Growth Mechanisms Of Oxide Scales Research Articles

Effect of gas composition on the oxide scale growth mechanisms in a ferritic steel for solid oxide cell interconnects

The oxidation behavior of a ferritic steel Fe-23Cr-0.5Mn-0.6 Nb-0.1Ti (at%) considered for application in solid oxide cell (SOC) stack interconnects was studied at 800 °C. The oxidation kinetics and oxide scale microstructure formed in Ar-20%O2, Ar-4%H2-4%H2O and Ar-1%CO-1%CO2 atmospheres, simulating the SOC operation environments, were investigated by thermogravimetry (TG) in conjunction with electron microscopy (SEM/TEM) and atom probe tomography (APT). In all three environments multilayered oxide scales formed, consisting of Mn-Cr spinel on top of Cr2O3 and an additional Nb-rich oxide layer at the chromia-alloy interface. The initially faster oxidation in the low pO2 gases was attributed to formation of porous chromia scales compared to a dense scale formed in the high pO2 (Ar-O2) atmosphere. APT revealed segregation of minor alloying elements (Mn, Nb and Ti) to chromia grain boundaries in all three simulated SOC environments in quantitatively similar amounts, suggesting their similar effect on the ionic transport through the oxide scale. The findings indicate that oxygen activity in the test gas plays a dominating role in governing the oxidation kinetics and the oxide scale microstructure of the studied ferritic steel.

The growth mechanism of oxide scale with Pt on NiCoCrAlY coating in water vapor at 1050∘C

In this study, we focus on the oxidation behavior of the oxide scale with Platinum (Pt) on free standing NiCoCrAlY coating in air with water vapor at high temperature. The Pt layer is deposited on the NiCoCrAlY coating surface by electroplating, and then heat treatment at 1000[Formula: see text]C with Argon (Ar) protection. Cycle oxidation of the oxide scale with Pt is conducted in air and air with water vapor at 1050[Formula: see text]C. The results indicated that the spinels (NiCr2O4) are formed on the surface of the alumina scale in air and air with water vapor. However, this kind of spinel is inhibited in the air with water vapor and the scale on the coatings with Pt presents a good adherence during oxidation process. The thermal stress and crack initiation and propagation will be discussed.

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.

Oxidation behaviour of Nimonic 263 in high-temperature supercritical water

ABSTRACTThe oxidation tests of the Nimonic 263 alloy exposed to deaerated supercritical water at 600–700°C under 25 MPa were carried out for up to 1000 h. Oxidation rate increased with an increase in temperature. The microstructure and phase composition of oxide scale were analysed by scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. It can be seen that a complex oxide structure formed on the surface of Nimonic 263 including an outer layer of Ni–Fe/Ni–Cr spinel oxide, Ni/Co hydroxide and TiO2 and an inner layer of a mixture of NiCr2O4 and Cr2O3 while the innermost layer is made up of Cr2O3. The MoO3 can be observed at 600°C but disappeared with the increasing temperature. The growth mechanism of oxide scale was discussed.

Steam Oxidation Behavior of Alloy 617 at 900 °C to 1100 °C

The steam oxidation behavior of solid solution strengthened alloy 617 at 900 °C-1100 °C was investigated. The oxidation products were characterized by scanning electron microscopy, X-ray diffraction, and energy-dispersive spectroscopy. The results show that the oxidation kinetics of alloy 617 in steam followed the parabolic oxidation law. The calculated activity energy of alloy 617 was 223.47 kJ/mol. The oxidation products were mainly composed of external and internal scales and prior oxides at grain boundaries. External oxide scales were MnCr2O4, TiO2, and Cr2O3. Internal oxidation scales and prior oxides were Al2O3 and some Cr2O3 dissolved into Al2O3. The growth mechanism of oxide scales on alloy 617 is proposed.

Determination of the oxide scale growth mechanism using 18O-tracer experiments in combination with Transmission Electron Microscopy and nanoscale Secondary Ion Mass Spectrometry

Two-stage 18O2/16O2 exposures can be used to investigate the effect that alloying elements, secondary phases, or surface treatments have on the high temperature oxidation behaviour of certain materials. During subsequent exposures to 16O2- and 18O2-rich atmospheres, 16O- and 18O-rich layers are formed. Analysis of the layers using Secondary Ion Mass Spectrometry (SIMS) depth profiling allows for conclusions to be drawn about the oxide scale growth mechanism. The conclusions are, however, not entirely unambiguous due to the limited lateral resolution of the technology. Rough surface topography and the thickness variation of the oxide scale over the analysed volume add to the ambiguity of the findings. In this study, an Fe-20%Cr alloy was exposed to both 18O- and 16O-rich environments at 850°C. Two methods were used to analyse the thermally grown Cr2O3 scale: (1) traditional SIMS depth profiling and (2) preparation of a cross-sectional lamellae for Transmission Electron Microscopy (TEM), which, subsequently, was analysed in a NanoSIMS. The NanoSIMS 16O and 18O elemental maps were then superimposed on the TEM image. In comparison with traditional SIMS depth profiling, the nanoSIMS elemental maps reveal detailed information about local oxide growth in different parts of an oxide scale. Moreover, a clear 16O/18O interface can be seen in the nanoSIMS maps, which is not the case in the sputter depth profiles. The findings of this study show that the aforementioned issues associated with sputter depth profiling can be eliminated by mapping a cross-section of an oxide scale using high resolution nanoSIMS.

High temperature oxidation kinetics of amorphous silicoboron carbonitride monoliths and silica scale growth mechanisms determined by SIMS

The highly dense, amorphous silicoboron carbonitride (Si-B-C-N) monoliths provide superior oxidation resistance above 1600°C, but the in-depth oxidation kinetics are unknown. Here oxidation kinetics of amorphous Si-B-C-N monoliths was investigated by one-step oxidation (in flowing 16O-rich air) over 1500–1700°C. Coincidently, the growth mechanisms of oxide scales on ceramic surfaces were studied by oxidizing ceramics in 18O-rich air followed by 16O where 16O was used as an equivalent isotopic tracer and the distributions of 16O and 18O in oxide scales were determined directly by secondary ion mass spectrometry (SIMS). The thickness changes of silica scales fit well to parabolic rules at 1500°C (32.5μm2h−1) and 1600°C (86.1μm2h−1) within 16h, but are irregular at 1700°C. The calculated Arrhenius activation energy is ca. 116kJmol−1 for 1500–1600°C/16h oxidation. The relative concentration profiles of 16O from SIMS analysis indicate growth of oxide scales is mainly by oxygen lattice diffusion inside scales during oxidation of the amorphous Si-B-C-N ceramics.

Study on growth behaviour of oxide scale and its effects on tribological property of nano-TiO2 additive oil-in-water lubricant

Oil-in-water (O/W) emulsion is widely applied as a lubricant during hot rolling process. For reducing environmental impact caused by O/W emulsion, a novel nano-TiO2 additive O/W lubricant with improved stability and reduced coefficient of friction (COF) is developed. Little research, however, has been focused on the growth behaviour of oxide scale formed on the workpiece surface and its effects on the tribological property of nano-TiO2 additive O/W lubricant. In the current work, oxidation tests of high strength steel were conducted to investigate the growth behaviour of oxide scale and obtain different surface morphologies of the oxide scale. Ball-on-disk tribological tests under a temperature of 80°C were carried out to investigate the effects of surface morphology of oxide scale on the tribological behaviour under nano-TiO2 additive O/W lubrication. The performance of the new developed lubricant was evaluated by hot rolling process. The growth mechanism of oxide scale and the modified oxide scale growth model were proposed. Tribological results indicate that oxide scale plays an important role in the tribological behaviour of nano-TiO2 additive O/W lubricant. A reduction in surface roughness can result in a decrease of COF, while the COF will increase if the surface roughness reduces too much because some of the nano-TiO2 particles cannot go through the contact area to play a positive role in lubrication.

Effect of thermal treatment on the corrosion resistance of Type 316L stainless steel exposed in supercritical water

There are still unknown aspects about the growth mechanism of oxide scales formed on candidate stainless steel fuel cladding materials during exposure in supercritical water (SCW) under the conditions relevant to the Canadian supercritical water-cooled reactor (SCWR). The tendency for intermetallic precipitates to form within the grains and on grain boundaries during prolonged exposure at high temperatures represents an unknown factor to corrosion resistance, since they tend to bind alloyed Cr. The objective of this study was to better understand the extent to which intermetallic precipitates affects the mode and extent of corrosion in SCW. Type 316L stainless steel, used as a model Fe–Cr–Ni–Mo alloy, was exposed to 25MPa SCW at 550°C for 500h in a static autoclave for this purpose. Mechanically-abraded samples were tested in the mill-annealed (MA) and a thermally-treated (TT) condition. The thermal treatment was conducted at 815°C for 1000h to precipitate the carbide (M23C6), chi (χ), laves (η) and sigma (σ) phases. It was found that although relatively large intermetallic precipitates formed at the scale/alloy interface locally affected the oxide scale formation, their discontinuous formation did not affect the short-term overall apparent corrosion resistance.

Oxidation behavior of Zr-containing Ti2AlNb-based alloy at 800 °C

The oxidation behavior of Ti–22Al–(27–x)Nb–xZr (x=0, 1, 6) alloys at 800 °C for exposure time up to 100 h was examined. It is shown that oxidation rate of experimental alloys obeys the parabolic kinetics. Ti–22Al–26Nb–1Zr alloy demonstrates more excellent oxidation resistance than the other two alloys. The main oxidation products are TiO2, Al2O3 and AlNbO4 phases for all these alloys. For the Ti–22Al–26Nb–1Zr alloy, Zr addition can modify the growth mechanism of oxide scale, which can effectively hinder the diffusion of oxygen. Whereas, reaction of Zr with oxygen leads to the formation of ZrO2 precipitates for the Ti–22Al–21Nb–6Zr alloy, which promotes the oxygen ingress into the substrate. Meanwhile, oxidation affected zones, including internal-oxidation layer and oxygen-enriched zone, are present beneath the outmost oxide scale. The difference in these zones is derived from the phase constitution in the starting Ti–22Al–(27–x)Nb–xZr (x=0, 1, 6) alloys.

A comparative study of pure nickel and the Ni–CeO2 nanocrystalline coatings: microstructural evolution, oxidation behavior, and thermodynamic stability

In the present work, a novel method of ultrasonic-assisted double-pulsed electro deposition was innovatively used to prepare nanocrystalline Ni coatings from a modified Watts–Nickel electrolyte without and with the addition of CeO2 nanoparticles. The dynamic oxidation kinetics of pure nickel and the Ni–CeO2 coatings was conducted in air at 650 and 850 °C. To clarify the beneficial effects of CeO2 addition on surface morphologies, phase composition and microstructural evolution, the as-received coatings were investigated using various analytical techniques of FE-SEM, XRD, XPS, and HR-TEM observations. In addition, both EDX maps and TG–DSC scans were also carried out for further evaluating the effective bonding effect that generated from the Ce-rich precipitated phase on the inhibition of coarsening growth and textural transformation. Meanwhile, an analytical model for predicting the growth mechanism of oxide scales was successfully validated for the comparison of their different oxidation behaviors. Experimental results illustrated that interfacial boundaries existed in nanocrystalline Ni coatings were favorable for embedding CeO2 nanoparticles to make dispersion strengthening. The existence of well-distributed CeO2 phase in the coating was beneficial to achieve finer-grained size of Ni grains for improvements of structural densification. It exhibited super oxidation resistance of the Ni–CeO2 coatings as compared to that of pure nickel, which was mainly attributed to the Ce-rich oxide scales that acted as a passive layer to fully cover on the coating surface. Furthermore, the precipitation of the Ce-rich phase along high-angle grain boundaries would make great contributions to promote the bonding interactions with high-energy defective regions for effectively completing those cracking boundaries, thereby achieving denser microstructure attached with excellent thermodynamic stability of the Ni–CeO2 coatings.

Lanthanum Effect on the Isothermal High Temperature Oxidation Behavior at 1,000 °C of a Phosphoric Acid-Treated AISI 304 Stainless Steel

Many studies have shown that a phosphoric-acid treatment improves the high temperature oxidation resistance in air of some alloys. Interestingly, though, the phosphoric-acid treatment generates a structural modification of the steel surface which is catastrophic for the high-temperature oxidation behavior at 1,000 °C. The aim of our work was to test the effect of a reactive element sol–gel coating on high-temperature oxidation resistance of phosphoric acid-treated AISI 304 steel. The oxide scale growth mechanisms were studied by exposing La-coated and uncoated phosphoric acid-treated 304 steel samples to high-temperature conditions in air. A phosphoric-acid treatment modified the structural composition and the surface morphology of the AISI 304 steel by the formation of a FeH2P3O10 structure, leading to hematite formation and to a breakaway phenomenon. Lanthanum coating, after initial phosphoric-acid treatment, led to the formation of LaCrO3 which limited through-scale cracking and reduced the growth of iron oxides.

Effect of Phosphoric Acid Treatment on Isothermal High Temperature Oxidation Behaviour of AISI 304 Stainless Steel at 800°C

Phosphoric acid treatment is used as a way to improve the high temperature oxidation resistance of a chromia-forming AISI 304 steel. Chromia-forming steels are excellent candidates to resist to high temperature oxidizing atmospheres because of the formation of protective oxide scales. The oxide scale growth mechanisms are studied by exposing phosphoric acid-treated and untreated 304 steel samples to high temperature conditions in air. The analyses were carried out by means of thermogravimetry, and in situ X-ray diffraction (XRD). The experimental results show that the phosphoric acid treatment does not have a beneficial effect on cyclic high temperature oxidation (up to 70h of the oxidation test) of AISI 304 steel because of growth of a layer mainly formed by external cation diffusion which grows very quickly. The isothermal high temperature oxidation of this steel at 800°C in air shows a very fast initial iron oxidation towards the external interface, allowing to chromium element to be more available to the internal interface to form a continuous chromia layer, thus causing the establishment of a parabolic oxidation regime and leading to a beneficial reduction of the oxidation rate (after 70h of the oxidation test).

High Temperature Oxidation of TiAl and TiAl8Nb Alloys in Air

Isothermal oxidation of two g-TiAl-based intermetallic alloys: Ti48Al and Ti46Al8Nb alloys was studied in synthetic air at 1073-1223 K for up to 240 hrs. Mass change per unit area for the oxidized samples followed approximately the parabolic rate law. The kp values for the studied temperature interval were in the range from 7.2×10-13 to 1.8×10-11 g2cm-4s-1. The activation energy for oxidation of Ti48Al alloy in air at 1073-1223 K was Ea = 165±12 kJ/mol. Niobium addition to Ti48Al alloy in the amount of 8% increased its oxidation resistance. Structure and chemical composition of the oxidation products, and morphology of the oxidized samples were investigated using XRD, SEM-EDS, and TEM. The oxide scales formed on Ti48Al and Ti46Al8Nb alloys were well adherent to the metallic substrates and exhibited a multilayer structure. Depending on the oxidation temperature and the alloy composition, the scale consisted of variable amounts of TiO2 and Al2O3. Additionally TiN, and niobium rich particles were also identified in the appropriate oxide scale. To understand the growth mechanism of oxide scale formed on Ti46Al8Nb alloy, two stage oxidation experiment was performed using 16O2/18O2, followed by SNMS and TEM-EDS. Particular attention was paid to the use of TEM in order to precisely characterize the reaction products on the Ti46Al8Nb alloy.

Manganese Effect on Isothermal High Temperature Oxidation Behaviour of AISI 304 Stainless Steel

Chromia-forming steels are excellent candidates to resist to high temperature oxidizing atmospheres because they form protective oxide scales. The oxide scale growth mechanisms are studied by exposing AISI 304 stainless steel to high temperature conditions in air, and the analyses were carried out by means of thermogravimetry and in situ X-rays diffraction. The in situ XRD analyses carried out during high temperature AISI 304 steel oxidation in air reveals the accelerated growth of iron-containing oxides such as hematite Fe2O3 and iron-chromite FeCr2O4, when the initial germination of the oxide layer contains the presence of a manganese-containing spinel compound (1000°C). When the initial growth shows the only chromia formation (800°C), hematite formation appears differed in time. Protection against corrosion is thus increased when the initial germination of manganese-containing spinel oxide is inhibited in the oxide scale.

Evaluation of Laves-phase forming Fe–Cr alloy for SOFC interconnects in reducing atmosphere

A Laves-phase forming Fe–Cr alloy was evaluated as interconnects for solid oxide fuel cells (SOFCs) in reducing atmosphere (in H 2-H 2O). The oxide scale growth was compared between Laves-phase forming alloy and typical stainless steel (SUS430). The oxide scale growth rates were decreased in the Laves-phase forming alloy, and the area-specific resistance (ASR) of oxidized Laves-phase forming alloy showed the lower values than that of SUS430. The temperature dependence of 1/ASR for the oxidized alloy was different between Laves-phase forming alloy and SUS430. The oxygen diffusivity in the oxide scale was also evaluated by the stable isotope oxygen ( 18O 2) diffusion in the scale. The chemical diffusion coefficients of isotope oxygen in the oxide scale showed the smaller value for the Laves-phase forming alloy ( D = 7.0 × 10 −13 cm 2 s −1) than that for SUS430 ( D = 4.6 × 10 −12 cm 2 s −1) at 1073 K. A relatively high diffusivity of oxygen was estimated in the Mn–Cr spinel oxide on the top surface of oxide scales. Inward diffusion of oxygen and outward diffusion of cation in the oxide scale were discussed to consider the oxide scale growth mechanism.

Enhanced oxidation of the 9%Cr steel P91 in water vapour containing environments

The short term (∼100 h) oxidation behaviour of the 9%Cr steel P91 was studied at 650 °C in N 2–O 2–H 2O gas mixtures containing a relatively low oxygen level of 1%. The oxidation kinetics were measured thermogravimetrically and the oxide scale growth mechanisms were studied using H 2 18O-tracer with subsequent analyses of oxide scale composition and tracer distribution by MCs +-SIMS depth profiling. The corrosion products were additionally characterised by light optical microscopy, SEM-EDX and XRD. It was found that the transition from protective, Cr-rich oxide formation into non-protective mixed oxide scales is governed by the ratio H 2O (g)/O 2 ratio rather than the absolute level of H 2O (g). The results of the tracer studies in combination with the data obtained from experiments involving in situ gas changes clearly illustrated that under the prevailing conditions the penetration of water vapour molecules triggers the enhanced oxidation and sustains the high growth rates of the poorly protective Fe-rich oxide scale formed in atmospheres with high H 2O (g)/O 2 ratios. The experimental observations can be explained if one assumes the scale growth to be governed by a competitive adsorption of oxygen and water vapour molecules on external and internal surfaces of the oxide scales in combination with the formation of a volatile Fe-hydroxide during transient oxidation. The formation of the non-protective Fe-rich oxide scales is suppressed in atmospheres with low H 2O (g)/O 2 -ratios, and the healing of any such scale is promoted.

Nanocrystalline ceria imparts better high–temperature protection

Nanomaterials have attracted considerable interest with numerous technological developments in the last decade. Nanomaterials exhibit different physicochemical properties compared with their bulk counterparts because the diameters of the nanoparticles are less than the Bohr exciton radius. Cerium oxide based materials have been extensively studied for various technological applications. In the present study, the application of nanocrystalline cerium oxide for improvement of high–temperature–oxidation resistance of stainless steel has been studied. The role of coating of nanocrystalline cerium oxide towards improvement of high–temperature–oxidation resistance has been investigated and compared with that of the micrometre–sized cerium oxide particles. It was observed that nanocrystalline ceria improved high–temperature–oxidation resistance of AISI 304 stainless steel to a large extent compared with the micrometre–sized ceria coating. Nanocrystalline ceria coating decreased the isothermal parabolic rate constant of oxidation by more than two orders of magnitude compared with that of the bare alloy. The resistance to oxide scale spallation was also found to improve with the coating of cerium oxide nanoparticles. Secondary ion mass spectroscopy (SIMS) study of nanocrystalline ceria–coated and oxidized specimens revealed the presence of nanoceria at the outermost oxide surface, indicating a change in the oxide scale growth mechanism from outward cation diffusion to inward oxygen diffusion in the bare alloy at high temperature in dry air. The oxide scale morphology was studied using a scanning electron microscope (SEM) while a focused ion beam (FIB) technique was used to study the oxide–alloy interface. X–ray photoelectron spectroscopy (XPS) study of nanocrystalline ceria showed the presence of Ce 3+ and Ce 4+ oxidation states. It is proposed that the presence of the Ce 3+ oxidation state in nanocrystalline ceria improves the oxidation resistance of stainless steel, and the related mechanisms are discussed.

Use of oxygen isotope to study the transport mechanism during high temperature oxide scale growth

The examination of high temperature (HT) oxide scale growth mechanisms was performed using secondary ion mass spectrometry (SIMS) and secondary neutral mass spectrometry (SNMS), in conjunction with 16O2/18O2 HT oxidation experiments. Cr2O3, NiO, ZrO2 and Al2O3 were studied because they constitute excellent representative thermally grown oxide scales: they grow by cationic diffusion (Cr2O3, NiO), anionic diffusion (ZrO2) or mixed anionic-cationic diffusion (Al2O3). The oxidation tests were performed first in 16O2 and subsequently in 18O2 at several temperatures (600–1000°C for NiO, 600°C for ZrO2, 1000°C for Cr2O3 and 1100°C for Al2O3). The oxygen isotope distribution observed by SIMS and SNMS profiles are discussed and related with the HT oxidation mechanisms proposed in the literature.

Application of TEM and SNMS in the study of thermally grown alumina scales

The oxidation behaviour of several alumina-forming alloys containing reactive elements was investigated at 1100°C in air under atmospheric pressure. Analytical techniques were applied to characterise the corrosion products and to understand the role of reactive elements on the alumina scale growth. Secondary neutral mass spectrometry was used to determine the in-depth compositional profile of the element composing the oxide scale, as well as the oxygen isotope, after sequential oxidation experiments using 16O2 and 18O2 enriched environments. Transmission electron microscopy revealed the microstructure of the oxide scale and located the reactive elements within the Al2O3 scale or at the metal–oxide interface. The combination of both techniques led to a better understanding of the oxide scale growth mechanism.