2\text{O} \rightarrow \text{Fe}^{\text{II}}\text{Fe}^{\text{III}}\_2\text{(OH)}\_{12}\text{SO}\_4\cdot\text{8H}\_2\text{O} + \frac{1}{2}\text{Fe}^{2+} + \frac{3}{2}\text{Mg}^{2+} \text{ (3)}$$

This writing shows that for this [MgII]/[FeII] ratio, all the Mg2+ ions could be released into the solution, more likely during the ageing procedure where GR(Cl−) transforms to GR(SO4<sup>2</sup>−). It can then be forwarded that in this first case, the SO4-LDH is close to GR(SO4<sup>2</sup>−) and contains a very small proportion of Mg(II). The GR-2 structure is obtained since Mg(II) cations are preferentially found in the solution and in the small amount of the remaining GR(Cl−) (or more exactly Cl-LDH).

In contrast, for the higher [MgII]/[FeII] ratios of 1 and 3, an important amount of Mg(II) is necessarily incorporated in the solid phase, which implies that the GR-1 structure is favored leading to the predominance of the Cl-LDH similar to GR(Cl−). Both GR(Cl−) and

iowaite are characterized by a (Fe,Mg)(II) to Fe(III) cation ratio of 3:1 [14,16], which implies that all divalent cations should be incorporated into the solid phase in the considered experimental conditions. For instance, for the highest [MgII]/[FeII] ratio considered here, the precipitation reaction of the Cl-LDH can be written, neglecting the small amount of GR(SO4 <sup>2</sup>−) that forms, as:

$$\frac{3}{2}\text{Fe}^{2+} + \frac{9}{2}\text{Mg}^{2+} $$

$$\rm{+2Fe^{3+} + 16OH^- + 2Cl^- + 4H\_2O \to Fe^{II}\_{1.5}Mg^{II}\_{4.5}Fe^{III}\_2(OH)\_{16}Cl\_2 \cdot 4H\_2O} \tag{4}$$

However, Mg(II) cations also promoted the formation of magnetite. Looking to reaction (4), it is seen that in the considered experimental conditions, which correspond to a (Fe,Mg)(II) to Fe(III) cation ratio of 3:1, the precipitation of a Cl-LDH having the same (Fe,Mg)(II) to Fe(III) cation ratio of 3:1 does not leave any divalent cations in the solution. In a previous study [11], it was demonstrated that in this case, the ageing of the suspension led to the formation of magnetite. Moreover, it must be noted that the experimental conditions considered here correspond to an [OH] to [FeII+MgII+FeIII] ratio of 3 to 2. Reaction (4) requires an [OH] to [FeII+MgII+FeIII] ratio of 4 to 2. Consequently, both divalent and trivalent cations are in excess with respect to the OH− ions available. It can then be forwarded that the excess Fe(II) and Fe(III) cations react with water molecules to form a small proportion of magnetite, according to the following reaction:

$$\text{Fe}^{2+} + 2\text{Fe}^{3+} + 4\text{H}\_2\text{O} \rightarrow \text{Fe}^{\text{II}}\text{Fe}^{\text{III}}\text{\textsuperscript{2}\text{O}\_4 + 8\text{H}^+}\tag{5}$$

The present findings can be connected with more applied aspects of marine corrosion and cathodic protection of steel structures. Actually, the concentration of Mg2+ in seawater is important, about 0.053 mol/kg [10]. Therefore, the smallest [MgII]/[FeII] ratio of 1/3 considered here would correspond to a Fe2+ concentration of 0.16 mol/kg in the bulk seawater, which is rather high. However, GR(SO4 <sup>2</sup>−) is the main GR compound identified in the corrosion product layers formed on steel immersed in seawater [1,2,5–7]. At the vicinity of the steel/seawater interface, where the Fe2+ cations are produced, the [MgII]/[FeII] ratio is necessarily lower than in the bulk seawater and it can be forwarded that the formation of GR(SO4 <sup>2</sup>−) only takes place close to the steel surface.

Our results also explain more clearly why an anodic polarization favors the formation of GR(SO4 <sup>2</sup>−) with respect to any other Fe(II,III) mixed valence compounds [1,7]. An anodic polarization decreases the interfacial [MgII]/[FeII] ratio and thus prevents the influence of Mg2+ cations.

In contrast, pyroaurite, the Mg(II)-Fe(III) CO3-LDH was observed on a steel surface under cathodic protection [9]. In this case, due to the low dissolution rate of iron, the [MgII]/[FeII] ratio is necessarily higher, even at the steel/seawater interface. The increase of the interfacial pH associated with the cathodic polarization tends to favor GR(CO3 <sup>2</sup>−) with respect to GR(SO4 <sup>2</sup>−) [8] even if Mg2+ cations are not present. However, the formation of the Mg(II)-Fe(III) LDH rather than the Fe(II,III) LDH confirms that Mg(II) cations can favor the formation of LDH phases characterized by the GR1-structure, i.e., Cl-LDH and CO3-LDH.
