Reactive-element effect of electrodeposited Y2O3 oxide films on the oxidation of Fe−25Cr and Fe−25Cr−10Al alloys

Thin Y₂O₃ films were deposited by the electrochemical deposition-pyrolysis process on Fe–25Cr and Fe–25Cr–10Al alloys. The influence of the films on the oxidation behavior of the alloys was studied at 850°C and 1000°C. The results showed that Y₂O₃ films remarkably decreased the oxidation rate of Cr₂O₃-forming alloys and spallation of the scales, but they did not decrease the oxidation rate of the Al₂O₃-forming alloys, although they do reduce the spallation of Al₂O₃ scales. Y₂O₃ films remarkably change the morphology of the scales on both alloys, depending on the oxidation temperatures. These results show that the reactive-element effects of Y₂O₃ films on the Cr₂O₃ formers and Al₂O₃ formers are different.     

The effect of aluminum as an alloying addition or as an implant on the high-temperature oxidation of Ni-25Cr

The oxidation behavior of aluminum-implanted Ni-25Cr and Ni-25Cr containing 1 wt.% Al has been studied at 1000°C and 1100°C in oxygen. As did Y alloying addition or Y-implantation, 1 wt.% Al added to Ni-25Cr prevented nodular formation of Ni-containing oxides, improved spalling resistance of the scale upon cooling to a similar degree, and eliminated the formation of large voids between the alloy and the scale at the oxidation temperature. However, the Al addition did not alter the rate of growth of the Cr₂O₃ scale, nor did it change the growth direction. Al-implantation produced no effect even when the maximum concentration and depth of penetration were adjusted to be identical with those of the yttrium in the Y-implanted alloy. The implications of these results concerning the reactive element effect are discussed.     

The oxidation of Ni-20 wt. % Cr-2ThO2

The oxidation of TD NiCr (Ni-20 wt. % Cr-2ThO₂) has been investigated between 900 and 1200°C, and the oxidation behavior is compared with Ni-30 wt. % Cr and Co-35 wt. % Cr alloys. All alloys develop Cr₂O₃ scales but the weight changes obtained for the NiCr and CoCr alloys show an increase with time whereas above 1000° C the TD NiC shows a progressive loss in weight from the evaporation of CrO₃ from the scale, and the reaction products appear to be formed mainly at the alloy-scale interface. However, no mechanism for its formation has been established.     

A TEM study of the oxide scale development in Ni-Cr-Al alloys

In the present study the isothermal oxidation behaviours of Ni-10Cr-5Al, Ni-20Cr-5Al and Ni-30Cr-5Al alloys were investigated. The alloys were oxidised in air for 50 h at 1000 °C. Analytical transmission electron microscopy was used to characterize the morphology, structure and composition of the oxide scale. The oxide formed adjacent to the alloy was α-Al₂O₃ such that the higher was the Cr content of the alloy the easier was its formation. The Ni-30Cr-5Al alloy formed a complete layer of α-Al₂O₃ in the initial stages of oxidation through ‘oxygen gettering’ by Cr. A decrease in scale thickness and an increase in scale adherence were observed with an increase in Cr content from 10 to 30 wt.%.     

Comparison of Oxidation Behavior and Electrical Properties of Doped NiO- and Cr2O3-Forming Alloys for Solid-Oxide, Fuel-Cell Metallic Interconnects

The goal of this paper was to determine if NiO-forming alloys are a viable alternative to Cr₂O₃-forming alloys for solid-oxide fuel-cell (SOFC) metallic interconnects. The oxide-scale growth kinetics and electrical properties of a series of Li- and Y₂O₃-alloyed, NiO-forming Ni-base alloys and La-, Mn-, and Ti-alloyed Fe–18Cr–9W and Fe–25Cr base ferritic Cr₂O₃-forming alloys were evaluated. The addition of Y₂O₃ and Li reduced the NiO scale growth rate and increased its electrical conductivity. The area-specific-resistance (ASR) values were comparable to those of the best (lowest ASR) ferritic alloys examined. Oxidation of the ferritic alloys at 800°C in air and air+10% H₂O (water vapor) indicated that Mn additions resulted in faster oxidation kinetics/thicker oxide scales, but also lower oxide scale ASRs. Relative in-cell performance in model SOFC stacks operated at 850°C indicated a 60–80% reduction in ASR by Ni+Y₂O₃, Ni+Y₂O₃, Li, and Fe–25Cr+La,Mn,Ti interconnects over those made from a baseline, commercial Cr₂O₃-forming alloy. Collectively, these results indicate that NiO-forming alloys show potential for use as metallic interconnects.     

Oxide scale adhesion and impurity segregation at the scale/metal interface

The chemistry at scale/metal interfaces was studied using scanning Auger microscopy after removal of the scale in ultra-high vacuum using an in situ scratching technique. Al₂O₃ and Cr₂O₃ scales formed between 900°C and 1100°C on Fe-18 wt.% Cr-5 wt.% Al and on Ni-25 wt.% Cr alloys, respectively, were investigated. The adhesion of these scales was determined qualitatively by way of micro-indentation and scratching on the surface oxide. All of the alumina scales fractured to the same degree to expose the metal surface, regardless of the oxidation temperature. The chromia-forming alloy on the other hand, developed more adherent scales at lower oxidation temperatures. About 20 at.% sulfur was found at the metal surface in all cases, and its presence was not only detected on interfacial voids, but also on areas where the scale was in contact with the alloy at temperature. Results from this study clearly demonstrated that sulfur as an alloying impurity does segregate to the scale/alloy interface. However, for alumina scales and chromia scales, the effect of this segregation on oxide adhesion is noticeably different.     

Cyclic and isothermal oxidation behavior on some Ni-Cr alloys

Additions of 3 wt.% Mn and 3 wt.% Si were made to Ni-20Cr. These alloys, along with Ni-20Cr and Ni-40Cr were oxidized for 100 1-hr cycles at 1100°C and 50 1-hr cycles at 1200° C. Oxidation behavior was judged by sample weight and thickness change, metallography, x-ray diffraction, and electron microprobe analysis. These tests showed that Ni-40Cr and Ni-20Cr-3Si were about the same and were the most oxidation-resistant alloys. Ni-20Cr-3Mn was not as oxidation resistant, especially at 1200° C. Ni-20Cr was far less oxidation resistant than any of the other alloys. The Ni-40Cr and Ni-20Cr-3Si relied on a protective layer of Cr₂O₃ for their oxidation resistance. A SiO₂ layer was noted beneath the Cr₂O₃ layer on the Ni-20Cr-3Si, but had apparently only a second-order effect. The source of improved protection of the Ni-20Cr-3Mn was apparently the formation of a relatively adherent MnCr₂O₄ layer at the metal-oxide interface.     

The influence of ion-implanted yttrium on the selective oxidation of chromium in Co-25 wt.% Cr

Specimens of Co-25 wt.% Cr, Co-25 wt.% Cr-1 wt.% Y, and yttriumimplanted Co-25 wt.% Cr alloy were oxidized at 1000°C in 1 atm O₂. The implantation dosage ranged between 10¹⁶ to 10¹⁸ ions/cm². The unimplanted binary alloy oxidized to a duplex Co-rich scale, but the Y-containing ternary alloy formed a continuous Cr₂O₃ layer. When the implantation dosages were lower than a nominal 10¹⁸ ions/cm², the alloy failed to develop a similar continuous Cr₂O₃ layer as that observed with the Y-containing alloy. A temporarily stable external Cr₂O₃ scale was formed on the most heavily implanted specimen (1×10¹⁸ Y⁺/cm²). This Cr₂O₃ scale consisted of very fine-grained oxide, which is permeable to the outward transport of Cr and Co. Internal oxidation pretreatment of the ion-implanted specimens converting the Y metal to its oxide prior to the oxidation experiment, can enhance the development of an external Cr₂O₃ scale, but this scale is also unstable. Results suggest that the selective oxidation of chromium in an ordinarily non-Cr₂O₃ -forming alloy can be due to the reactive element oxides acting as preferential nucleation sites on the alloy surfaces, but the subsequent growth of these scales may require a continuous supply of reactive elements in the alloy.     

The Effects of Reactive-Element,Ion-Implantation-Induced Amorphous Layers on the Oxidation of Co-12Cr and Ni-12Cr Alloys

Nickel-chromium (Ni-12Cr, wt.%) andcobalt-chromium (Co-12Cr, wt.%) alloys were ionimplanted with 150 keV yttrium to fluences that rangedbetween 2 × 10¹⁶ and 1 ×10¹⁷ ions/cm². The influence ofthe implantation on the microstructure of the alloy wasdetermined. The effect of the highest dose implantationon the alloys' oxidation response at 1000°C, 48 hrwas measured. Both alloys contained an amorphous surface phase as a result of this fluence and one ofthe effects of oxidation was to recrystallize theamorphized alloy in the first few minutes of oxidation.The lower doses of 2 × 10¹⁶ions/cm² were sufficient to cause amorphization of both the Ni-12Cr andthe Co-12Cr. The implantation reduced the isothermalmass gain by a factor of 25% for the Ni-12Cr, but had anegligible effect on the Co-12Cr alloy. Short-term oxidation experiments at 600°C showed viatransmission electron microscopy that, in the absence ofthe yttrium implant, the Ni-12Cr alloy forms NiO in thefirst minute of oxidation and the Co-12Cr alloy forms CoO and CoCr₂O₄.The implanted Ni-12Cr, on the other hand (1 ×10¹⁷Y⁺/cm²), formsrecrystallized Ni-Cr, Y₂O₃, andNiO in the near-surface region, while the implantedCo-12Cr alloy forms CoO, CoCr₂O₄, and a recrystallized intermetallic alloy fromthe amorphized region.     

Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. I. Influence of C,Mn, and Si

The oxidation behavior of Ni-Cr alloys (34 and 20 wt.% Cr) was investigated between 850 and 1200°C in oxygen for a maximum duration of about 70 hr. The oxide-growth mechanism is a diffusion process controlled by either outward diffusion of chromium in Cr₂O₃ (Ni-34Cr alloy) or by an increase in grain size (Ni-20Cr alloy). In the case of the Ni-34Cr alloy, low values of chromium diffusion were found for the growth of Cr₂O₃ by taking into account the general equation of Wagner. The influence of impurities (Si, C, Mn, Ni) diffusing from the underlying alloy is analyzed because of their doping effect in the outer oxide scales.     

Influence of Pre-oxidation on the Corrosion and Mechanical Strength of Fe–25Cr and Fe–25Cr–20Ni Alloys at 973 K

The influence of pre-oxidation on the corrosion and mechanical strength of Fe–25Cr and Fe–25Cr–20Ni alloys was investigated in N₂–0.1SO₂ at 973 K with and without mechanical loadings. About 0.1μm-thick Cr₂O₃ scales formed on the Fe–25Cr alloy by pre-oxidation in Ar. However, spinel oxides of (Fe,Cr)₃O₄ remained on the Cr₂O₃ and voids formed at the oxide/metal interface on Fe–25Cr–20Ni after pre-oxidation in Ar. The preformed oxides are very effective in preventing corrosion of the alloy surfaces. The preformed oxides are also beneficial to increase the strength of the alloys in corrosive environments. The effects of pre-oxidation on Fe–25Cr are stronger than those of Fe–25Cr–20Ni due to the different characteristics of the preformed oxides.     

The effect of preoxidation on the sulfidation of Ni-20Cr (2–5)Al Alloys

Ni-20Cr alloys with 2, 3.5, and 5 wt.% Al have been preoxidized up to 100 hr at 1000°C in dry H₂, in H₂/23% H₂O and in air and subsequently exposed to an H₂/5% H₂S atmosphere at 750° C. During the preoxidation treatment different types of oxide scales were formed which affect the sulfidation protection in different ways. Optimum results were obtained for alloys with 3.5 and 5 wt.% Al after 20 hr exposure to dry H₂ at 1000°C. A thin Al₂O₃ scale is formed which decreases the sulfur attack by more than one order of magnitude. Preoxidation conditions for Ni-20Cr-2Al alloys in H₂ and for Ni-20Cr-2Al and Ni-20Cr-3.5Al in H₂/H₂O were observed to be less effective. No improvement was found for preoxidation in air or for Ni-20Cr-5Al alloys preoxidized in H₂/H₂O.     

Oxidation behavior of Ni-Cr-1ThO2 alloys at 1000 and 1200°C

The oxidation behavior of Ni-13.5-33.7Cr-1ThO₂ alloys in flowing oxygen at 150 Torr was investigated in the temperature range 1000–1200°C. Gravimetric measurements of the oxidation kinetics have been combined with microstructural studies of the reacted samples in order to evaluate the reaction mechanisms. The oxide products formed on the alloys were a function of Cr content, sample surface preparation, reaction time, and temperature. The presence of ThO₂ appears to produce two effects during alloy oxidation. First, enhanced Cr diffusion to the alloy surface results in rapid formation of a Cr₂O₃ subscale beneath NiO on Ni-13.5Cr-1ThO₂ and selective oxidation of Cr for Ni-22.6Cr-1ThO₂. Second, the mechanism of formation of Cr₂O₃ is apparently different from that for simple Ni-Cr alloys, resulting in about an order of magnitude reduction in the Cr₂O₃ growth rate. The oxidationvaporization of Cr₂O₃ to CrO₃ becomes rate controlling for the higher Cr alloys after only a few hours of exposure at 1200°C.     

Effect of Y2O3 additions on the plasticity of sintered Cr2O3

We have carried our constant strain-rate compression tests on polycrystalline Cr₂O₃ and Cr₂O₃ doped with 0.09 wt. % Y₂O₃ to establish whether there exists an effect of Y₂O₃ on the plasticity of Cr₂O₃. This study is motivated by previous work on the oxidation of alloys containing reactive-element additions. In that work, it has been observed that the addition of oxygen-active elements, such as Y to alloys that form Cr₂O₃ or Al₂O₃ oxide layers upon exposure at high temperature, strongly enhances the adhesion of the oxide layer to the base alloy as compared with alloys without reactive-element additions. We have found that at 1200°C (1) chromia exhibits limited plasticity at high temperatures, and (2) the presence of Y in the oxides does not enhance plasticity compared with addition-free oxides.     

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.     

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.     

The influence of superficially applied oxide powders on the high-temperature oxidation behavior of Cr2O3-forming alloys

The effects of superficially applied CeO₂, mixed rare earth oxides, Co₃O₄, and Cr₂O₃ powders on the isothermal and cyclic oxidation of Ni-Cr alloys and the effects of CeO₂ and MgO powders on the isothermal oxidation of Fe-25 wt.% Cr have been studied over the temperature range 940–1150°C in pure oxygen and dry air. The rates of oxidation of both the Ni- and Fe-base alloys were markedly reduced by the application of CeO₂ powder. The presence of CeO₂ also improved the scale adherence and resulted in marked changes in the oxidation morphology. The presence of Co₃O₄ or Cr₂O₃ powders on Ni-Cr alloys or MgO on Fe-Cr also produced changes in the oxidation morphology but did not decrease the rate of oxidation. These results are interpreted in terms of the influence of the oxide powders on the development of scale microstructure and their effectiveness in decreasing grain boundary transport in Cr₂O₃.This paper is based in part on the Ph.D. thesis of G. M. Ecer (1975) and in part on the M.S. thesis of R. B. Singh (1977).     

Oxidation of ternary Co-Cr-W alloys

Oxidation rates in air at 1000–1250°C are reported for a series of Co-Cr-W alloys with 34–40 wt. % Cr and up to 10 wt. % W. Alloys with larger W contents exhibited slower oxidation rates and their parabolic rate constants agreed well with those for binary and ternary, Cr₂O₃ protected, Ni-base and Co-base alloys in the Co-Cr and Ni-Cr-W systems. The resulting scales were characterized by optical and scanning electron metallography, and electron microprobe analysis. The favorable effect of W additions to a Cr₂O₃-forming Co-Cr base alloy was the opposite of that reported for Ni-Cr-W alloys. The resupply of Cr to a Cr-depleted matrix beneath a protective CrO₃ scale is achieved by the dissolution (denuding) of Cr-rich second phases in the Co-Cr-W alloys. Thus, the internal oxidation of Cr beneath the Cr₂O₃ scale is avoided for high W alloys. No catastrophic failure by liquid phase formation was observed for high-W alloys oxidized 20 hr at 1250°C.     

The corrosion behavior of Ni-Nb alloys in a mixed gas of H2-H2O-H2S

The corrosion behavior of Ni-Nb alloys containing up to 40 wt.% Nb was studied over the temperature range of 550–800°C in a mixed H₂/H₂O/H₂S gas. The scales formed on all alloys were multilayered. The outer scale was single-phase Ni₃S₂, while the structure and constitution of the inner scale depended on alloy composition and reaction conditions. Internal oxidation has been found in Ni-20Nb and Ni-30Nb, external oxidation has been observed on Ni-34Nb. Platinum markers were located at the interface between the outer scale and inner scale. The decrease in corrosion rate with increasing Nb content may be attributed to the presence of increasing amounts of Ni-Nb double sulfides as well as to the presence of Nb₂O₅ in the inner region of the scale.     

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.