Oxidation Behavior In Air Research Articles

Oxidation of Hf2SnC and Nb2SnC in air in the 400-600°C temperature range

The oxidation behavior in air of fully dense, predominantly (92+vol.%) single phase, samples of Nb2SnC and Hf2SnC was studied in the 400–600°C temperature range. The oxidation products in both cases were SnO, SnO2 and either Nb2O5 or HfO2. The oxidation is characterized by an initial incubation period, the duration of which decreased with increasing temperatures, followed by a period of near-linear oxidation kinetics. Oxidation at temperatures as low as 400°C for 72 hr resulted in the total disintegration of bulk samples, and their total conversion to oxides. Comparison with published results indicate that the oxidation kinetics of Hf2SnC are poorer than those of Hf or HfC. Similarly, Nb2SnC is more prone to oxidation than pure Nb metal.

Influence of yttrium as alloying element on the oxidation at 1100°C of a model alumina-forming alloy

In order to study the effect of yttrium as an alloying element, 0.093 wt% yttrium was incorporated into the model Fe-20Cr-5Al alloy to give the FeCrAl-0.1Y substrate. During isothermal oxidation and thermal cycling tests, we have observed that yttrium has a beneficial effect on the oxidation behavior in air at 1100°C. In situ X-Ray diffraction allowed to demonstrate that yttrium favors a preferential crystallographic orientation of the α-Al2O3 scale. Glancing angle X-ray diffraction on a sample oxidized during 1000 hours under thermal cycling conditions indicates that yttrium is located at the internal interface as Y3A15O12. We think that the beneficial effect of yttrium on high temperature oxidation of the model alloy is due to its location at the internal interface and to the new crystallographic structure of the scale induced by the presence of Y3Al5Ol2.

Coatings of Nb-based Alloy by Cr and/or Al Pack Cementations and Its Oxidation Behavior in Air at 1273-1473 K

An Nb–5Mo–15W alloy (mass%) was surface-treated in vacuum by Cr and/or Al pack cementations, and the oxidation behavior of the coated alloys was investigated in air at temperatures up to 1473 K for 32.4 ks. The Al packed alloy formed a coating layer with the outer NbAl2 and the inner Nb3Al, and showed poor oxidation resistance. The Cr packed alloy formed a coating with outer α-Cr and inner Cr2Nb layer, and showed good oxidation resistance up to 1373 K. The alloy packed with Cr and then Al formed a coating layer with outermost α-Cr(Al), outer α-Cr(Nb), and inner Cr2Nb layers, and this coated alloy showed good oxidation resistance up to 1473 K by forming an Al2O3 scale.

224 二次元炭素繊維強化炭素複合材料の空気酸化挙動 : 熱膨張特性への影響

224 二次元炭素繊維強化炭素複合材料の空気酸化挙動 : 熱膨張特性への影響

Studies on the Air-oxidation Behavior of Uranium Dioxide I. Phase transformation from (U1-yGdy)O2 to (Ul-yGdy)3OS

The effect of dopant on the phase transformation from UO2 to U3O8 was investigated using X-ray diffraction (XRD) anlysis. Sample powders of Gd-doped stoichiometric UO2 of various dopant contents (y=0-0.16 in U1-yGdyO2) were oxidized at 250, 305, 340, 420 and 700 °C for 4 hours under continuous air flow and the structural changes were measured. The fractions of (U1-yGdy)O2, (U1-yGdy)4O9, (U1-yGdy)O37 and (U1-yGdy)O38 were identified from the peak intensity of each phase. It is confirmed that a tendency of retardation or inhibition by Gd content in the oxidation reaction from UO2 to U3O8.

Oxidation behavior of a PVD-processed Mg-10.6Zr alloy

The oxidation behavior in air of a physical vapor-deposited (PVD) Mg–10.6Zr (wt. %) alloy was studied in the 325–450°C temperature range. The oxidation rate of this alloy remains low at temperatures below 375°C. However, at higher temperatures, the alloy experienced extremely high oxidation rates, which can even lead to disintegration of the sample. Oxidation is controlled by fast inward oxygen transport along the “open boundaries” of the alloy, leading to the formation of cracks throughout the sample, and subsequent formation of a thin MgO at crack interfaces. The MgO layer remains protective while coarsening of zirconium precipitates at the open boundaries does not take place. Thickening of Zr precipitates over a critical size induces impairment of the MgO layer at crack interfaces, facilitating inward oxygen diffusion. The volume increase resulting from the formation of new oxide at open boundaries favors decohesion of open boundaries, leading to accelerated oxidation.

Influence of the alloy grain size on the oxidation behaviour of PM2000 alloy

The influence of the alloy grain size on the oxidation behaviour in air between 900 and 1400 °C of oxide dispersion strengthened PM2000 alloy was studied. The grain size of the alloy affects the morphology, the composition of the alumina scale as well as the size, composition and volume fraction of nodules formed on the alumina scale, but has only a small influence in the kinetics. In the fine-grained PM2000, grain boundaries act as rapid pathways for aluminium diffusion, leading to the establishment of a purer alumina scale than that formed on coarse-grained material. On the coarse-grained alloy, there is enough time for the nucleation of oxides other than alumina. These oxides, impeding alumina grain growth, can lead to the formation of a fine-grained alumina layer. The outward cation and inward oxygen diffusion in this fine-grained alumina layer may contribute to both the slight increase in mass-gain and the increased tendency to form nodules at the outer surface.

Influence of water vapour on the oxidation behaviour of titanium aluminides

The influence of water vapour on the oxidation of a technical γ-TiAl based alloy at temperatures of 700 and 750 °C is discussed. The present paper focuses on the oxidation behaviour in dry air and air containing 10 vol.% H 2O with respect to oxidation kinetics and microstructural investigations of the oxide scales and the subsurface zones. The presence of water vapour led to a significant acceleration in the oxidation kinetics as well as to a change in the crystal morphology and microstructure of the oxide scales formed. The results indicate that the kinetics of the reaction of water molecules with the rutile phase and a change in its defect structure are important parameters in the mechanism. The subsurface zone formed in water vapour-containing atmospheres consisted (in addition to the cubic Al-depleted layer) of a lamellar layer of Al 2O 3 and Ti due to either internal oxidation or decomposition of the cubic phase. Both, the oxide scale and the depletion layer are important regarding the mechanical behaviour of a TiAl component at high temperatures which was also investigated.

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 α-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.

High temperature oxidation of HFPD thermal-sprayed MCrAlY coatings

NiCrAlY and CoNiCrAlY powders were thermal-sprayed using the high frequency pulse detonation method (HFPD) onto AISI 310 austenitic stainless steel samples in order to obtain dense, adherent high temperature oxidation resistant coatings. The cyclic oxidation behaviour in air of both types of coatings was determined at a maximum temperature of 1273 K. The porosity, internal oxidation (after the first 8-h oxidation cycle), hardness, coating thickness and substrate-coating adherence were not significantly modified by the high temperature oxidation cycles. The surface phase composition was evaluated throughout the tests using X-ray diffraction and scanning electron microscope techniques revealing the formation of a continuous and highly protective alumina layer. The oxidation kinetics of both coatings can be characterized by parabolic rate constants, which are very similar to those of pure alumina.

Oxidation behaviour in air of thin alloy 601 fibres

The oxidation behaviour in air of 12 μm diameter continuous alloy 601 fibres has been studied using thermo-gravimetric analysis (TGA) for kinetics identification and transmission electron microscopy (TEM) for determination of the nature of the oxide layers. The TGA allows two stages in the formation of the oxide layer to be distinguished: the first stage corresponds to the growth of a continuous layer of NiO above a discontinuous sub-layer of Cr2O3 whereas the second stage is attributed to the parabolic growth of the Cr2O3 sub-layer, from the time it becomes continuous. A third stage can be observed for high oxidation temperatures. The TEM observations of oxide layers formed after 30 min at 650, 750 and 900°C confirm these results. One common characteristic of these 3 oxidation conditions is the appearance of large cavities under the oxide layer. These cavities seem to be the consequence of the oxidation mechanism of Cr and of the particular morphology of the material (i.e. small diameter cylinders).

Oxidation behaviour in air of thin alloy 601 fibres

The oxidation behaviour in air of 12 μm diameter continuous alloy 601 fibres has been studied using thermo-gravimetric analysis (TGA) for kinetics identification and transmission electron microscopy (TEM) for determination of the nature of the oxide layers. The TGA allows two stages in the formation of the oxide layer to be distinguished: the first stage corresponds to the growth of a continuous layer of NiO above a discontinuous sub-layer of Cr2O3 whereas the second stage is attributed to the parabolic growth of the Cr2O3 sub-layer, from the time it becomes continuous. A third stage can be observed for high oxidation temperatures. The TEM observations of oxide layers formed after 30 min at 650, 750 and 900°C confirm these results. One common characteristic of these 3 oxidation conditions is the appearance of large cavities under the oxide layer. These cavities seem to be the consequence of the oxidation mechanism of Cr and of the particular morphology of the material (i.e. small diameter cylinders).

Thermal and electrochemical studies of carbons for Li-ion batteries: 1. Thermal analysis of petroleum and pitch cokes

Thermal analyses involving simultaneous thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were used to study the air oxidation of petroleum (fluid and needle cokes) and coal-tar pitch cokes. The intent of this study is to understand the relationship between the structure of carbonaceous materials and their thermal oxidation behavior in air (Part 1). A correlation between the thermal oxidative properties of cokes and their electrochemical Li-intercalation performance is discussed in the following paper (Part 2). The carbon samples were heat-treated at temperatures up to 2800°C, and three thermal parameters were determined — ignition temperature ( T i), temperature maximum ( T m) in the DTA curves, and the temperature at which 15% carbon weight loss was attained ( T 15). The measurements showed trends that are consistent with prior reports that the active sites on the surface and not the total surface area (TSA) are responsible for the thermal behavior of the carbons. Because of the difference in the graphitizability of petroleum and pitch cokes that was obtained by heat treatment, variations in the thermal parameters were observed. Needle cokes are the most easily graphitized and this is reflected in the higher values of the thermal parameters compared to the fluid and pitch cokes.

Mechanisms of scale formation on new alumina forming γ-TiAl alloys studied by MCs+-SIMS

Two Ag-containing γ-titanium aluminides, Ti–48Al–2Ag and Ti–50Al–2Ag (concentrations in at.%), have been studied in respect to their long-term oxidation behaviour in air at 800°C. The exposures up to 6000 hours showed that the Ag additions promote formation of long-term protective alumina surface scales on γ -TiAl alloys. Extensive analyses of the oxidation products using optical metallography, SEM/EDX, XRD, EPMA and SIMS revealed that Ag stabilises the Z-phase (Ti5Al3O2) in the sub-scale depletion layer thereby preventing formation of α2-Ti3Al as well as Ti-rich nitrides, which are responsible for the destruction of alumina surface scales in common γ-TiAl alloys. The best results were obtained for Alloy Ti–50Al–2Ag; even after 6,000h exposure the thickness of the alumina based scale on this alloy was only around 0.5 to 1.5 μm.The results indicate that for optimum oxidation resistance careful adjustment of the Al and Ag content is necessary because otherwise massive Ag precipitation occurs at the Z-phase–alloy interface which leads to localized destruction of the protective scale.

The Failure of Protective Oxides on Plasma-Sprayed NiCrAlY Overlay Coatings

The oxidation behavior in air of air-plasma sprayed (APS) overlay coatingsof Ni–25Cr–6Al–Y have been studied at 1100°C. Aprotective alumina scale developed after 5- to 10-hr exposure with, initially,parabolic growth kinetics. With protracted exposures (>100 hr),subparabolic behavior developed, associated with aluminum depletion withinthe coating caused, principally, by internal oxidation of the low-densityAPS structure. This depletion caused intrinsic chemical failure, manifestedby the formation of a layer of Cr,Al,Ni-rich oxide beneath the residualalumina layer. Associated with this process of chemical failure was theformation of a layer of porous Ni,Cr-rich oxide above the aluminalayer. Oxide spallation occurred by delamination within this layer duringcooling; the spallation sites tended to lie above protuberances in theunderlying coating. Initial spallation occurred at a critical temperaturedrop, which decreased rapidly with increasing exposure time. A nonrigorousmodel of this spallation process has been developed which envisages thatdelamination occurs by the propagation of an oxide void under the action ofout-of-plane tensile stresses developed during cooling. Agreement with thespallation data is encouraging and shows that the deterioration ofspallation resistance with exposure time arises not only because oxidethickness increases but also because the maximum void size within the porousoxide layer increases.

Effect of Cr- and Y-implantation on oxidation behaviour of powder metallurgical Ni3 Al

The effect of Cr- and Y-implantation on the isothermal and cyclic oxidation behaviour in air of a boron doped Ni3 Al alloy at 585 and 900°C for up to 200 h is discussed. Yttrium implantation proved to be detrimental, causing slightly higher oxidation rates at both temperatures, while the scale formed at 900°C on Y implanted Ni3 Al was prone to spallation. After 200 h at 585°C (isothermal or cyclic oxidation) the scale consisted of an outer NiO layer, an intermediate layer of pure Ni, and an internal oxidation zone. At 900°C the scale consisted of an outer NiO layer, an intermediate Ni and Al rich oxide zone, and an inner thin alumina layer. A similar scale pattern was observed for the non-implanted material, however, the oxide scale formed on the non- implanted material did not show any significant susceptibility to scale cracking and spallation. Chromium implantation on the other hand was beneficial in that it decreased the oxidation rates at both temperatures owing to the formation of a continuously healing alumina layer, induced by the presence of chromium. No spallation of the oxide scale occurred, even after 200 cycles at 900°C.

Effect of La2O3 particles on the oxidation of electrodeposited nickel films

Electrodeposited Ni-La2O3 composite films with nanometer-size La2O3 oxide inclusions were fabricated by the codeposition of nickel with La2O3 particles. The comparative oxidation behavior in air at 900 and 1000 degrees C of nickel coated with the Ni-La2O3 composite and films with and without nickel-plating was studied by TGA, AE, SEM/EDX, EPMA, and TEM/EDX. In general, the Ni-La2O3 composite-coated nickel had the slowest rate and the best resistance to thermal cycling. AE tests revealed that cracking events in NiO scales on Ni-La2O3 composite-coated nickel was significantly reduced in comparison to that of the scale on nickel-coated nickel during thermal cycling at 900 degrees C. SEM investigation showed that the La2O3-free NiO scale was composed of outer coarse columnar grains and inner equiaxed ones. By contrast, the scale on the Ni-La2O3 composite-coated nickel consisted of only fine equiaxed NiO grains. The scale on the La2O3-free samples was characterized by cracks that originated at the scale-substrate interface and spanned the scale thickness. By contrast, no scale cracks formed at the La2O3-doped NiO scale-substrate interface, but small voids were created at the triple points of the grain boundaries of NiO. In the La2O3-doped NiO scale, segregation of La ions to the NiO grain boundaries near the scale-surface was observed by EDX microanalyses in the TEM. It is believed that the La ions segregated at the grain boundaries of NiO led to an increase in the cohesion between nickel oxides and in a reduction of the scaling rate and the formation of scale with fine equiaxed crystal structure by blocking the outward and lateral growth of scale. The latter was due to the predominant outward diffusion of nickel along NiO grain boundaries being inhibited effectively by the segregated La ions. The mechanism of the effect of the added La2O3 particles on the nickel electrodeposits is discussed in detail.

On the Mechanism of High-Temperature Oxidation of ODS Superalloys: Significance of Yttrium Depletion Within the Oxide Scales

The oxidation behavior in air of two ODSsuperalloys, the chromia-forming PM 1000 and thealumina-forming PM 2000, was investigated attemperatures between 900 and 1300°C. For both alloysgrowth rates of the oxide scales obey a subparabolic timedependence and an Arrhenius-type temperature dependenceof the rate constants is observed. Diffusion of yttriumto the surface of the oxide scales leads to almost complete depletion of this element within theoxide scales of both alloys. Scale growth then takesplace by simultaneous inward diffusion of oxygen andoutward diffusion of cations. At this stage of the oxidation process, the scale-growth mechanismis similar for both ODS alloys, at least in aqualitative way, to conventional (non-ODS) alloys of thesame base composition.

Influence of TiC additions on the oxidation behaviour of Si3N4-based ceramics

The oxidation behaviour in air of Si3N4 ceramics containing Y2O3 and Al2O3 as sintering aids, with and without a dispersion of TiC particles, has been studied between 1200 and 1400 °C. The influence of TiC additions on the oxidation kinetics is discussed in comparison with the results obtained for the Si3N4–Y2O3–Al2O3 reference material. However, in all cases and within the experimental temperature range, the oxidation kinetics were observed to be of a parabolic type. The microstructure of the oxide scales formed, which has been characterized by scanning and transmission electron microscopy, was observed to depend on the oxidation temperature and the initial composition of the material. © 1998 Chapman & Hall

A Comparison of the High-Temperature Oxidation of 17Cr-2Ni and 18Cr-8Ni Austenitic Stainless Steels at 973 K

The oxidation behavior in air at 973 K of theaustenitic stainless steels 18Cr8Ni and17Cr-2.5Ni-10.5Mn-2Cu-0.17N (low-nickel content), wasstudied in a thermobalance. The steels were heated fromroom temperature up to 973K at 10 K min-1, oxidizedfor 80 hr and then cooled to room temperature at 80 Kmin-1. The two steels had the same weightgains, 0.18 mg cm-2, which is equivalent tooxidation layers about 1.15 μm theoretical thickness. In both cases, thegeneral shapes of the ΔWS-1 (mgcm-2) curves vs. t (hours) were parabolic,but X-ray diffraction of the oxidized surfaces, surfaceand crosssection optical microscopy and SEM observations and EDSmicroanalysis show important differences betweenthem.