Field sludge characterization obtained from inner of pipelines

Physicochemical characterization of sludge obtained from refined hydrocarbons transmission pipeline was carried out through Mössbauer spectroscopy and X-ray diffraction. The Mössbauer and X-ray patterns indicate the presence of corrosion products composed of different iron oxide and sulfide phases. Hematite (α-Fe₂O₃), magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃), magnetic and superparamagnetic goethite (α-FeOOH), pyrrhotite (Fe_1−xS), akaganeite (β-FeOOH), and lepidocrocite (γ-FeOOH) were identified as corrosion products in samples obtained from pipeline transporting Magna and Premium gasoline. For diesel transmission pipeline, hematite, magnetite, and magnetic goethite were identified. Corrosion products follow a simple reaction mechanism of steel dissolution in aerated aqueous media at a near-neutral pH. Chemical composition of the corrosion products depends on H₂O and sulfur inherent in fluids (traces). These results can be useful for decision-making with regard to pipeline corrosion control.     

Microstructural changes in particles detected during the transformation from β-FeOOH to α-Fe2O3 in dense aqueous suspensions

The hydrothermal transformation from β-FeOOH to α-Fe₂O₃ in dense aqueous suspensions, obtained by partial neutralization of concentrated FeCl₃ solution with concentrated NaOH solution, was investigated. Mössbauer spectroscopy was used for the phase analysis of samples, as well as the complementary techniques XRD and FT-IR. The size and morphology of the particles were inspected with FE-SEM. At the beginning of the crystallization process very fine β-FeOOH particles were formed, which transformed to α-Fe₂O₃ (end-product) with a prolonged time of heating. A small amount or traces of α-FeOOH as an intermediate phase were detected by FT-IR and FE-SEM. Gradual formation of α-Fe₂O₃ double spheres with ring was monitored. Double spheres with ring were formed by the aggregation mechanism. The orientation effect of α-Fe₂O₃ subunits in double spheres with ring was observed. α-Fe₂O₃ double spheres with ring also showed two narrow particle size distributions.     

Short-term atmospheric corrosion of mild steel at two weather and pollution monitored sites

Results are recorded of topographical details, weight loss and corrosion product analysis for short-term atmospheric exposure of mild steel, in relation to weather and pollutant factors. Various structured corrosion features, e.g. doughnut-like, were noted. The major corrosion product found was γ-Fe₂O₃,H₂O, with some α-Fe₂O₃,H₂O and α-FeOOH.Wet initial conditions of exposure and subsequent high levels of humidity, were found to have the dominant overriding effect in promoting corrosion losses. Smoke was shown to be capable of exerting a very strong influence upon the effective corrosivity of atmospheric sulphur dioxide.     

A Mössbauer spectroscopic study of rust formed during simulated atmospheric corrosion

Steel coupons were subjected to 100% relative humidity and were inoculated every day with 100 μl of 0.01 N solutions of NaCl, Na₂SO₄, LiCl or CsCl. The first solid rust constituent that formed contained significant amounts of both γ-FeOOH and ferrihydrite. In contrast, only γ-FeOOH was observed in the rust formed during atmospheric corrosion and during wet-dry cycling with distilled water in the laboratory. The ferrihydrite in time was converted to γ-FeOOH, α-FeOOH, and γ-Fe₂O₃. The fractions of ferrihydrite + γ-FeOOH in the rust formed as a function of time during atmospheric exposure and during rusting in the laboratory environment were the same in the two cases.     

The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media

The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media was monitored by X-ray diffraction (XRD), ⁵⁷Fe Mössbauer and Fourier transform infrared (FT-IR) spectroscopies and field emission scanning electron microscopy (FE SEM). Acicular and monodisperse α-FeOOH particles were precipitated at a very high pH by adding a tetramethylammonium hydroxide solution to an aqueous solution of FeCl₃. The XRD analysis of the samples precipitated in the presence of Zn²⁺ ions showed the formation of solid solutions of α-(Fe, Zn)OOH up to a concentration ratio r = [Zn]/([Zn] + [Fe]) = 0.0909. ZnFe₂O₄ was additionally formed in the precipitate for r = 0.1111, whereas the three phases α-FeOOH, α-Fe₂O₃ and ZnFe₂O₄ were formed for r = 0.1304. In the corresponding FT-IR spectra, the FeOH and FeO stretching bands were sensitive to the Zn²⁺ substitution, whereas the FeOH bending bands of α-FeOOH at 892 and 796 cm⁻¹ were almost insensitive. The Mössbauer spectra showed a high sensitivity to the formation of α-(Fe, Zn)OOH solid solutions which were monitored on the basis of a decrease in B_hf values in dependence on Zn-doping. A strictly linear decrease in B_hf for α-FeOOH doped with Zn²⁺ ions was measured up to r = 0.0291, whereas for r = 0.0476 and higher there was a deviation from linearity. The presence of α-(Fe, Zn)OOH, α-Fe₂O₃ and ZnFe₂O₄ phases in the samples was determined quantitatively by Mössbauer spectroscopy. Likewise, Mössbauer spectroscopy did not show any formation of the solid solutions of α-Fe₂O₃ with Zn²⁺ ions. FE SEM showed a strong effect of Zn-doping on the elongation of acicular α-FeOOH particles (500–700 nm in length) up to r = 0.1111. For r = 0.1304 the sizes of ZnFe₂O₄ particles were around 30–50 nm, and those of α-Fe₂O₃ particles were around 500 nm, whereas a relatively small number of very elongated α-(Fe, Zn)OOH particles was observed. A possible mechanism of the formation of α-(Fe, Zn)OOH, α-Fe₂O₃ and ZnFe₂O₄ particles was suggested.     

An electrochemical study of phase-transitions in rust layers

Isolated rust layers have been investigated by electrochemical methods to find out whether their reduction and re-oxidation can affect the atmospheric corrosion of iron. At potentials below 0 mV, first a thin Fe²⁺-containing surface layer is formed on top of the γ-FeOOH. This reduced surface layer can dissolve into the cell electrolyte at acid pH, or at lower potentials the Fe²⁺-ions can react with γ-FeOOH to Fe₃O₄. The formation of magnetite could be followed by in-situ magnetic measurements. The reduced surface layer can easily be oxidized back to γ-FeOOH, magnetite can partly be oxidized to γ-Fe₂O₃.     

Oxidation behavior of NixFe1−x(OH)2 in Cl-containing solution

The addition of Ni leads to the formation of protective rust layer on steel and subsequently high corrosion resistance of steel in Cl⁻-containing environment. α-FeOOH, β-FeOOH, γ-FeOOH and Fe₃O₄ are formed mainly on steels exposed to Cl⁻-containing atmosphere. It is expected that systematic investigation of the effect of Ni(II) on the formation process of each oxide in solution should lead to elucidation of the role of Ni in the formation of anticorrosive oxide layer. This study reports the oxidation behavior of Ni_xFe_1−x(OH)₂ in Cl⁻-containing solution at two different pH regions (condition I under which solution pH is allowed to decrease and condition II under which solution pH is maintained at 8) where γ-FeOOH and Fe₃O₄ are predominantly formed, respectively, upon the oxidation of Fe(OH)₂. In the presence of Ni(II) in the starting solution, the formation of Ni(II) doped β-FeOOH with very low crystalline was facilitated and the formation of γ-FeOOH was suppressed with increasing Ni(II) content and with increasing oxidation rate of Fe(II). Ni(II) was found to have Fe₃O₄-suppressing effect under condition II.     

Corrosion resistance of the Dhar iron pillar

The corrosion resistance of the 950-year old Dhar iron pillar has been addressed. The microstructure of a Dhar pillar iron sample exhibited characteristics typical of ancient Indian iron. Intergranular cracking indicated P segregation to the grain boundaries. The potentiodynamic polarization behaviour of the Dhar pillar iron and mild steel, evaluated in solutions of pH 1 and 7.6, indicate that the pillar iron is inferior to mild steel under complete immersion conditions. However, the excellent atmospheric corrosion resistance of the phosphoric Dhar pillar iron is due to the formation of a protective passive film on the surface. Rust analysis revealed the presence of crystalline magnetite (Fe_3−xO₄), α-Fe₂O₃ (hematite), goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH) and phosphates, and amorphous δ-FeOOH phases. The rust cross-section revealed a layered structure at some locations.     

Iron oxide hollow spheres: Microwave–hydrothermal ionic liquid preparation,formation mechanism,crystal phase and morphology control and properties

This paper reports on the microwave–hydrothermal ionic liquid method for the synthesis of a variety of iron oxide nanostructures such as α-FeOOH hollow spheres, β-FeOOH architectures and α-Fe₂O₃ nanoparticles. The formation mechanism for α-FeOOH hollow spheres is discussed. The effects of the reaction parameters on the morphology and crystal phase of the final product are studied. The relationship between the morphology and crystal phase of the product is discussed. A general thermal transformation strategy is designed to prepare α-Fe₂O₃ hollow spheres using α-FeOOH hollow spheres as the precursor and template. By thermal treatment of the as-prepared α-FeOOH hollow spheres, α-Fe₂O₃ hollow spheres showing good photocatalytic activity are obtained. And by autocatalysis of the adsorbed available Fe(II) on the α-FeOOH surfaces, Fe₃O₄ hollow spheres are also obtained.     

Characterization of Delhi iron pillar rust by X-ray diffraction, Fourier transform infrared spectroscopy and Mössbauer spectroscopy

Rust samples obtained from the region just below the decorative bell capital of the Delhi iron pillar (DIP) have been analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Mössbauer spectroscopy. The identification of iron hydrogen phosphate hydrate in the crystalline form by XRD was unambiguous. Very weak diffraction from the oxyhydroxides/oxides of iron was observed indicating that these phases are most likely to be present in the amorphous form in the rust. The present XRD analysis of rust obtained from an inaccessible area of the DIP has also been compared with earlier analyses of DIP rust obtained from regions accessible to the public. FTIR indicated that the constituents of the scale were γ-, α-, δ-FeOOH, Fe_3−xO₄ and phosphate, and that the scale was hydrated. The unambiguous identification of the iron oxides/oxyhydroxides in the FTIR spectrum implied that they are present in the amorphous state, as XRD did not reveal these phases. The FTIR results have also been compared with earlier FTIR spectroscopic results of atmospheric rust formation. Mössbauer spectroscopy indicated that the rusts contained γ-FeOOH, superparamagnetic α-FeOOH, δ-FeOOH and magnetite, all in the amorphous form. The Mössbauer spectrum also confirmed that iron in the crystalline iron hydrogen phosphate hydrate, whose presence was confirmed by XRD, was in the ferric state indicating that it was a stable end corrosion product.     

Formation and properties of BaxFe3−xO4 with spinel structure by mechanochemical reaction of α-Fe2O3 and BaCO3

Magnetic Ba_xFe_3−xO₄ (x  0.23) with spinel structure was fabricated by ball milling of mixture of BaCO₃ and nonmagnetic α-Fe₂O₃ powders, and the molar ratio of BaCO₃ and α-Fe₂O₃ is 1:6. In the milling process, a mechanochemical reaction took place between BaCO₃ and α-Fe₂O₃, and Ba cation incorporated into α-Fe₂O₃ with rhombohedral structure to form a α-(Fe,Ba)₂O₃ solid solution. The Ba content in the α-(Fe,Ba)₂O₃ increased with increasing milling time, when the Ba content exceeded a limited solubility, the α-(Fe,Ba)₂O₃ transformed into a phase of Ba_xFe_3−xO₄ with spinel structure, where the Ba cation occupied an octahedral site or tetrahedral site. The product obtained in the balling process was different from that prepared in the annealing process at atmospheric pressure, which was BaFe₂O₄ with orthorhombic structure. Accompanying the crystal structure transition from α-(Fe,Ba)₂O₃ to Ba_xFe_3−xO₄, the magnetic properties also changed from nonmagnetism into ferromagnetism. The saturation magnetization was 53.3 emu/g and coercivity was 113.7 Oe. The mechanism of transitions of the crystal structure was discussed in the present work.     

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.     

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.     

Steel corrosion behaviour in carbonated alkali-activated slag concrete

Steel bars embedded in an alkali-activated slag (AAS) concrete were exposed (after curing for 28 days) to an accelerated carbonation test (3% CO₂, 65% relative humidity (RH), and 25 °C temperature) and a laboratory environment (0.03% CO₂, 65% RH, and 25 °C). Ordinary Portland cement (OPC) was also tested for comparative purposes and exposed to identical experimental conditions. The corrosion behaviour of uncarbonated and carbonated AAS and OPC concretes was tested for different times, performing corrosion potential, linear polarization resistance, and electrochemical impedance spectroscopy measurements. Corrosion products were analysed using the Mössbauer technique. The main corrosion products found were magnetite (Fe₃O₄), wüstite (FeO), and goethite (α-FeOOH).     

Atmospheric corrosion of hot and cold rolled carbon steel under field exposure in Saudi Arabia

Carbon steel (hot and cold rolled) specimens have been exposed to the action of different atmospheres at 20 test sites distributed in Saudi Arabia and was investigated in terms of environmental factors such as average temperature, average relative humidity and deposition rates of atmospheric pollutants (Cl⁻ and SO₂). Applying the standard ISO 9223 norm aggressiveness of the atmospheres corresponding to 0the different test sites has been determined. Calculations of corrosion rates were made via loss of weight and characterization of the corrosion products formed on samples has been carried out by means of X-ray diffraction (XRD). The major constituent of the rust formed in marine and marine-industrial environment is goethite (α-FeOOH). These samples also show the presence of a large proportion of lepidocrocite (γ-FeOOH) and small amounts of ferrihydrite and maghemite (γ-Fe₂O₃). In the case of urban and rural samples goethite is the major constituent of corrosion layers. The rust formed under the urban environment also contains large amounts of ferrihydrite and in a lesser proportion, of goethite and maghemite.     

Role of oxide films in stress corrosion cracking of mild steel in nitrate solutions

Time to failure of iron with varying carbon content was determined during stress corrosion at 75°C in various nitrate solutions. Ability of these solutions to passivate iron was estimated. It has been suggested that the high rate of the intergranular stress corrosion cracking of mild steel is connected with the formation of the passive γ-Fe₂O₃ film and its sensitivity to the depassivating action of steel constituents such as carbon, segregated at grain boundaries. It is thought that the Fe₃O₄ film also protects the grain boundaries.     

The reductive dissolution of γ-FeOOH in corrosion scales formed on cast iron in near-neutral waters

Corrosion scales which form on cast iron in oxygenated, near-neutral waters at 50°C contain green rust, Fe₂O₄ and γ-FeOOH. Upon de-oxygenation of the water, γ-FeOOH → Fe₃O₄ occurs by dissolution-precipitation as a cathodic reaction in the corrosion cell. Mechanisms of corrosion reactions are postulated in which it is proposed that the amount of γ-FeOOH is controlled by a balance between its rate of precipitation and its rate of reductive dissolution. The mechanism explains the eventual predominance of Fe₃O₄ in scales formed in oxygenated waters and the absence of γ-FeOOH from scales formed in waters with [O₂] < ～ 1 ppm.     

Influence of the prior oxide film on the growth and structure of oxides formed on Fe---26Cr alloys at 600°C

A Fe---26 Cr alloy has been oxidized at 600°C in 5 × 10⁻³, 5 × 10⁻² and 5 × 10⁻¹ torr oxygen to examine the influence of the prior oxide film on the growth and structure of oxides formed at high temperature. Different prior oxides were produced either by electropolishing or by annealing the electropolished specimen in vacuum at 600°C. Auger electron spectroscopy (AES) showed the Cr content of the prior oxide film to be increased from 50 to 95% during annealing, and electron diffraction indicated a change in oxide structure from amorphous to crystalline. At 5 × 10⁻³ torr, electropolished Fe---26 Cr oxidizes faster than the vacuum annealed specimens because the amorphous prior oxide gives rise to a finer-grained cubic oxide with more grain boundary easy diffusion paths for cation transport. From AES and electron back-scattering Fe⁵⁷ Mössbauer spectroscopy it is concluded that this cubic oxide is a duplex layer of inner γ-Cr₂O₃ and outer Fe₃O₄. The oxidation rate slows markedly when nucleated α-Fe₂O₃ covers the cubic oxide. With increased oxidation time Fe₃O₄ converts to α-Fe₂O₃ and the γ-Cr₂O₂ to α-Cr₂O₃. Annealed Fe---26 Cr oxidizes slower primarily because of a lower cation transport through a coarser-grained cubic oxide rather than because of a higher Cr content in the prior oxide. α-Fe₂O₃ nucleates at an earlier stage in the oxidation and essentially stifles the reaction. The extent of Cr incorporation into any of the Fe oxides produced in 5 × 10⁻³ torr oxygen is small ( 5%). Increasing the oxygen pressure from 5 × 10⁻³ to 5 × 10⁻² and 5 × 10⁻¹ torr has little effect on the mechanism of oxidation of vacuum annealed Fe---26 Cr, except that the overall extent of oxidation is less because of earlier α-Fe₂O₃ formation and, after a few hours of oxidation, up to 20% Cr is incorporated into the α-Fe₂O₃ lattice. On electropolished Fe---26 Cr at 5 × 10⁻² and 5 × 10⁻¹ torr oxygen nodules of α-Cr₂O₃ form and continue to grow both at grain boundaries and within the grains. Possible mechanisms for this nodule formation, which is exclusive to electropolished specimens oxidized at the higher pressures, are considered.     

Morphology of rust phases formed on weathering steels in various laboratory corrosion tests

The morphology and growth characteristics of rust phases formed on ASTM A-588 weathering steel in three different types of laboratory tests—accelerated atmospheric exposure simulation tests (AAEST), salt fog test, and continuous immersion test in plain as well as salt water—are analyzed using microstructural information obtained from representative exposed specimens studied in a scanning electron microscope (SEM). The ultimate and most dominant phase in the AAEST was α-FeOOH whereas an amorphous phase designated as amorphous bulk (AB) appeared as “cotton bolls” in the adherent, sedimentary layer formed on the steel surface during continuous immersion. Crystalline phases α-, δ-, and γ-FeOOH as well as γ-Fe₂O₃.H₂O were found developed on top of the first-formed sedimentary amorphous layer, containing another amorphous phase designated as amorphous mix (AM). Magnetite was the dominant phase obtained in the salt fog test. It forms in layers and seems to transform to α-FeOOH through formation of whiskers and rods on its surface. Sandy grains of γ-Fe₂O₃.H₂O were also seen in the rusts obtained in this test.     

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.