Room-temerature oxidation of Ni(110) at low and atmospheric oxygen pressures

Oxidation of atomically clean (110) nickel single crystals has been studied at room temperature and in pure oxygen or air at pressures from 1×10^–9 torr to atmospheric. X-ray photoelectron (XPS) and Auger electron spectroscopic (AES) data indicate that the standard regimes of dissociative chemisorption, oxide nucleation, and oxide lateral growth to coalescence were observed at low pressures. After the NiO layer coalesced at low pressures, exposure of the sample to atmospheric oxygen or air did not cause further growth of the oxide thickness at room temperature. Instead the growth of a high-energy shoulder on the O 1s XPS peak indicated the formation of Ni(OH)₂ on the surface. The presence of the hydroxide is consistent with high-resolution, electronenergy-loss spectroscopy (HREELS) data and chemical shifts in the Ni 2p spectra. While the oxide thickness is constant, the hydroxide thickness increased with exposure and time at high pressure. Surface analysis and lowpressure techniques are appropriate for the study of room-temperature, ambient-oxide formation and allow a determination of the kinetics and reaction products critical to the passivation of Ni.     

Reports of Meetings

Abstract

The resistance of nickel/2·5% thoria alloy to high-temperature oxidation and sulphidation has been evaluated, in comparison with that of nickel and a commercial nickel/20% chromium alloy. in sulphur-containing simulated fuel-ash and gaseous environments the resistance of the nickel/thoria approached that of nickel/chromium. The observed dependence of corrosion rate on temperature is explicable on the basis of thermodynamic consideraiions. In a simulated vanadium-containing fuel-ash the nickel/thoria alloy retained the good resistance of nickel, but in oxidation by quiescent air it showed little improvement. However, in temperature-cycled conditions the thoria dispersion conferred excellent resistance to spalling of the oxide.     

Selective oxidation of FeCr alloys in the 295–450 K temperature range

A series of iron-chromium alloys were oxidized for 10² to 6×10⁴ s in air and in the 295–500 K temperature range. Room-temperature oxidation of iron, chromium, antimony, and copper were also conducted at extended times. Oxidation characteristics such as oxide thickness and composition of the oxide and of the underlying alloy were evaluated from measurements by electron spectroscopy for chemical analysis (ESCA). An initial selective oxidation of chromium with a concomitant chromium depletion in the alloy was found. This initial oxidation step is followed by growth of an outer, iron rich oxide which causes the former chromium depletion to vanish. Apparent activation energies extracted from parabolic oxidation kinetics (295–500 K) of the investigated metals were found to be in the 10–20 kcal/mole range.     

Esca-studies of the composition profile of low temperature oxide formed on chromium steels—I. Oxydation in dry oxygen

ESCA (Electron Spectroscopy for Chemical Analysis) has been used to analyse the composition of oxide films formed on chromium steels with 6–20%Cr at room temperature in pure oxygen at 0·2 atm. during different times from 2 min to 46 h. Ion etching was used to clean the surface before oxidation and to investigate the composition profile by successive stripping of the oxide layer.It is demonstrated that the total chromium content in the oxide is proportional to the concentration in the metal, but that it varies with depth inside the film. At the oxide-gas boundary the chromium concentration is less than in the metal, while chromium is enriched at the metal phase boundary. It is suggested that the oxide consists of (Fe,Cr)₂O₃ in the outermost zone and FeCr₂O₄ at the inside, with the reservation that the method is not sensitive to deviations from stoichiometry.The thickness of the layer has been determined as 25 Å from the intensity ratio of the deconvoluted ESCA peaks of the metallic and oxidized states. The thickness and the composition profile of the film do not vary noticeably with the oxidation time.     

Oxidation of nickel base alloys strengthened by different hardening effects

Abstract

Three nickel base alloys strengthened by different hardening effects were investigated by thermogravimetry in air under isothermal conditions. The alloys investigated were γ′-Ni₃ (Al, Ti)-hardening alloy 80A (75Ni, 21Cr, 2·5Al, 1·7Ti, DIN No. 2·4952),solid solution hardened alloy C22 (59Ni, 21Cr, 13Mo, 3·5 Fe, 2·8W, DIN No. 2·4602) and a new high nitrogen containing and nitride hardening alloy N (61Ni, 27Cr, 10W, 1·4Ti, 0.2N). Tests were conducted in air between 900 and 1100° C for 48 h. Parabolic oxidationrates were determined and the formation of the oxide layer was investigated by optical microscopy and SEM. Oxidation data showed that the hardening mechanism has almost no influence on the oxidation kinetics. All of the alloys investigated formed chromia layers. After initial transient stateoxidation, the kinetics followed a parabolic law. Alloy 80A had the highest oxidation rate of the investigated alloys, which is attributed first to its lower chromium content and second to the formation of chromium carbides. At grain boundaries, internal oxidation, mainly of aluminium andtitanium, took place. The Al and Ti contents of alloy 80A were too low for the formation of a protective inner oxide layer of one of the two elements to take place. Alloy C22 showed the best resistance to oxidation since its chromium content of 21% is close to that for the minimum in the kineticsof oxide formation that has been found for binary Ni–Cr alloys. Additionally, there were no chromium rich precipitates to shift this chromium content to values that would result in higher oxidation rates. The nitride-containing alloy N contained a higher chromium content of 26%, whichled to a higher oxidation rate than that for alloy C22. A certain amount of inner oxidation took place, especially at coarse Cr₂N precipitates. Conclusions are presented about the optimised chemical composition of chromia laye-forming nickel base alloys for minimised oxidationrate.     

Studies on the Mechanism,Structure and Microhardness of Ni-W Alloy Electrodeposits

The electrodeposition of Ni-W alloy has been studied on the glassy carbon electrode by the cyclic voltammetry and potentiostatic step methods. It has been found that electrodeposition of Ni-W alloy involves an intermediate valence tungsten oxide which inhibits hydrogen evolution. Ni-W alloy electrodeposition occurs by a mechanism involving progressive nucleation followed by three dimensional growth.

The structures of nickel-tungsten alloy deposits were analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD results from Ni-W alloy deposits reveal a face-centered cubic solid solution, the microstructure of the deposits exhibit (111) preferred orientation. The lattice constant and microhardness of Ni-W alloy deposit increase as the tungsten content increases, the XPS results of Ni-W alloy deposits indicate that the nickel and tungsten of the deposits exist in the metallic state, but the Ni-W alloy deposit with a tungsten content of 40.7% is an intermetallic compound. The XPS results of the deposit with tungsten content of 40.7% show that the atomic ratio of Ni to W is 4:1, so β-Ni₄ W alloy can be obtained by electrodeposition and its microhardness (Hv) is as high as 672.8.     

Oxidation of a chromia-forming nickel base alloy at high temperature in mixed diluted CO/H2O atmospheres

Corrosion of a chromia-forming nickel base alloy, Haynes 230^®, has been investigated under impure helium containing a few Pa of CO and H₂O at 900 °C. It has been found that this alloy reacts simultaneously with CO and H₂O. Oxidation by CO has been revealed to occur mainly in the first hours. CO diffuses through the scale via short-circuit pathways and oxidizes Al, Cr and Si at the oxide/metal interface. Kinetics of CO oxidation has been investigated and several rate limiting steps are proposed. In the long term, H₂O is the major oxidant of chromia-forming nickel base alloys in impure helium.     

The oxidation of incoloy 800 in moist air containing hcl(g) at 800°C

The oxidation of a commercial high nickel alloy, Incoloy 800 (32Ni-21Cr-46Fe) has been studied in a slowly-flowing environment of air containing 500 vpm HCl (g) and 13,000 vpm H₂O (g) at a working temperature of 800°C for times up to 250 h. Severe spalling of the oxide corrosion products were observed upon cooling and this behaviour was compared to that observed after oxidation in HCl-free moist air. The effect of temperature cycling between the working temperature and ambient was also studied. Gravimetric, metallographic, electron probe micro-analysis and X-ray diffraction results are presented along with observations of morphological detail revealed using scanning electron microscopy. The results indicate that apart from having a serious effect on oxide adhesion, the presence of a halogen-rich region at the alloy-oxide interface significantly affects the transport of metal ions, principally chromium, iron and titanium to the oxidising surface.     

High-temperature oxidation of a single crystal Ni-base superalloy

The isothermal oxidation behaviour of a single crystal Ni-base superalloy (CMSX2) between 900 and 1300°C in air at atmospheric pressure or in pure static oxygen at 0.2 atm. pressure has been studied for test durations between 8 to 1000 hours. The extremely low oxidation rate did not permit accurate measurements of oxidation kinetics. However, oxide scale examinations by means of SEM, EDS, TEM, X-ray diffraction, RHEED, SIMS and metallography allowed to discuss the oxidation mechanisms of the studied alloy. At temperature higher than 1100°C, the very good oxidation resistance is due to the growth of a thin Al₂O₃ scale while at lower temperature it may be attributed to the growth of a subscale of a complex oxide (Cr, Ti, Al) (Ti, Ta)O₄ possessing a rutile structure. The effect of chemical homogeneity was studied in comparing the behaviour of as-cast and homogenized specimens.     

Failure and oxidation phenomena in Fe-35Ni-18Cr-2Si heating-element wire

The oxidation behavior of an Fe-35Ni-18Cr-2Si alloy commonly used as a resistance heating-element material has been studied in the temperature range 1000–1300°C. Techniques employed included thermogravimetry at constant temperatures, measurements of chemical changes in the protective oxide film, and metallography. The oxidation behavior was found to approximate a parabolic law and data obtained gave an estimate of the activation energy for oxidation of 125 kJ/mole (30 kcal/mole) at around 1000°C and 339 kJ/mole (80 kcal/mole) at higher temperatures. The concentration of elements in the protective oxide was found to be both temperature and time dependent. Chromium became concentrated at the expense of nickel and iron at the lower temperature (<1100°C) and was found to evaporate out of the oxide at higher temperatures and longer times, thereby increasing both the nickel and iron concentrations. On the basis of changes in the chemical composition of the oxide film, together with scaling, the method of failure of heating-element wire by the development of hot spots can be explained. At failure both long-term (low temperature 1000°C) and shorter term tests (temperature > 1000° C) were found to have oxidized to the same depth. This can be explained on the basis of a depletion of chromium having occurred at the oxide-alloy interface, and that on scaling, insufficient chromium was present to reform a protective film.     

Short-Term Oxidation Studies on Nicrofer-6025HT in Air at Elevated Temperatures for Advanced Coal Based Power Plants

Several advanced air separation unit (ASU) designs being considered for use in coal gasification rely on the use of solid state mixed ionic and electronic conductors. Nicrofer-6025HT, a nickel-based alloy, has been identified as a potential manifold material to transport the hot gases into the ASUs. In the current study, isothermal oxidation tests were conducted on Nicrofer-6025HT in the temperature range of 700–900 °C for up to 24 h. The evolution of oxide scale was evaluated using SEM, XRD, and XPS. The composite surface oxide layer that formed consisted of an outer chromia-rich scale and an inner alumina scale. For the longer times at the higher temperatures evaluated, a NiCr₂O₄ spinel phase was located at the interface between the alumina and chromia. Based on the experimental results a four-step oxidation model was proposed.     

The oxidation behaviour of austenitic FeCrNi alloys containing dispersed phases

The high temperature oxidation behaviour of FeCrNi austenitic alloys containing 1% Ti which, in some cases, had been converted into an oxide dispersion has been examined. The oxide dispersions were produced by an internal oxidation treatment using a 50/50 Cr/Cr₂O₃ powder mixture in a sealed quartz capsule at 1100°C: the samples were not in direct contact with the powders. Generally, the effect of the dispersed oxide was much less pronounced than in corresponding nickel-free, ferritic alloys. Nevertheless, the time-to-breakaway of the protective Cr₃O₃ scale which developed on Fe18CrNi alloys was substantially increased, although the differences between the untreated and the internally oxidized alloys reduced with increasing nickel content. An Fe14Cr20Ni alloy did not show any improvement after internal oxidation. Unlike the ferritic alloys, no coarsening of the dispersoid phase was observed during exposure.     

Substrate Depletion Analysis and Modeling of the High Temperature Oxidation of Binary Alloys

To predict concentration changes in binary alloy due to preferential oxidation during high temperature oxidation, a model based on the flux balance of the oxidized element at the moving oxide/alloy interface has been developed. These changes are driven by the alloy oxidation rate k _c . The model is numerically solved considering different initial alloy concentration profiles and a possible diffusion enhancement effect close to the alloy surface. After validating the model by comparison with Wagner’s analytical solution for Ni_30wt%–Pt alloy oxidised at 850 °C, we show that the effects of an initial alloy depletion due to the presence of a passive oxide layer are cancelled after very short oxidation times. We also show that a diffusion acceleration due to work-hardening in the vicinity of the alloy surface induces an inflexion point in the depletion profile.     

Studies on the oxidation behavior of Inconel 625 between 873 and 1523 K

The oxidation behavior of Inconel 625 during the early stages (<150 min) has been studied at oxygen pressures (P_{O 2}) of 0.12 kPa (0.9 torr) and 101.3 kPa (760 torr) in the temperature range of 1323 K to 1523 K by using TGA and between 873 and 1523 K by using XPS, AES, and EDS. The TGA results correlated well with those obtained by surface analysis of the oxide films. The results of XPS and AES analysis suggested that two distinctly different oxidation mechanisms operate, depending on the temperature of oxidation. Enrichment of the oxide films with respect to Cr₂O₃ occurs above 873 K, the degree of enrichment peaking at about 1200 K such that the oxide films formed at temperatures close to this consist almost exclusively of Cr₂O₃. At temperatures above 1300 K, the oxides of two minor alloying components, Nb and Ti, have been found to be present in the oxide films in significant proportions. The results have been discussed on the basis of the relative thermodynamic stabilities of the competing oxide phases and the diffusivities of the alloying elements in Inconel 625.     

Oxidation kinetics of a Pb-64 at.% in single-phase alloy

The solid-state oxidation kinetics of a Pb-64 at. % In (50 wt. %) single-phase alloy were studied from room temperature to 150°C using an AES (Auger Electron Spectroscopy) depth profiling technique. The general oxidation behavior of this alloy is different from that of a Pb-3 at.% In alloy but similar to that of a Pb-30 at.% In alloy. The oxide formed on this alloy is almost pure In oxide (In₂O₃) with the possible existence of some In suboxide near the oxide/alloy interface. At room temperature, oxidation of the alloy follows a direct logarithmic law, and the results can be described by the model proposed previously by Zhang, Chang, and Marcotte. At temperatures higher than 75° C, rapid oxidation occurred initially followed by a slower parabolic oxidation at longer time. These data were described quantitatively by the model which assumes the existence of short-circuit diffusion in addition to lattice diffusion in the oxide as proposed by Smeltzer, Haering, and Kirkaldy. The effects of alloy composition on the oxidation kinetics of (Pb, In) alloy are also examined by comparing the data for Pb-3, 30, and 64 at. % In alloys.     

A comparison of the effects of small additions of various second elements on the oxidation of nickel at 1200°C

The oxidation behaviour of Ni-4.2% Mo, Ni-4.0% W, Ni-2.5% Al, Ni-4.2% V and Ni-5.0% Cr (all wt. %) at 1200 °C in flowing oxygen at 1 atm. pressure has been studied using various techniques. In particular, the solubility of the second element in NiO, its distribution across the NiO scale and the effects of these on the oxidation rates and scale morphologies have been examined. The oxidation rates of all the alloys are greater than that of nickel, although for Ni-4.2% Mo, where incorporation of internal oxide into the scale does not occur and molybdenum does not dope the oxide, the small increase in weight gain during oxidation Compared with that for nickel is due to internal oxide formation only. As the internal oxide particles pileup at the alloy/oxide interface, they exert a blocking effect to outward diffusion of Ni²⁺ ions, especially in the later stages of oxidation. Ni-4.0% W behaves similarly, although a few internal oxide particles are incorporated into the scale and a small amount of doping of the oxide ensures that it thickens at a slightly faster rate than the scale on nickel and for Ni-4.2% Mo. The oxidation rates of the other alloys are significantly faster than that of nickel and increase in the order Ni-2.5% Al, Ni-4.2% V, Ni-5.0% Cr. These increased rates are largely caused by increases in the total cation vacancy concentrations in the scales, although internal oxide formation can make a significant contribution to the oxidation kinetics. The influence on the oxidation behaviour of a number of factors, namely doping of the scale, internal oxidation, dissociation of NiO and transport of gaseous oxygen within the scale, blocking effects in the oxide and at the alloy/oxide interface, and grain growth of the oxide, are considered in detail.     

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.     

Oxidation behaviour of Ni–Cr based alloy containing Si during high temperature application in an oil burner

Abstract

In certain modern oil burners, the combustion reaction is started in a flame tube. In the combustion atmosphere, the tube material is exposed to high temperatures and temperature changes. Nickel–chromium alloys are used to meet the requirement of high oxidation resistance. The paper presents the results on the oxidation behaviour of the silicon containing alloy 603 exposed to a low NO_x burner at temperatures of 950 and 1000°C up to 2000 h. Beneath a chromia scale silica precipitates formed at the beginning of exposure, which grew laterally establishing a nearly continuous interlayer. The interlayer disintegrated during the continued exposure. A thinner chromia scale was observed for alloy 603 compared with the scales observed for aluminium containing nickel–chromium alloys. This was attributed to a pronounced scale spallation. After 1000 h at 1000°C catastrophic oxidation of alloy 603 occurred involving Mo oxide and internal chromium oxide.     

Effects of temperature variations on oxidation of iron–20% chromium alloys at 1200 K in Ar–20% O2- Cl2 gas mixtures

Oxidation of an Fe-20%Cr alloy has been investigated at 1200 K in gas mixtures of Ar-20%O₂ with additions of 0.05 to 1.0%Cl₂ with temperature cycles to room temperature at 12 h intervals. The metals experience accelerated oxidation which proceeds via a chloride vapor transport mechanism and produces a porous oxide scale. The temperature cycles trigger accelerated oxidation in gas mixtures which would not produce accelerated oxidation under isothermal conditions. The results indicate that chlorine and temperature cycles have a synergistic effect on the oxidation rates.     

The mechanism of oxidation of dilute NiAl alloys at 800–1200°C

The oxidation behaviour of dilute NiAl alloys at 800–1200°C in flowing oxygen at 1 atm pressure has been studied using kinetic measurements, optical and scanning electron microscopy and electron probe micro-analysis. The oxidation rates of Ni0.5 to 4%Al alloys are greater than the corresponding values for nickel at 1000 and 1200°C, but less at 800°C. The increased rates at the higher temperatures are largely due to increases in the total cation vacancy concentration in the scale, although internal oxide formation can make a significant contribution to the oxidation rate. The decreased rates at 800°C are almost certainly due to a build-up of Al₂O₃ particles at the oxide/alloy interface. The roles played in the oxidation processes by doping, internal oxidation, blocking effects in the oxide, dissociation of NiO and gaseous transport of oxygen within the scale are considered in detail and related to the oxidation rates of the various alloys.