Oxide-gas Interface Research Articles

Oxygen tracer studies of the oxidation of niobium

SIMS was used to locate O18 in oxide scales grown on niobium specimens oxidized at 600, 700, 780, and 825°C. Analyses were performed in depth profile from the oxide-gas interface and in step-scan across polished sections. The measured distributions confirm that oxygen is the most mobile species in T -Nb2O5 and gave no evidence that pore diffusion was important in the growth of compact scales. In these scales, which grow essentially at a parabolic rate, O18 had a sigmoidal profile across the scale, plateauing near the oxide-gas interface. The O18 content of the plateau was typically less than 90% of the O18 content of the oxidizing atmosphere. At temperatures below 720°C the measured O18 distributions are compatible with a paralinear growth mode of the layered scales. In this mode the reaction rate is controlled by the diffusional properties of the oxide layer in contact with the specimen.

Comparison of the kinetics and morphologic properties of titanium, Ti-1.5Ni and Ti-2.5Cu during oxidation in pure oxygen between 600 and 820°C

The kinetics and morphologic oxidation properties of titanium, Ti-1.5Ni and Ti-2.5Cu were compared. Titanium and Ti-1.5Ni have a similar behavior, concerning the kinetics and the oxide micro structure. Copper additions decrease the oxidation rate of titanium. The oxide scales formed on Ti-Cu are thinner and less cracked than those obtained on Ti or Ti-1.5Ni. Copper is found in the oxide scale of Ti-Cu, whereas nickel is not found in the oxide scale of Ti-Ni. The oxidation of titanium and its alloys is controlled chiefly by diffusional phenomena in the oxide scale. Thus the alterations of the oxide scale structure and the slower oxidation rate of Ti-2.5Cu can be attributed to the copper which diffuses towards the gas-oxide interface.

The influence of an intermediate annealing treatment on the oxidation of copper and nickel

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics of copper and nickel in 1 atm oxygen at 800 and 1100°C, respectively, using a semiautomatic microbalance. Changes in scale morphology and composition have been investigated using various physical techniques. The outer CuO layer formed on copper during oxidation dissociates very rapidly on annealing to give CU2O and oxygen since the partial pressure of oxygen in the gas is below the dissociation pressure of CuO but above that of Cu2O at 800°C. The CuO layer is quickly re-formed on reoxidation in oxygen. There are relatively few other changes in the oxide morphologies of either metal during annealing, although the small grains present in the scale adjacent to the metal after oxidation are able to grow. During reoxidation both metals show a reduction in oxidation rate constant because of the decrease in total cation vacancy concentration in the scale and the reduced cation vacancy gradient across the scale brought about by the reduction in oxygen partial pressure at the oxide-gas interface during annealing. The reoxidation rate constants following annealing approach those recorded prior to annealing as the equilibrium cation vacancy levels in the scales are reestablished in the oxidizing environment. Rosenberg's method for analysis of the kinetics of reoxidation has enabled the equilibrium concentrations and diffusion coefficients of cation vacancies in CU2O and NiO during oxidation in 1 atm oxygen at the appropriate temperatures to be estimated approximately. These show reasonable agreement with literature values.

Oxygen-18 and deuterium profiling in thick films on Fe-9% Cr alloys by 3 MeV nuclear microprobe

The oxidation of Fe-9% Cr alloys with different Si contents has been studied in dry and wet CO 2. C 18O 2 and D 2O were used as tracers and detected by the 18O ( p, α) 11N and 2D( 3He , p) 4 He reactions at 0.84 and 0.75 MeV respectively using a scanning ion microbeam. With 0.04 wt.% Si most of the 18O was taken up at the outer oxide-gas interface. Higher silicon levels lowered the oxidation rate and the outward diffusion of iron. No difference was found between 18O profiles from material oxidized in wet or dry CO 2. Deuterium was found only in the inner Fe-Cr spinel layer of material oxidized in CO 2-D 2O mixtures.

The room temperature oxidation of Cu (111): Pressure effects

Ellipsometry has been used to follow the oxidation of the (111) face of a copper single crystal at 21 °C for oxygen pressures from 0.03–760 Torr. The pressure dependence is complex, with the short term oxide film thickness increasing to a limiting value as pressure increases, whereas the long term thickness goes through a maximum as pressure increases. The low pressure growth kinetics, as well as previously published contact potential measurements, can be described using the Cabrera–Mott theory for the formation of very thin oxide films modified to include the effects of space charge in the oxide. The rate determining step is taken to be the formation of cation vacancies at the gas–oxide interface. It is suggested that a monolayer of CuO forms on top of the Cu2O film at high oxygen pressures with a concomitant chance in mechanism.

The annealing and re-oxidation of oxidized Cu [sbnd]Ni alloys

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics in dry oxygen of Cu 10%Ni and Cu 24%Ni at 800°C and Cu 80%Ni at 1000°C using a semi-automatic microbalance. Changes in scale morphology and composition have been investigated using various techniques. Oxidation of Cu 10%Ni and Cu 24%Ni produces external scales consisting of a thick, inner Cu 2O layer and a thin, outer CuO layer, together with nickel-rich internal oxide in the adjacent alloy. Oxidation of Cu 80%Ni yields an external scale consisting of a thick, inner NiO layer and a thin, outer CuO layer, together with a few nickel-rich internal oxide particles in the adjacent alloy. During annealing, the outer CuO layers on the three alloys dissociate to give Cu 2O and oxygen. With Cu 80%Ni only, further dissociation to give copper metal takes place after long annealing times. In all cases, the annealing treatment leads to a reduction in the cation vacancy gradient across the scale and a reduction in the total vacancy level in the scale due to the reduction in oxygen activity at the oxide-gas interface. In Cu 10%Ni and Cu 24%Ni, internal oxide formation during annealing stimulates dissociation of the Cu 2O scale near the oxide-alloy interface to give copper metal and oxygen. Similarly, internal oxide formation stimulates dissociation of the NiO layer on Cu-80%Ni, although to a lesser extent than for the copper-rich alloys. During re-oxidation, the kinetics are partly determined by the extent of scale thinning during the prior annealing treatment, giving more rapid weight gains than expected from those recorded during the initial oxidation period. Nevertheless, particularly for Cu-80%Ni, where the scale thinning is relatively small, the reduction in the cation vacancy gradient across the scale and the reduction in the total vacancy level in the scale do result in a reduced oxidation rate compared with the rate recorded during the initial oxidation period.

The annealing and re-oxidation of oxidized Cu [sbnd]Ni alloys

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics in dry oxygen of Cu 10%Ni and Cu 24%Ni at 800°C and Cu 80%Ni at 1000°C using a semi-automatic microbalance. Changes in scale morphology and composition have been investigated using various techniques. Oxidation of Cu 10%Ni and Cu 24%Ni produces external scales consisting of a thick, inner Cu 2O layer and a thin, outer CuO layer, together with nickel-rich internal oxide in the adjacent alloy. Oxidation of Cu 80%Ni yields an external scale consisting of a thick, inner NiO layer and a thin, outer CuO layer, together with a few nickel-rich internal oxide particles in the adjacent alloy. During annealing, the outer CuO layers on the three alloys dissociate to give Cu 2O and oxygen. With Cu 80%Ni only, further dissociation to give copper metal takes place after long annealing times. In all cases, the annealing treatment leads to a reduction in the cation vacancy gradient across the scale and a reduction in the total vacancy level in the scale due to the reduction in oxygen activity at the oxide-gas interface. In Cu 10%Ni and Cu 24%Ni, internal oxide formation during annealing stimulates dissociation of the Cu 2O scale near the oxide-alloy interface to give copper metal and oxygen. Similarly, internal oxide formation stimulates dissociation of the NiO layer on Cu 80%Ni, although to a lesser extent than for the copper-rich alloys. During re-oxidation, the kinetics are partly determined by the extent of scale thinning during the prior annealing treatment, giving more rapid weight gains than expected from those recorded during the initial oxidation period. Nevertheless, particularly for Cu 80%Ni, where the scale thinning is relatively small, the reduction in the cation vacancy gradient across the scale and the reduction in the total vacancy level in the scale do result in a reduced oxidation rate compared with the rate recorded during the initial oxidation period.

The use of SIMS to investigate the incorporation of oxygen in CoO scales

SIMS was used to locate O18 in a thick oxide scale grown on a cobalt specimen oxidized first in O16 and subsequently in O18. Analyses were performed in depth profile from the oxide-gas interface and in step scan across a polished cross section. SIMS was found to be well suited to this type of oxygen tracer study. Eighty percent of the O18 was located in the inner quarter of the scale with a maximum value of 70% 10–15 μ from the metal-oxide interface. The distribution of the remaining O18 was essentially uniform in the outer 3/4 of the scale at an average value of 4.6%. There was, however, a layer 150 nm thick highly enriched in O18 at the oxide-gas interface. The O18 in the outer 3/4 of the porous scale is probably the result of normal exchange processes. The form of the distribution near the metal-oxide interface cannot be explained at this time.

Étude du comportement à l'oxydation de l'alliage tA6Zr5D (685) à haute température

The oxidation behaviour of the TA6Zr5D alloy was studied between 600 and 900°C in pure oxygen at a pressure of 76 Torr. Kinetics results showed that this alloy has a better oxidation resistance than pure titanium. The oxide scale had a typical structure which consisted of successive layers parallel to the oxide-gas interface; therefore, the stratification was less marked than in the case of pure titanium. Concerning the composition of the oxide scale, the aluminium was essentially localized close to the outer interface while the zirconium was gathered at the inner interface. The X-ray diffraction experiments only revealed the presence of rutile and α alumina. The influence of the different additions on the oxidation behaviour of the TA6Zr5D alloy is discussed in comparison with observations carried out for the cases of pure titanium, TA6V4 alloy and binary Ti-Al alloys.

On the redox kinetics and chemical diffusivity of wüstite in CO-CO2 mixtures

Intraphase redox kinetics were monitored across the wustite, Fe1−xO, phase of the iron-oxygen system in CO2-CO and CO2-CO-Ar mixtures at 1062 and 1106° C. Kinetic data expressed in terms of the chemical diffusion coefficient and the equilibration time as a function of composition strongly indicated the importance of a reaction at the gas-oxide interface in the overall reequilibration of a specimen. Both the chemical diffusion coefficient and the equilibration time were found to be inversely proportional to the deviation from stoichiometry. Surface rate constants calculated from initial linear oxidation data were found to decrease as the deviation from stoichiometry increased.

Oxidation of titanium between 25°C and 400°C

The oxidation of Ti between 25°C and 400°C has been studied in an ultra high vacuum system with ellipsometry, Auger spectroscopy and surface potential difference. The surface potential difference is primarily sensitive to the adsorbed layer of oxygen, the Auger spectrometer is sensitive to the oxygen and Ti concentration at the oxide-gas interface and the ellipsometer is sensitive to the film thickness. Consequently, it was possible to follow the various processes separately during oxidation and oxide dissolution. Oxygen adsorbs in a percusor state as a molecule which dissociates and adsorbs in the atomic state. Reaction of atomic oxygen with Ti at the oxide-gas interface causes the stoichiometry to vary with the temperature, oxygen pressure and film thickness. At low temperatures and oxygen pressure the initial film growth follows a logarithmic time law. The predominant transport species is cationic. After initial oxidation, at higher temperatures and oxygen pressure, the predominant transport species is anionic. At constant temperature and oxygen pressure the oxide grows until the rate of growth is equal to the rate of dissolution into the metal. At this point the film thickness remains constant with time. Doping the Ti surface with Au results in a decrease in the oxide dissolution process due to blocking of interstitial diffusion paths by Au atoms.

On the Formation of Very Thin Oxide Films on Metals: Contact Potential Measurements During the Oxidation of Different Crystallographic Faces of Copper

The low temperature oxidation of copper is treated in terms of a model in which the surface reaction involving the formation of copper ion vacancies at the gas-oxide interface is rate controlling. The predictions of the model are compared with experimental observations of the change in contact potential difference when clean (100), (110), and (111) copper surfaces are oxidized at room temperature. A value of 0.75±0.03 eV for the activation energy for formation of a copper ion vacancy is derived from the process of fitting experiment to theory. The interpretation of the observed changes in contact potential difference is made in terms of surface states located at the metal-oxide interface and at the gas-oxide interface. A reaction rate anisotropy that is in agreement with the observations of Young, Cathcart, and Gwathmey follows naturally from the model.

Morphologies of Nonadherent Al2O3 and Matching Substrate Surfaces

The performance of most oxidation resistant alloys and coatings is markedly improved if the oxide scale strongly adheres to the substrate surface. Consequently, in order to develop alloys and coatings with improved oxidation resistance, it has become necessary to determine the conditions that lead to spallation of oxides from the surfaces of alloys. In what follows, the morphological features of nonadherent Al2O3, and the substrate surfaces from which the Al2O3 has spalled, are presented and related to oxide spallation.The Al2O3, scales were developed by oxidizing Fe-25Cr-4Al (w/o) and Ni-rich Ni3 (Al,Ta) alloys in air at 1200°C. These scales spalled from their substrates upon cooling as a result of thermally induced stresses. The scales and the alloy substrate surfaces were then examined by scanning and replication electron microscopy.The Al2O3, scales from the Fe-Cr-Al contained filamentary protrusions at the oxide-gas interface, Fig. 1(a). In addition, nodules of oxide have been developed such that cavities were formed between the oxide and the substrate, Fig. 1(a).

Formation of Very Thin Oxide Films on Metals: Contact Potential Measurements during the Oxidation of (100) Cu

The coupled currents approach of Fromhold and Cook as applied to the formation of very thin oxide films on metals is modified to allow for the reflection of electrons which have tunneled to the gas–oxide interface. Reaction of the tunneling electrons with adsorbed oxygen atoms (present at low concentration) rather than with adsorbed oxygen molecules is suggested as a possible, although not exclusive, physical basis for the low capture probability. This modification leads to the hypothesis that neither a virtual electronic or ionic equilibrium may prevail across the oxide during low-temperature oxidation studies. Experimental measurements of the change in the contact potential difference between a (100) Cu single crystal and a high purity Au polycrystal during exposure to oxygen are compared with the predicted electric potential difference cross the oxide; the agreement between theory and experiment is good. A similar comparison between Rhodin's thickness measurements and predictions of the above model for the oxidation of (100) Cu shows poor quantitative agreement.

Kinetics of oxidation of copper at low temperatures under the influence of externally induced current flow

Investigations have been carried out on the kinetics of oxidation of copper in dry air under the action of externally applied current of both polarities in the temperature range of 75–101°C. The rate of oxidation was found to increase or decrease depending upon the direction of the applied current. The results for the retarding field application are discussed with reference to the William and Hayfield's logarithmic equation within the experimental range of temperature. The current is found to alter the concentration of vacant cationic sites in the oxide migrating/sec due to electrolysis of the oxide. The results for the accelerating field application are discussed with reference to Mott and Cabrera's parabolic equation for the thin film range. The change from logarithmic to parabolic law due to the application of current is probably due to a change in the mechanism of the process. Due to excess availability of electrons at the oxide-gas interface, the rate controlling step is changed from availability of electron at the oxide-gas interface to the field induced diffusion which is the basis of Mott's parabolic equation.

Oxidation kinetics of copper in the thin film range

Investigations have been carried out on the thin film oxidation of Copper in the temperature range 65–120°C. Both annealed and 25% cold deformed samples were employed. The results are discussed with reference to the logarithmic equation developed by William and Hayfield as well as Uhlig, considering the arrival of electrons at the oxide-gas interface by thermionic emission as the rate controlling step. The concentration of electron trapping centers in the oxide and the activation energy are found to be higher in case of cold worked material. The number of Fermi electrons arriving at the barrier per unit time as calculated from the oxidation data is found to be lower in the deformed Copper.

The interaction of the alloy niobium-25Titanium with air, oxygen and nitrogen I. The unusual oxidation behavior of Nb-25Ti at 1000 ° C

The oxidation behavior of Nb-25 wt.% Ti (39 at.% Ti) in air and oxygen at 1000 °C has been studied. The oxidation kinetics are linear in air with a pressure dependence of approximately 0.4. In oxygen the kinetics are in agreement with the parabolic rate law after about 8 h and are less dependent on the gas pressure. Oxidation in either air or oxygen results in the development of a unique substructure in the alloy. The external oxide layer at the alloy-oxide interface consists of TiO 2 and Ti 2Nb 10O 29. At the oxide-gas interface TiNb 2O 7 is observed formed by the solid state reaction of TiO 2 and Ti 2Nb 10O 29. Immediately below the external oxide layer, a gas-hardened zone consisting of two phases is observed. One of the phases is niobium-rich, the other is titanium-rich. The hardened zone is consistently thicker in specimens oxidized in oxygen than in air for equal exposure times. In either atmosphere, the growth of the hardened zone is diffusion controlled. Below the hardened zone, a small, but finite transition zone is observed followed by an extensive region containing a titanium oxide internal precipitate. The growth of this zone is also diffusion controlled, but the kinetics are complex since a portion of the zone is continuously being consumed by the inward growth of the gas-hardened zone. The oxidation behavior of the alloy, which is similar in some respects to that described by other investigators for Nb-Cr alloys, can be divided into two interrelated processes, dissolution of oxygen in the alloy and growth of an external scale. The poorer oxidation resistance of the alloy in air is assumed to be due to the interaction of nitrogen during the conversion of metal to external oxide. The substructure developed during exposure in air or oxygen is the same and is due to oxygen dissolution.

Nonequilibrium Chemical Reactivity of Polycrystalline Iron Foils

It is shown that continuous oxide films form on iron specimens in contaminated hydrogen atmospheres in which the metal is the thermodynamically favored phase; the explanation being that (1) oxygen in the gas does not come to equilibrium with the hydrogen but reacts with the iron, (2) the high activation energy of nucleation prevents reduction at the oxide-gas interface, and (3) hydrogen cannot diffuse through the continuous oxide film to the oxide-metal interface. The nonequilibrium nature of the oxidation is demonstrated by the finding that simultaneously with the formation of the continuous oxide films, bulk iron oxides on iron specimens become reduced. Also, reduction of the continuous oxide films proceeds readily when iron nuclei are formed by the addition of small quantities of iron carbonyl to the hydrogen stream.

Electrolysis of SiO2 on Silicon

The application of an external voltage across a layer of SiO2 on silicon causes a reduction in the thickness of the oxide layer by electrolysis when the applied voltage exceeds the theoretical cell voltage of the cell O2, Pt/SiO2/Si and the electric field is oriented such that the silicon–silicon dioxide interface is negative with respect to the oxide–gas interface. A measurement of the stopping voltage reveals that four equivalents are involved in the reaction, thus indicating that O= ions or some similarly charged defect could be involved in the electrolysis. Further evidence shows that the transport of uncharged oxygen species is negligible during the electrolysis of SiO2.

General Concepts of Gas-Metal Reactions

Abstract A metal in a gas atmosphere at high temperature is a complex chemical system. The metal usually is not pure but contains in addition to metallic impurities O, N, H, C, S, etc. The gas atmosphere also is usually complex, containing (in addition to O2) N2, H2, CO2, H2O, etc, A wide variety of chemical and physical phenomena occur in and on the metal surface in a typical gaseous environment at high temperature. The various phenomena are systematized into five zones of activity: metal, metal-oxide interface, oxide, oxide-gas interface, and gas. The general concepts are considered by setting down classification schemes of diverse observations and phenomena. Eight classification systems are given, then discussed. These are (1) type of reaction, (2) sign and magnitude of free energy change, (3) extent of reaction, (4) crystal structure, (5) empirical and theoretical rate laws, (6) physical and chemical properties of metal and oxide, (7) topology of metal and reaction product and (8) effect of reaction on metal or environment.