Effects of water vapour on isothermal oxidation of chromia-forming alloys in Ar/O2 and Ar/H2 atmospheres

Fe9Cr, Fe17Cr and Fe25Cr alloys were subjected to isothermal oxidation in Ar/O₂ and Ar/H₂ atmosphere at 700 °C as high temperature corrosion for 48 h. Oxidation weight change measurement showed increasing Cr content reduced the oxidation rate. The oxidized Cr alloys were analysed using SEM, TEM and XRD. The addition of water vapour accelerates the onset of breakaway oxidation kinetics for Fe9Cr. The presence of water vapour promotes internal oxidation of Cr within Fe9Cr. For Fe17Cr and Fe25Cr, the water vapour effect is not significant due to the large Cr reservoir due to continue growth of Cr₂O₃.     

Stability of MnCr2O4 spinel and Cr2O3 in high temperature carbonaceous environments with varied oxygen partial pressures

In this investigation, Cr₂O₃ and MnCr₂O₄ were comparatively tested at 1050 °C in carbonaceous environment with varied oxygen partial pressures. MnCr₂O₄ exhibits much better resistance to carbonaceous attack than Cr₂O₃. The carburization rate of MnCr₂O₄ decreases sharply with increasing oxygen partial pressures. The oxygen partial pressures have less effect on the carburization resistance of Cr₂O₃. The increased resistance of MnCr₂O₄ to carburization is attributed to the dissolution of MnO into Mn-Cr-O spinel lattices with elevated oxygen partial pressures, which retards the decomposition and carburization of Mn-Cr-O spinel. The thermodynamic equations defining the carburization stability of MnCr₂O₄ and Cr₂O₃ are modified.     

Ion-plated Al-Al2O3 films as diffusion barriers between NiCrAlY coating and orthorhombic-Ti2AlNb alloy

Ion-plated Al-Al₂O₃ cermet films were fabricated as diffusion barriers between NiCrAlY coating and orthhombic-Ti₂AlNb alloy. The oxidation and interdiffusion behaviour of coatings with and without diffusion barrier were investigated in isothermal and cyclic oxidation tests at 800 °C. The results indicated that substantial interdiffusion and rapid oxidation degradation occurred in the coated specimens without diffusion barrier. With Al-Al₂O₃ diffusion barriers, deferred interdiffusion and improved oxidation resistance was observed. Among them, duplex coating containing 1Al-Al₂O₃ interlayer exhibited the best performance. Coefficient of diffusion hindering and factor of reaction hindering were proposed to compare and quantify the efficiency of the diffusion barriers.     

Oxidation and interfacial fracture behaviour of NiCrAlY/Al2O3 coatings on an orthorhombic-Ti2AlNb alloy

Al₂O₃ diffusion barriers of various thicknesses have been fabricated by filtered arc ion plating between the NiCrAlY coating and the O-Ti₂AlNb alloy. Isothermal oxidation tests and three-point bend tests have been conducted to investigate the influence of the Al₂O₃ diffusion barriers on the oxidation and interfacial fracture behaviour of the coatings. The results indicate that the Al₂O₃ diffusion barrier defers interdiffusion and gives oxidation resistance of the NiCrAlY coatings. The thickness of the Al₂O₃ interlayer not only influences the oxidation behaviour but also affects the interfacial fracture properties. Additionally, thermal exposure affects the critical load in three-point bend tests.     

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.     

Effect of tin on high-temperature sulphidation of Fe-Cr alloys in H2/H2S atmospheres

The high temperature sulphidation behaviour of Fe-46Cr-xSn (x = 0; 0.2; 0.5; 1; 2) alloys has been studied at temperatures of 1073, 1173 and 1273 K in H₂/H₂S mixtures with different sulphur vapour partial pressures of 10⁻¹, 10⁻³ and 10⁻⁵ Pa. Thermogravimetric studies in combination with scanning electron microscope (SEM), with energy dispersive spectrometer (EDS), and X-ray diffraction (XRD) techniques, have displayed a significant influence of the sulphur partial pressure on the composition and growth rate of the sulphide scale. The results have shown that addition of tin increases the sulphidation rate of Fe-46Cr alloys but not considerably (except at temperatures of 1073 and 1173 K combined with sulphur partial pressure of 10⁻⁵ Pa). The metallic core of the studied samples was enriched in tin and iron, moreover tin was found in the internal layer close to the metallic core as metallic Fe_xSn_y inclusions with tin concentrations of up to 12 at.%.     

Al2O3-ZrO2 mixed oxide composite incorporated aluminium rich zinc coatings for high wear resistance

Corrosion resistance and wear resistance are the two important parameters for high performance of zinc galvanic coating. In the present work, the improvement of these two characteristics was achieved by the incorporation of Al₂O₃-ZrO₂ mixed oxide composite in the coating. Al₂O₃-ZrO₂ mixed oxide composite was synthesized from ZrOCl₂·8H₂O. Aluminium rich zinc coatings with high sliding wear resistance was developed from a galvanic bath containing the mixed oxide. Based on the performance of the coating during physicochemical and electrochemical characterization, the concentration of mixed oxide composite in the bath was optimized as 0.50 wt% Al₂O₃-0.50 wt% ZrO₂. While rich in Al-metal content in the coating caused high corrosion resistance, the incorporation of the mixed oxide improved structural characteristics of the coating resulting in high wear resistance also. The coating was nonporous in nature and even the interior layers had high stability. The coatings have potential scope for high industrial utility.     

A novel ultrafine-grained Ni3Al with increased cyclic oxidation resistance

An ultrafine-grained (UFG) Ni₃Al was fabricated by annealing an electrodeposited Ni–Al composite in vacuum at 600 °C for 2 h. The UFG Ni₃Al, compared to a compositional-similar but coarse-grained (CG) alloy prepared by arc-melting, exhibited a greatly increased cyclic oxidation resistance at 900 °C. Microstructural investigation showed that the CG alloy grew a scale with a high susceptibility to buckling and cracking because of the formation of large voids at the scale/metal interface, but that the UFG alloy grew an adherent scale, because its typical structure prevented the formation of the interface void during oxidation.     

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.     

Effect of sintering temperature on corrosion properties of sol–gel based Al2O3 coatings on pre-treated AZ91D magnesium alloy

A novel anti-corrosion sol–gel based Al₂O₃ coating was developed on the AZ91D magnesium alloy. The morphology, microstructure and composition of the coatings were investigated by scanning electron microscope coupled with energy dispersive spectroscopy, Fourier transform infrared spectrum analysis, X-ray diffraction, thermo-gravimetric and differential thermal analysis. The corrosion resistance of the coatings in 3.5 NaCl wt.% solution was studied using electrochemical measurements. The results demonstrated that a homogeneous Al₂O₃ coating could be obtained and the sol–gel coated samples sintered at 380 °C had the best corrosion resistance properties as compared to the specimens sintered at 120 and 280 °C.     

Hot corrosion behaviour of Yb2Zr2O7 ceramic coated with V2O5 at temperatures of 600-800 °C in air

To better understand the hot corrosion behaviour of Yb₂Zr₂O₇ ceramic in molten V₂O₅, hot corrosion experiments were performed in a temperature range of 600-800 °C in air. Different reaction products of ZrV₂O₇, YbVO₄ and m-ZrO₂ were identified depending upon the hot corrosion conditions, for example, ZrV₂O₇ and YbVO₄ at 600 °C for 2 h and 8 h; ZrV₂O₇, m-ZrO₂ and YbVO₄ at 700 °C for 2 h; m-ZrO₂ and YbVO₄ either at 800 °C for 2 h or at 700-800 °C for 8 h. The hot corrosion reaction mechanisms were further discussed based on the thermal instability of ZrV₂O₇ at elevated temperatures.     

High temperature oxidation resistance of FeCrAl alloys covered with ceramic SiO2–Al2O3 coatings deposited by sol–gel method

One-, three- and five-layer SiO₂–Al₂O₃ coatings were deposited on a FeCrAl alloy basis by the sol–gel method. Sols in which the molar ratio of tetraethoxysilan to aluminium tri-sec-butoxide was 1:1 and 1:3 were used.As the samples were being soaked at T = 1200 °C for t = 700 h the mass of the samples increased. Thermal shock (T = 1200 °C, 10,000 cycles) causes greater degradation of the surface than soaking at a constant temperature. The XPS and EDS results show that the composition of the top layer of the coatings changes during high temperature oxidation and thermal shock. The outward movement of aluminium cations results in surface enrichment with aluminium, particularly for the single-layer coatings. The measured energies of bonds Si 2p and Al 2p in the multilayer coatings indicate that a structure of aluminosilicates with a composition between that of mullite and that of sillimanite forms during sintering.     

Recession behavior of Yb2Si2O7 phase under high speed steam jet at high temperatures

Recession behavior of Yb₂Si₂O₇ phase was examined under high speed steam jet environment between 1300 °C and 1500 °C. Yb₂SiO₅ phase was formed on the bulk surface by the decomposition of Yb₂Si₂O₇ phase and the elimination of silica component at elevated temperatures. The phase ratio of Yb₂SiO₅/Yb₂Si₂O₇ increased up to 1400 °C and then decreased above 1400 °C. The relative intensity of 2 2 0 peak for Yb₂Si₂O₇ phase increased with increasing the temperatures. Fine grains were generated on the bulk surface at 1300 °C. The phase decomposition caused on the grain interior. A porous structure was formed on the bulk surface during the test at 1400 °C. Surface cracks were generated for 1400 °C test sample. A smooth surface was generated on the surface of 1500 °C test sample. The triple points of the grains were bridged with a glassy phase.     

Low-temperature atomic layer deposition of Al2O3 thin coatings for corrosion protection of steel: Surface and electrochemical analysis

ToF-SIMS, XPS, voltammetry and EIS investigation of the anti-corrosion properties of thin (10, 50 and 100 nm) alumina coatings grown by atomic layer deposition at 160 °C on steel is reported. Surface analysis shows a thickness-independent Al₂O₃ stoichiometry of the coating and trace contamination by the growth precursors. The buried coating/alloy interface has iron oxide formed in ambient air and/or resulting from the growth of spurious traces in the initial stages of deposition. Electrochemical analysis yields an exponential decay of the coating porosity over four orders of magnitude with increasing thickness, achieved by sealing of the more defective first deposited 10 nm.     

Discontinuous oxidation and erosion-oxidation of a CeO2-dispersion-strengthened chromium coating

A CeO₂-dispersion-strengthened chromium coating was developed on a carbon steel using a two-step process: prior electrodeposition of a Ni-CeO₂ nanocomposite film and subsequent chromization using a conventional pack cementation method. Compared to the CeO₂-free coatings prepared on the carbon steel without and with pre-electrodeposition of a pure Ni film, the CeO₂ dispersed chromium coating offered profoundly improved discontinuous oxidation resistance at 900 °C in 5% O₂ + N₂ and in 5% O₂ + 1000 ppm SO₂ + N₂, and erosion-oxidation resistance in a laboratory-scale fluidized-bed combustor (FBC), mainly because of the development of a denser, less wrinkled and more adherent chromia scale.     

Platinum-induced oxidation of chromium in O2 at 800 °C

The effects of porous Pt on the oxidation of Cr at 800 °C have been studied with the ¹⁸O-SIMS technique, gas phase analysis and XPS. In oxide areas with Pt a pronounced inward oxygen transport takes place and a substantial oxide growth near the Cr substrate is observed. In oxide grown on areas without Pt the counts of CrO ions in SIMS and the binding energy of O (1s) in XPS depend on the distance from the area with Pt. The experimental observations are believed to be a consequence of a high dissociation efficiency of O₂ on areas with Pt in combination with a high diffusivity of O in external and internal oxide surfaces on areas both with and without Pt.     

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.     

Microstructure, spallation and corrosion of plasma sprayed Al2O3–13%TiO2 coatings

The electrochemical corrosion behaviours of the steel substrates coated with three different plasma sprayed Al₂O₃–13%TiO₂ coatings were studied in this paper. The three kinds of Al₂O₃–13%TiO₂ coatings were conventional ME coating, nanostructured NP coating and NS coating. There were micro cracks, laminar splats and straight columnar grains in ME coating. For the two nanostructured coatings, the laminar microstructure and columnar grains were not obvious. The NP coating had the highest hardness and spallation resistance. Electrochemical corrosion behaviour of the three coatings was mainly investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in aqueous Na₂SO₄ solution.     

Role of Al2O3 layer in oxidation resistance of Cu-Al dilute alloys pre-annealed in H2 atmospheres

Copper was alloyed with small amounts of Al (0.2, 0.5, 1.0 and 2.0 mass%) to improve the oxidation resistance. Copper (6 N) and the Cu-Al alloys were oxidized at 773-1173 K in 0.1 MPa oxygen atmosphere after hydrogen annealing at 873 K. Continuous very thin Al₂O₃ layers were formed on the surface of all Cu-Al dilute alloys during the hydrogen annealing. Oxidation resistance of Cu-Al alloys was improved especially for Cu-2.0Al at 773-973 K, while it decreases on increasing the oxidation temperature. Cu-Al alloys followed the parabolic rate law at 1173 K, but most of other cases do not at and below 1073 K. Oxidation resistance for Cu-Al alloys was found relevant to the maintenance of the thin Al₂O₃ layer at the Cu₂O/Cu-Al alloy interface.     

B2O3 modified SiC–MoSi2 oxidation resistant coating for carbon/carbon composites by a two-step pack cementation

To protect carbon/carbon (C/C) composites against oxidation, a B₂O₃ modified SiC–MoSi₂ coating was prepared by a two-step pack cementation. The microstructure and the oxidation resistant property of the coating were studied. The results show that, the as-received coating is a dense structure, and is composed of α-SiC, β-SiC and MoSi₂. The B₂O₃ modified SiC–MoSi₂ coating has excellent oxidation resistant property, and can protect C/C composites from oxidation at 1773 K in air for more than 242 h. The failure of the coating was considered to arise from the existence of the penetration cracks in the coating during the slow cooling from 1873 to 673 K.