The preparation of magnetite, goethite, hematite and maghemite of pigment quality from mill scale iron waste

The long term growth of the protective rust layer formed on weathering steel by atmospheric corrosion during a quarter of a century

Using Fourier Transform InfraRed (FTIR) spectroscopy, Raman spectroscopy, X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM), we characterize the structure and/or morphology of hematite (α-Fe2O3) particles with sizes of 7, 18, 39 and 120 nm. It is found that these nanoparticles possess maghemite (γ-Fe2O3)-like defects in the near surface regions, to which a vibrational mode at 690 cm−1, active both in FTIR and Raman spectra, is assigned. The fraction of the maghemite-like defects and the net lattice disorder are inversely related to the particle size. However, the effect is opposite for nanoparticles grown by sintering of smaller hematite precursors under conditions when the formation of a uniform hematite-like structure throughout the aggregate is restricted by kinetic issues. This means that not only particle size but also the growth kinetics determines the structure of the nanoparticles. The observed structural changes are interpreted as size-induced α-Fe2O3
				↔
				γ-Fe2O3 phase transitions. We develop a general model that considers spinel defects and absorbed/adsorbed species (in our case, hydroxyls) as dominant controls on structural changes with particle size in hematite nanoparticles, including solid-state phase transitions. These changes are represented by trajectories in a phase diagram built in three phase coordinates—concentrations of spinel defects, absorbed impurities, and adsorbed species. The critical size for the onset of the α
				→
				γ phase transition depends on the particle environment, and for the dry particles used in this study is about 40 nm. The model supports the existence of intermediate phases (protohematite and hydrohematite) during dehydration of goethite. We also demonstrate that the hematite structure is significantly less defective when the nanoparticles are immersed in water or KBr matrix, which is explained by the effects of the electrochemical double layer and increased rigidity of the particle environment. Finally, we revise the problem of applicability of IR spectroscopy to the lattice vibrations of hematite nanoparticles, demonstrating that structural comparison of different samples is much more reliable if it is based on the Eu band at about 460 cm−1 and the spinel band at 690 cm−1, instead of the A2u/Eu band at about 550 cm−1 used in previous work. The new methodology is applied to analysis of the reported IR spectra of Martian hematite.

Size-dependent structural transformations of hematite nanoparticles. 1. Phase transition.

Composition and protective ability of rust layer formed on weathering steel exposed to various environments

Corrosion is a subject of interest to interdisciplinary research communities, combining fields of materials science, chemistry, physics, metallurgy and chemical engineering. In order to understand mechanisms of corrosion and the function of corrosion inhibitors, the reactions at the interfaces between the corrosive electrolyte and a steel surface, particularly at the initial stages of the corrosion process, need to be described. Naturally, these reactions are strongly affected by the nature and properties of the steel surfaces. It is however seen that the majority of recent corrosion and corrosion-inhibition investigations are limited to electrochemical testing, with ex situ analysis of the treated steels (post-exposure analysis). The characterization of materials and their surface properties, such as texture and morphology, are not being considered in most studies. Similarly, in situ investigations of the initial stages of the corrosion reactions using advanced surface characterization techniques are scarce. In this review, attention is brought to the importance of surface features of carbon steels, such as texture and surface energy, along with defects dislocation related to mechanical processing of carbon steels. This work is extended to a critical review of surface analytical techniques used for characterization of carbon steels in corrosive media with particular focus on examining steel surfaces treated with corrosion inhibitors. Further, emerging surface analysis techniques and their applicability to analyse carbon steels in corrosive media are discussed. The importance of surface properties is commonly addressed by surface scientists as well as researchers in other chemistry fields such as nanotechnology, fuel cells, and catalysis. This article is expected to appeal to a broad scientific community, including but not limited to corrosion scientists, material chemists, analytical chemists, metal physicists, corrosion and materials engineers.

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Carbon steel corrosion: a review of key surface properties and characterization methods

Characterization of the passive film formed on weathering steels

Clarification of the Mechanism of the Dealloying Phenomenon

Oxide scales formed on metals at elevated temperatures may be different, both chemically and structurally, from the scales on the metal once it has cooled to room temperature. This paper discusses Raman spectroscopy instrumentation for the in-situ identification of scales formed on metal surfaces exposed to gaseous environments. The results of an experimental program to characterize scales formed on two commercial iron-chromium alloys, AISI 446 and 502, in air and oxygen environments are also presented.

Oxide Scale Formation on Iron-Chromium Alloys in Elevated Temperature Air Environments

Infrared and Raman spectroscopy are two analytical tools which have recently been applied to the study of corrosion products formed on metal surfaces. The two techniques are complementary and give structural information similar to that provided by X-ray and electron diffraction. However, they are unlike X-ray and electron diffraction in that they do not require crystalline solids to provide structural identification. Weathering steels are an important class of metals which form noncrystalline corrosion products. This report describes infrared and Raman spectra obtained from a weathering steel and compares the spectra to those obtained for carbon steel which had been exposed to the same environment.

An infrared and Raman spectroscopy study of the corrosion products on carbon steel and weathering steel

Infrared and Raman spectroscopy were used to study the oxidation of Fe3O4 on surfaces of weathering steel, carbon steel, and pure iron. Two sets of samples were polished and coated with an Fe3O4 paste. One set was moistened daily with distilled water and allowed to freely dry in air. The second set was treated in a similar manner except that every other day, a 0.1M Na2SO4 solution was used in place of the distilled water solution. In the set of samples treated only with distilled water, Fe3O4 was oxidized to γ-FeOOH on both the weathering and carbon steel surface, but oxidation of Fe3O4 was not observed on the pure iron surface. On all of the samples treated with 0.1M Na2SO4, Fe3O4 was oxidized initially to γ-FeOOH which converted to α-FeOOH after several weeks. The initial oxidation of Fe3O4 is proposed to occur by a reaction with H2O2 produced from the reduction of O2.

Robert H. Heidersbach

Papers

Characterization of the passive film formed on weathering steels

Clarification of the Mechanism of the Dealloying Phenomenon

Oxide Scale Formation on Iron-Chromium Alloys in Elevated Temperature Air Environments

An infrared and Raman spectroscopy study of the corrosion products on carbon steel and weathering steel

The Oxidation of Fe3O4 on Iron and Steel Surfaces