Oxide-alloy Interface Research Articles

The oxidation mechanism of Ni3Al containing yttrium

The high-temperature oxidation behavior of Ni3Al (Ni-13.2 wt.% Al) with and without additions of 0.5 wt.% yttrium has been studied over the range of 900–1200°C in air. None of the commonly accepted rate laws were followed by the kinetics. Although the weight gains of samples containing yttrium were consistently 10–20% greater than those without yttrium, the steady-state scaling rates were identical. A quantitative x-ray diffraction technique was used to determine the kinetics of growth of the protective alpha-alumina layer (one of several oxides formed). The alumina growth followed the parabolic rate law under all conditions studied. The rate-controlling transport process in alumina was the enhanced diffusion of oxygen down grain boundaries. The presence of yttrium as nickel-rich intermetallics promoted the formation of nickel aluminate (spinel). A marked increase in scale adherence was observed for short times. At longer times, however, the outer layer of spinel and unreacted nickel oxide spalled off along with some of the inner alumina layer. Loss of adherence was caused by a complex yttrium-aluminum oxide which formed by the solid-state reaction of yttria and alumina. The poor scale adherence on Ni3Al was due to the formation of voids at the alloy-oxide interface. These voids concentrated the athermal stresses above the oxide-to-metal adherence strength. The voids were produced as a result of the selective oxidation of aluminum resulting from a “Kirkendall” effect in the substrate. During the selective oxidation process, a vacancy flux directed from the matrix to the metaloxide interface resulted in a supersaturation of vacancies. Equilibrium was maintained by the condensation of excess vacancies. The presence of yttrium as either nickel-rich intermetallics or internal oxide prevented the voids from forming. The yttrium-rich particles relieved the matrix of vacancy supersaturation by providing vacancy sinks. The chemical nature of the particles does not seem important. A necessary and sufficient condition for an effective vacancy sink appears to be the presence of an incoherent boundary between particle and matrix.

The role of yttrium in high-temperature oxidation behavior of Ni-Cr-Al alloys

Oxidation of five nickel-chromium-aluminum alloys with yttrium additions of between 0.005 and 0.7wt.% has been studied in the temperature range 800–1200°C in oxygen at pressures of 1, 10, and 720 Torr. The yttrium additions improve the oxidation behavior of the nickel-chromium-aluminum alloys, and generally an addition of 0.1 or 0.2 wt.% yttrium gives the best improvement in the oxidation resistance. In this case, the oxidation kinetics indicate asymptotic approach toward zero scale growth with time. This is suggested to be caused by the formation of subgrains in the alloy which (a) provide enhanced diffusion of aluminum to the surface and (b) increase the number of oxide nucleation sites. Preferred oxidation of aluminum occurs, resulting in the formation of an α-Al 2O3 layer. Additions of more than 0.3 wt.% yttrium result in preferential grain boundary oxidation and a convoluted alloy-oxide interface. This effect, “key-on effect” along with the formation of an aluminum and yttrium double oxide, produces increased oxide adherence for these alloys.

The influence of aluminum additions on the oxidation of Co-Cr alloys at 1000 and 1200�C

The isothermal oxidation of Co-Cr-Al alloys, containing 10–30 % Cr and 1 or 4.5% Al in 1 atm flowing oxygen at 1000 and 1200°C has been studied by thermogravimetric methods, optical metallography, electron probe microanalysis, and scanning electron microscopy. The addition of 1 % Al to Co-10% Cr and Co-15% Cr has little effect on the over-all oxidation rate, although there is increased internal oxidation and the outer-inner scale thickness ratio is decreased. The oxidation rate is controlled largely by Co2+ ion diffusion out through the entire scale, with oxygen gas transport across voids, spinel blocking effects and doping in the inner layer probably playing subsidiary roles. With Co-15 %-Cr-1%Al, limited healing by Cr2O3 increases progressively with time at the alloy-oxide interface. An addition of 1 % Al to Co-30 %Cr assists the formation of an initially protective Cr2O3-rich surface layer by internally oxidizing, thereby allowing more of the chromium to diffuse to the surface and form an external scale. This Cr2O3 layer tends to lift and crack open, enabling CoO-rich scales to form on the exposed alloy. Co-15%Cr-4.5% Al produces a protective α-Al2O3 layer on certain surface regions, sometimes with an overlying Cr2O3 layer and internal α-Al2O3 particles in the underlying alloy. In other regions, rapidly growing CoO-rich nodules develop from the outset, or after early lifting and fracture of the α-Al2O3 scale. Generally, the presence of 28% Cr and 4.5% Al is sufficient to ensure an external scale of α-Al2O3, the chromium acting as an oxygen “getter.” If such scale fractures, healing is very rapid.

A study of the Cr-depleted surface layers formed on four Cr-Ni steels during oxidation in steam at 600� C and 800� C

Three commercial steels with 18% Cr-11% Ni, 19% Cr-25% Ni, and 19% Cr-35% Ni and a laboratory-made 20% Cr-35% Ni steel have been oxidized in steam at 600°C and 800°C. Two types of oxides are distinguished: a thin layer rich in Cr2O3 and patches of thick oxide of Fe, Ni, Cr spinels. The Cr concentration profile in the steels under the first type has been determined by microprobe analysis of ground and pickled or annealed specimens oxidized for various times. The variation of the Cr concentration in the alloy at the alloy-oxide interface with surface treatment, time, and temperature is discussed. It is concluded that one reason for the more extensive nodular growth on pickled or annealed specimens is their greater Cr depletion after short time oxidation. The interdiffusion coefficients in the Cr-depleted zones have been determined on ground and annealed specimens oxidized at 800°C. The acceleration of the diffusion rate due to grinding is clearly demonstrated. A fair agreement with diffusion data from diffusion couples is found.

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.

The oxidation of Fe [sbnd]Ni alloys

The oxidation behaviour of Fe Ni alloys is divided into four major compositional ranges on the basis of the type of scale formed: 0–2%Ni, 2–35%Ni, 35–80%Ni, 80–100%Ni. The oxidation type and oxidation rate are correlated with scale composition, nickel enrichment at the alloy-oxide interface, and the Fe Ni O ternary phase diagram.

The identification of thin healing layers at the base of oxide scales on Fe [sbnd]Cr base alloys

Electron probe microanalysis in plan and scanning electron microscopy have been used to study the alloy-oxide interface after oxidation of Fe Cr Si and Fe Cr Y alloys in 1atm oxygen at 1000–1300°C. The asymptotic scaling rate and scale adhesion characteristics of the Si-alloys are proved to be associated with a virtually complete SiO 2 layer, readily visible and analysable on certain specimens, at the base of the main Cr 2O 3 scale. The evidence for the corresponding behaviour of the Y-alloys is less clear-cut but adds some support to the concept of a blocking layer at or near the interface. Further observations on the adhesion characteristics and porosity of scales on such alloys are presented.

The role of nickel in the high-temperature oxidation of Fe-Cr-Ni alloys in oxygen

The oxidation of several largely austenitic Fe-Cr-Ni alloys in 1 atm oxygen at 800–1200°C has been studied thermogravimetrically, metallographically, and in detail by electron probe micro analysis. Fe-Cr-Ni alloys of this type are protected by Cr2O3-healed scale, which thickens slower than on the corresponding binary Fe-Cr and Ni-Cr alloys, presumably because nickel and iron ions dope the Cr2O3 more effectively together than singly and/or because the alloy composition and ability to absorb cation vacancies are such as to produce a smaller vacancy activity gradient or level in the scale, or voids within it. The scale adhesion, as on Ni-Cr alloys, is generally good after long times, at least partly due to the convoluted alloy-oxide interface, in some cases to large intergranular Cr2O3-rich stringers, and possibly to the general specimen mechanical properties. Nonprotective stratified scale development is relatively unusual and often produces nickel-rich, alloy-particle-containing nodules, as on Fe-Ni alloys. Careful selection of ternary and more complex alloys with appropriate alloy interdiffusion coefficients and oxygen solubilities and diffusivities should permit development of materials with the best compromise between ease of Cr2O3 establishment, avoidance of breakaway, and readiness of scale healing.

Two-phase scale formation on Cu-Ni alloys

The three main types of scaling behaviour of Cu-Ni alloys in O at 800°C are typified byalloys containing 80, 55 and 10% Ni. Oxidation kinetics are presented and the three types of distributions of NiO, Cu 2O and CuO are illustrated by metallography and electron probe microanalysis. The growth mechanisms of the scales are discussed in terms of competing fluxes of Ni atoms outwards and O atoms inwards at the alloy-oxide interface.

Observations of the metal oxide interface for a Fe-Cr alloy

The partial spalling that occurs upon cooling after high temperature oxidation has permitted the direct observation of the alloy-oxide interface, as well as the separated interfaces of the alloy and the oxide. The large depth of focus of the scanning electron microscope has shown a striking difference between the true area of contact and the nominal geometrical interfacial area, both within the alloy grains and at the grain boundaries. The relevance of these observations to the determination of the rate controlling step and the diffusing species in the oxidation process is discussed.

Role of Minor Elements In the Oxidation of Metals★

Abstract An analysis is made of the role of impurities in the oxidation of metals. The various impurities are considered in terms of their effects on the physical and chemical structure of the oxide, oxide-metal interface and metal. Eight types of effects are discussed and where experimental data are available these data are used to illustrate the nature of the effect. The eight types of effects are: (1) formation of new oxide of impurity atoms in outer layer, (2) formation of mixed oxides or spinels, (3) formation of impurity atom oxide in inner layer, (4) change in conductivity of oxide film, (5) change in physical properties and adhesive properties of oxide and oxide-alloy interface, (6) formation of inclusions and other defects in the metal, (7) concentration of impurities at the grain boundaries of the metal and (8) surface oxide-impurity atom reaction with volatile reaction product. 3.7.2