Characterization of Iron Oxides Commonly Formed as Corrosion Products on Steel

For fundamental studies of the atmospheric corrosion of steel, it is useful to identify the iron oxide phases present in rust layers. The nine iron oxide phases, iron hydroxide (Fe(OH)₂), iron trihydroxide (Fe(OH)₃), goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), feroxyhite (δ-FeOOH), hematite (α-Fe₂O₃), maghemite (γ-Fe₂O₃) and magnetite (Fe₃O₄) are among those which have been reported to be present in the corrosion coatings on steel. Each iron oxide phase is uniquely characterized by different hyperfine parameters from M?ssbauer analysis, at temperatures of 300K, 77K and 4K. Many of these oxide phases can also be identified by use of Raman spectroscopy. The relative fraction of each iron oxide can be accurately determined from the M?ssbauer subspectral area and recoil-free fraction of each phase. The different M?ssbauer geometries also provide some depth dependent phase identification for corrosion layers present on the steel substrate. Micro-Raman spectroscopy can be used to uniquely identify each iron oxide phase to a high spatial resolution of about 1 μm. This revised version was published online in August 2006 with corrections to the Cover Date.     

Characterization of initial atmospheric corrosion of conventional weathering steels and a mild steel in a tropical atmosphere

The phases and compositions of the corrosion products of a mild steel (A-36) and two weathering steels (A-588 and COR 420) formed after 3 months exposure to the tropical marine atmosphere of Panama were examined using FTIR and Mössbauer spectroscopy. The results show that amorphous or crystallized iron oxyhydroxides goethite α-FeOOH and lepidocrocite γ-FeOOH are early corrosion products. Maghemite γ-Fe₂O₃ and magnetite Fe₃O₄ have also been identified and found to be prominent components for steels exposed to the most aggressive conditions. The formation of akaganeite β-FeOOH was observed when chlorides were occluded within the rust. FTIR showed the presence of hematite α-Fe₂O₃ in one sample.     

The growth of the passive film on iron in 0.05 M NaOH studied in situ by Raman micro‐spectroscopy and electrochemical polarisation. Part I: near‐resonance enhancement of the Raman spectra of iron oxide and oxyhydroxide compounds

Raman spectroscopy, in principle, is an excellent technique for the study of molecular species developed on metal surfaces during electrochemical investigations. However, the use of the more common laser wavelengths such as the 514.5‐nm line results in spectra of less than optimal intensity, particularly for iron oxide compounds. In the present work, near‐resonance enhancement of the Raman spectra was investigated for the iron oxide and iron oxyhydroxide compounds previously reported to be present in the passive film on iron, using a tuneable dye laser producing excitation wavelengths between 560 and 637 nm. These compounds were hematite (α‐Fe₂O₃), maghemite (γ‐Fe₂O₃), magnetite (Fe₃O₄), goethite (α‐FeOOH), akaganeite (β‐FeOOH), lepidocrocite (γ‐FeOOH) and feroxyhyte (δ‐FeOOH). Optimum enhancement, when compared to that with the 514.5‐nm line, was obtained for all the iron oxide and oxyhydroxide standard samples in the low wavenumber region (<1000 cm⁻¹) using an excitation wavelength of 636.4 nm. Particularly significant enhancement was obtained for lepidocrocite, hematite and goethite. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of Cu and Ni on the morphology and composition of the rust layer of steels exposed to industrial environment

Four samples of steels with alloying elements were exposed to an industrial environment during 1,955 days, aiming to elucidate the effect of the alloying elements Cu and Ni on the resistance of weathering steels to corrosion processes. The samples were characterized with optical microscopy, scanning electron microscopy (SEM), powder X-ray diffraction (XRD), saturation magnetization measurements and with energy dispersive (EDS), infrared, Mössbauer and Raman spectroscopies. All the steels originated orange and dark corrosion layers; their thicknesses were determined from the SEM images. EDS data of such rust layers showed that the alloying element content decreases from the steel core towards the outer part of the rust layer. Moreover, in the dark rust layer some light-gray regions were identified in the W and Cu-alloy steel, where relatively higher Cr and Cu contents were found. XRD patterns, infrared, Raman and Mössbauer spectra (298, 110 and 4 K) indicated that the corrosion products are qualitatively the same, containing lepidocrocite (γFeOOH; hereinafter, it may be referred to as simply L), goethite (αFeOOH; G), feroxyhite (δ′FeOOH; F), hematite (αFe₂O₃; H) and magnetite (Fe₃O₄; M) in all samples; this composition does not depend upon the steel type, but their relative concentrations is related to the alloying element. Mössbauer data reveal the presence of (super)paramagnetic iron oxides in the corrosion products. Saturation magnetization measurements suggest that feroxyhite may be an occurring ferrimagnetic phase in the rust layer.     

A new method for preparation of magnetite from iron oxyhydroxide or iron oxide and ferrous salt in aqueous solution

In this study, a new method is proposed for the preparation of Fe₃O₄ from iron oxyhydroxides (goethite, akaganeite, lepidocrocite, feroxyhyte and ferrihydrite) or iron oxide (hematite) and ferrous salt in aqueous solution. The product is magnetite with various particle sizes. Products are characterized by X-ray powder diffraction, IR spectra and vibrating sample magnetometery.     

Analysis of atmospheric corrosion products of steel and coated steel by means of scattering Mössbauer spectrometry

Wearthering steels treated with and without zinc phosphate solution were exposed to atmosphere for 15 years and rust layers produced on the steels were analysed by scattering Mössbauer spectrometry (CEMS and XMS). γ-FeOOH, fine α-FeOOH, 5Fe₂O₃·9H₂O, γ-Fe₂O₃ and Fe₃O₄ were identified to be present in the rust formed on the steel without phosphate coating. Large particles of γ-Fe₂O₃ and Fe₃O₄ formed on the uncoated steel exposed to atmosphere in a position facing north on vertical plane. The layer structure of rust was affected by the position. The thin rust layer formed on the phosphate + carylite resin coated steel was considered to consist of γ-FeOOH, fine α-FeOOH, and fine γ-Fe₂O₃.     

Atmospheric corrosion in the Gulf of México

The corrosion products on steels exposed at two sites in Campeche, México and one site at Kure Beach, USA, have been investigated to determine the extent to which different marine conditions and exposure times control the oxide formation. The corroded coupons were analyzed by Mössbauer, Raman and infrared spectroscopy as well as X‐ray diffraction, in order to completely identify the oxides and map their location in the corrosion coating. The coating compositions were determined by Mössbauer spectroscopy using a new parameter, the relative recoilless fraction (F-value) which gives the atomic fraction of iron in each oxide phase from the Mössbauer sub‐spectral areas. For short exposure times, less than three months, an amorphous oxyhydroxide was detected after which a predominance of lepidocrocite (γ-FeOOH), and akaganeite (β-FeOOH) were observed in the corrosion coatings with the fraction of the later phase increasing at sites with higher atmospheric chloride concentrations. The analysis also showed that small clusters of magnetite (Fe₃O₄), and maghemite (γ(Fe₂O₃), were seen in the micro-Raman spectra but were not always identified by Mössbauer spectroscopy. For longer exposure times, goethite (α-FeOOH), was also identified but little or no β-FeOOH was observed. It was determined by the Raman analysis that the corrosion products generally consisted of inner and outer layers. The protective layer, which acted as a barrier to slow further corrosion, consisted of the α-FeOOH and nano-sized γ-Fe₂O₃ phases and corresponded to the inner layer close to the steel substrate. The outer layer was formed from high γ-FeOOH and low α-FeOOH concentrations.     

Study of ferrous corrosion products on iron archaeological objects by electron backscattered diffraction (EBSD)

The corrosion of iron-based archaeomaterials in anoxic environments leads mainly to Fe(II) compounds, like the hydroxychloride β-Fe₂(OH)₃Cl, chukanovite Fe₂(OH)₂CO₃ or siderite FeCO₃. The understanding of the mechanisms then necessarily implies a thorough investigation of the chemical, mechanical and morphological characteristics of the Fe(II)-based layer that develops between the metal surface and the environment. In the peculiar case of Fe(II) compounds, generally very reactive towards O₂, the main concern is to prevent any transformation by air during the analysis. The EBSD technique is adapted on a scanning electron microscope (SEM) where the samples are analysed under vacuum and consequently sheltered from air. Different options offered by EBSD for phase characterisation and microstructural study were tested for the first time on the rust layers of two archaeological iron nails. Results were confronted to those obtained by micro-Raman spectroscopy, which was used as reference method. Magnetite, Fe(II) hydroxychloride β-Fe₂(OH)₃Cl and siderite were analysed successfully but improvements have to be brought for the study of other compounds such as iron oxyhydroxides and chukanovite. The choice of experimental parameters in our approach as well as the potentialities and limits of the technique for this kind of application are discussed.     

Spectroscopic studies of the corrosion of model iron electrodes in carbonate and phosphate buffer solutions

We have extended our unenhanced (non-SERS) Raman spectroscopic investigations to include a study of the corrosion of an iron electrode in carbonate and phosphate buffer solutions. The measurements have been supported by electrochemical investigations (via cyclic voltammetry), enabling oxidation and reduction reactions to be systematically followed at variable applied potentials. In a carbonate buffer (pH = 9.4) the surface oxidation led to the formation of a ‘green rust’ (a hydrated hydroxy-carbonate), followed by the α- and β-forms of FeOOH and an underlying magnetite layer formed on the cathodic (reduction) cycle. In a phosphate buffer (pH = 7.7) the surface was passivated by hydrated phosphates of iron [identified as FePO₄ · xH₂O and Fe₃(PO₄)₂·8H₂O]. The formation of oxides (Fe₂O₃ and Fe₃O₄) were inferred from voltammetry, but spectral identification was more difficult because of broad, ill-defined spectra. Despite the challenges of using unenhanced Raman spectroscopy, we believe that the effort was worthwhile, the reactions identified being more likely to be relevant to real electrochemical environments. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman study of a deuterated iron hydroxycarbonate to assess long‐term corrosion mechanisms in anoxic soils

In several contexts such as cultural heritage, oil and gas or nuclear waste disposal, the long‐term corrosion mechanisms of iron in anoxic soils are studied. For this purpose, corrosion layers formed on ferrous archaeological artefacts from the site of Glinet (16th century, Normandy, France) were characterised. The main phases identified are siderite (FeCO₃), chukanovite (iron hydroxycarbonate: Fe₂(OH)₂CO₃ and magnetite (Fe₃O₄). In order to provide reliable Raman references for further studies on carbonated systems, the iron hydroxycarbonate (chukanovite) was synthesised on iron discs. The corrosion mechanisms were investigated by re‐corroding the archaeological samples in a deuterated solution. Raman characterisation on cross sections inside the layer revealed the presence of deuterated chukanovite, allowing the deuterium tracing of the spreading of the corrosion. A set of chukanovite samples was synthesised with various D/H ratios. Using these reference data, the proportion of deuterated chukanovite in re‐corroded artefacts was evaluated, and the corrosion rate was estimated as less than 1.6 µm/year. Copyright © 2010 John Wiley & Sons, Ltd.     

Kinetics and structural studies of the atmospheric corrosion of carbon steels in Panama

The corrosion of a carbon steel was studied in different atmospheres at sites in the Republic of Panama. The weight loss (corrosion penetration) suffered by the carbon steel is related to time by a bilogarithmic law. Mössbauer spectroscopy indicated the rust was composed of non-stoichiometric magnetite (Fe_3-xO₄), maghemite (γ-Fe₂O₃), goethite (α-FeOOH) of intermediate particle size, lepidocrocite (γ-FeOOH) and superparamagnetic particles. Magnetite formation is related to the alternating dry--wet cycles. Goethite is related to corrosion penetration by a saturation type of behavior, following a Langmuir type of relationship. Goethite in rust protects steel against further atmospheric corrosion.     

Raman and IR spectroscopy study of corrosion products on the surface of the hot‐dip galvanized steel with alkaline mud adhesion

The corrosion products formed on hot‐dip galvanized steel sheets for the automobile application with adhesion of alkaline mud containing different Cl⁻ ion contents are investigated by means of Raman and infrared (IR) spectroscopy. Results show that the Cl⁻ ion content in alkaline mud has great influence on the corrosion behavior of the galvanized steel. The Cl⁻ ions are responsible for the formation of the Zn₅Cl₂(OH)₈· H₂O layer on the surface of the steel at the early stage of corrosion. The rest of the Cl⁻ ions then penetrate and interrupt corrosion product layer resulting in pitting corrosion. Subsequently, the red corrosion product of α‐FeOOH (shaped as needle‐like structure) is formed, which then transforms into black rust of Fe₃O₄ (having a shape of slim needle). It is interesting to find out that pitting depth is inversely proportional to the Cl⁻ ion content. However, corrosion rate decreases with the increase of the Cl⁻ ions in mud. Copyright © 2009 John Wiley & Sons, Ltd.     

Initial oxidation of polycrystalline Permalloy surface

X-ray photoelectron spectroscopy (XPS) and work-function measurements were used in combination to investigate the initial steps of Permalloy (Ni₈₀Fe₂₀) oxidation at room temperature. They showed that, after oxygen saturation, the surface is covered by nickel oxide (NiO), nickel hydroxide (Ni(OH)₂) and iron oxides (Fe_xO_y), and there is no preferential oxidation. Iron oxidation proceeds through the formation of FeO (Fe²⁺) followed with Fe₂O₃ growth (Fe³⁺). The oxidation is governed by a dissociative Langmuir-type oxidation: the sticking coefficient is decreasing over oxygen exposure. Oxidation continues by oxygen dissolution into the first layers to form a nano-oxide of about 8 Å in thickness.     

Raman spectroscopic studies of the corrosion of model iron electrodes in sodium chloride solution

We have explored the un‐enhanced Raman spectra of both single and twin electrodes in 3.5% NaCl solution (at ambient temperatures) over a range of applied potentials (between 20 and 200 mV) and times (between 0 and 5 h). Under these conditions, we observed the initial formation of ‘green rust’ (hydroxychlorides and/or hydroxycarbonates), followed by the formation of magnetite (Fe₃O₄) and then a mixture of the α‐ and γ‐FeOOH (goethite and lepidocrocite, respectively). These data are consistent with a model for corrosion during which the initially formed magnetite is either covered, or replaced, by layers of the FeOOH oxidation products. Fitting of the data as a function of time and potential shows that, although the product range is independent of potential, the relative kinetics of formation of magnetite and its subsequent conversion to the γ‐FeOOH were potential and time dependent. Analysis by mapping of the dry corroded surface showed a variety of species, including green rust, some Fe(OH)₃, as well as the γ‐FeOOH, and possibly some β‐FeOOH. But no surface magnetite was found, indicating that this material had been either covered up or converted to FeOOH. We noted several complications during this work, including the interference of resonance effects (on the Raman intensities) and the heterogeneity of the corrosion process (and hence distribution of species on the corroded surface). However, we believe that the use of un‐enhanced Raman methods has led to conclusions more likely to be relevant to ‘real’ corrosion processes. Copyright © 2007 John Wiley & Sons, Ltd.     

Raman study of ultrathin Fe3O4 films on GaAs(001) substrate: stoichiometry,epitaxial orientation and strain

The growth and characterization of high‐quality ultrathin Fe₃O₄ films on semiconductor substrates is a key step for spintronic devices. A stable, single‐crystalline ultrathin Fe₃O₄ film on GaAs(001) substrate is obtained by post‐growth annealing of epitaxial Fe film with thicknesses of 5 and 12 nm in air. Raman spectroscopy shows a high ability to convincingly characterize the stoichiometry, epitaxial orientation and strain of such ultrathin Fe₃O₄ films. Polarized Raman spectroscopy confirms the unit cell of Fe₃O₄ films is rotated by 45° to match that of the Fe (001) layer on GaAs, which results in a built‐in strain of − 3.5% in Fe₃O₄ films. The phonon strain‐shift coefficient(−126 cm⁻¹) of the A_1g mode is proposed to probe strain effect and strain relaxation of thin Fe₃O₄ films on substrates. It can be used to identify whether the Fe layer is fully oxidized to Fe₃O₄ or not. Copyright © 2011 John Wiley & Sons, Ltd.     

Phase transformations in CaCO3/iron oxide composite induced by thermal treatment and laser irradiation

Calcium carbonate (CaCO₃)/iron oxide composites were synthesized through a simple one‐step impregnation procedure by mixing iron oxide nanoparticles (γ‐Fe₂O₃ and Fe₃O₄) of about 6 nm in size and CaCO₃ microparticles (Φ = 2 µm–8 µm, vaterite phase). The morphology and structural properties of CaCO₃, iron oxide nanoparticles and CaCO₃/iron oxide composites were characterized as a function of low iron content (0 %w to 3.2 %w) by scanning electron microscopy and transmission electron microscopy, X‐ray diffraction and ⁵⁷Fe Mössbauer spectrometry. The phase transformations induced by thermal treatment and laser irradiation were investigated in situ by X‐ray thermodiffraction (XRTD) and Raman spectroscopy. We have shown that the phase transformations observed by XRTD are also observed under laser irradiation as a consequence of the absorption of the laser irradiation by iron oxide nanoparticles. Copyright © 2012 John Wiley & Sons, Ltd.     

Raman spectra of apatites: La10−x Si6−y (Al,Fe)y O26±δ

Raman spectra of eight polycrystalline apatites of the general formula La_10−xSi_6−yM′_yO_{26 ± δ} with M′ = Al or Fe were obtained at 300 K. Raman spectra of La₁₀Si₄Fe₂O₂₆ and La_9.83Si_4.5Al_1.5O₂₆ were investigated in the range 80–1000 K and 80–623 K, respectively. Tentative assignments of bands to stretching and bending modes of SiO₄ tetrahedra and to M'O vibrations are proposed. Except for the two new bands, which appear around 700 cm⁻¹ when Al is replaced by Fe, only some band broadenings and relative intensity changes are observed as a function of the rate of O5 or La vacancies. Most of the bands soften and broaden continuously when raising the temperature. This is an indication that the Al‐ and Fe‐substituted apatites do not undergo any structural change up to 1000 K. Above 1000 K, the broad and weak shoulder observed at 850 cm⁻¹ for La₁₀Si₄Fe₂O₂₆ is replaced by a strong band at 868 cm⁻¹, suggesting that SiO₄ tetrahedra undergo a structural modification. All compounds show the same residual band broadening at 80 K. This suggests that there is a small rate of static disorder preferentially related to the solubility of Al and Fe in the Si sublattice rather than to other defects. Moreover, the observation of FeO modes indicates that the dynamics of the solid solution obeys the so‐called two‐mode behavior. The occurrence of FeO stretching vibrations 150 cm⁻¹ lower than for those of SiO suggests that the coordination number of iron could be larger than 4, particularly for the Fe⁴⁺ species. Copyright © 2006 John Wiley & Sons, Ltd.     

A study of nanostructures formed in the hydrogen reduction of Fe(OH)<Subscript>3</Subscript>

A complex study of the hydrogen reduction of nanosized iron hydroxide Fe(OH)₃ at 400°C was performed. It was shown that, during the reduction of Fe(OH)₃ to iron metal α-Fe, intermediate compounds such as Fe(OH)₂, α-FeOOH, β-FeOOH, γ-FeOOH, δ-FeOOH, and FeO are formed along with stable iron oxides α-Fe₂O₃, γ-Fe₂O₃, and Fe₃O₄. A scheme of chemical and structural transformations that occur in the reduction of nanosized Fe(OH)₃ is presented. The scheme takes into account the possibility of the bifurcation mechanism of reaction development.     

Micro‐Raman investigations on inclusions of unusual habit in a commercial tanzanite gemstone

The vibrational properties of submillimetre size inclusions of unusual habit in a commercial tanzanite gemstone were investigated by confocal Raman microspectroscopy with the aim of probing both their chemical composition and crystal structure. Highly contrasted Raman spectra were recorded in confocal conditions from several inclusions incorporated at different depths, ranging between a few microns to some tens of microns beneath the gemstone surface. The observed spectral features were identified as specific markers of hematite (α‐Fe₂O₃). Their unambiguous assignment has been inferred by comparing our experimental findings with the literature data recorded either in single crystals of hematite or in other iron oxides and oxyhydroxides. Our results rule out the presence of any pseudomorphic variety of hematite in the investigated gemstone, while confocal micro‐Raman spectroscopy definitively proved itself as a very reliable, relatively costless and noninvasive tool for unambiguous identification of subsurface regions of gemstones. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of Cooling and Storage Properties of a Gas-Filled Linear Paul Trap Coupled to a Time-of-Flight Mass Spectrometer for Mass Measurements of Exotic Nuclei

The conversion properties of 15 different combinations of rust converters have been studied. The results indicate that the rust layer without conversion is composed of spinel phase, akaganeite, magnetically ordered goethite, and lepidocrocite. On the other hand, besides the mentioned iron oxides and oxyhydroxides, the converted-rust layer consists of iron phosphate and iron tannate. Mössbauer spectrometry has allowed us to classify the degree of conversion of the different formulations according to both the tannic acid content and the mixture of the isopropyl and terbutanol alcohol. It was found that there is a specific mixture of alcohol for each tannic concentration for which the power of conversion is greatly enhanced. It is also concluded that non-stoichiometric spinel phase underwent the greater transformation of about 90%, followed by magnetically blocked goethite, in which 30% of it was transformed. In contrast, akaganeite seems to be not noticeably affected by the converters. It was not possible to assess the effect on lepidocrocite.