Transformation of green rust (Cl−) into different oxyhydroxides in aqueous solution containing molybdenum ions

To investigate the influence of the addition of molybdenum ions on the transformation of green rust GR(Cl^?) comprising Fe(II) and Fe(III) into different oxyhydroxides, suspensions of GR(Cl^?) containing molybdenum ions were oxidized by passing nitrogen gas containing oxygen through the suspensions. X‐ray diffraction (XRD) was used for identifying the solid particles formed by oxidation. The results showed that in the presence of molybdenum ions, the formation of β‐FeOOH and α‐FeOOH was enhanced as compared to that of γ‐FeOOH in the GR(Cl^?) suspensions. This implied that the molybdenum ions interacted with the chloride ions obtained from GR(Cl^?) and induced the formation of β‐FeOOH that contained chloride ions. Transmission electron micrographs of the solid particles showed that the particle size of the resulting oxyhydroxides was considerably reduced because of the addition of the molybdenum ions. The pH and oxidation–reduction potential (ORP) of the aqueous solution were also measured during the oxidation. The addition of molybdenum ions was found to cause characteristic changes in the pH and ORP in the initial stage of the oxidation.     

Corrosion behaviour of Ti–Ta–Sn alloy systems in 0.9% NaCl solution

Two alloys of varying contents of tantalum (Ta) and tin (Sn) were prepared and homogenized to evaluate their microstructural and electrochemical characteristics. The microstructural features were revealed through optical microscopy and X‐ray diffraction methods. The formation and stability of passive film was studied by open circuit potential, potentiodynamic polarization and electrochemical scratch tests. The electrochemical impedance spectroscopy results simulated with equivalent electrical circuit suggested bilayer structure of outer porous and inner barrier oxide films. The quantitative data showed thick inner barrier oxide film retarded electrochemical reactions at low ‘Ta’ and ‘Sn’ concentration. The increased percentage of ‘Ta’ and ‘Sn’ deteriorated barrier properties by agglomeration of Ta₂Sn₃ and Ta_0.15Ti_0.85 precipitates within grains and at the grain boundaries.     

Morphology of corrosion products of steel in concrete under macro‐cell and self‐corrosion conditions

In this paper investigations into the formation of specific corrosion products during the process of chloride induced corrosion of steel in concrete are presented. The extension of corrosion products within concrete was established by means of X‐ray tomography analyses. Then a detailed analysis of the nature of corrosion products has been conducted by means of Raman micro‐spectroscopy and energy dispersive spectroscopy. Results emphasize two different corrosion patterns. The first one is composed of shallow cavities, where mostly magnetite and goethite were identified, traducing aerated to moderate aerated conditions in these media. The second pattern was identified as deep, needle‐like pits, where chlorinated‐iron‐oxides phases were present associated with more or less important chloride enrichments. The presence of these particular species is indicating low redox and low pH conditions within these pits.     

Influence of tungstate ions on transformation of green rust to ferric oxyhydroxide via aqueous solution investigated by in situ X-ray absorption spectroscopy

Influence of tungstate ions on the transformation of chloride-containing green rust (GR(Cl⁻)) to fine goethite (α-FeOOH) particles due to the oxidation reaction via aqueous solution at about 300 K was investigated by in situ measurements of X-ray absorption spectra. Results showed that the transformation rate of GR(Cl⁻) in the suspension containing 5 mol% W was lower than that in the suspension without tungstate ions. Almost all tungstate ions in the suspension were adsorbed on GR(Cl⁻) and α-FeOOH. It is probable that the adsorption of tungstate ions reduces the transformation rate of GR(Cl⁻) and also leads to the precipitation of fine particles.     

Early‐stage oxidation behavior of Co‐rich high‐temperature alloys

The long‐term oxidation performance of an alloy is critically linked to the early‐stage oxidation behavior of high‐temperature alloys. This study investigates early‐stage oxidation behavior in terms of oxidation kinetics, scale evolution, and residual stresses developed within a scale of the commercially available cobalt‐rich alloys: HAYNES® 188, 6B, 25, and HR‐160® and a newly developed nitride‐dispersion strengthened NS‐163® alloy (HAYNES®, HR‐160®, NS‐163® are registered trademarks of Haynes International, Inc). Short‐term isothermal oxidation exposures were conducted in flowing air at 982 °C for durations of 1–50 h. Oxidation kinetics was assessed by weight‐change behavior, which showed that 188 alloy exhibited the lowest weight‐gain, while for similar times HR‐160 alloy underwent weight‐loss. SEM/EDS analysis was performed to characterize oxides formed in these alloys, while stresses developed in the oxides of different alloys were measured using synchrotron X‐ray radiation. The results in this paper clearly demonstrated the effects of alloy composition on the scale evolution and the amount of stresses developed in oxides.     

Oxidation mechanisms of Cr‐containing steels and Ni‐base alloys at high‐temperatures –. Part I: The different role of alloy grain boundaries

It is essential for materials used at high‐temperatures in corrosive atmosphere to maintain their specific properties, such as good creep resistance, long fatigue life and sufficient high‐temperature corrosion resistance. Usually, the corrosion resistance results from the formation of a protective scale with very low porosity, good adherence, high mechanical and thermodynamic stability and slow growth rate. Standard engineering materials in power generation technology are low‐Cr steels. However, steels with higher Cr content, e.g., austenitic steels, or Ni‐base alloys are used for components applied to more severe service conditions, e.g., more aggressive atmospheres and higher temperatures. Three categories of alloys were investigated in this study. These materials were oxidised in laboratory air at temperatures of 550°C in the case of low‐alloy steels, 750°C in the case of an austenitic steel (TP347) and up to 1000°C in the case of the Ni‐base superalloys Inconel 625 Si and Inconel 718. Emphasis was put on the role of grain size on the internal and external oxidation processes. For this purpose various grain sizes were established by means of recrystallization heat treatment. In the case of low‐Cr steels, thermogravimetric measurements revealed a substantially higher mass gain for steels with smaller grain sizes. This observation was attributed to the role of alloy grain boundaries as short‐circuit diffusion paths for inward oxygen transport. For the austenitic steel, the situation is the other way round. The scale formed on specimens with smaller grain size consists mainly of Cr₂O₃ with some FeCr₂O₄ at localized sites, while for specimens with larger grain size a non‐protective Fe oxide scale is formed. This finding supports the idea that substrate grain boundaries accelerate the chromium supply to the oxide/alloy phase interface. Finally, in the Ni‐base superalloys deep intergranular oxidation attack was observed, taking place preferentially along random high‐angle grain boundaries.     

Effect of carbonation process on the passivating products of zinc in Ca(OH)2 saturated solution

The effect of carbonation process on the passivating layer of zinc in Ca(OH)₂ saturated solution was studied. The investigation was performed by means of corrosion potential, corrosion current density, and impedance measurements. To analyze the changes in the passivating layer, X‐ray diffraction, Raman spectroscopy and scanning electron microscopy (SEM) were utilized. The results obtained indicate that the layer of calcium hydroxyzincate (Ca[Zn(OH)₃]₂ · 2H₂O) (CHZ), which determines the passivity state of zinc both in Ca(OH)₂ saturated solution and in concrete, is destroyed by the carbonation process, in agreement with previous results obtained for galvanized steel embedded in concrete. X‐ray diffractometry and Raman spectroscopy showed that CHZ reaction with CO₂ leads to the formation of Zn₅(CO₃)₂(OH)₆ (hydrozincite) and CaCO₃ (calcite). SEM observation confirms the deep transformation in the passivating layer caused by carbonation. Corrosion potential and corrosion current density measurements show that zinc maintains its passive state also after carbonation. However, impedance measurements indicate that hydrozincite has lower passivating properties than calcium hydroxyzincate in the carbonated solution.     

Formation of ‘ferric green rust’ and/or ferrihydrite by fast oxidation of iron(II-III) hydroxychloride green rust

Fast oxidation processes of iron(II-III) hydroxychloride green rust GR(Cl⁻), Fe^II₃Fe^III(OH)₈Cl · 2H₂O, were simulated by two different methods. The first one consisted in using a strong oxidiser, namely H₂O₂. The main end product, analysed by X-ray diffraction and Mössbauer spectroscopy, is an iron(III) compound, designated as “ferric GR(Cl⁻)”, characterised by a layered structure identical to that of normal GR(Cl⁻). The second method consisted in decreasing the initial concentrations of reactants, thus increasing the proportion of dissolved O₂. Suspensions of GR(Cl⁻), obtained by mixing 4 × 10⁻³ M NaOH with FeCl₂ solutions for various R^′=[Cl⁻]/[OH⁻] values, oxidised rapidly into ferrihydrite-like compounds.     

Oxidation mechanism of the Inconel 601 alloy at high temperatures

The Inconel 601 alloy oxidation was performed in air, in the temperature range 1000–1150 °C, during 90 h. Kinetic results show that the parabolic behavior is always followed in this temperature range. The Arrhenius plot of the k_p values shows two different activation energies. Between 1000 and 1050 °C the activation energy is E_a1 = 160 ± 10 kJ/mol. In the 1050–1150 °C temperature range a higher value is calculated E_a2 = 252 ± 20 kJ/mol. The E_a2 value and the X‐ray diffraction (XRD) results and scanning electron microscope (SEM) energy dispersive X‐ray spectroscopy (EDS) examinations are in accordance with a scale growth mechanism limited by a growing Cr₂O₃ scale acting as a diffusion barrier. In the 1000–1050 °C temperature range the activation energy is lower and the structural analyses show that the oxide scale is not only composed of Cr₂O₃. Then, the oxide scale is composed of titanium oxides (TiO₂ and Ti₂Cr₇O₁₇) and chromia mixed together. A doping effect of the chromia scale by titanium can be envisaged. Our results also show the presence of some Mn_1.5Cr_1.5O₄ at the external interface. This external subscale spalls off easily during cooling after the highest temperature oxidation tests. Nevertheless, XRD results and SEM–EDS observations show that the Cr₂O₃ scale remains very adherent on the substrate and can give a good oxidation protection. This good adherence can be related to the presence of a low amount of aluminum in the Inconel 601 alloy composition.     

Influence of silicon species on the transformation of green rust I(Cl) in aqueous solution by oxidation

X-ray diffraction (XRD) and solution analysis were used for characterizing the influence of different silicon species on oxidation of green rust (GRI(Cl⁻)) suspension. While addition of silicon to metallic iron enhanced the formation of β-FeOOH, GRI(Cl⁻) in aqueous solution oxidized into lepidocrocite and oxidation was delayed in presence of silica and silicate species as noticed from potential, pH, and dissolved oxygen (DO) measurements. Transmission electron micrographs showed that the particle size of lepidocrocite was reduced due to silicate addition. The influence of silicate was attributed to its adsorption on GRI(Cl⁻) and lepidocrocite particles as confirmed from ICP-AES analysis of supernatant solution.     

X‐Ray photoelectron spectroscopic investigations of powder coated steel

Polyester powder coatings are used for corrosion protection of different materials. XPS studies were performed in order to characterize such coatings on steel surfaces as a pre‐treatment for following applications like gluing. Different surface treatments by alkaline cleaner, plasma treatment (Plasma Treat® process), and the use of a silicoater (Pyrosil® process) were investigated. In dependence of such treatments, the resulting surfaces are found to be hydrophilic or hydrophobic. Although this behavior can also be investigated by contact‐angle measurements, the aim of our work is the element‐specific characterization of chemical bonding states (speciation).