**1. Introduction**

Green rust compounds (GR) are common and important corrosion products of steel exposed to marine environments [1]. They are mixed valence Fe(II,III) hydroxysalts and a particular case of layered double hydroxide (LDH). LDH compounds can be based on various divalent and trivalent cations, for instance, Mg(II), Ni(II), Zn(II), Al(III), Cr(III), etc., and can incorporate various monovalent and divalent anions, e.g., Cl<sup>−</sup>, SO4<sup>2</sup><sup>−</sup>, and CO3<sup>2</sup><sup>−</sup>. Actually, when carbon steel is immersed in seawater, the sulfated green rust GR(SO4<sup>2</sup>−) with composition FeII4FeIII2(OH)12SO4·8H2O is the first corrosion product that forms [2]. As it contains mainly Fe(II) cations, GR(SO4<sup>2</sup>−) is readily oxidized by dissolved O2, a process that leads to Fe(III)-oxyhydroxides and/or magnetite (Fe3O4) [3,4]. This process explains why the corrosion product layer formed on carbon steel permanently immersed in seawater is mainly composed of (at least) two strata. First, an inner dark stratum is present at the metal surface. It contains the Fe(II)-based corrosion products (e.g., the sulfated green rust) forming from the dissolution of the metal. Second, an orange-brown outer stratum is present on top of the dark inner stratum. It contains mainly Fe(III)-oxyhydroxides resulting from the oxidation of Fe(II)-based corrosion products [2,5–7].

**Citation:** Refait, P.; Duboscq, J.; Aggoun, K.; Sabot, R.; Jeannin, M. Influence of Mg2+ Ions on the Formation of Green Rust Compounds in Simulated Marine Environments. *Corros. Mater. Degrad.* **2021**, *2*, 46–60. https://doi.org/10.3390/ cmd2010003

Received: 18 December 2020 Accepted: 26 January 2021 Published: 31 January 2021

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The formation of the carbonated green rust GR(CO3<sup>2</sup>−), i.e., FeII4FeIII2(OH)12CO3·2H2O, is favored when a cathodic polarization is applied to steel [8]. As a result, pyroaurite was observed at the surface of steel structures under cathodic protection [9]. This compound is similar to GR(CO3<sup>2</sup>−), with Mg2+ cations substituted for Fe2+ cations. The formation of pyroaurite is the consequence of the presence of Mg2+ ions in seawater ([Mg2+] ~ 0.053 mol/kg, the second most abundant cation after Na+ [10]). This finding suggests that, even at the open circuit potential (OCP), some Mg2+ cations could be incorporated in the crystal structure of green rust compounds, thus influencing more or less importantly the nature and properties of various components of the corrosion product layer. The main aim of the present study was to determine whether Mg2+ ions could indeed have an important role on the formation of GRs, and in particular GR(SO4<sup>2</sup>−), a question that has not ye<sup>t</sup> been addressed.

In the present study, GR(SO4<sup>2</sup>−) was formed by precipitation from dissolved Fe(II) and Fe(III) species, i.e., no metal (Fe0) was used. The GR was then prepared by mixing a solution of Fe(III), Fe(II), and/or Mg(II) salts (chlorides and/or sulfates) with a solution of NaOH. This precipitation reaction is assumed to mimic the process leading from the dissolved species produced by the corrosion of steel to the GR compound. It corresponds to the first step of the formation of the corrosion product layer that covers steel surfaces immersed in seawater [1,2]. The aim of the study was then to determine the effects of dissolved Mg(II) species on the precipitation reaction. For that purpose, Mg2+ cations were partially or totally substituted for Fe2+. The solid phases obtained for various Mg(II):Fe(II) substitution ratios were characterized by X-ray diffraction (XRD), immediately after precipitation or after one week of ageing. To simulate a marine environment, the overall chloride and sulfate concentrations were adjusted at values typical of seawater.

#### **2. Materials and Methods**

#### *2.1. Synthesis of (Fe,Mg)II-FeIII LDH*

Five precipitates, called M0-M4 were precipitated by mixing a solution (100 mL) of FeCl3·6H2O, FeCl2·4H2O and/or MgCl2·4H2O, NaCl and Na2SO4·10H2O with a solution (100 mL) of NaOH. All the chemicals had a purity higher or equal than 99%. The experiments were performed at room temperature (RT = 22 ± 1 ◦C).

The considered concentrations are given in Table 1. They are expressed with respect to the overall amount of the solution, i.e., 200 mL, and are based on previous work [11]. The overall chloride concentration is 0.55 mol/L, whereas the sulfate concentration is 0.03 mol L−1. They are both similar to the Cl− and SO4<sup>2</sup>− concentrations characteristic of seawater [10]. M0 is the reference experiment performed without Mg(II). M1-M3 are experiments performed with increasing Mg(II):Fe(II) concentration ratios, i.e., 1:3 for M1, 1:1 for M2, and 3:1 for M3. M4 is the experiment performed without Fe(II).


1 M4s: Specific experiment without Fe(II) and Cl<sup>−</sup>.

M4s is an additional experiment performed without Fe(II) and Cl− ions, i.e., using Mg(II) and Fe(III) sulfates and omitting NaCl.

The precipitation reaction of GR(SO4<sup>2</sup>−) can be written as follows:

$$4\text{Fe}^{2+} + 2\text{Fe}^{3+} + 12\text{OH}^- + \text{SO}\_4^{2-} + 8\text{H}\_2\text{O} \rightarrow \text{Fe}^{\text{II}}\_4 \cdot \text{Fe}^{\text{III}}\_2 \text{ (OH)}\_{12} \text{ SO}\_4 \cdot 8\text{H}\_2\text{O} \tag{1}$$

According to this reaction, stoichiometric conditions correspond to [FeII]/[OH−] = 1/3, [FeII]/[FeIII] = 2, and [FeII]/[SO42−] = 4. The experimental conditions considered to precipitate M0 correspond to [FeII]/[OH−] = 1/2, [FeII]/[FeIII] = 3, and [FeII]/[SO42−] = 4, i.e., to an excess of Fe(II) with respect to Fe(III) and OH<sup>−</sup>. As observed in [11], this situation leads to an excess of dissolved Fe(II) (and SO4<sup>2</sup>−) species in the solution and hinders the formation of magnetite Fe3O4. The precipitation reaction is then, for the experimental conditions considered in the present study (omitting Cl− and Na+ ions that do not participate in the reaction though present in the solution):

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

The suspensions were stirred for 1 min and aged 1 week at RT in a flask filled to the rim. The flask was then hermetically sealed to avoid any oxidation by air of the precipitates. The aged precipitates were finally filtered for analysis by XRD. They were sheltered from air with a plastic membrane during filtration to avoid the oxidation of the obtained GR compounds. The pH of the suspensions after ageing was measured close to neutrality (6.5 to 7.3). The pH of the solution has an influence on the evolution of the precipitate during the ageing procedure. The experimental conditions of the present study were chosen to avoid the transformation of GR to magnetite [11].

Additional experiments were performed similarly to analyze the unaged precipitate. In this case, the suspension was filtered immediately after the 1 min-stirring.
