The oxidation of cobalt—tungsten alloys

The oxidation behaviour of CoCrW alloys containing from 0–25%Cr and up to 30%W in oxygen at 900–1100°C has been studied. In CoW alloys there is a slight reduction in the oxidation rate as the tungsten content is increased, hwoever this is much mor emarked in Co15CrW alloys. Tungsten has little effect in Co25CrW alloys. On the binary alloys and CoCrW alloys which do not form Cr₂O₃, the scale has two layers: an outer, tungsten-free layer of columnar-grained CoO, and an inner layer of CoO containing CoWO₄ precipitates together with CoCr₂O₄ particles in the ternary alloys. The relative thicknesses of the two layers and the distribution of the constituents in the inner layer depends in temperature and alloy composition. The CoWO₄ and CoCr₂O₄ particles appear to be responsible for the reduction in oxidation rate by a blocking mechanism in the inner layer. There is some evidence to suggest that tungsten additions to Co?25%Cr alloys assist the exclusive formation of Cr₂O₃.     

The isothermal oxidation of Co-Cr alloys in 760 Torr oxygen at 1000°C

The isothermal oxidation of Co-Cr alloys containing 0–30% Cr in 760 Torr oxygen at 1000° C has been studied kinetically and by appropriate physical techniques. Chromium additions to cobalt increase the parabolic oxidation rate to an almost constant level from 2 to 15% Cr, while further additions to 20–30% Cr decrease the rate. All the alloys produce a virtually pure CoO layer outside a layer containing Co-Cr spinel particles in a Cr³⁺ -doped CoO matrix. The variation of oxidation rate with alloy chromium content is explained in terms of the complex interplay of doping, blocking of cation transport by voids and spinel particles and short circuiting by transport of dissociative oxygen across these voids and other processes, internal oxidation making a negligible direct contribution to weight gain. Complete spinel layers are never quite developed under the conditions studied, although formation of spinel does slow the oxidation rate. The improved protection eventually obtained at higher chromium levels is produced by the tendency to form a Cr₂O₃ healing layer.     

Optimization of Cr-Content for High-Temperature Oxidation Behavior of Co–Re–Si-Base Alloys

The oxidation behavior of Co-17Re-xCr-2Si alloys containing 23, 25, 27 and 30 at.% chromium at 1,000 and 1,100 °C were investigated. Alloy Co–17Re–23Cr–2Si showed a poor oxidation resistance during exposure to laboratory air forming a two-layer external scale and a very thin discontinuous Cr₂O₃ layer at the oxide/substrate interface. The outer layer of the oxide scale consisted of CoO, whereas the inner layer was a porous mixture of CoCr₂O₄ spinel particles in a CoO matrix. The oxide scale was found to be non-protective in nature as the vaporization of Re-oxide took place during oxidation. An increase of chromium content from 23 at.% to 25 at.% improved significantly the alloy oxidation resistance; a compact protective Cr₂O₃-scale formed and prevented the rhenium oxide evaporation. The oxidation behavior of alloys containing 27 at.% and 30 at.% chromium were quite similar to that of Co–17Re–25Cr–2Si. The oxidation mechanism for Co–17Re–25Cr–2Si alloy was established and the subsurface microstructural changes were investigated by means of EBSD characterization.     

The effect of silicon and manganese on the oxidation mechanism of Ni-20 Cr

Additions of 3% silicon or manganese to Ni-20 Cr reduced the oxidation rate, whereas additions of 1% had little effect. Three percent silicon alloys formed an inner scale of SiO₂, and 3% manganese alloys formed an inner spinel layer of essentially pure MnCr₂O₄. The experimentally determined solid-state growth rate of NiCr₂O₄ was about 1000 times slower than the growth rate for Cr₂O₃. It has been established that the protective layer on Ni-20 Cr (Nichrome alloys) is the spinel and not Cr₂O₃ as previously postulated. The mechanism for scale growth is discussed for Ni-20 Cr alloys.This work was performed at Stanford Research Institute, Menlo Park, Calif. and was supported by the National Aeronautics amd Space Administration, Contract NAS 3-11165.     

On the oxidation of Co?35 w% Cr at high temperatures

The oxidation behaviour of coupon and spherical specimens of Co-35 w/o Cr have been studied in the temperature range 900–1300°C and at oxygen pressures from 0.2 to 760 torr. For coupon specimens the oxidation is approximately parabolic at and below 1100°C. Protective scales of Cr₂O₃ are formed on flat surfaces. Corners and edges show enhanced oxidation, and in these regions the surface oxide also contains CoCr₂O₄. Oxidation of spherical specimens is nonparabolic and the oxide scales consist of Cr₂O₃, CoCr₂O₄, and interspersed particles of CoO. During initial oxidation spherical specimens oxidize faster than coupon specimens.     

Mapping of the oxidation products in the ternary Co-Cr-Al system

The kinetics and products of oxidation of alloys in the Co-Cr-Al system have been studied and four mechanisms of oxidation identified. For the first mechanism, the rate-controlling process is cation diffusion of cobalt cations through CoO, and the main oxidation product is CoO. In the second mechanism, cation diffusion through CoO is still rate controlling, but the oxidation is strongly inhibited by an inner discontinuous spinel layer. The major oxidation products are CoO and CoCr₂O₄. The third mechanism of oxidation consists of preferential oxidation of chromium coupled with internal oxidation of aluminum while the fourth mechanism is the preferential oxidation of aluminum with the subsequent formation of Al₂O₃ scales, providing the best oxidation resistance. From the oxidation data obtained, an oxide map from which the oxidation behavior of various alloys may be deduced is drawn for the Co-Cr-Al system at 1100°C.     

The effect of small amounts of silicon on the oxidation of high-purity Co-25 wt. % Cr at elevated temperatures

Some investigators have reported that Co-25 wt.% Cr oxidizes slowly at temperatures in the range 1000–1200°C forming a protective Cr₂O₃ scale; and this is the normal behavior of cobalt-base superalloys. Others have reported very rapid oxidation, forming a two-layer scale: an outer CoO layer and an inner mixture of Cr₂O₃ and CoCr₂O₄ particles in a CoO matrix. This investigation shows that the principal reason for this behavior is the purity of the material; it appears that the rapid mode of oxidation is the intrinsic behavior for high purity material. The most probable impurity to produce the slower mode is silicon, and it is shown that as little as 0.05 wt. % Si is sufficient to change the mode of oxidation provided sufficient oxygen is also present in the alloy: it seems probable therefore that a fine dispersion of SiO₂ is responsible.     

The oxidation of cobalt-chromium-titanium alloys at 1000°C

Cobalt-based alloys containing 3,5,10,15, and 20% Cr with 1 and 3% Ti were oxidized at 1000°C in slowly flowing oxygen gas. In general, titanium additions decreased the oxidation rate with the most pronounced effect being observed at the 10% Cr level. Titanium accelerated the formation of Cr₂O₃ layers at the metal-oxide interface. Faceted Cr_xTi_yO_z spinel particles were found at the metal-oxide interface which varied in composition according to microprobe results. There was no evidence of spalling on the Co-Cr-Ti alloys studied in contrast to the severe spalling normally encountered in Ni-Cr-Ti alloys. Distinct morphological differences existed on the outer CoO layer of the 1% Ti alloys in comparison to the O and 3% Ti alloys.     

Silicon contamination effects in the oxidation of carbide-containing cobalt-chromium alloys

Austenitic Co-25Cr(wt pct) and two phase γ + M₇ C₃ alloys of composition Co-25Cr-xC were reacted with pure, dry oxygen at 1000°C. All alloys reacted according to relatively fast parabolic kinetics if they were prepared without silicon contamination. However, if the alloys were contaminated with silicon during annealing in silica ampoules, or if 0.05 wt pct silicon was added to Co-25Cr-1.0C, parabolic oxidation kinetics two orders of magnitude lower resulted. In the case of the rapid reactions, the scales consisted of an inner layer of CoCr₂O₄ + CoO overlaid by CoO. The slow reactions corresponded to growth of a thin scale of Cr₂O₃ overlaid by CoCr₂ O₄. In the latter case the selective oxidation of chromium led to chromium carbide dissolution in a subsurface zone of the two-phase alloy, but the rates were the same as for the single-phase alloy. Consideration of gas phase conditions in the silica annealing ampoules showed that p_SiO values were high enough to transfer substantial amounts of silicon to the alloy if p_O2 was low enough. This situation arose in well evacuated ampoules where oxygen was consumed by reaction with alloy chromium, or in titanium gettered capsules. In contrast, annealing the alloys under moderate oxygen pressures led to the growth of a protective oxide film which prevented silicon contamination of the oxide surface. It is concluded that the presence or absence of carbon in Co-25Cr is irrelevant to the oxidation mechanism and that the silicon effect is critical. An approximate diffusion analysis shows that bulk alloy properties are not affected by the silicon, and it is concluded that silicon has its effect at the alloy surface, by promoting Cr₂O₃ nucleation.     

Air Oxidation of FeCoNi-Base Equi-Molar Alloys at 800–1000°C

The oxidation behavior of FeCoNi, FeCoNiCr, and FeCoNiCrCu equi-molar alloys was studied over the temperature range 800–1000 °C in dry air. The ternary and quaternary alloys were single-phase, while the quinary alloy was two-phase. In general, the oxidation kinetics of the ternary and quinary alloys followed the two-stage parabolic rate law, with rate constants generally increasing with temperature. Conversely, three-stage parabolic kinetics were observed for the quaternary alloy at T 900°C. The additions of Cr and Cu enhanced the oxidation resistance to a certain extent. The scales formed on all the alloys were triplex and strongly dependent on the alloy composition. In particular, on the ternary alloy, they consist of an outer-layer of CoO, an intermediate layer of Fe₃O₄, and an inner-layer of CoNiO₂ and Fe₃O₄. Internal oxidation with formation of FeO precipitates was also observed for this alloy, which had a thickness increasing with temperature. The scales formed on the quaternary alloy consisted of an outer layer of Fe₃O₄ and CoCr₂O₄, an intermediate layer of FeCr₂O₄ and NiCr₂O₄, and an inner layer of Cr₂O₃. Finally, the scales formed on the quinary alloy are all heterophasic, consisting of an outer layer of CuO, an intermediate-layer of CuO and Fe₃O₄, and an inner-layer of Fe₃O₄, FeCr₂O₄, and CuCrO₂. The formation of Cr₂O₃ on the quaternary alloy and possibly that of CuCrO₂ on the quinary alloy was responsible for the reduction of the oxidation rates as compared to the ternary alloy.     

Kinetics and mechanism of high-temperature oxidation of dilute cobalt-chromium alloys

Kinetics of oxidation of Co-Cr alloys containing 0.4%–15% Cr was studied as a function of temperature (1273–1573 K) and oxygen pressure (4 × 10²–10⁵Pa). The oxidation process was found to be approximately parabolic and faster than that for pure cobalt. The scales are double-layered and consist of a compact outer CoO layer and a porous inner layer containing CoO slightly doped by chromium and spinel CoCr₂O₄. The oxidation mechanism was investigated by means of platinum markers and the¹⁸O isotope. The scale on the alloys containing less than 1% Cr grows exclusively by outward diffusion of cobalt, while that on the alloys containing more chromium—with a significant contribution of inward oxygen transport from atmosphere. This transport is not a lattice diffusion, but proceeds presumably through microfissures resulting from the secondary process of perforating dissociation of the outer scale layer.     

Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium

Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the corrosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr₂O₃-rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr₂O₃, resulting in a single-layered oxide film dominantly composed of Cr₂O₃. The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds.     

Comparison of isothermal and cyclic oxidation behavior of twenty-five commercial sheet alloys at 1150°C

Twenty-five commercial nickel-, iron-, and cobalt-base sheet alloys incorporating chromium or chromium and aluminum additions for oxidation resistance were tested at 1150°C in air for 100 hr in both isothermal and 1-hr cyclic furnace exposures. The alloys were evaluated by sample specific weight change, by type of scale formed, by amount and type of spall, and by sample thickness change and microstructure. In isothermal steady-state oxidation, four types of controlling oxides were observed depending on alloy composition: NiO, Cr₂O₃-chromite spinel, ThO₂-blocked Cr₂O₃, and Al₂O₃-aluminate spinel. The latter three types are considered protective. In the Cr₂O₃-forming alloys, however, scale vaporization is a critical factor in determining the parabolic scaling rate based on paralinear oxidation. In cyclic oxidation the alloys which form Cr₂O₃-chromite spinel scales were degraded severely when sufficient chromite spinel developed to trigger spalling. The cyclic behavior of the other three types of alloys does not differ greatly from their isothermal behavior. If chromite spinel formation is minimal, the thinner the oxide formed, the less the tendency to spall. Factors contributing to a thin scale are low isothermal scaling rates; reactive element additions, such as thorium, lanthanum, and silicon; and scale vaporization. Scale vaporization may, however, lead to catastrophic oxidation at high gas velocities or low pressures or both. A tentative mass-balance approach to scale buildup, scale vaporization, and scale spalling was used to calculate the critical oxidation parameter—the effective metal thickness change. In general, this calculated thickness change agrees with the measured change to within a factor of 3 if a correction is made for grain boundary oxidation. The calculated thickness change parameter was used to rate the oxidation resistance of the various alloys under isothermal or cyclic conditions. The best alloys in cyclic furnace oxidation tests were either Al₂O₃-aluminate spinel formers or Cr₂O₃ formers with ThO₂ blockage.     

Evaluation of High Temperature Oxidation Behaviour of Nanostructured Cr/Co–Al Coatings

The high temperature oxidation behavior of sputtered Cr/Co–Al coatings fabricated by DC/RF magnetron sputtering on a superalloy substrate has been studied in the present work. The microstructural features and phase formation of the as-deposited coatings were characterized by FE-SEM, AFM, and XRD, respectively. Weight-change measurements were made to calculate the cyclic oxidation kinetics of the coated superalloy exposed to air at 900 °C. It was observed that the corrosion rate of sputtered Cr/Co–Al coated superalloy is lower than that of the uncoated superalloy owing to the formation of continuous, dense, adherent and protective oxide scales over the surface of the coatings. The protective oxide scales in the corroded coatings were basically the thin layer of Cr₂O₃, CoO, Al₂O₃ and CoCr₂O₄, which provide protection to the base superalloy at high temperature.     

The oxidation of Fe-19.6Cr-15.1Mn stainless steel

The oxidation behavior in air of Fe-19.6Cr-15.1Mn was studied from 700 to 1000°C. Pseudoparabolic kinetics were followed, giving an activation energy of 80 kcal/mole. The scale structure varied with temperature, although spinel formation occurred at all temperatures. At both 700 and 800°C, a thin outer layer of -Mn₂O₃ formed. The inner layer at 700°C was (Fe,Cr,Mn)₃O₄, but at 800°C there was an intermediate layer of Fe₂O₃ and an inner layer of Cr₂O₃ + (Fe, Cr,Mn)₃O₄. Oxidation at 900°C produced an outer layer of Fe₃O₄ and an inner layer of Cr₂O₃+(Fe,Cr,Mn)₃O₄. Oxidation at 1000°C caused some internal oxidation of chromium. In addition, a thin layer of Cr₂O₃ formed in some regions with an intermediate layer of Fe₃O₄ and an outer layer of (Fe,Mn)₃O₄. A comparison of rates for Fe₃O₄ formation during oxidation of FeO as well as for the oxidation of various stainless steels, which form spinels, gave good agreement and strongly suggests that spinel growth was rate controlling. The oxidation rate of this alloy (high-Cr) was compared with that of an alloy previously studied, Fe-9.5Cr-17.8Mn (low-Cr) and was less by about a factor of 12 at 1000°C and by about a factor of 100 at 800°C. The marked differences can be ascribed to the destabilization of wustite by the higher chromium alloy. No wustite formation occurred in the high-Cr alloy, whereas, extensive wustite formed in the low-Cr alloy. Scale structures are explained by the use of calculated stability diagrams. The mechanism of oxidation is discussed and compared with that of the low-Cr alloy.     

Oxidation of Fe-8Cr-14Ni-Al-Si-Mn from 700 to 1000°C

This paper reports an investigation into reducing the Cr concentration in commercial-grade stainless steels while maintaining oxidation protection at elevated temperatures. Aluminum and Si were added as partial substitute alloy elements to enhance the reduced operation protection resulting from Cr concentration reduced by approximately 50 pct of that found in stainless steels. The goal of this study was to determine the oxidation mechanism of such an Fe, Al-Si alloy: Fe-8Cr-14Ni-1Al-3.5Si-1Mn. During the initial oxidation period the protection resulted from a thin film of Al₂O₃ over an Fe and Cr spinel. Long-term oxidation protection resulted from the gradual formation of a Cr sesquioxide (Cr₂O₂) inner oxide layer. Eventually an outer oxide layer formed that was a mixed composition spinel of Cr and Mn (MnO · Cr₂O₃). The Al₂O₃, which was part of the original protective layer flaked off early in the oxide testing, and the aluminum oxide that formed later appeared as an internal oxide precipitate.     

Modification of the oxidation behavior of high-purity austenitic Fe-14Cr-14Ni by the addition of silicon

The oxidation behavior of Fe-14Cr-14Ni (wt.%) and of the same alloy with additions of 1 and 4% silicon was studied in air over the range of 900-1100° C. The presence of silicon completely changed the nature of the oxide scale formed during oxidation. The base alloy (no silicon) formed a thick outer scale of all three iron oxides and an internally oxidized zone of (Fe,Cr,Ni) spinels. The alloy containing 4% silicon formed an outer layer of Cr₂O₃ and an inner layer of either (or possibly both) SiO₂ and Fe₂SiO₄. The formation of the iron oxides was completely suppressed. The oxidation rate of the 4% silicon alloy was about 200 times less than that of the base alloy, whereas the 1% silicon alloy exhibited a rate intermediate to the other two alloys. The actual ratio of the oxidation rates may be less than 200 due to possible weight losses by the oxidation of Cr₂O₃ to the gaseous phase CrO₃. The lower oxidation rate of the 4% silicon alloy was attributed to the suppression of iron-oxide formation and the presence of Cr₂O₃, which is a much more protective scale.     

Effect of chromium on early stages of oxidation of Ni3Al alloys at 600°C

Initial oxide formation at 600°C in air on Ni₃Al alloys with and without chromium additions was studied by TEM. Significant lateral nickel diffusion (apparently stress-induced) occurred in both alloys producing bands of nickel and nickel oxide-enriched hillocks. Chromium additions clearly alleviate dynamic embrittlement in Ni₃Al; chromium additions were previously assumed to affect the oxidation process. Chromium additions significantly reduced the oxidation rate of the alloy. However, a continuous film of pure Cr₂O₃ had not yet formed after 45 sec oxidation. Grain boundaries preferentially oxidized to form Al₂O₃ or Cr₂O₃ and rejected nickel along both the surface and the grain boundary, deeper into the specimen. The dramatic effect of chromium on improving the ductility of Ni₃Al when tested at high temperature in air is apparently a result of a process that occurs at or near the tip of a propagating crack that is both faster and on a finer scale than that studied here.     

The effect of particle size of alumina dispersions on the oxidation resistance of Ni-Cr alloys

The oxidation behavior of Ni-Cr alloys with various chromium concentrations and particle sizes of a dispersion of 10 vol.% Al₂O₃ was observed in 1 atm of oxygen at 1000°C. This study was intended to determine the critical chromium concentration to form a protective Cr₂O₃ oxide layer for different Al₂O₃ particle sizes. The oxidation rate of Ni-Cr alloys containing 10 vol.% Al₂O₃ followed a parabolic rate law and a Cr₂O₃ protective layer continuously formed when the oxidation rate decreased rapidly. Times to form a continuous and protective Cr₂O₃ layer during the initial oxidation shortened as the size of the dispersion decreased. The critical chromium concentration to form a protective Cr₂O₃ layer in the oxide scale was 69 wt.% and was related strongly to the particle size of the Al₂O₃ dispersion.     

Water Vapour Effect on the Oxidation Mechanism of a Cobalt-Based Alloy at High Temperatures (800–1,100 °C)

A cobalt-based Phynox alloy was oxidized in the 800–1,100 °C temperature range. The alloy oxidation was consistent with a growth mechanism limited by the diffusion process in a growing Cr₂O₃ oxide scale. Water vapour enhanced the alloy oxidation rate and scale porosity. Thermal cycling tests at 900 and 1,000 °C showed that water vapour reduces the outer Mn_1.5Cr_1.5O₄ subscale adherence, but the chromia scale adherence was not affected. These temperatures permited a rapid chromium supply from the substrate to form a continuous chromia scale. At 1,100 °C thermal cycling conditions led to scale spallation and chromium depletion in the alloy. In dry air, weight losses were recorded due to cobalt and molybdenum oxidation, giving CoCr₂O₄ and CoMoO₄. In wet air, the initial porous chromia scale permited nickel and cobalt oxidation, leading to Ni₅Co₃O₈ and CoCr₂O₄ formation and resulting in bad adherence during thermal cycling.