Two-stage Oxidation Experiments Research Articles

Raman study of oxide layers on zirconium alloys using 18O tracers

ABSTRACT In this work, samples of zirconium alloys Zr1Nb and Zircaloy-4W were subjected to two-stage oxidation experiments – at first in H2 16O solution and then in H2 18O steam. Raman spectroscopy was used to analyse distribution of 18O isotope in corrosion layers of the alloys. Layered distribution of 18O was observed in case of Zr1Nb samples; Zircaloy-4W had 18O present only at metal/oxide interface. This observation is attributed to differing kinetic transitions, which influence transport of oxygen through oxide layer.

Growth Mechanisms of Oxide Scales on Two-phase Co/Ni-base Model Alloys between 800 °C and 900 °C

Elementary processes during the oxidation of single crystal Co/Ni-base model alloys between 800 °C and 900 °C were investigated in the present study. The influence of the base element (Co or Ni) on the oxidation rate was elucidated by continuous thermogravimetry in synthetic air for 100 h. After exposure, the multilayered oxide scales were characterized by scanning electron microscopy combined with electron probe micro analysis. The substitution of Co by Ni caused reduced oxidation rates at 900 °C. At lower temperatures an inverse trend with Co-rich alloys demonstrating lower mass gains was observed. The development of diffusion-limiting scale sections that lead to overall thinner oxide layers could be proved for the Co-base superalloy without Ni at 800 °C and 850 °C. The growth of AlN as well as the unwanted phase transformation of the γ/γ′ microstructure into Co3W persisted in alloys with high Co levels for all investigated temperatures. Material transport at 900 °C was elucidated by two-stage oxidation experiments in normal oxygen and 18O-enriched atmospheres. Transport of O2 via microchannels could be concluded for all compositions. The provided insights contributed to deeper understanding of essential processes during the formation of barrier layers in the scale.

High-Temperature Oxidation of Zircaloy-4 in Air Studied with Labeled Oxygen and Raman Imaging

High-temperature oxidation of zirconium-based cladding materials is expected to be the primary cause of the fuel assemblies’ degradation in spent fuel storage pool loss-of-cooling accidents. Here, high-temperature Zircaloy-4 oxidation has been studied through two-stage oxidation experiments using 18O isotope. Particular attention is paid to the effect of a low-temperature pre-oxidation scale which aims to simulate the corrosion scale existing on spent fuel cladding. Raman imaging was used to investigate the 18O distribution in the scales exposed either to 18O2 or to 18O2 + N2 atmosphere at high temperature. Results were assessed against more conventional SIMS mapping. The use of the 18O isotope tracer technique associated with micro-Raman mapping of the scales is demonstrated to be a powerful method to investigate the transport properties in the scales and will help to gain understanding of the kinetic differences between different pre-oxides.

The use of micro-Raman imaging to measure 18O tracer distribution in thermally grown zirconia scales

High temperature Zircaloy-4 oxidation has been studied through two-stage oxidation experiments using 18O isotope. 18O distribution in the oxidation scales was investigated by micro-Raman imaging. First, zirconia standards with known 18O content were prepared and analyzed for calibration purposes. Then, to assess the method on a simple case, 18O tracer penetration patterns were recorded after a 800°C two-stage oxidation. Finally, oxidation at 850°C of a Zircaloy4 cladding tube pre-corroded at 425°C to simulate corrosion during normal reactor operation was studied. These preliminary results give insights on the protective nature of the first formed zirconia scale in case of an accident.

Atomic transport mechanisms in thin oxide films grown on zirconium by thermal oxidation, as-derived from 18O-tracer experiments

Two-stage oxidation experiments using 16O and 18O isotopes were performed to reveal the governing atomic transport mechanism(s) in thin (thickness <10nm) oxide films grown during the initial stages of dry thermal oxidation of pure Zr at 450K. To this end, bare (i.e. without a native oxide) Zr(0001) and Zr(101¯0) single-crystalline surfaces were prepared under ultra-high vacuum conditions by a cyclic treatment of alternating ion-sputtering and in vacuo annealing steps. Next, the bare Zr surfaces were oxidized at 450K and at pO2=1×10−4Pa, first in 16O2(g) and subsequently in 18O2(g). The 18O-tracer depth distributions in the oxide films were recorded by time-of-flight secondary ion mass spectrometry. It was concluded that the early stage of the oxidation process is governed by oxygen transport to the metal/oxide interface through the lattice and along the grain boundaries of the nanosized oxide grains whereas, on continuing oxidation, only oxygen lattice transport controls the oxidation process. An oxide-film growth mechanism is proposed.

On the Mechanism of Cu&lt;sub&gt;2&lt;/sub&gt;O Oxidation at High Temperatures

Oxidation mechanism of nonstoichiometric cuprous oxide (Cu2 yO) has been studied as a function of temperature (973-1273 K) and oxygen pressure (1-105 Pa), using microthermogravimetric, marker and two-stage oxidation techniques. The last method consisted in using heavy oxygen isotope 18O in order to confirm the results, obtained with marker technique. It has been found that in early stages of reaction not exceeding 30 hours, CuO formation on the surface Cu2 yO follows approximately cubic rate law. In later stages, during long-term oxidation, lasting hundreds of hours, oxidation process follows strictly parabolic kinetics with the rate, being independent of Cu2 yO pre-treatment. Marker and two-stage oxidation experiments have shown that the growth process of CuO layer proceeds by the outward diffusion of cations, clearly indicating that cation sublattice of CuO is predominantly defected.

The Role of Zr in the High-Temperature Oxidation of Fe&lt;sub&gt;3&lt;/sub&gt;Al

The paper describes an examination of the effect of the addition of zirconium as a third element on the heat-resisting properties and explains the high temperature oxidation mechanism of Fe3Al intermetallic compounds. The Fe3Al and Fe3Al-0,05Zr specimens have been isothermally oxidized in the temperature range of 1173-1473 K in synthetic air for 100 hrs. The formed oxide layer, about 1,5-2 μm thick, was Al2O3. An examination of the cross-sectioned scales by SEM-EDS showed that the alumina layer consisted of a small inner columnar layer and an outer equiaxed grain layer. Additionally, very fine (50-150 nm) oxide grains rich in Zr, further identified as ZrO2, were found across the alumina scales. To understand the role of Zr on the growth mechanism of α–Al2O3 oxide scale on Fe3Al materials, two-stage oxidation experiments were performed (16O2/18O2), followed by SIMS and TEM-SAD observations. Particular attention was paid to the use of TEM in order to precisely characterize the products on samples prepared using the FIB (Focused Ion Beam) method. A combination of analytical techniques revealed that ZrO2 particles, most of which were formed along alumina grain boundaries, enhanced oxygen diffusion along grain boundaries due to oxygen-deficient composition of zirconium oxide (ZrO2-y).

Mechanisms Involved by Reactive Elements upon High Temperature Chromia Scale Growth

The influence of Y 2 O 3 , Pr 2 O 3 , Nd 2 O 3 , Sm 2 O 3 and Yb 2 O 3 coatings on Fe-30Cr alloy oxidation behaviour was investigated at 1000°C in air under atmospheric pressure. Isothermal exposures indicated that the Y 2 O 3 coating was the most protective after 100 hours. Pr 2 O 3 , Nd 2 O 3 and Sm 2 O 3 coatings were less effective, but the less beneficial effect was observed when Yb 2 O 3 coating was applied onto the Fe-Cr alloy surface. Two-stage oxidation experiments in 16 O 2 and then 18 O 2 were performed to get information about the chromia growth phenomena with and without reactive elements. The 18 O-tracer distribution was determined by secondary ion mass spectrometry (SIMS) and sputtered neutral mass spectrometry (SNMS). The experiments performed on uncoated samples clearly demonstrated that chromia growth mechanism was controlled by chromium cationic diffusion, whereas on reactive element coated samples the external diffusion of chromium ions was not predominant.

Study of the high‐temperature oxidation behaviour of Ti and Ti4Nb with SNMS using tracers

The effect of niobium and water on the high-temperature oxidation behaviour of pure titanium and Ti4Nb (at.%) was studied. An oxidation apparatus was designed that allowed not only the performance of two-stage oxidation experiments using synthetic air isotopically enriched in 18O2 and 15N2 but also the control of humidity using either H2 16O or H2 18O. Oxidation experiments were performed in dry and humid atmospheres at 800 °C. The oxide scales formed were analysed by secondary neutral mass spectrometry. The results obtained show that niobium significantly reduced the oxidation rate of titanium and also suppressed the influence of water. Moreover, the presence of niobium stabilizes the formation of a nitride layer formed at the scale/metal interface. In humid atmospheres water is the main oxygen source for the oxidation process. This phenomenon has to be taken into consideration when interpreting two-stage oxidation experiments and discussing oxidation mechanisms. Copyright © 2000 John Wiley & Sons, Ltd.

Study of the high-temperature oxidation behaviour of Ti and Ti4Nb with SNMS using tracers

The effect of niobium and water on the high-temperature oxidation behaviour of pure titanium and Ti4Nb (at.%) was studied. An oxidation apparatus was designed that allowed not only the performance of two-stage oxidation experiments using synthetic air isotopically enriched in 18O2 and 15N2 but also the control of humidity using either H2 16O or H2 18O. Oxidation experiments were performed in dry and humid atmospheres at 800 °C. The oxide scales formed were analysed by secondary neutral mass spectrometry. The results obtained show that niobium significantly reduced the oxidation rate of titanium and also suppressed the influence of water. Moreover, the presence of niobium stabilizes the formation of a nitride layer formed at the scale/metal interface. In humid atmospheres water is the main oxygen source for the oxidation process. This phenomenon has to be taken into consideration when interpreting two-stage oxidation experiments and discussing oxidation mechanisms. Copyright © 2000 John Wiley & Sons, Ltd.

The isothermal and cyclic high temperature oxidation behaviour of Ti–48Al–2Mn–2Nb compared with Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr

The isothermal and cyclic oxidation behaviour of Ti–48Al–2Mn–2Nb (at%) were studied at high temperatures in air in comparison with the intermetallic alloys Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr. Tests were performed in air between 800 and 900°C. At 800°C Ti–48Al–2Mn–2Nb showed an excellent oxidation resistance under isothermal and cyclic conditions, comparable with Ti–48Al–2Cr–2Nb, and superior to Ti–48Al–2Cr. At 900°C the isothermal oxidation rate of Ti–48Al–2Mn–2Nb was similar as found for Ti–48Al–2Cr–2Nb, but much lower as that of Ti–48Al–2Cr. Upon cooling the oxide scale formed on Ti–48Al–2Mn–2Nb was prone to spallation. During the cyclic oxidation at 900°C, a steady state condition is reached for both niobium bearing materials, with a net linear mass loss rate, due to spallation and (re-)growth of the oxide scale. The linear mass loss rate for the Ti–48Al–2Mn–2Nb was higher than that of Ti–48Al–2Cr–2Nb, indicative of a higher susceptibility for spallation. During the initial stage of oxidation of all tested materials a complex multi-phased and multi-layered scale was formed consisting of α-Al 2O 3, TiO 2 (rutile), TiN and Ti 2AlN. After longer exposure times the outer scale was dominated by TiO 2. In case of the niobium containing materials no loss of protectivity of the oxide scale was found during the growth of the outer TiO 2 layer (under isothermal conditions). Two-stage oxidation experiments with isotope tracers were performed to study the oxidation mechanism in more detail.

SNMS and GDOES studies of the oxidation behavior of TiAl modified by niobium addition

TiAl-based intermetallic alloys are promising candidates as structural materials for high temperature applications. However, industrial application is hindered by insufficient oxidation resistance at temperatures above 700°C in air. The oxidation resistance can be improved by the addition of ternary and quaternary alloying elements, such as niobium. In several studies it has been demonstrated that this element can reduce the oxidation rate dramatically although the underlying mechanism is not yet fully understood. In the present study the influence of niobium on the high temperature oxidation behavior at 800°C of Ti-48Al-2Cr was investigated. Niobium was added by alloying as well as by ion implantation. Some specimens were pre-oxidized prior to ion implantation. Thus, it could be demonstrated that niobium is not only active when present in the bulk alloy, but also when located in the initially formed corrosion scale. Moreover, the implantation experiments revealed that the often suggested doping mechanism of the titania lattice by Nb 5+ ions cannot play an important role explaining the beneficial effect of Nb. The morphology and composition of the scales formed during oxidation were studied by glow discharge optical emission spectroscopy (GDOES) and secondary neutral mass spectrometry (SNMS). The latter technique was also used in combination with two-stage oxidation experiments using the isotope tracers 18 O and 15 N.

TEM and SNMS studies on the oxidation behaviour of NiCrAlY-based coatings

The effect of minor Ti additions content (0% Ti, 0.4% Ti, 1% Ti, 2% Ti) on the oxidation behaviour of Ni-20Cr-10Al-0.4Y (in weight-%) model alloys was investigated in the temperature range 950°C to 1100°C up to 200 h in Ar - 20% O 2 . Alloy microstructure, oxide scale morphology and microstructure of the scale were characterized by SEM/EDX and TEM. The growth mechanisms of the alumina scales formed on the model alloys were studied by two-stage oxidation experiments with 18 O 2 -tracer and subsequent SNMS-analyses. The microstructural observations were correlated with the oxide scale properties in respect to growth rates and spalling resistance. which was tested during cyclic oxidation.

The high-temperature oxidation behaviour of Ti-47Al-2Cr-0.2Si and Ti-48Al-2Cr-2Nb compared with Ti-48Al-2Cr

The oxidation behaviour of Ti-47Al-2Cr-0.2Si and Ti-48Al-2Cr-2Nb (at%), with a fine-grained near-gamma microstructure was studied at 700 and 800 °C in comparison with Ti-48Al-2Cr. Isothermal and cyclic oxidation tests were performed either in humidified synthetic air or in static laboratory air. Ti-48Al-2Cr-2Nb showed the lowest oxidation rate and the highest spallation resistance. For Ti-47Al-2Cr-0.2Si a low isothermal oxidation rate but pronounced susceptibility for oxide spallation was found. The niobium-free and silicon-free alloy Ti-48Al-2Cr showed a high isothermal oxidation rate; however, spallation was between the two extremes represented by Ti-48Al-2Cr-2Nb and Ti-47Al-2Cr-0.2Si. During oxidation a complex multiphased and multilayered scale was formed on the surface of the alloys. The oxidation of the alloys initially resulted in the formation of α-Al2O3, TiO2 (rutile), Ti2AlN, and TiN. After longer exposure times the outer scale of Ti-47Al-2Cr-0.2Si was dominated by fast growing TiO2 crystals and an Al-depletion layer was formed at the scale/metal interface. In the case of Ti-48Al-2Cr-2Nb, however, no Al-depletion zone was found at the scale/ metal interface after longer exposure times, but a narrow layer slightly enriched in Nb. The outer scale also consisted of a TiO2 layer but with a crystal density and crystal size significantly smaller than in the case of Ti-48Al-2Cr and Ti-47Al-2Cr-0.2Si. Two-stage oxidation experiments with isotope tracers were performed to study the oxidation mechanisms. Some attempts to explain the effect of niobium and silicon on the oxidation resistance of TiAl-based intermetallics are discussed.

Evaluation of growth mechanisms of protective oxide layers on metals by modelling 18O-profiles in high temperature two-stage oxidation experiments

Typical two-stage oxidation experiments in high temperature oxidation studies on metals are analyzed on the basis of a two-dimensional model. This model describes the oxide growth by simultaneous diffusion of oxygen and metal ions via volume and grain boundary diffusion while the grain size is allowed to vary over the oxide layer. The 18O profile can be calculated and compared with experiments if the growth rates at the oxide/gas and metal/oxide interfaces can be individually determined. The same applies to the grain size if the growth rate is measured in which case the number of independent parameters is reduced drastically. The parameters needed then for this simulation are the volume diffusivity and the grain boundary diffusivity as well as the grain boundary width and the electronic transport number. The practical applicability of this model is demonstrated on the basis of published results, where the oxide layer grows by inward transport of oxygen. Assuming the electronic transport number and the correlation factor to be one, the grain size as a function of location as well as the 18O-profile is calculated and compared to experiment using three independent parameters only.

Theory of oxygen tracer diffusion along grain boundaries and in the bulk in two-stage oxidation experiments. Part II: analysis of type B regime

In part I of this work a model of 18 O transport in a growing oxide film with parallel grain boundaries has been proposed, and analysis of type A and type C regimes has been carried out. In this paper the analysis is extended to the intermediate, type B regime. We derive an analytical solution of the model, which describes both B and C type regimes as particular cases, and calculate tracer penetration profiles along grain boundaries depending on the drift velocity and time. The results of penetration profile analysis are presented as kinetic diagrams allowing to predict kinetic regimes of 18 O transport under various conditions. We also discuss methods of profile processing for diffusion parameters determination. The analytical solutions obtained and the kinetic diagrams constructed can be used for the treatment of experimental profiles measured in two-stage oxidation experiments

Theory of oxygen tracer diffusion along grain boundaries and in the bulk in two-stage oxidation experiments. Part I: formulation of the model and analysis of type A and C regimes

Based on a simple model of oxide film growth, we derive equations for oxygen tracer diffusion during the second stage in two-stage oxidation experiments. Using this model, we analyze tracer penetration profiles depending on temperature, time and the atomic mechanism of oxygen transport. In Part I the high-temperature A and the low-temperature C regimes of diffusion are considered. The results are presented as kinetic diagrams convenient for designing and interpreting experiments. Part II of this work will be devoted to the intermediate regime B

Transport in α-Al 2O 3 scales on FeAl and NiAl alloys at 1100°C

The nature of transport processes occurring during the growth of oxides at 1100°C on Fe-25 wt% Al and Ni-32 wt% Al has been investigated. Both materials develop protective α-Al 2O 3 scales which have been characterized by SEM, RHEED and Auger electron spectroscopy. Two-stage oxidation experiments have been conducted using 16O 2 and 18O 2 gases; imaging SIMS (secondary ion mass spectrometry) has been used to locate the different isotopic species. The SIMS data (profiles from areas as small as 3.5 μm×3.5 μm, and images with 0.1 μm lateral resolution) illustrate the complexity of the growth processes, and indicate areas where scale growth has taken place predominantly by cation or oxygen ion diffusion.

Analysis of oxygen-18 tracer profiles in two-stage oxidation experiments (I): predominant oxygen diffusion in the growing scale

Typical two-stage oxidation experiments in high-temperature oxidation studies on metals are analyzed. Two cases of predominant oxygen diffusion in the scale are studied: pure volume diffusion and simultaneous transport via grain boundaries and via the bulk. An analytical expression for the growth of the oxide layer is given for the assumption that the chemical potential of the oxygen varies linearly over the oxide layer. The numerical treatment of the differential equation is improved so that the calculation is possibly faster and/ or more accurate compared to a method given in the literature. The experimental profiles are described by four parameters, the grain boundary width, the grain radius, and the volume and grain boundary diffusivities. Two equations correlating these parameters can be extracted from the profiles. Two benchmark tests are described for testing the program. An analytical solution is presented which approximately describes the distribution of O-18 in the oxide layer for pure volume diffusion. Experimental SIMS profiles on Fe-Cr-Al alloys are explored on the basis of our calculation.

SIMS investigations on the growth mechanisms of protective chromia and alumina surface scales

The oxidation behaviour of the oxide-dispersion strengthened (ODS) high-temperature alloys MA 956 (an aluminium oxide former) and MA 754 (a chromium oxide former) has been compared with that of two model alloys, Fe-20Cr-5Al and Ni-25Cr. The morphology and composition of the oxide scales were investigated by metallography, X-ray diffraction analysis and scanning electron microscopy. For analysis of the oxide layer growth mechanisms, twostage oxidation experiments with18O as tracer were used, the distribution of the oxygen isotopes in the oxide scale being determined by SIMS. The ODS alloys show a more selective oxidation than the two model alloys; moreover, the protective oxides on the ODS alloys have a lower growth rate and better adhesion than those on the two model alloys. From the SIMS investigations it can be deduced that the improved properties of the layers on the ODS alloys result from a change in the transport processes in the protective layer; whereas the aluminium and chromium oxide films on the conventional alloys grow by cation and anion transport, the scales on the ODS alloys grow almost exclusively by anion transport. It is shown that the observed properties of the oxide scales on the ODS alloys can be explained by this change in transport mechanism.