Improvement in the oxidation resistance of TiAl by preoxidation in a TiO2-powder pack

The resistance of TiAl coupons to cyclic oxidation at 1300 K in a flow of purified oxygen under atmospheric pressure has been significantly improved by preoxidation. The preoxidation was performed by heating the specimens, buried in rutile powder and encapsuled in a silica tube under a vacuum of 1.3×10^–3 Pa, at 1200 K for 100ks. The scales formed by preoxidation are very adherent, and the mass changes are very small for at least up to 20 cycles (400 h) of subsequent oxidation. This excellent oxidation resistance is attributable to the formation of scales very rich in alumina by preoxidation under a very low oxygen partial pressure. A similarly good oxidation resistance was obtained when the specimens were buried in rutile powder and preoxidized in Ar gas under atmospheric pressure at 1300 K for 100 ks.     

High-temperature corrosion and mechanical properties of protective scales on Incoloy 800H: The influence of preoxidation and ion implantation

Coatings, obtained by preoxidation of Incoloy 800H at low PO₂ show good sulphidation resistance due to the higher chromia content in the oxide scale. Yttrium-ion implantation of Incoloy 800H has also a beneficial effect on sulphidation, if preoxidation is applied. The reason for this is presumably the segregation of yttrium to grain boundaries of the oxide. Furthermore, the oxidation kinetics of Incoloy 800H are independent of the partial pressure of the oxygen. Mechanical testing of the preformed oxide scale/substrate combinations in air at 600°C by means of constant-extension-rate experiments shows that preoxidation at low partial pressures of oxygen leads to earlier scalecracking.     

High temperature oxidation of Mo–Si–B alloys: Effect of low and very low oxygen partial pressures

The oxidation mechanism of a Mo–Si–B alloy in two different oxygen partial pressure ranges was investigated between 820 and 1200 °C. Oxygen partial pressures between 10^?19 and 10^?12 bar were applied in order to suppress Mo oxide formation. Weight gain kinetics were determined resulting from simultaneous external and internal oxidation. Silica scale formation was found to lead to a droplet shape because of the high evaporation rates of B₂O₃ and limited wetting of the silica. In the oxygen partial pressure range 10^?6–10^?4 bar Mo–Si–B alloys suffer from severe degradation due to continuous formation of volatile MoO₃. Catastrophic oxidation was observed as a consequence of the formation of a highly porous and non-protective silica scale.     

Oxidation behavior of alloy HK40 in H2?H2O atmosphere

In the present paper, the isothermal oxidation behavior of alloy HK40 in H₂? H₂O atmosphere in the temperature range 850–950 °C was systematically investigated by means of TGA, XRD, SEM, and EDS. The results demonstrated that the oxidation of alloy HK40 under low oxygen partial pressure obeyed a cubic instead of parabolic rate law. The values of the oxidation rate constant k at 850, 900, and 950 °C were 0.0079, 0.01886 and 0.04031 mg³/(cm⁶ h) at 850, 900, and 950 °C, respectively. The scales were composed of MnCr₂O₄ and Cr₂O₃. MnCr₂O₄ mainly existed in the form of blades in the outer part of the scale and its amount gradually increased with increasing temperature. In addition, there were silica particles along the scale/metal interface and indications were found that they contribute to the improvement of scale spallation resistance at high temperature as found by scratch tests.     

Oxidation Behavior and Oxide Layers of Ti–50Al Intermetallics by Preoxidation in High-Pressure Oxygen

The oxidation resistance of Ti–50Al intermetallics is improved bypreoxidation for 1, 4, or 16 hours in high-pressure, pure oxygen(3.9 atm) at 900°C. Specimens preoxidized for 1 hr exhibit betteroxidation resistance than others. Prolonged preoxidation time candeteriorate the oxidation resistance and reduce the parabolic-lineartransition time during subsequent cyclic oxidation in 800°C air. Theoxide-mound occurrence is an important factor for evaluating theeffectiveness of the preoxidation treatment in oxygen. The formationmechanism of Z-phase (Ti₅Al₃O₂) in the Al-depleted layer beneath theflat oxide scale and that beneath the oxide mound are also proposed inthis study.     

Oxidation behaviour of zirconium diboride–silicon carbide ceramic composites under low oxygen partial pressure

The oxidation behaviour of ZrB₂-based ceramics under low oxygen partial pressure ranged from 0.5 to 1.5 kPa was investigated. Low oxygen partial pressure was found to have remarkable effect on phase composition of the surface and the structures of oxide scale. And the thickness and microstructures of oxide scale was characterized by using scanning electron microscopy and X-ray diffraction analysis. The results indicate that the oxidation mechanism of ZrB₂-based ceramics changes under low oxygen partial pressure, and the oxidation resistance increases with the reduction of oxygen partial pressure.     

Isothermal and cyclic oxidation behaviour of hot-pressed MSi2 compounds (with M = V,Ti, Cr)

The oxidation resistance of MSi₂ compounds with M = V, Ti, Cr was investigated from 450 to 950 °C in air under isothermal and cyclic conditions. Vanadium, chromium and titanium disilicide were not subjected to the pest phenomenon at 650 °C over 800 1-h cycles. The results demonstrated very low weight gains regardless of the testing conditions. Oxidation tests were also performed over long duration (1000 h) to identify the oxidation products. The MSi₂ compounds were all subjected to the simultaneous oxidation of M and Si despite the formation of a protective silica scale. Increasing the duration of oxidation enhanced the protective properties of the silica scale. Therefore, short-term measurements (by thermogravimetry) did not allow an extrapolation of the MSi₂ lifetime. The formation of molten V₂O₅ induced a higher oxidation rate of VSi₂ and delayed the establishment of the protective silica scale compared with CrSi₂ and TiSi₂.     

Oxidation of Ni-base alloys in atmospheres with widely varying oxygen partial pressures

The oxidation behavior of the Ni-base alloys IN 617, IN 713 LC, Ni20Cr, and Ni20Cr+Si has been investigated in the temperature range from 850°C to 1000°C in air and at low-oxygen partial pressure p(O₂) (10^–19 to 10^–16 bar). With the exception of alloy IN 713 LC, the materials show no influence of p(O₂) on the oxidation mechanisms and the kinetics. This result can be explained by the formation of a dense Cr₂O₃ layer, the growth rate of which is controlled by the Cr ion interstitial concentration in Cr₂O₃ at the phase boundary oxide/alloy and the mobility of Cr ions in Cr₂O₃. For the alloy IN 713 LC which develops a dense Al₂O₃ layer in air, a modified transition mechanism at low p(O₂) leads to the formation of Cr₂O₃ at the surface and a strong internal oxidation of Al.     

Effect of molybdenum on high-temperature corrosion of Fe–Al intermetallics

Fe41Al4Mo and Fe32Al19Mo (at.%) intermetallics were prepared by self-propagating high-temperature synthesis (SHS) combined with arc melting (AM) and vacuum annealing. Corrosion resistance was evaluated on the basis of sulphidation and oxidation tests. Sulphidation experiments were carried out in the flowing H₂/H₂S mixtures mainly at a temperature of 1273 K and at sulphur pressures 10⁻³–0.3 Pa. Reaction progress was followed thermogravimetrically. The extent of corrosion attack increased with sulphur pressure, temperature and molybdenum concentration. Oxidation tests were carried out in air in isothermal (at 1073 and 1273 K) and thermal cycling conditions (24-h cycles at 1223 K). The results of kinetic studies, metallographic observations as well as phase and chemical analysis of corrosion products indicated limited resistance of the investigated FeAlMo alloys to sulphidation or oxidation at elevated temperatures.     

High-Temperature Corrosion Resistance of Al–Re and Re Coatings

The oxidation behavior in air at 1473 K, and sulfidation behavior in a H2S–H2 gas mixture with a sulfur partial pressure of 10^–2 Pa at 973 K of Al–Re coated CMSX-4 were studied. Investigation on the sulfidation behavior of the Re-coated CMSX-4 was carried out in a H2S–H2 gas mixture with a sulfur partial pressure of 10^–2 Pa at 973 K. The experimental results show that a Re-rich alloy layer was formed between an -Al₂O₃ layer and the inner concentration zone of Ta and Ti for the CMSX-4 single crystal alloy with an Al–Re coating after oxidation. The Re-rich alloy layer containing Cr, W, Ni, Co, and Mo effectively inhibited the outward diffusion of alloying elements and the inward diffusion of Al. The Al/Re-coated CMSX-4 single crystal alloy had excellent sulfidation resistance; the Re-rich alloy layer, containing W, Ti, Ta, and Mo, which formed during the sulfidation process and located between the alumina scale and the CMSX-4 base alloy, plays a role in inhibiting the outward diffusion of alloying elements. The sulfidation resistance of CMSX-4 single-crystal alloy is greatly improved by a Re coating on the CMSX-4 alloy surface; this is attributed to a Re–Cr–W alloy layer that retarded the outward diffusion of cations and the oxide layer containing Ta, Ti, and Al, which inhibited the inward penetration of sulfur.     

Production of Si by vacuum carbothermal reduction of SiO<Subscript>2</Subscript> using concentrated solar energy

Using concentrated solar radiation as the energy source of high-temperature process heat, the carbothermal reduction of silica to silicon was examined thermodynamically and demonstrated experimentally at vacuum pressures. Reducing the system pressure favors Si(g) formation, enabling its vacuum distillation. Experimentation in a solar reactor was performed in the range 1,997–2,263 K at ∼3×10⁻³ bar with mixtures of charcoal and silica directly exposed to radiative flux intensities equivalent to 6,500 suns, yielding Si purities ranging from 66.1–79.2 wt.%. The Si purity increased with temperature. Solid characterizations showed SiC and SiO as important reaction intermediaries.     

Corrosion of alloy 800H and the effect of surface-applied CeO2 in a sulphidizing/oxidizing/carburizing environment at 700°C

The corrosion behavior of a wrought austenitic Fe-20Cr-32Ni steel, Alloy 800H, was studied in a simulated coal-gasification atmosphere at 700°C for exposure times up to 2500 hr. The influence of preoxidation and CeO₂-surface application followed by preoxidation on the corrosion resistance of this material was assessed. The improvement in the corrosion resistance due to preoxidation of the blank material was small, whereas the effect of the CeO₂-treatment was significant. This difference is thought to be due to better scale adherence in the case of CeO₂-surface application.     

The corrosion of pure niobium in oxidizing,sulfidizing, and oxidizing-sulfidizing gas mixtures at 600–800°C

The corrosion of pure niobium has been studied at 600–800°C in various environments as part of a study of the corrosion resistance of its alloys with iron, cobalt, and nickel to atmospheres of low-oxygen and/or high-sulfur activities. The results have shown that not only the sulfidation but also the corrosion in mixed atmospheres and particularly the oxidation under low oxygen pressures of pure niobium are quite slow, with kinetics rather similar in the three types of gas mixtures used. The good corrosion resistance of niobium to attack by oxygen under low pressures is quite interesting because this element is corroded very rapidly by oxygen under high oxygen pressures, due to the formation of the nonprotective highest oxide Nb₂O₅ as a main corrosion product.     

Effect of Co on oxidation and hot corrosion behavior of two nickel-based superalloys under Na2SO4–NaCl at 900 °C

Nickel-based superalloys with and without Co by partial replacement of W were prepared using double vacuum melting. A comparison of the oxidation in air and hot corrosion behaviors under molten 75wt.%Na₂SO₄+ 25wt.%NaCl at 900 °C were systematically investigated. The results showed that partial replacement of W with Co promoted the formation of chromia scale and consequently decreased the oxidation rate. Besides, the addition of Co also retarded the internal oxidation/nitridation of Al and consequently promoted the growth of Al₂O₃ scale, which further decreased the scaling rate and improved the adhesion of scale. Moreover, the addition of Co also further improved the hot corrosion resistance under molten Na₂SO₄–NaCl salts.     

TEM Observations of the Initial Oxidation Stages of Nb-Ion-Implanted TiAl

Coupon specimens of TiAl were implanted with Nb ions at an acceleration voltage of 50 kV with a dose of 10²¹ ions m.^–2 They were then slightly oxidized during heating to 900 or 1200 K, or at 1200 K for 3.6 ksec (1 hr) in a flow of purified oxygen under atmospheric pressure. The implanted specimens and oxidized specimens were characterized and observed by AES, X-ray diffractometry, SEM, TEM, EDS, and EPMA. Implantation improves the oxidation resistance significantly by forming virtually -Al₂O₃ scales. The implanted layer is about 75 nm thick; the outer part of 30-nm thickness is -Ti phase and the rest of 45-nm thickness is amorphous. Heating to 900 K in O₂ results in partial crystallization of the amorphous layer to Ti₅Al₃O₂ (Z-phase) and to 1200 K results in oxide scales of 270 to 400 nm thickness consisting mainly of Al₂O₃. The fraction of Al₂O₃ in the scale increases toward the substrate. Oxidation at 1200 K for 3.6 ksec results in Al₂O₃-rich scales of about 400-nm thickness. The oxide grain size is very fine, about 80 nm in size, and becomes smaller toward the outer scale surface. This implies that implantation enhanced the nucleation of Al₂O₃ grains relative to the growth of TiO₂ grains. This finding and the formation of -Ti phase are thought to be responsible for the excellent oxidation resistance obtained.     

Oxidation rates of niobium and tantalum alloys at low pressures

Niobium and tantalum alloys have excellent properties for use in high-temperature, space-power applications, but must be protected from oxidation that would result from exposure to air in ground-evaluation tests. The oxygenuptake/oxidation rates of three alloys, Nb-1Zr, PWC-11, and ASTAR-811C were measured at oxygen partial pressure of 10^–6 and 10^–7 torr at temperatures up to 1350 K. No visible oxide film was observed, and the oxidation rate was found to be linearly proportional to pressure and exponentially proportional temperature. A thin molybdenum coating on Nb–1Zr was a barrier to lowpressure oxidation at 773 K.     

Active Oxidation of Liquid Silicon: Experimental Investigation of Kinetics

Small scale laboratory experiments on the oxidation of liquid silicon have reproduced important features of the industrial refining of liquid silicon: active oxidation led to the formation of amorphous silica spheres as a reaction product. The boundary condition for active oxidation in terms of maximum oxygen partial pressure in the bulk gas was found to lie between 2·10^?3 and 5·10^?3?atm at T?=?1,500?°C. The active oxidation of liquid silicon had linear kinetics, and the rate was proportional to bulk oxygen partial pressure and the square root of the linear gas flow rate, consistent with viscous flow mass transfer theory. Classical theory for unconstrained flow over a flat plate led to mass transfer rates for SiO_(g) which were 2–3 times slower than observed. However, computational fluid dynamic modeling to take into account the effects of reactor tube walls on flow patterns yielded satisfactory agreement with measured volatilization rates.     

Oxidation kinetics of a Ni–Cu based cermet at high temperature

The oxidation kinetics of a cermet composed of Ni–Cu alloy and nickel ferrite was studied by thermogravimetry at 960 °C under oxygen in the range 0.5–77 kPa. After an initial mass increase up to 15 g/m² due to oxidation of surface metallic particles, the mass change was attributed to both outwards NiO growth and internal oxidation. Above 40 g/m², the NiO scale thickness remained constant and the oxidation kinetics followed a complete parabolic law. The variations of the kinetic rate with oxygen partial pressure allowed to propose mechanisms, rate-controlling steps and kinetic laws in both transient and long term oxidation periods.     

Effects of preformed alumina scales on the behavior of FeCrAl alloys in simulated coal-gasifier atmospheres

Iron-based mechanical alloys containing 3.2–6.6 Al, 16.0–24.7 Cr, 0.5 Ti, and 0.5 Y₂O₃ (mass%) were preoxidized in air at 1373 K for 10–180 min. Alumina scales were formed. Scales were isolated and examined in a high-voltage transmission electron microscope. Specimen sections were examined in a scanning electron microscope. Specimens were exposed to atmospheres exhibiting oxygen activities from 1.5×10^–13 to 1.4×10^–14 and sulfur activities from 7.4×10^–7 to 3.6×10^–5 at 1123, 1223, and 1323 K. Alumina, chromia, and iron sulfide are thermodynamically stable under these conditions. Sulfidation did not occur at 1223 or 1323 K. Specimens exposed at 1123 K were sulfidized and formed a scale containing iron, chromium, and sulfur. The period before the onset of sulfidation was dependent on the preoxidation time. A model is developed to account for the temperature dependence of the sulfidation in terms of the alloy-interdiffusion rate of aluminum and the activity of the sulfidizing species at the scale-gas interface.     

Oxidation Resistance of Vacuum Plasma Sprayed CoNiCrAlY Coatings Modified by Filtered Cathodic Vacuum Arc Deposition Aluminizing

The vacuum plasma sprayed CoNiCrAlY coatings are modified by filtered cathodic vacuum arc deposition aluminizing. The microstructure and oxidation resistance of the coatings are investigated. The parabolic law is obeyed for the aluminized coatings after oxidation at 1100 °C for 100 h. Its parabolic kinetic constant is 0.080 mg²/cm⁴ h, which is lower than that of as-sprayed coatings. The continuous and dense Al₂O₃ scale is formed earlier due to the increase of Al concentration, and the spinels hardly exist. The oxidation resistance is improved obviously after filtered cathodic vacuum arc deposition aluminizing.