J.-M. R. Génin

Mechanisms of formation and structure of green rust one in aqueous corrosion of iron in the presence of chloride ions

The crystal structure of the Fe(II)-Fe(III) hydroxychloride known as Green Rust one (GR) was investigated by X-ray diffraction and confirmed to be analogous with that of iowaite. It is a rhombohedral crystal [R3m, a=0.3190(1)nm, c=2.385(6)nm] consisting of Fe(OH)2 like-hydroxide sheets which alternate regularly with interlayers composed of Cl ions and H2O molecules and follow the stacking sequence AcBiBaCjCbAk..., where A, B, C are OH layers, a, b, c Fe layers and i, j, k interlayers. By means of transmission Mossbauer spectroscopy analyses at 20K of the solid phases formed during the oxidation of Fe(OH)2 into GR(Cl), it was demonstrated that the composition of the GR compound varied continuously from FeII3FeIII(OH)8Cl\nH2O, with n probably equal to 2, to approximately FeII2.2FeIII(OH)6.4Cl\nH2O. The formation of GR(Cl) involves an in situ incorporation of the Cl ions from the solution into the interlayers of GR(Cl) and a corresponding oxidation of Fe(II) to Fe(III) without any structural changes. This mechanism, which allows the oxidation of iron at the only cost of chloride intercalation, explained why GR formation would be favoured vs other possibilities such as direct ferric oxyhydroxide or magnetite formation. To confirm this assumption, in situ transmission Mossbauerspectroscopy analyses were performed on iron coupons polarised potentiostatically in KCl solutions of pH about 9. At a potential of 0.55VSHE a ferrous hydroxide layer is formed on the metal while at 0.35VSHE the corrosion product is mainly composed of GR.

The preparation and thermodynamic properties of Fe(II)-Fe(III) hydroxide-carbonate (green rust 1) ; Pourbaix diagram of iron in carbonate-containing aqueous media

Abstract Carbonate-containing green rust 1, GR1(CO32−), is prepared by oxidation of Fe(OH)2 in aqueous solution. Ferrous hydroxide is precipitated from NaOH and FeSO4·7H2O solutions and carbonate ions are added as a Na2CO3 solution. For sufficiently large concentrations of sodium carbonate, SO42− ions do not play any role during the oxidation process and, at the end of the first stage of reaction, Fe(OH)2 oxidizes into GR1(CO32−). In the second stage of reaction, GR1(CO32−) oxidizes into α-FeOOH goethite except when the transformation of ferrous hydroxide is partial, which leads to the formation of magnetite. From the X-ray diffraction analysis of GR1(CO32−), lattice parameters of its hexagonal cell are found to be a = 3.160 ± 0.005 A and c = 22.45 ± 0.05 A . From the Mossbauer analysis of the stoichiometric GR1(CO32−), which leads to a Fe2+:Fe3+ ratio of 2:1, the chemical formula is established to be: [Fe4(II)Fe2(III)(OH)12][CO3·2H2O]. The 78 K Mossbauer spectrum of the compound can be fitted with three quadrupole doublets, two Fe2+ doublets d1 and D2 corresponding to isomer shifts (IS) of 1.27 and 1.28 mm s−1 and quadrupole splittings (QS) of 2.93 and 2.67 mm s−1, respectively, and one Fe3+ doublet D3 with an IS of 0.47 mm s−1 and QS of 0.43 mm s−1. These three doublets were already used to fit the Mossbauer spectrum of chloride-containing GR1(Cl−) [see J.M.R. Genin et al., Mat. Sci. Forum8, 477 (1986) and J.M.R. Genin et al., Hyp. Int. 29, 1355 (1986)]and therefore are characteristic of GR1 compounds. From the recording of electrode potential E and the pH of the suspension versus time during the oxidation, the standard free enthalpy of formation of stoichiometric GR1(CO32−) is estimated to be ΔG °f = − 966.250 cal mol−1. Knowing the chemical formula and ΔG °f of GR1(CO32−) the Pourbaix diagram of iron in carbonate-containing aqueous solutions is drawn.

The oxidation of ferrous hydroxide in chloride-containing aqueous media and pourbaix diagrams of green rust one

The oxidation of a ferrous hydroxide precipitated by mixing solutions of ferrous chloride and caustic soda depends on the ratio R′ = Cl−/OH− of the initial concentrations of the reactants. The mechanisms of oxidation for 1 < R′ ≤ 3 are related to the existence of the ferrous-ferric compound green rust 1 (GR1). The direct recording of the pH and the electrode potential allows the trigger value of R′c = 87 to be determined, which is found to correspond to the stoichiometric conditions of obtention of GR1. By coupling this information with that obtained from the three iron sites R1, R2, R3 detected by Mossbauer spectroscopy, the formula of GR1 is set up at 3Fe(OH)2 · Fe(OH)2Cl · nH2O. Thus GR1 contains an equal number of Cl− and Fe3+ ions. The standard chemical potential μ0 is determined to be −509,500 ± 500 cal mol−1 for n = 0. Eh/pH Pourbaix diagrams specific to the oxidation of iron in chloride-containing aqueous media, including GR1, are drawn.

Surface chemistry and structural properties of mackinawite prepared by reaction of sulfide ions with metallic iron

Tetragonal FeS1−x mackinawite, has been synthesized by reacting metallic iron with a sodium sulfide solution and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), transmission Mossbauer spectroscopy (TMS) and X-ray photoelectron spectroscopy (XPS). Based on XRD and TEM analyses, synthetic mackinawite exhibits crystallization and is identical to the natural mineral. Unit cell parameters derived from XRD data are a = b = 0.3670 nm and c = 0.5049 nm. The bulk Fe:S ratio derived from the quantitative dispersive energy analysis is practically 1. XPS analyses, however, showed that mackinawite surface is composed of both Fe(II) and Fe(III) species bound to monosulfide. Accordingly, monosulfide is the dominant S species observed at the surface with lesser amount of polysulfides and elemental sulfur. TMS analysis revealed the presence of both Fe(II) and Fe(III) in the mackinawite structure, thus supporting the XPS analysis. We propose that the iron monosulfide phase synthesized by reacting metallic iron and dissolved sulfide is composed of Fe(II) and S(-II) atoms with the presence of a weathered thin layer covering the bulk material that consists of both Fe(II) and Fe(III) bound to S(-II) atoms and in a less extent of polysulfide and elemental sulfur.

Nomenclature of the hydrotalcite supergroup: natural layered double hydroxides

Abstract Layered double hydroxide (LDH) compounds are characterized by structures in which layers with a brucite-like structure carry a net positive charge, usually due to the partial substitution of trivalent octahedrally coordinated cations for divalent cations, giving a general layer formula [(M1-x2+Mx3+)(OH)2]x+. This positive charge is balanced by anions which are intercalated between the layers. Intercalated molecular water typically provides hydrogen bonding between the brucite layers. In addition to synthetic compounds, some of which have significant industrial applications, more than 40 mineral species conform to this description. Hydrotalcite, Mg6Al2(OH)16[CO3]·4H2O, as the longest-known example, is the archetype of this supergroup of minerals. We review the history, chemistry, crystal structure, polytypic variation and status of all hydrotalcite-supergroup species reported to date. The dominant divalent cations, M2+, that have been reported in hydrotalcite supergroup minerals are Mg, Ca, Mn, Fe, Ni, Cu and Zn; the dominant trivalent cations, M3+, are Al, Mn, Fe, Co and Ni. The most common intercalated anions are (CO3)2-, (SO4)2- and Cl-; and OH-, S2- and [Sb(OH)6]- have also been reported. Some species contain intercalated cationic or neutral complexes such as [Na(H2O)6]+ or [MgSO4]0. We define eight groups within the supergroup on the basis of a combination of criteria. These are (1) the hydrotalcite group, with M2+:M3+ = 3:1 (layer spacing ~7.8 Å); (2) the quintinite group, with M2+:M3+ = 2:1 (layer spacing ~7.8 Å); (3) the fougèrite group, with M2+ = Fe2+, M3+ = Fe3+ in a range of ratios, and with O2- replacing OH- in the brucite module to maintain charge balance (layer spacing ~7.8 Å); (4) the woodwardite group, with variable M2+:M3+ and interlayer [SO4]2-, leading to an expanded layer spacing of ~8.9 Å; (5) the cualstibite group, with interlayer [Sb(OH)6]- and a layer spacing of ~9.7 Å; (6) the glaucocerinite group, with interlayer [SO4]2- as in the woodwardite group, and with additional interlayer H2O molecules that further expand the layer spacing to ~11 Å; (7) the wermlandite group, with a layer spacing of ~11 Å, in which cationic complexes occur with anions between the brucite-like layers; and (8) the hydrocalumite group, with M2+ = Ca2+ and M3+ = Al, which contains brucite-like layers in which the Ca:Al ratio is 2:1 and the large cation, Ca2+, is coordinated to a seventh ligand of ‘interlayer’ water. The principal mineral status changes are as follows. (1) The names manasseite, sjögrenite and barbertonite are discredited; these minerals are the 2H polytypes of hydrotalcite, pyroaurite and stichtite, respectively. Cyanophyllite is discredited as it is the 1M polytype of cualstibite. (2) The mineral formerly described as fougèrite has been found to be an intimate intergrowth of two phases with distinct Fe2+:Fe3+ ratios. The phase with Fe2+:Fe3+ = 2:1 retains the name fougèrite; that with Fe2+:Fe3+ = 1:2 is defined as the new species trébeurdenite. (3) The new minerals omsite (IMA2012-025), Ni2Fe3+(OH)6[Sb(OH)6], and mössbauerite (IMA2012-049), Fe63+O4(OH)8[CO3]·3H2O, which are both in the hydrotalcite supergroup are included in the discussion. (4) Jamborite, carrboydite, zincaluminite, motukoreaite, natroglaucocerinite, brugnatellite and muskoxite are identified as questionable species which need further investigation in order to verify their structure and composition. (5) The ranges of compositions currently ascribed to motukoreaite and muskoxite may each represent more than one species. The same applies to the approved species hydrowoodwardite and hydrocalumite. (6) Several unnamed minerals have been reported which are likely to represent additional species within the supergroup. This report has been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association, voting proposal 12-B. We also propose a compact notation for identifying synthetic LDH phases, for use by chemists as a preferred alternative to the current widespread misuse of mineral names.

The mechanisms of oxidation of ferrous hydroxychloride β-Fe2(OH)3Cl in aqueous solution: The formation of akaganeite vs goethite

Abstract β-Fe2(OH)3Cl is precipitated by mixing FeCl2 · 4H2O and NaOH aqueous solutions with a large excess of ferrous chloride. This excess is determined by the experimental ratio R′ of the initial concentrations of reactants (R′ = [Cl−]/[OH−]or 2 × [FeCl2 · 4H2O]/[NaOH]with [NaOH]= 0.4 mol l−1) which varies from 5 to 9. Whatever the value of R′, the oxidation of β-Fe2(OH)3Cl first gives rise to the standard chloride-containing green rust one GR1(Cl−) with formula [FeII3FeIII(OH)8]+ [Cl · nH2O]−. Then, due to the large chloride concentration of the solution (from 2 mol l−1 to 3.6 mol l−1), the oxidation of GR1(Cl−), which usually leads to γ-FeOOH lepidocrocite, produces an over-chlorinated GR1 compound, GR1(C1−)∗, with the general composition of [FeII3 − xFeIII1 + x(OH)8](1 + x)+ [Cl1 + x · (n − y)H2O](1 + x)−. The excess x of intercalated Cl− ions increases with R′ i.e. with the chloride concentration, up to a maximum around 1 3 . Moreover, the oxidation process of these over-chlorinated GR1(C1−)∗ compounds changes with R′. It produces α-FeOOH goethite when R′ ⪯ 6.0 but the formation of this compound is not connected to the modifications occurring in the precursor GR1 compound since it is induced by a dissolved Fe(II) species, the complex FeCl2aq. In contrast, the formation of akaganeite, which is obtained along with goethite when R′ ≥ 6.5 and alone when R′ ≥ 8.0, is to be correlated to the increase of the chloride-content of the GR1(C1−1)∗ compound. Finally, the measurements of the electrode potential and pH of the solution at the equilibrium conditions between GR1(C1−) and β-Fe2(OH)3Cl allow to estimate the standard chemical potential of the ferrous hydroxychloride at μ0[β-Fe2(OH)3Cl] = − 219 600 cal mol−1.

The mechanism of oxidation of ferrous hydroxide in sulphated aqueous media: Importance of the initial ratio of the reactants☆

Abstract In simulated aqueous corrosion processes of iron in sulphated media, the most important factor is the ratio, R, of the initial concentration of Fe2+ and SO42− to OH− ions which separates the basic and acidic media at a value of 1 2 . Mossbauer spectroscopy, coupled with direct recording of the pH and the electrode potential, has identified six characteristic values of R and a basic sulphate, named sulphated ferrous hydroxide (SFH) and also determined the chemical formula of green rust 2 (GR2). Apart from GR2, two other ferrous-ferric intermediate compounds, a basic compound and a hydrated magnetite, the oxidation of which leads to delta FeOOH and magnetite respectively, are suspected. The various mechanisms of oxidation are established based on the rigorous analysis of the evolution of the composition of the end products of oxidation with respect to R.

Effect of orthophosphate on the oxidation products of Fe(II)-Fe(III) hydroxycarbonate: the transformation of green rust to ferrihydrite

Abstract Hydroxycarbonate green rust GR(CO32−) has been synthesized by oxidation of aqueous suspensions of Fe(OH)2 by aeration at the air–liquid interface, in the presence of HCO3− ions at pH 7.5 to 9. During the oxidation of GR(CO32−), ferrihydrite formed first and then turned into goethite by dissolution and precipitation. The oxidation of GR(CO32−) in the presence of orthophosphate ions, which were added as Na2HPO4 · 7H2O salt, also involved the formation of ferrihydrite but not that of goethite, because the dissolution of ferrihydrite is inhibited by the adsorption of phosphate ions on its surface. The oxidation was slowed down because of the suppression of the catalytic effect of iron(III) hydroxide on the oxidation of Fe(II). In anoxic conditions without phosphate, a mixture of GR(CO32−), goethite, and ferrihydrite was observed to transform spontaneously into a mixture of siderite and magnetite. It is thermodynamically consistent, which shows that GR(CO32−) is metastable with respect to the two-phase system FeCO3–Fe3O4. In the presence of phosphate, this transformation was inhibited and GR(CO32−) did not transform in anoxic conditions. Anionic phosphate species dissolved in solution did not give rise to a corresponding GR, i.e., phosphate species did not substitute for carbonate inside the interlayers of the GR. Moreover, iron phosphates did not appear, neither during the oxidation of GR(CO32−) in the presence of oxygen nor in anoxic conditions.

Formation of the Fe(II)-Fe(III) hydroxysulphate green rust during marine corrosion of steel

Rust layers formed on steel sheet piles immersed 1 m above the mud line for 25 years were analysed by Raman spectroscopy, scanning electron microscopy and elemental X-ray mappings (Fe, S, O). They consist of three main strata, the inner one mainly composed of magnetite, the intermediate one of iron(III) oxyhydroxides and the outer one of hydroxysulphate green rust GR(SO42−). Simulations of GRs formation in solutions having large [Cl−]/[SO42−] ratios revealed that the hydroxysulphate GR(SO42−) was obtained instead of the hydroxychloride GR(Cl−), as demonstrated by X-ray diffraction and transmission Mossbauer spectroscopy analyses. Measurements of the [S], [Fe] and [Cl] concentrations allowed us to establish that GR(SO42−) formed along with a drastic impoverishment of the solution in sulphate ions; the [Cl−]/[SO42−] ratio increased from 12 to 240. The GR, acting like a “sulphate pump”, may favour the colonisation of the rust layers by sulphate reducing bacteria.

Conversion electron Mössbauer spectroscopy and X-ray diffraction studies of the formation of carbonate-containing green rust one by corrosion of metallic iron in NaHCO3 and (NaHCO3 + NaCl) solutions

Abstract The corrosion of α-iron in 0.1 mol 1−1 NaHCO3 solutions, with or without additional NaCl, leads to the formation of a deep-green homogeneous layer which covers the metallic surface. It is analysed by X-ray diffraction (XRD) and conversion electron Mossbauer spectroscopy (CEMS) and proves to be made of carbonate-containing green rust one, GR1(CO32−), an Fe(II)-Fe(III) hydroxide-carbonate with chemical formula [Fe4 staggered(II)Fe2 staggered(III)(OH)12][CO3·2H2O]. If left in solution, the green rust layer oxidizes into α-FeOOH goethite. The corrosion process is: Fe → GR1(CO32−) → α-FeOOH, without previous formation of ferrous hydroxide, as expected from the Pourbaix diagram of iron in carbonate-containing aqueous media. If removed from the solution and oxidized in the air, the green rust layer oxidizes into a mixture of ferrihydrite or δ-FeOOH, i.e. poorly crystallized hydrated ferric oxide, and of a compound that could be called ferric green rust which keeps, in spite of the oxidation of the Fe(II) ions, the original stacking sequence. Iron samples corroded in (0.1 mol1−1 NaHCO3 + 4 mol1−1 NaCl) solutions are also covered with carbonate-containing green rust one layers and the chloride-containing green rust one, [Fe3 staggered(II)Fe(III)(OH)8][Cl·nH2O]is not observed even though the ClHCO3-ratio of the solution is as large as 40:1.