Ph. Refait

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.

Coprecipitation thermodynamics of iron(II–III) hydroxysulphate green rust from Fe(II) and Fe(III) salts

Iron(II–III) hydroxysulphate GR(SO42−) was prepared by precipitating a mixture of Fe(II) and Fe(III) sulphate solutions with NaOH, accompanied in most cases by iron(II) hydroxide, spinel iron oxide(s) or goethite. Its [Fe(II)]/[Fe(III)] ratio determined by transmission Mossbauer spectroscopy was 2±0.2, whatever the initial [Fe(II)]/[Fe(III)] ratio in solution. Proportion of Fe(OH)2 increased when the initial [Fe(II)]/[Fe(III)] ratio increased, whereas proportion of α-FeOOH or spinel oxide(s) increased when this ratio decreased. GR(SO42−) is metastable vs. Fe3O4 except in a limited domain around neutral pH. Precipitation from solutions containing both Fe(II) and Fe(III) dissolved species seems to favour GRs formation with respect to stable systems involving iron (oxyhydr)oxides.

Electrodeposition of Zn–Mn alloys on steel from acidic Zn–Mn chloride solutions

Abstract The potentiostatic electrodeposition of zinc–manganese alloys on steel from MnCl 2 and ZnCl 2 solutions, was studied in an aqueous KCl+H 3 BO 3 matrix. Cyclic voltammetry was used to determine the potential ranges where the redox processes of interest were taking place. The obtained Zn and Mn containing films were investigated by means of scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction. The effects of potential deposition and stirring were studied. At E =−1.65 V/SCE without stirring, it proved possible to obtain Zn–Mn deposits constituted of a single e phase, with approximately 13 at.% Mn. Decreasing the potential or agitating the electrolyte reduced the Mn proportion in the films. At 10.3 at.% Mn, the film obtained is composed of HCP Zn–Mn e and monoclinic ζ (MnZn 13 ) phases. At 3.4% Mn, the film consists of three phases, Zn along with the two previous Zn–Mn phases. All films have a cauliflower morphology and appear to be compact and homogeneous except for the films with the largest Mn contents (at.% Mn>11) where columnar deposits are obtained. Polyethylene glycol was tested as a potential additive. Its main effects were a decrease in the Mn content and inhibition of the ζ phase formation.

The behaviour of phosphate ions as corrosion inhibitor for Fe(II)-Fe(III) hydroxycarbonate green rust

The oxidation of Fe(II)-Fe(III) hydroxycarbonate green rust GR(CO 3 2− ) in the presence of phosphate ions known as corrosion inhibitor is monitored by transmission Mossbauer spectroscopy. In the absence of phosphate, the first product of oxidation, previously called “amorphous active FeOOH” was identified as ferrihydrite which transformed by dissolution to goethite. The oxidation of GR(CO 3 2− ) in the presence of phosphate ions, which were added as hydrated phosphate salt, also involved the formation of ferrihydrite but not that of goethite. The dissolution of ferrihydrite is inhibited by the adsorption of phosphate ions as confirmed by X-ray photoelectron spectroscopy. The oxidation was slowed down and interpreted as due to the suppression of the catalytic effect of iron(III) hydroxide upon the oxidation of Fe(II).

Mechanisms of Formation and Transformation of Ni-Fe Hydroxycarbonates

The mechanisms of the transformation of (Ni,Fe)(OH)2 precipitates in carbonated aqueous solutions were studied. The reactions were monitored by measuring the redox potential of the aqueous suspension, and end products were studied by Mossbauer spectroscopy, X‐ray diffraction and Raman spectroscopy. The oxidation processes were compared to those occurring without Ni, that is when the initial hydroxide is Fe(OH)2. Schematically, the oxidation of Fe(OH)2 involves two intermediate compounds, the carbonated GR of formula Fe4IIFe2III(OH)12CO3 ⋅ 2H2O, and ferrihydrite, before to lead finally to goethite α‐FeOOH. It proved possible to prepare Ni(II)‐Fe(III) hydroxycarbonates with ratios Fe/Ni from 1/6 to 1/3. When the Fe/Ni ratio is larger than 1/3, a two stage oxidation process takes place. The first stage leads to a Ni(II)‐Fe(II)‐Fe(III) hydroxycarbonate. The second stage corresponds to the oxidation of the Fe(II) remaining inside the hydroxycarbonate and leads to a mixture of Ni(II)‐Fe(III) hydroxycarbonate wi...

Synthesis and Reactivity of Mg(II)-Fe(II)-Fe(III) Hydroxycarbonates

Green rust-like compounds (GRs) were discovered as natural minerals in various hydromorphic soils, where anoxic conditions allow their stability. They may control some redox processes in aquifers and participate to the transformation of various pollutants. Since Mg(II) cations are present in the fields where GRs were discovered, a partial substitution of Mg(II) to Fe(II) leading to intermediate compounds between GRs and usual Mg(II)-Fe(III) hydroxysalts is suspected. Mg(II)-Fe(II)-Fe(II) hydroxycarbonates can be obtained as intermediate oxidation products of (Mg, Fe)(OH)2 in carbonate-containing aqueous media obeying to [Fe 4(1−x) II Mg 4x II Fe 2 III (OH)12]2+ · [CO 3 2− · nH2O]−2. TMS spectra at 12 K are similar to those of GRs, i.e., two quadrupole doublets, one due to Fe(II) with a large isomer shift δ = 1.29 mms−1 (with respect to α-iron at room temperature) and quadrupole splitting ΔE Q = 2.76 mms−1, the other one due to Fe(III) with smaller hyperfine parameters δ = 0.49 mms−1 and ΔE Q = 0.44 mms−1. Fe(II) ions oxidise rapidly into Fe(III) with dissolved O2. The reactivity is similar to that of Fe(II)-Fe(III) hydroxysalts GR, and thus the potential of Mg(II)-Fe(II)-Fe(III) compounds for reducing pollutants.