Reactive Element Effect Research Articles

Practical Aspects of the Reactive Element Effect

In alumina and especially chromia forming alloys containing RE's in form of oxide dispersions the size and distribution of the RE is of equal importance for obtaining optimum oxidation properties as the actual type (Y 2 O 3 , La 2 O 3 , CeO 2 ) and/or exact amount (0.1-1 wt.-%) of the dispersion. In conventional cast or wrought alloys the effective distribution of the RE will strongly depend on the form in which it prevails in the alloy, e.g. dissolved or in form of an intermetallic compound. The mechanism of RE incorporation into the surface scale can be affected by presence of other reactive minor alloying elements, such as Ti. Common alloy impurities such as C and/or N, if present in sufficiently high amounts, lead to formation of RE-carbo/nitride precipitates which become embedded in the growing oxide scale. Oxidation of the embedded precipitates upon long term exposure leads to microcracking and subsequent increased oxidation rates.

Oxidation behaviour of FeAl intermetallics: effects of reactive elements on cyclic oxidation properties

The cyclic oxidation behaviour of doped and undoped FeAl intermetallics was studied at 1000–1200°C. It was found that the scale spallation resistance of FeAl was remarkably improved by reactive element (Zr and/or Y) additions. The exposure time in each cycle also affected the scale spallation tendency of undoped FeAl. Cycling with short exposure times changed the scale spallation mechanisms and improved the spallation resistance. Electron microscopy, microanalysis, and X-ray diffractometry were used to study the scale spallation processes. The effects of reactive element additions on scale spallation mechanisms are also discussed based on experimental observations and stress calculations.

Contribution of solid electrochemical techniques to high temperature oxidation studiest

Solid electrochemical techniques have been applied to high temperature oxidation studies in experiments and concept. The principles of the parabolic oxidation law and solid electrolyte cells have been derived from the same origin developed by Carl Wagner. Oxygen sensors and pumps using solid electrolyte cells are very useful for the study of oxidation kinetics as a function of oxygen partial pressure. A tip-type solid electrolyte CO2 sensor is very sensitive to detect the evolution of CO2 gas, and is useful for the analysis of high temperature oxidation of metal carbides. A mechanistic model proposed for the reactive element effect of chromia-forming alloys has been confirmed by the measurements and data analysis of thermodynamic properties, electrical conductivity and diffusivity of YCrO3 on the basis of solid electrochemical techniques and defect structures.

Oxidation behaviour of FeAl intermetallics.The effects of Y and/or Zr on isothermal oxidation kinetics

The isothermal oxidation behaviour of doped and undoped Fe–37 at% Al intermetallic compounds in an ambient atmosphere over the temperature range of 1000–1200°C was studied. The oxidation products were identified by X-ray diffraction (XRD). Characterisation of the specimens after oxidation was conducted using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The rate constants of the isothermal oxidation were determined using three methods. The effective diffusion coefficients for the oxidation of doped and undoped Fe–37Al are discussed based on oxide grain size and the effects of reactive elements in lattice and grain boundaries. The effects of reactive elements on the isothermal oxidation kinetics of Fe–37Al were analysed based on oxide morphologies and microstructures.

Computer simulation of the reactive element effect in NiO grain boundaries

Diffusion within the grain boundaries of ceramics is an important mechanism for the growth of oxide films at moderate temperatures. Experiments show that the addition of impurities can drastically reduce the rate at which the film grows. We have investigated this by atomistic computer simulation. Since the migration energies of grain boundary processes in ionic systems are too high for a conventional molecular dynamics simulation to be used, we have used a modified simulation. The structure of the boundary is equilibrated and a vacancy introduced. A migration trajectory is chosen and a small force pulling the ion along this trajectory added to the ion that is to hop. A counter-force is added to the remaining ions to prevent the whole cell moving and the velocities are scaled to remove the energy introduced by doing work on the moving ion. The effect of this is to enable the hopping trajectory to be plotted out and the migration energy found. We have investigated the effect of Mg 2+, Ca 2+, Sr 2+ and Ba 2+ impurities on the migration energies and diffusion pathways of cation vacancies in the {310}/[001] and {410}/[001] tilt grain boundaries of NiO at moderate temperatures. We show that there is a correlation between the size of the impurity and its favoured position within the boundary. The presence of impurities increases the migration energies and alters the diffusion pathways. We conclude that impurities will bind vacancies to themselves, reducing the rate of diffusion.

Effects of a Reactive Element on isothermal and cyclic oxidation of chromia-forming alloys: SEM/EDX,TEM and SIMS investigations.

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), secondary neutral mass spectrometry (SNMS) and X-ray diffraction (XRD) were used to study the reactive element effect on chromia-forming alloys. The reactive element, neodymium, was introduced as an oxide film at the surface of the alloys. The analyses were performed during the early stages of oxidation at 1,273 K. Uncoated and Nd2O3-coated alloys have been oxidised for 1, 5, 30, 60, 120 minutes and 50 hours in air at atmospheric pressure. Chromia growth mechanisms were studied by two-stage 16O2/18O2 oxidation exposures followed by SIMS and SNMS analyses. Chromia grains quickly grew on uncoated samples, whereas they slowly developed on Nd2O3-coated specimens. A neodymium-containing phase rapidly evolved from Nd2O3, to NdCrO3 and then NdTi21O38. Indeed, the main phase evolution appeared during the first 60 minutes of the oxidation process. Chromia growth mechanism was not changed after 1 hour of oxidation because Nd was not yet incorporated into Cr2O3 scales. During the early stages of oxidation, Nd was mainly concentrated in the outer part of the scale composed of a spinel phase, Mn1.5Cr1.5O4. After two hours of oxidation, Nd was incorporated inside the chromia scale, leading to inward diffusion of oxygen. These results clearly demonstrated that the incorporation of the reactive element in the chromia scale as grain boundary segregant is the main explanation of the reactive element effect in the case of chromia-forming alloys.

The effect of H2-anneal on the adhesion of Al2O3 scales on a Fe3Al-based alloy

The cyclic (1,100°C, air) and isothermal (1,000°C, O2) oxidation behavior of a Fe-28Al-5Cr (at%) alloy, with and without a prior H2-anneal heat treatment at 1,200°C for 100 h, was studied. Changes in interfacial chemistry were evaluated using Scanning Auger Microscopy after removal of the oxide film in ultra high vacuum. This was achieved by making a scratch on the specimen surface, which caused spallation of the film at various locations along the scratch. The scale thickness and the temperature drop at which spallation took place during cooling were utilized to semi-quantitatively compare the adherence of the scales. Porosity at the scale–alloy interface and the scale microstructure were determined from scanning electron microscope observations. It was found that H2-anneal greatly improved scale adhesion and resulted in a pore-free and sulfur-free interface. The effects were similar to that of a 0.1 at% Zr-containing alloy, except that the improvement in scale adhesion was not as great as that from Zr doping. This implies that oxide/alloy interfaces are not intrinsically strong and the effect of reactive elements, such as Zr, is more than preventing impurity from segregating to the interface. Results are also compared with the effect of H2-anneal on other model alloys, such as NiCrAl, FeCrAl and NiAl, and on single crystal superalloys.

Segregation of Neodymium in Chromia Grain-Boundaries during High-Temperature Oxidation of Neodymium Oxide-Coated Chromia-Forming Alloys

The influence of MOCVD reactive-element-oxide (REO) coatings (Nd2O3) onhigh-temperature, chromia-forming alloy oxidation was investigated. REOcoatings decreased steel oxidation rates and greatly enhanced oxide scaleadherence. Uncoated and coated F17Ti samples were oxidized over thetemperature range 700–1050°C in air at atmospheric pressure. SIMSexperiments were performed on oxidized-coated samples in order to determinethe RE distribution through the oxide scale. Nd was distributed across theoxide layers with a higher concentration in the outer part of thescale. Transmission-electron microscopy (TEM) investigations were performedto more precisely locate the RE through the scale. Transverse crosssections, prepared on oxidized Nd2O3-coatedFe–30Cr (model system), showed that Nd, associated with Cr and O,segregated at chromia grain boundaries. It is thought that this is the maincause of the beneficial effects usually ascribed to the RE inchromia-forming alloys. The effect of chromia grain-boundary segregation onchromia growth mechanism and its influence on the reactive-element effect(REE) are discussed.

High-temperature oxidation behaviour of EN-1.4301 stainless-steel after surface Ce deposition by a modified CVD method

The oxidation of EN-1.4301 (18Cr–8Ni) stainless-steel specimens after Ce deposition at 1173 K under synthetic air is studied. Cerium deposition was carried out by pyrolysis of Ce(NO3) aerosols. Thermogravimetric results show that total mass gain after 22 h of oxidation is one order of magnitude smaller than for the untreated specimen, indicating that the deposition treatment is very effective in increasing the oxidation resistance of EN-1.4301 stainless steel. After oxidation the scale is composed mainly of Cr2O3 and Cr–Mn spinels. Preoxidation in air of the stainless steel prior to Ce deposition does not alter significantly the composition of the oxide scale except for a slight increase in iron content and a reduction in mass gain and thickness of the oxide layer. Rutherford backscattering spectrometry data confirm these results and show a homogeneous in-depth distribution of Ce along the oxide scale. The presence of cerium oxide particles in the scale, in contrast to the presence of perovskite phases on depositing lanthanum, suggests that the silica layer found at the alloy/scale interface could be responsible for the reactive element effect. Copyright © 2000 John Wiley & Sons, Ltd.

The Effects of Molybdenum on the High-Temperature Oxidation Resistance of Thin Foils of Fe–20Cr–5Al at Very High Temperatures

Thin foils of Fe–20Cr–5Al alloys are susceptible to breakawayoxidation once the aluminum content of the substrate has fallen below somecritical value. The combined addition of 0.1 wt.% lanthanum and 0, 1, or 2wt% molybdenum has a beneficial effect on the high-temperature oxidation ofsuch foils. Lanthanum has the well-known reactive-element effect on adhesionof the protective alumina scale, thereby increasing the time to onset ofbreakaway oxidation, while, for alloys containing molybdenum, breakawayoxide spreads relatively slowly over the specimen in comparison to alloysthat contain no molybdenum. In particular, molybdenum-containing alloys areable to develop a protective Cr2O3 layer at the breakawayoxide–substrate interface. Conversely, molybdenum-free alloys form aninternal-oxide zone in the substrate adjacent to this interface, rather thana Cr2O3 layer, so breakaway oxide spreads rapidly. A martensitic phase isobserved in the substrate adjacent to the breakaway oxide formed on Fe–20Cr–5Al–La specimens, which means that theα-phase has transferred to the γ -phase at the temperature ofthe oxidation test (1150°C). Conversely, α-phase is retained inthe molybdenum-containing alloy, even after breakaway takes place, sincemolybdenum, which is a strong ferrite former, is enriched in the alloyadjacent to areas of breakaway oxide. The diffusion rate of chromium isslower in the γ than in the α-phase so a continuouschromium-rich oxide layer, which is effective in inhibiting breakawayoxide from spreading, cannot be established at the breakawayoxide–substrate interface for the molybdenum-free alloys.

The effect of impurities on vacancy migration at NiO grain boundaries

Diffusion within ceramics is an important mechanism for the growth of oxide films. Experiments show that the addition of impurities can drastically reduce the rate at which the film grows. This, the reactive element effect, is very useful for corrosion protection. Grain boundaries are regions of high diffusivity and high defect concentration and are important for studying this effect. We present the results of atomistic simulations, using a modified molecular dynamics method, to calculate the effect of neutral impurities upon the activation energies and diffusion pathways of vacancy migration at {310} and {410} tilt grain boundaries of NiO. We show that there is a correlation between the size of the impurity ion and its favoured position within the boundary. The presence of impurities increases the activation energies and diffusion pathways to a greater extent for the {310} boundary than for the {410} boundary.

Grain boundary character distribution in oxides formed on (100) and (111) nickel single crystals coated with ceria gel

A numerical analysis of texture data was conducted to assess the grain boundary character distribution in oxides formed at 1073 K on the (100) and (111) crystal faces of nickel coated with ceria gel. As input data were used the orientation distribution functions of oxide grains derived from three basic pole figures, measured by the X-ray technique. As an output of the calculations, the grain boundary misorientation functions and frequencies of coincidence site lattice boundaries were obtained. The results revealed that the substrate crystallographic orientation exerted an essential influence on oxide grain boundary structure. The presence of ceria introduced some changes; however, they are rather small and cannot explain the existing differences in oxide growth kinetics. An implication of this finding for the explanation of the `reactive element effect' is discussed.

The Effect of Lanthanum on the Scales Developed on Thin Foils of Fe-20Cr-5Al at Very High Temperatures

A study has been undertaken of the effects oflanthanum on the oxidation of thin foils of Fe-20Cr-5Alin air at 1150°C. The addition of lanthanum causesthe time to breakaway to increase from about 24 hr for Fe-20Cr-5Al to over 400 hr. Oxidationof the lanthanum-containing alloy occurs in threestages. During the first stage, anα-Al2O3 layer is establishedand thickens with time until the aluminum in the foil is depleted sufficiently for alayer of Cr2O3 to become stableand develop at the scale-alloy interface. This continuesto thicken at a relatively slow rate until breakawayoccurs. The main emphasis in the present paper has been anexamination and analysis of the scale established on thelanthanum-containing alloy in cross section in theanalytical transmission electron microscope (TEM), after an exposure period that coincides with thesecond stage of oxidation, prior to breakaway. The scaleat that time consists of three layers. The outer layeris composed of equiaxed Al2O3grains. The intermediate and inner layers consist of columnarAl2O3 grains and equiaxedCr2O3 grains, respectively.Numerous voids are observed in the oxide grainboundaries and at the intermediate-inner layerinterface. Lanthanum segregates in the oxide grain boundaries andits concentration increases toward the outermost surfaceof the scales. These results are consistent with the“dynamic segregation model” to account for the effects of reactive elements on thegrowth of Al2O3 scales.

Reactive element effects on the ionic transport processes in Cr 2O 3 scales

A study of the ionic and electronic partial conductivity of Cr 2O 3 scales selectively grown on Ni–20%Cr alloy with and without yttria additions in hydrogen water vapour at 900°C, was carried out by the asymmetry polarisation technique. The results revealed that there were two main regions for the temperature dependence of conductivity in the range 300-900°C. Above 700°C, the Cr 2O 3 scale was an intrinsic electronic conductor with a corresponding activation energy of 1.7 eV. Below 700°C, the Cr 2O 3 scale was a mixed ionic and electronic conductor. The activation energy was 0.6 eV for electronic conductivity and 0.3 eV for ionic conductivity. The ionic transference number decreased as the temperature or applied voltage increased. The data strongly suggest that the presence of the reactive element in the scale reduces the concentration of mobile species, thus lowering the ionic transport number.

Substrate and bond coat compositions: factors affecting alumina scale adhesion

Thermally grown oxide scales that form beneath ZrO 2 top coats play an important role in determining the performance of thermal barrier coatings. Numerous factors, including the composition of both the alloy substrate and the bond coat, affect adhesion of the α-Al 2O 3 scale. Three areas of focus in the formation of an `ideal', adherent scale encompass: (1) migration of Al and other elements in the metal substrate, (2) segregation of elements to the metal–scale interface and the scale grain boundaries, and (3) generation of stresses in the scale. Examples of the effects of reactive elements, Pt, indigenous S, and reaction temperature on scale adhesion are discussed.

Ion implantation for high temperature corrosion protection

Ion implantation has only limited potential as a corrosion protective treatment in high temperature technological applications. Far more important is the indirect contribution of ion implantation to corrosion science as a research tool to study the influence of various elements on the corrosion behaviour of materials. It can be a very efficient technique in the frame of screening tests for evaluating the effect of possible alloying additions in materials. Additionally, it can also play a role to study the underlying corrosion mechanisms. In the present paper this is illustrated with some high temperature corrosion studies. Notably, its use for studying the mechanisms underlying the so-called reactive element effect; for developing more corrosion resistant TiAl-based intermetallic alloys and to address mechanistic questions concerned with mixed oxidation/sulphidation. It is concluded that ion implantation is a powerful tool in high temperature corrosion studies, and can thus be very helpful for material development.

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.

The Effect of Impurities on the Alumina Scale Growth: An Alternative View

High temperature oxidation of Fe-Cr-Al and Fe-Cr-Al-Y alloys is usually accompanied by the formation of a protective scale of {alpha}-Al{sub 2}O{sub 3}. Good oxidation resistance of an alloy requires the slow growth rate of the oxide and the ability of the scale to withstand temperature changes without spalling or cracking. Small alloying additions of reactive elements such as yttrium are well known to improve oxide adherence and change the morphology of the oxide layer. While the scale formed on yttrium-free alloy is typically convoluted or wrinkled and poorly adherent, the yttrium-containing alloy produces a flat and adherent oxide. Improvements in the scale adhesion are now widely considered to be a result of sulfur gettering by yttrium additions. The major point of this sulfur effect is in the assumption that without reactive elements sulfur segregation from the bulk alloy to the scale-alloy interface causes a weakening of the oxide-metal bond. In discussing the reactive element-sulfur interaction, surprisingly not much attention has been given to the fact that yttrium additions not only improve adherence, but they also substantially change oxide morphology. The widely reported effect of yttrium in eliminating oxide wrinkling is though to be of secondary importance. On the other hand,more » it is known that the flat alumina layer is usually adherent, while the wrinkled scale typically fails in cyclic oxidation conditions. From various results it can be concluded that sulfur in the alloy is not responsible for the structural differences between adherent and non-adherent oxide. If so, there might be other species causing the scale wrinkling and/or affecting adherence. In performing the work described here the authors were searching for an alternative explanation of the reactive element effect and the possible role of impurities other than sulfur.« less

Standard free energy change of formation per unit volume: a new parameter for evaluating nucleation and growth of oxides, sulphides, carbides and nitrides

The present paper introduces a thermodynamic parameter, the standard free energy changes of formation of oxide, sulphide, carbide and nitride per unit volume, as a criterion for comparing the formation tendency of these compounds. The diagrams for the standard free energy change of formation of common oxides, sulphides, carbides and nitrides per unit volume vs temperature have been calculated and established based on the available thermodynamic data. It is believed that these diagrams can provide better explanations to some oxidation phenomena including the effects of reactive elements on the selective oxidation of Cr2O3 and Al2O3.

The effect of reactive elements on the oxidation behaviour of Fe23Cr5Al alloys at high temperatures: I. Kinetics and thermal cycling

The oxidation behaviour of Fe-23Cr-5Al-based alloys unmodified and modified by incorporation, using various procedures, of reactive elements and by implantation of sulphur was investigated at temperatures up to 1673 K, under both isothermal and thermal cycling conditions. The reactive elements, in general, slightly enhanced the oxidation rate of the studied alloy and improved the scale resistance to spalling, these effects being dependent on a few factors. Implanted sulphur did not exert significant influence under studied conditions.