Control of polymorphism in Al2O3 scale formed by oxidation of alumina-forming alloys

The selective oxidation of specific components in alumina-forming alloy such as CoNiCrAlY under precisely regulated oxygen partial pressures (P_O2) can be used to control polymorphism in Al₂O₃ scale formed on the alloy. Dense, smooth α-Al₂O₃ scale was formed rapidly by treatment at 1323 K under a thermodynamically determined P_O2, where both aluminum and chromium in the alloy were oxidized and elements such as cobalt and nickel were not oxidized. By contrast, under a higher P_O2 all the components in the alloy were oxidized, the transformation was obviously retarded, and (Co,Ni)(Al,Cr)₂O₄ was produced.     

Cyclic oxidation of Ti3AlC2 at 1000–1300 °C in air

The cyclic-oxidation behavior of Ti₃AlC₂ was investigated at 1000–1300 °C in air for up 40 cycles. It was revealed that Ti₃AlC₂ had excellent resistance to thermal cycling. The cyclic oxidation of Ti₃AlC₂ basically obeyed a parabolic law. In all cases, the scales were dense, resistant to spalling and highly stratified. The inner continuous α-Al₂O₃ layer was well adhesive, while the outermost layer changed from rutile TiO₂ at temperatures below 1100 °C to Al₂TiO₅ at 1200 and 1300 °C, respectively. At 1300 °C, a mechanical-keying structure of inner Al₂O₃ to the Ti₃AlC₂ substrate formed, which improved the resistance to scale-spallation.     

Characterisation of fresh surface oxidation films formed on pure molten magnesium in different atmospheres

This paper examines the early stages of surface oxidation of liquid magnesium under argon, air, and air mixed with protective fluorine-bearing gases. Surface film characteristics such as morphology, thickness and composition are determined. In all cases except argon the film was locally uniform with no evidence of specific nuclei.In air, the film thickness was 15-150 nm. Under fluorine-bearing gas mixtures the surface film was a mixed fluoride and oxide and more even: 70-100 nm thick under SF₆ and 30-50 nm under 1,1,1,2-tetrafluoroethane. The latter had a substantially lower O:F ratio. Complete conversion of available fluorine into the film was indicated.     

Influence of manganese on iron oxyhydroxides and oxides formed in aqueous solution

Structural analysis techniques such as X-ray diffraction and anomalous X-ray scattering were used for characterizing the influence of manganese on iron oxyhydroxides and oxides formed from green rust (GR) in an aqueous solution. The results showed that the formation of Fe₃O₄ was enhanced by the addition of manganese ions during the conversion of GR2 to α-FeOOH and Fe₃O₄. The results obtained from anomalous X-ray scattering showed that manganese was present both in α-FeOOH and Fe₃O₄ particles. The incorporation of manganese in α-FeOOH appears to induce the distortion of the atomic-scale structure of α-FeOOH particles formed in an aqueous solution.     

Atomic-scale structure and morphology of ferric oxyhydroxides formed by corrosion of iron in various aqueous media

Fine particles of corrosion products consisting of one kind of ferric oxyhydroxide, either γ-FeOOH (lepidocrocite) or α-FeOOH (goethite), were directly prepared by dipping pure iron into aqueous solutions containing sodium chloride, sodium sulfate, or their mixed salts with sodium bicarbonate. Quantitative X-ray structural analysis using an in-house X-ray diffraction apparatus has been used for characterizing the atomic-scale structure of these ferric oxyhydroxide particles. The morphology of the γ-FeOOH and α-FeOOH particles was observed by transmission electron microscopy (TEM), and their bonding structures were analyzed using Fourier transform infrared spectroscopy (FT-IR). The realistic atomic-scale structures in the γ-FeOOH and α-FeOOH particles were estimated by fitting the interference functions with the help of the reverse Monte Carlo (RMC) simulation technique. The results showed that the linkages of fundamental FeO₆ octahedral units in the particles were deviated from the ideal crystal structure. The structural deviation is believed to be due to the incorporation of foreign anions during the formation of these particles in the aqueous solutions. The resultant atomic-scale structures in the γ-FeOOH and α-FeOOH particles were correlated with their morphology and bonding structure.     

In situ characterization by localized electrochemical impedance spectroscopy of the electrochemical activity of microscopic inclusions in an X100 steel

A localized electrochemical impedance spectroscopy (LEIS) technique was used to characterize in situ the micro-electrochemical activity of inclusions contained in an American Petroleum Institute (API) X100 steel in a near-neutral pH solution. It is found that there exists an electrochemical heterogeneity between inclusions and the adjacent steel matrix. Consequently, a galvanic couple is formed to result in the locally preferential dissolution. The local electrochemical activity of the inclusion depends on its composition. A Si-enriched inclusion is associated with a high electrochemical activity, and the preferential dissolution of the inclusion generates a local microvoid, whose further dissolution initiates a corrosion pit. An aluminum oxide-enriched inclusion is more stable than the adjacent steel matrix. The preferential dissolution would occur on the steel, causing the “drop-off” of the inclusion and generating a corrosion pit.     

Ex-situ and in-situ X-ray diffractions of corrosion products freshly formed on the surface of an iron-silicon alloy

Ex-situ X-ray diffraction measurements of a small amount of samples extracted from wet corrosion products freshly formed on a pure iron and iron-2 mass% silicon surfaces have been conducted using synchrotron radiation for clarifying the formation process of corrosion products. The results showed that γ-FeOOH was formed on the outer side of wet corrosion products formed on the surface of the pure iron by sodium chloride solution, while γ-FeOOH, α-FeOOH, Fe₃O₄, and green rusts were formed on the inner side. On the other hand, in comparison to the case of the pure iron, a significant formation of β-FeOOH was observed in the iron-silicon alloy. Influences of silicon alloying on corrosion products formed by aqueous solution containing sulfate ions were also observed. Furthermore, in-situ diffraction measurements by a conventional X-ray source were conducted for analyzing corrosion products formed on the pure iron and iron-silicon alloy surfaces by cyclic exposure to wet and dry atmospheres. The results obtained by the in-situ diffraction and ex-situ diffraction measurements on the corrosion products were consistent.     

The nucleation and growth of metastable pitting on pure iron

The nucleation and growth of metastable pitting on pure iron surface in NaNO₂ + NaCl solution were investigated by potentiodynamic tests. The current fluctuations appear as cluster and overlapped. The active sites for nucleation depend mainly on the surface geometry. The growth behaviour of pure iron is different from stainless steels and carbon steels. On pure iron surface, many pits pile up together to form huge damage area. Pits are shallow, do not grow deeply and pits array one by one along the direction of abrasion grooves. The growth of stable pitting is different from metastable pitting.     

An ESEM in situ investigation of initial stages of the KCl induced high temperature corrosion of a Fe–2.25Cr–1Mo steel at 400 °C

The initial oxidation of a low-alloyed steel (Fe–2.25Cr–1Mo) in the presence of small amounts of KCl(s) have been investigated through ESEM in situ exposure and analysis at 400 °C. The samples were also characterized by XRD, SEM/EDX and FIB. The present study shows the corrosive nature of KCl towards the low alloyed steel. It is concluded that the initial KCl distribution is important and that a KCl/FeCl₂ liquid phase film forms on large parts of the oxide surface in the presence of KCl. It is proposed that Cl increases the oxidation rate (by decorating oxide grain boundaries) and decreases the oxide scale adhesion.     

Polymorphism of PrIr2Si2 – In situ XRPD experiments and theoretical calculations

We have confirmed polymorphism of PrIr₂Si₂ and performed the in situ high-temperature X-ray powder diffraction (XRPD) experiments focused on the dynamics of the crystallographic phase transition from the high-temperature CaBe₂Ge₂-type phase to the low-temperature ThCr₂Si₂ phase above 250 °C. A double-exponential time evolution of this phase transformation has been observed at 325 °C. We have also performed density functional calculations to analyze why the low-temperature crystallographic modification becomes the ground state crystal structure. The important role of the c/a ratio in this process can be argued from results of calculations.     

Three dimensional microstructure study of oxide scale formed on a high-speed steel by means of SEM, FIB and TEM

Characteristics of the oxide scale formed on a high-speed steel (HSS) material in the temperature range 550–650 °C were examined. The surface morphologies of oxidised sample indicate that the temperature has a significant influence on the oxidation behaviour of the HSS samples. Differential oxidation is expected to occur due to high-alloyed components in the material. The carbide-free matrix has a good oxidation resistance due to the dissolved chromium; while vanadium rich carbide (MC) regions were oxidised heavily because of high free energy at the carbide/matrix interface and low thermal stability of the MC carbides. FIB/TEM cross-section observation shows that the oxide scale formed on the surface has a duplex-layer structure with vanadium oxides covering the top of the scale.     

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.     

In situ impedance spectroscopy study of the electrochemical corrosion of Ti and Ti–6Al–4V in simulated body fluid at 25 °C and 37 °C

The corrosion resistance of Ti and Ti–6Al–4V was investigated through electrochemical impedance spectroscopy, EIS, potentiodynamic polarisation curves and UV–Vis spectrophotometry. The tests were done in Hank solution at 25 °C and 37 °C. The EIS measurements were done at the open circuit potential at specific immersion times. An increase of the resistance as a function of the immersion time was observed, for Ti (at 25 °C and 37 °C), and for Ti–6Al–4V (at 25 °C), which was interpreted as the formation and growth of a passive film on the metallic surfaces.     

Temperature dependence of oxide scale formation on high-Cr ferritic steels in Ar–H2–H2O

Four high chromium ferritic steels were oxidized in Ar/H₂/H₂O at temperatures between 500 and 900 °C. Polished specimens of all steels formed iron-rich oxides at temperatures below 600 °C, whereas increasing the temperature resulted in local formation of protective chromia scales. As the temperature was raised further, the specimens were totally covered with chromia scales. For the higher chromium steels this was also observed at 900 °C but not for the steel with a chromium content of 16%. The temperature dependence of the oxidation rates is governed by the competing diffusion processes in the alloy and the growing scales.     

Influence of phosphate species on green rust I transformation and local structure and morphology of γ-FeOOH

To clarify the role of phosphate in the formation of corrosion products, the transformation of GRI(Cl⁻) with the addition of phosphate was characterized through XRD, TEM, and solution analysis. Electrochemical analysis showed that the transformation of GRI(Cl⁻) was delayed and the size of the final products, i.e., γ-FeOOH was reduced in the phosphate added case. X-ray absorption spectroscopy indicated that the neighboring Fe–Fe coordination number of FeO₆ octahedral unit in γ-FeOOH was decreased. These effects of phosphate are attributed to its adsorption on GRI(Cl⁻) and nucleated γ-FeOOH that prevented particle growth during oxidation process.     

Effect of Dy on oxide scale adhesion of NiAl coatings at 1200 °C

The cyclic oxidation behaviour of β-NiAlDy coatings produced by electron beam physical vapour deposition (EB-PVD) was investigated. For the Dy-free NiAl coating, numerous voids developed at the Al₂O₃ scale/NiAl interface and sulfur was found to segregate at the void surfaces, leading to early spallation of the scale. The addition of Dy prevented sulfur segregation and void formation at the scale/NiAl interface. As a result, the scale grown on the NiAlDy coating remained adherent and no spallation occurred even after 400 h cyclic oxidation. The 0.05 at.% Dy doped coating revealed lower oxidation rate than the more Dy doped coatings.     

Corrosion of Ti3AlC2 at 800–1100 °C in Ar–0.2% SO2 gas atmosphere

Ti₃AlC₂ was corroded between 800 and 1100 °C in an Ar–0.2% SO₂ gas atmosphere according to the equation: Ti₃AlC₂ + O₂ → rutile-TiO₂ + α-Al₂O₃ + (CO or CO₂). The scales that formed on the Ti₃AlC₂ were thin and rich in α-Al₂O₃, whose growth rate was exceedingly slow. The TiO₂ was present either as the outermost surface scale or a mixture inside the α-Al₂O₃-rich scale. In the Ti₃AlC₂, the activity and diffusivity of Ti were low, whereas those of Al were high. This was the main reason for the superior corrosion resistance of Ti₃AlC₂ over TiAl.     

A fundamental aspect of the growth process of alumina scale on a metal with dispersion of CeO2 nanoparticles

The dispersion of CeO₂ nanoparticles resulted in a decrease of the oxidation rate of an ultrafine-grained Ni₂Al₃ at 1000 °C. The reason is explained as follows. During oxidation many Ce ions are released from the CeO₂ nanoparticles that are enveloped by the inward growing α-Al₂O₃ from the scale/metal interface due to an increased solubility. The Ce ions transport outward along grain boundaries of the scale, retarding the diffusion of Al ions for the thickening of the outer θ-Al₂O₃. This explanation is consistent with an observation that many CeO₂ nano-precipitates appeared mainly in the near-surface zone of the formed alumina scale.     

Corrosion behavior of commercially pure Mg and ZM21 Mg alloy in Ringer’s solution – Long term evaluation by EIS

Evaluation of the corrosion behavior of commercially pure magnesium (CP-Mg) and ZM21 Mg alloy immersed in Ringer’s solution for 92 h by electrochemical impedance spectroscopy (EIS) is addressed. The formation of a compact layer of well-developed rod-like aragonitic CaCO₃ crystals and its subsequent thickening with increase in immersion time offers a higher corrosion protective ability for ZM21 Mg alloy. The formation of a mud-crack pattern and a large number of clusters of needle-like crystals offers a relatively lower corrosion resistance for CP-Mg. The study suggests that ZM21 Mg alloy is a promising candidate material for the development of degradable implants.     

Oxidation behaviour of Al2O3–TiC–Co composites at 800–1000 °C in air

The oxidation behaviour of nanometre and micrometre sized Al₂O₃–TiC–Co composites is investigated at 800–1000 °C in air for 25 h. The oxidation resistance of nanometre sized samples is better than of micrometre sized. Phase compositions and microstructures were studied by XRD and SEM. The values of general rate constant k and oxidation exponent n are dependent on oxidation temperature and composites. The oxidation kinetics followed a rate that is slightly faster than the parabolic-rate law at 800–1000 °C. The activation energy of the nanometre sized is higher than of micrometre sized in the range of 800–1000 °C.