Formation, fast oxidation and thermodynamic data of Fe(II) hydroxychlorides

Iron(II) hydroxide and hydroxychloride precipitates were obtained by mixing FeCl₂ · 4H₂O and NaOH aqueous solutions with various concentration ratios R′ = [Cl⁻]/[OH⁻] = 2 [FeCl₂]/[NaOH] at [NaOH] = 0.4 mol L⁻¹. They were analysed by Infrared spectroscopy after 24 h of ageing at room temperature. Fe(OH)₂ was obtained alone only for the smallest values of R′, typically R′ ? 1.16. β-Fe₂(OH)₃Cl formed as soon as R′ ? 1.40 and was obtained alone for R′ ? 2.25. The initial precipitates were oxidised by addition of a small amount of hydrogen peroxide (5 mL of an aqueous solution containing approximately 30 vol% H₂O₂) instead of O₂. The action of H₂O₂ on Fe(OH)₂ gave rise to δ-FeOOH as already reported. Its action on Fe(II) hydroxychlorides gave rise to akaganéite β-FeO_1−2x(OH)_1+xCl_x. A transformation of the two-phase system found at R′ = 1.5 after long ageing times (6 months) was observed and β-Fe₂(OH)₃Cl remained alone. This slow transformation of Fe(OH)₂ into β-Fe₂(OH)₃Cl may explain why β-Fe₂(OH)₃Cl was only reported as a corrosion product on iron archaeological artefacts. Finally, the respective domains of stability of Fe(OH)₂ and β-Fe₂(OH)₃Cl were demarcated and an estimation of the standard Gibbs free energy of formation of β-Fe₂(OH)₃Cl could be given: .     

Mechanisms of long-term anaerobic corrosion of iron archaeological artefacts in seawater

An iron ingot immersed during 2000 years at 12 m depth in the sea has been examined with the help of a combination of microscale techniques. This methodology allowed us to show that the main phase precipitated during the immersion is an iron hydroxychloride (β-Fe₂(OH)₃Cl) that is characteristic of corrosion in anoxic and chlorinated medium. Moreover locally on the external part of the corrosion products sulphur containing phases have been identified as mackinawite (FeS) in nanocrystalline or slightly oxidised state. The presence of this phase could be explained by the activity of sulphate-reducing bacteria. The presence of β-Fe₂(OH)₃Cl could be interpreted via a thermodynamic modelling taking into account the environmental conditions.     

Buried iron archaeological artefacts: Corrosion mechanisms related to the presence of Cl-containing phases

Deterioration after excavation of archaeological iron artefacts buried in soil is often associated with the presence of Cl-containing phases in corrosion products, leading to serious problems for conservation of metallic objects of cultural heritage. Thus, in order to better understand the corrosion process related to the presence of chlorine, some high-resolution techniques of materials characterisation are implemented. Particularly this paper shows the great utility of the combination of micro-X-ray diffraction, micro-Raman spectroscopy and micro-X-ray absorption spectroscopy. The analyses are realised on the cross sections of iron corroded objects excavated on archaeological sites dated from the 12th to the 16th century AD. In addition to the common oxyhydroxide containing chloride, akaganeite (β-FeOOH) often mentioned in the literature, a ferrous hydroxychloride β-Fe₂(OH)₃Cl was also found in the corrosion layers. In order to explain the corrosion system formed during burial, a corrosion mechanism including the presence of chlorine, is proposed.     

Anticorrosion properties of polyaniline-coated pigments in organic coatings

Four pigments of various morphology – specularite, α-Fe₂O₃; goethite, α-FeO(OH); talc, (Mg₃(OH)₂(Si₄O₁₀); and graphite, C – without and with polyaniline phosphate coating, were tested for the anticorrosion performance in coatings produced by epoxy binders on iron plates. The corrosion tests were carried out in a condenser chamber with continuous water condensation or humidity with sulfur dioxide, and in a salt mist cabinet. Polyaniline coating of pigments in all cases improved the anticorrosion properties. Graphite coated with polyaniline performed the best among eight systems under investigation. The role of graphite and polyaniline conductivity in the electron transfers associated with corrosion of iron has been proposed.     

Atmospheric corrosion of Galfan coatings on steel in chloride-rich environments

Galfan coatings on steel in laboratory exposures with predeposited NaCl and cyclic wet/dry conditions exhibit nearly the same corrosion products as after 5 years of marine exposure. A general scenario for corrosion product evolution on Galfan in chloride-rich atmospheres is proposed. It includes the initial formation of ZnO, ZnAl₂O₄ and Al₂O₃ and subsequent formation of Zn₆Al₂(OH)₁₆CO₃⋅4H₂O, and Zn₂Al(OH)₆Cl⋅2H₂O and/or Zn₅Cl₂(OH)₈⋅H₂O. An important phase is Zn₆Al₂(OH)₁₆CO₃⋅4H₂O, which largely governs the reduced long-term zinc runoff from Galfan. A clear influence of microstructure could be observed on corrosion initiation in the slightly zinc-richer η-Zn phase adjacent to the β-Al phase.     

X-ray diffraction phase analysis of crystalline copper corrosion products after treatment in different chloride solutions

The corrosion products Cu₂(OH)₃Cl, Cu₂O, and CuCl₂ were identified on the surface of copper plates after their four days treating in three different sodium chloride, sodium/magnesium, and sodium/calcium chloride solutions using X-ray diffraction powder analysis. However, the quantitative proportions of individual corrosion products differ and depend on the type of chloride solution used. Treating of copper plates only in the sodium chloride solution produced the mixture of corrosion products where Cu₂O is prevailing over the Cu₂(OH)₃Cl and CuCl₂ was not identified. The sample developed after treating of the cooper surface in the sodium/magnesium chloride solution contains Cu₂(OH)₃Cl and CuCl₂ prevailing over the Cu₂O, while the sample developed after treatment of copper in sodium/calcium chloride solution contains Cu₂(OH)₃Cl prevailing over CuCl₂ and Cu₂O was not identified.     

Corrosion product formation on Zn55Al coated steel upon exposure in a marine atmosphere

The formation of corrosion products on Zn55Al coated steel has been investigated upon field exposures in a marine environment. The corrosion products consisted mainly of zinc aluminium hydroxy carbonate, Zn_0.71Al_0.29(OH)₂(CO₃)_0.145·xH₂O, zinc chloro sulfate (NaZn₄(SO₄)Cl(OH)₆·6H₂O), zinc hydroxy chloride, Zn₅(OH)₈Cl₂·H₂O and zinc hydroxy carbonate, Zn₅(OH)₆(CO₃)₂ were the first three phases were formed initially while zinc hydroxy carbonate Zn₅(OH)₆(CO₃)₂ was formed after prolonged exposure in more corrosive conditions. The initial corrosion product formation was due to selective corrosion of the zinc rich interdendritic areas of the coating resulting in a mixture of zinc and zinc aluminium corrosion products.     

Mechanistic studies of corrosion product flaking on copper and copper-based alloys in marine environments

The mechanism of corrosion product flaking on bare copper sheet and three copper-based alloys in chloride rich environments has been explored through field and laboratory exposures. The tendency for flaking is much more pronounced on Cu and Cu–4 wt%Sn than on Cu–15 wt%Zn and Cu–5 wt%Al–5 wt%Zn. This difference is explained by the initial formation of zinc and zinc–aluminum hydroxycarbonates on Cu15Zn and Cu5Al5Zn, which delays the formation of CuCl, a precursor of Cu₂(OH)₃Cl. As a result, the observed volume expansion during transformation of CuCl to Cu₂(OH)₃Cl, and concomitant corrosion product flaking, is less severe on Cu15Zn and Cu5Al5Zn than on Cu and Cu4Sn.     

Effect of SO<Subscript>4</Subscript> <Superscript>2-</Superscript>, Cl<Superscript>–</Superscript> and NO<Subscript>3</Subscript> <Superscript>-</Superscript> anions on the formation of iron oxide nanoparticles via microwave synthesis

In the present work iron oxide nanoparticles have been prepared by microwave assisted synthesis with the influence of different precursor salts and synthesis of magnetite, hematite, Iron oxide hydroxide and maghemite nanoparticles. Synthesized iron oxide nanoparticles were characterized with Infrared Spectroscopy (FTIR), Field Emission Scanning Electron Microscopy (FESEM), and Energy-dispersive X-ray Spectroscopy (EDX). XRD measurements show that the peaks of diffractogram are in agreement with the theoretical data of magnetite, hematite, FeO(OH) (Iron oxide hydroxide) and maghemite. Crystallite size of the particles was found to be 33, 45, 36 and 43.5 nm for Fe₃O₄, α-Fe₂O₃, FeO(OH) and γ-Fe₂O₃. FESEM studies indicated that size of the particles is observed in the range of about 19.4 to 46.7 nm (Fig. 2a, average 32 nm), 29.1 to 67.6 nm (Fig. 2b average 45 nm), 29.1 to 40.8 (Fig. 2c average 36.6 nm), 29.1 to 80 nm (Fig. 2d average 43.5) for Fe₃O₄, α-Fe₂O₃, FeO(OH) and γ-Fe₂O₃ respectively. EDX spectral analysis reveals the presence of carbon, oxygen, iron in the synthesized nanoparticles. The FTIR graphs indicated absorption bands due to O–H stretching, C–O bending, C–H stretching and Fe–O stretching vibrations.     

The effect of synthetic zinc corrosion products on corrosion of electrogalvanized steel. II. Zinc reactivity and galvanic coupling zinc/steel in presence of zinc corrosion products

The reactivity of zinc under synthetic zinc patinas and the galvanic coupling in steel/patina/Zn are studied. Zn₅(OH)₆(CO₃)₂ and Na₂Zn₃(CO₃)₄⋅3H₂O inhibit zinc anodic dissolution in NaCl, while Zn₅(OH)₈Cl₂ H₂O and Zn₄(OH)₆SO₄ nH₂O do not. The galvanic current in steel/patina/NaCl/Zn is smaller as compared to steel/NaCl/Zn. The inhibiting effect decreases with time for Na₂Zn₃(CO₃)₄⋅3H₂O or Zn₄(OH)₆SO₄ nH₂O due to the transformation into Zn(OH)₂. In NaHCO₃, the polarity between zinc and steel can reverse. The effect of confinement on the cathodic current is stronger than the initial effect of patina which is explained by the instability of the patinas under rapid pH-increase.     

Characteristics of iron corrosion scales established under blending of ground, surface, and saline waters and their impacts on iron release in the pipe distribution system

Interior scales on PVC, lined ductile iron (LDI), unlined cast iron (UCI) and galvanized steel (G) were analyzed by XRD, RMS, and XPS after contact with varying water quality for 1 year. FeCO₃, α-FeOOH, β-FeOOH, γ-Fe₂O₃, Fe₃O₄ were identified as primary UCI corrosion products. No FeCO₃ was found on G. The order of Fe release was UCI > G ? LDI > PVC. For UCI, Fe release decreased as % Fe₃O₄ increased and as % Fe₂O₃ decreased in scale. Soluble Fe and FeCO₃ transformation indicated FeCO₃ solid was controlling Fe release. FeCO₃ model and pilot data showed Fe increased as alkalinity and pH decreased.     

Identification of corrosion products resulting from accelerated oxidation process

Abstract

Scanning electron microscopy analysis, X-ray powder diffraction and room temperature ⁵⁷Fe Mössbauer spectroscopy were used to identify the corrosion products of uncoated and coated low alloy steels (LAS) and low carbon steels (LCS) resulting from an accelerated steam oxidation test for 180 h at 660°C. From the Mössbauer spectral analysis, it was shown that in all cases, a series of iron compounds such as α-Fe₂O₃, Fe₃O₄, γ-Fe₂O₃, δ-FeOOH, α-FeOOH, Fe(OH)₂ and Fe(OH)₃ were formed, while XRD measurements revealed only the α-Fe₂O₃ and/or Fe₃O₄/γ-Fe₂O₃ phases. In the LAS uncoated sample, an amorphous phase with magnetic features is found. In the spectra of the borided samples and of the uncoated LCS, an additional doublet was observed, which reveals the presence of a superparamagnetic phase. From the relative areas of the subspectra, it is concluded that the boron aluminised sample underwent the lowest degradation. The mechanism proposed for corrosion products formation is based on the dissociation process.     

Effects of DBU, morpholine, and DMA on corrosion of low carbon steel exposed to steam

Effects of morpholine, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and dimethylamine (DMA) on oxidation kinetics and oxide phase formation/transformation of AISI 1018 steel at 120 °C were evaluated. Low carbon steel samples were exposed to steam in an autoclave containing amine added aqueous solution at pH of 9.5 for 1, 2, 4, 6, 8, and 12 h. Control samples exposed to plain steam and amines showed the highest and lowest weight loss respectively. Fourier Transform Infrared Spectrophotometry (FTIR) showed that DBU containing steam favored formation of magnetite (Fe₃O₄) while steam with DMA formed more α and γ-FeOOH. Transformation of magnetite to hematite (α-Fe₂O₃) was fastest for morpholine. Analysis of oxides morphology was done utilizing Scanning Electron Microscopy (SEM). Oxides formed in plain or DMA containing steam exhibited acicular particles of goethite/hematite (α-FeOOH/α-Fe₂O₃) compared to DBU containing steam that showed equiaxed particles of magnetite/maghemite (Fe₃O₄/γ-Fe₂O₃). Morpholine containing steam promoted agglomeration of thin sharp platelets into coarse flakes of hematite.     

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 Mössbauer 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.     

Mechanism and kinetics of iron extraction from high silica boehmite–kaolinite bauxite by hydrochloric acid leaching

The chemical and mineral compositions of bauxite recovered from the Severoonezhsk Bauxite Mine (Arkhangelsk region, Russia) were studied by XRD, ICP-OES, TG/DSC, SEM, TEM, and Mössbauer spectroscopy. The iron-containing minerals of the bauxites were found to comprise alumogoethite (α-Fe_1–xAl_xOOH), alumohematite (α-(Fe_1–xAl_x)₂O₃), alumoakaganeite (β-Fe_1–xAl_xO(OH,Cl)), and chromite (FeCr₂O₄). The efficiency of Fe extraction from the bauxite by HCl leaching was 82.5% at 100 °C, HCl concentration of 10%, solid/liquid ratio of 1:10, and the process duration of 60 min, with aluminum loss from the bauxites below 4.5% of the total Al contents in the bauxite. Analysis of the kinetics of the iron leaching process proved diffusion to be the limiting stage of the process at 90–100 °C. Bauxite residue after leaching presented traces of α-Fe_1–xAl_xOOH and β-Fe_1–xAl_xO(OH,Cl), and most of the iron content was in the FeCr₂O₄. In bauxite residue after HCl leaching, in addition to iron oxide, the contents of chromium and calcium oxides significantly decreased. The iron chloride liquor after leaching contained the rare earth elements (REE) of 6.8 mg/L Sc, 4.1 mg/L Ce and 2.3 mg/L Ga.     

On the mechanism of formation of zinc pack coatings

The mechanism of the formation of zinc (Zn) pack coatings is studied with scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). DSC showed that the coatings formation takes place in three steps. The initial step (at 193.9 °C) is endothermic and involves the transformation of α-NH₄Cl to β-NH₄Cl and the NH₄Cl decomposition to NH₃ and HCl. During the second step (at 248.6 °C), which is exothermic, Zn²⁺ salts are formed and especially ZnCl₂. Finally at 264.1 °C Zn is deposited by an endothermic reaction on the ferrous substrate through the decomposition of ZnCl₂. The as-cast Zn diffuses into the iron lattice forming the gamma (Γ-Fe₁₁Zn₄₀) and delta (δ-FeZn₁₀) phases. Al₂O₃ is not involved in the above-mentioned mechanism and acts only as filler.     

Morphology and phase composition of corrosion products formed at the zinc-iron interface of a galvanized steel

The corrosion products formed on the inner wall of pipes made of galvanized low carbon steel, exposed for ∼2 years to water flowing in a large household heating system, were analysed using X-ray diffraction, Mössbauer and Raman spectroscopic techniques, as well as metallographic techniques. Products grew in the form of large-sized tubercles that gradually developed causing base metal losses up to perforation of the steel pipe. Considerable differences in the phase composition were found between the products formed in contact with the steel and those constituting the outer part of tubercles. The former were mainly made of FeCO₃ (siderite), with small amounts of Zn₅(CO₃)₂(OH)₆ (hydrozincite), ZnCO₃ (smithsonite), (Fe,Zn)CO₃ mixed carbonate and CaCO₃ (calcite), the latter mainly by Fe(III) oxyhydroxide goethite. Both parts of the tubercles also contained small amounts of other ferric oxyhydroxides, γ-FeOOH (lepidocrocite) and β-FeOOH (akaganeite), and very small amounts of hematite. The procedures used proved effective for an adequate identification of both the iron-containing and iron-free compounds in the corrosion products as well as for suggesting a corrosion mechanism.     

Properties of Co-doped N-type FeSi2 processed by mechanical alloying

Iron-silicide was produced with a mechanical alloying process and consolidated through vacuum hot pressing. The as-milled powders were of metastable state and fully transformed into the ß-FeSi₂ phase through subsequent isothermal annealing. The as-consolidated iron silicides consisted of an untransformed mixture of α-Fe₂Si₅ and ?-FeSi phases and a partially transformed β-FeSi₂ phase was found in the low density compact. Isothermal annealing was carried out to induce transformation into a thermoelectric semiconducting β-FeSi₂ phase. The transformation behavior of the β-FeSi₂ was investigated utilizing DTA, SEM, and XRD analyses. Isothermal annealing at 830°C in vacuum led to a thermoelectric semiconducting β-FeSi₂ phase transformation, but some residual metallic α and ?-phases were unavoidable even after 96 hours of annealing. The iron silicide microstructures were investigated using SEM and TEM. The mechanical and thermoelectric properties of the β-FeSi₂ materials before and after isothermal annealing are characterized in this study.     

Effects of blending on surface characteristics of copper corrosion products in drinking water distribution systems

Copper scales formed over 6-months during exposure to ground, surface and saline waters were characterized by EDS, XRD and XPS. Scale color and hardness were light red-brown-black/hard for high alkalinity and blue-green/soft for high SO₄ or Cl waters. Cl was present in surface or saline copper scales. The Cu/Cu₂O ratio decreased with time indicating an e transfer copper corrosion mechanism. Cu₂O, CuO, and Cu(OH)₂ dominated the top 0.5-1 A° scale indicating continuous corrosion. Cu₂O oxidation to CuO increased with alkalinity, and depended on time and pH. Total copper release was predicted using a Cu(OH)₂ model.     

Oxidation of green rust suspensions containing different chromium ion species

The oxidation of hydrosulphate green rust (GR2(SO₄²⁻)) suspension containing different chromium ion species was investigated by X-ray diffraction, X-ray absorption spectroscopy and transmission electron microscopy. The pH, oxidation-reduction potential and amount of dissolved oxygen in aqueous solutions were measured during the reactions. The results show that the addition of Cr(III)₂(SO₄)₃ solution suppresses the transformation of GR2(SO₄²⁻) into iron oxyhydroxides and oxides in aqueous solution, while the addition of Na₂Cr(VI)O₄ solution promotes the transformation of GR2(SO₄²⁻) in which Cr(VI) is reduced to Cr(III); α-FeOOH particles were refined by the addition of the chromium ions.