Scale Growth Kinetics Research Articles

Oxide Scale Microstructure and Scale Growth Kinetics of the Hot-Pressed SiBCN-Ti Ceramics Oxidized at 1500 °C.

In this study, the SiBCN-Ti series ceramics with different Ti contents were fabricated, and the oxidation resistance and microstructural evolution of the ceramics at 1500 °C for different times were explored. The results show that with the increase in oxidation time, pores and bubbles are gradually formed in the oxide layer. When the oxidation time is less than or more than 4 h, the Ti(C, N) in the ceramics will maintain its initial structure or mostly transform to TiN. The introduction of Ti content can promote the formation of rutile silicate glass, thus healing the cracks and improving the oxidation resistance of the ceramics effectively.

Oxidation behaviour of Zr added hypereutectic Nb-19Si-5Mo-20Ti alloy

The influence of Zr (3 at%) addition on non-isothermal and isothermal oxidation behaviour of the Nb-19Si-5Mo-20Ti (base alloy) has been investigated. During heating in a thermogravimetric analyser to 1300 °C in air, the mass-gain begins at ∼600 °C, and shows a more gradual rise as well as a lower peak value for the Zr-containing alloy compared to the base alloy. Furthermore, the Zr-containing alloy exhibits superior oxidation resistance in isothermal test for 24 h at 1100 °C, but not at 900 °C and 1200 °C, probably due to the differences in oxide scale growth kinetics and defect density. The oxidation products have been identified as Nb2O5, SiO2, TiO2, MoO2, TiNb2O7, Ti2Nb10O29 for both the alloys, with ZrO2 being additionally observed in the Zr-containing alloy. Analysis of the isothermal oxidation kinetics suggests an initial stage of interface-reaction controlled rapid mass increase followed by parabolic behaviour, where oxygen diffusion through the oxide scale is apparently rate-controlling. Slow down to parabolic kinetics is ascribed to the formation of a SiO2-rich compact oxide scale. The oxide scale becomes protective due to reduced oxygen anion diffusivity in Nb2O5 having Mo, Ti and Zr as substitutional solute, along with the formation of ZrO2 and TiO2 with Pilling-Bedworth ratio less than 2.

Corrosion Behaviors of Ni–Cr Alloys in O2, H2O and H2O + O2 Gases at 700°C and the Effect of Temperature

Pure nickel and four binary Ni–Cr (5, 10, 20 and 30 wt%) alloys were exposed to Ar–20O2, Ar–20O2–20H2O and Ar–20H2O (vol.%) at 700°C. In dry and wet O2, three Ni–Cr alloys (5, 10 and 20 wt%) formed a three-layered structure: an external NiO layer, an inner oxide layer of NiO + Cr2O3 and an internal oxidation zone (IOZ) with Cr2O3 dispersed. The Ni–30Cr alloy formed a continuous chromia layer, with locally outer NiO islands. In pure water vapor, all alloys underwent internal oxidation, nickel metal expulsion and NiO formation. Compared with the results at 650°C, increasing the temperature to 700°C did not alter the morphology of the oxide scale, but enhanced the scale growth kinetics. A significant difference between these two temperatures was that the strong retarding effect of water vapor on NiO formation in oxygen at 650°C became insignificant at 700°C. However, in water vapor only condition, NiO formation remained much slower at both temperatures. The effects of water vapor and oxygen partial pressure on NiO formation of different alloys and the effect of temperature on scale growth kinetics and morphology were discussed.

Superior oxidation resistance of the chemically complex but structurally simple Ti-Al-Ta-Ce-Si-La-B-nitride

Alloying (Ti,Al)N with Ta, CeSi2 or LaB6 is beneficial for both hardness and thermal stability. Here we show that their different mechanisms allow for a cumulative improvement when alloyed together, which is especially pronounced for the oxidation resistance. During isothermal oxidation treatments in ambient air at 900 °C (for up to 25 h), Ti0.44Al0.44Ta0.12N allows for a parabolic scale growth rate constant kp of 6.039 × 10-5 µm2/s. This is reduced to 2.074 × 10-5 and even 0.399 × 10-5 µm2/s when alloying with 2 mol% CeSi2 respectively CeSi2 + LaB6 (1 mol% each). The oxide scale growth kinetics for the latter can be even better described by a logarithmic or cubic law. In the as-deposited state the CeSi2 and CeSi2 + LaB6 alloyed (Ti,Al,Ta)N are single-phase fcc structured providing an indentation hardness of 32.6 ± 1.5 and 37.8 ± 1.5 GPa, combined with an indentation modulus of 496 ± 22 and 496 ± 14 GPa, respectively. After vacuum-annealing at 1000 °C, their hardness is still 33.0 ± 1.6 and 34.8 ± 1.1 GPa, and noticeable formation of hexagonal AlN and TaNx phases can only be detected by X-ray diffraction when annealed at temperatures above 1200 °C.Using CeSi2 and LaB6 instead of their elemental form is furthermore beneficial for the target production itself, as Ce and La are highly reactive.

Electronic structure of α−Al2O3 grain boundaries containing reactive element segregants

It has long been known that the addition of small quantities (``doping'') of so-called reactive elements (REs) such as Y, Zr, and Hf to high-temperature ${\text{Al}}_{2}{\text{O}}_{3}$ scale-forming alloys improves oxidation resistance. The presence of reactive elements at grain boundaries lowers the growth rate of the $\ensuremath{\alpha}\text{\ensuremath{-}}{\text{Al}}_{2}{\text{O}}_{3}$ scales, but the cause of the reduced scale growth kinetics is not fully understood. Explanations based on steric effects and explanations based on reducing the grain boundary electronic conductivity have been proposed. We have used density functional theory to study the structural and electronic properties of two $\mathrm{\ensuremath{\Sigma}}7$ bicrystal grain boundaries containing Y, Hf, and Zr substitutional defects on Al sites. The presence of RE substitutional defects plays a minimal direct role in reducing the density of electronic states near the valence-band maximum. However, ${\mathrm{Hf}}^{4+}$ or ${\mathrm{Zr}}^{4+}$ substitutions at the grain boundary repel the positively charged oxygen vacancy ${\text{V}}_{\text{O}}^{2+}$. As ${\text{V}}_{\text{O}}^{2+}$ contributes a defect state above the valence-band maximum but below the Fermi energy, this indirectly lowers the density of current-carrying holes and thus the electronic conductivity of the grain boundary. Replacing ${\mathrm{Al}}^{3+}$ ions with ${\mathrm{Hf}}^{4+}$ or ${\mathrm{Zr}}^{4+}$ ions also makes the grain boundary positively charged, further reducing the hole density.

Cold spray for production of in-situ nanocrystalline MCrAlY coatings – Part II: Isothermal oxidation performance

Cold spray (CS) MCrAlY coatings have been widely explored as potential bond coats for thermal barrier coatings used on the hot-section components of modern gas turbines. In this study, NiCoCrAlTaY coatings were applied on CMSX-4 single-crystal Ni-base superalloy substrates using the CS technique. Nitrogen was used as process gas with low spray parameters to allow the production of in-situ nanocrystalline NiCoCrAlTaY coatings while using conventional powders of large grain size, as demonstrated in Part I of the study using scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). In this part of the study (Part II), the coated samples were examined under isothermal oxidation conditions at 1100 °C for 1 h to 500 h. The as-deposited NiCoCrAlTaY coatings, characterized by dense microstructure, low surface roughness, nanoscale grains, and a large number of crystallographic defects such as grain boundaries and dislocations, promoted the formation of a protective α-Al2O3 scale during short-term oxidation. Fast diffusion via these crystallographic defects facilitated Al diffusion in the coatings. This provides sufficient Al replenishment to the coating subsurface region, sustaining the growth of the α-Al2O3 scale during prolonged oxidation. The relatively low scale growth kinetics found in the study demonstrated enhanced oxidation performance of the CS nanocrystalline NiCoCrAlTaY coatings.

Microstructural and chemical constitution of the oxide scale formed on a pesting-resistant Mo-Si-Ti alloy

The oxidation behaviour of the pesting-resistant eutectic alloy Mo-20Si-52.8Ti was investigated by addressing the microstructural and chemical constitution by grazing incidence X-ray diffraction GIXRD and atom probe tomography APT of oxide scales formed at 800 and 1200 °C in order to understand their protective character. The duplex Si-Ti-oxide scale formed at 800 °C in air, i.e. within the so-called “pesting regime”, is considerably slow growing with a scale thickness of (11 ± 3) μm after 1000 h of exposure. It is composed of a Ti(+IV)-oxide with the chemical composition of approximately TiO2.1 confirmed by APT and SiO2 islands, which are most likely amorphous. The oxide scale formed at 1200 °C, a potential application temperature, is characterised by a top single-phase scale and an underlying duplex scale with a considerably increased thickness of (68 ± 11) μm in total. It consists of rutile TiO2 and some cristobalite SiO2 as determined by GIXRD. APT analysis of the top Ti-oxide scale confirms the stoichiometry TiO2.2. The underlying duplex scale comprising Ti- and Si-oxide is characterised by strongly off-stoichiometric compounds, namely TiO1.4 and SiO1.6. Initial linear oxide scale growth kinetics are found at 800 °C, which change to cubic kinetics for exposure times longer than 100 h. At higher temperatures of 1100 and 1200 °C, diffusion-controlled, almost parabolic kinetics prevail.

Effect of substrate alloy composition on the oxidation behaviour and degradation of aluminide coatings on two Ni base superalloys

The oxidation behaviour and microstructural evolution of aluminide coatings on the Ni base superalloys Rene 80 and alloy 625 were investigated during 1050 °C oxidation in air. Alumina based oxide scales formed on the coating surfaces of both alloys and only minor differences in scale growth and spallation kinetics were found. A coupled thermodynamic-kinetic model was employed to describe the effect of the base material on the depletion processes and microstructural changes in the aluminide coating thus providing a tool for coating life time prediction without very extensive test programs.

Transport mechanisms during the high-temperature oxidation of ternary \u03b3/\u03b3\u2032 Co-base model alloys

Over a decade ago, γ′-strengthened Co-base alloys were introduced as potential replacement for conventional Ni-base Superalloys. Insufficient resistance against high-temperature oxidation restricts the number of possible applications. The present study contributes to the understanding of elementary mechanisms such as material transport during extensive oxide scale formation on γ/γ′ Co-base alloys to explain their inferior oxidation behaviour. A clear dependency of the scale growth kinetics on W content and oxidation temperature is demonstrated by thermogravimetry and subsequent analysis of cross-sections. By means of electron backscattered diffraction (EBSD), the evolution of microstructures in the outer oxide layers were examined depending on the oxidation temperature. Sequential exposure of samples in 16O2- and 18O2-containing atmospheres proved counter-current material transport. The combination of focused ion beam (FIB) and secondary ion mass spectroscopy (SIMS) visualised the formation of new oxide phases mainly on the outer and inner interface of the oxide scale. An elaborate review of available transport paths for oxygen is given during the discussion of results. All experimental findings were combined to a coherent explanation of the inferior oxidation resistance of this relatively new class of high-temperature materials at temperatures above 800 °C.

Dense amorphous Si2BC1-4N monoliths resistant to high-temperature oxidation for hypersonic vehicle

Dense amorphous Si2BC1-4N monoliths were subjected to aggressive environments at high-temperatures to explore the oxide scale microstructure and growth kinetics. The effects of high-pressure sintering and crystallization on oxidation resistance were studied in detail. Dense amorphous Si2BC1-4N monoliths oxidize according to a parabolic rate law or linear equation, depend on the carbon content. Oxidation first attacks at the outermost surfaces of amorphous phases, subsequently proceeds at the interfaces of the crystallized nuclei and neighboring amorphous phases. High-temperature oxidation induces nucleation and thermo-oxidative coupling promotes the nucleation and growth of the nanocrystals.

Kinetics of oxide scale growth on a (Ti, Mo)5Si3 based oxidation resistant Mo-Ti-Si alloy at 900-1300∘C

Abstract High-temperature oxidation behaviour of Mo-40Ti-30Si (at.%) alloy was investigated in the temperature regime of 900-1300∘C in air. Isothermal weight change data recorded up to 100 h of exposure revealed parabolic weight gain kinetics at all the tested temperatures. The protective oxide scale composed with SiO2 (silica) and TiO2 (titania) forming a duplex oxide microstructure consisting of TiO2 particles embedded in the continuous SiO2 matrix. The oxide scale showed parabolic growth kinetics, and the activation energies for the scale growth were found to be 72.2 kJ/mol in 900-1200∘C and 324.9 kJ/mol in 1200-1300∘C. The kinetics of the protective scale growth on the alloy surface was mainly controlled by the growth of the silica film and the inward diffusion of oxygen through the duplex oxide layer.

Numerical Simulation of Heat Transfer and Scale Formation in a Reheat Furnace

In steel rolling mills, reheat furnaces are used to reheat slabs to high temperatures in a highly oxidizing environment; this results in the formation of iron oxide scale on the slab surface. Scale formation poses an ongoing material and economic loss to industry and should be minimized where feasible. The kinetics of scale growth are complex and still not fully understood. Previous studies that modeled scale formation with mathematical methods are limited to simple case studies. Here, computational fluid dynamics (CFD) is used to simulate slab reheat furnace operations and to investigate complicated physical phenomenon. This paper proposes a new numerical method to model scale growth under varying conditions/characteristics including temperature, gas atmosphere composition, and steel grade. This method uses a mixed linear‐parabolic equation to model both the initial surface reaction (and scale formation) and the subsequent solid‐state ion diffusion through the developed scale. This model can be used to predict the amount of scale that will form on a slab under certain conditions. Model predictions were found to be consistent with experimental data.

Cyclic Oxidation and Hot Corrosion Behavior of Plasma-Sprayed CoCrAlY + WC-Co Coating on Turbine Alloys

Components in energy-producing systems suffer a variety of degradation processes such as oxidation and molten salt-induced corrosion as a consequence of complex multi-component gaseous environment. Coatings provide a composition that will grow the protective scale at high temperatures having long-term stability. Plasma spraying was used to deposit CoCrAlY + WC-Co composite coatings on turbine alloys of Hastelloy X and AISI 321. The thermocyclic oxidation behavior of coated alloys was investigated in static air and in molten salt (Na2SO4-60%V2O5) environment at 700 °C. The thermogravimetric technique was used to approximate the kinetics of oxidation in 50 cycles, each cycle consisting of heating and cooling. X-ray diffraction and SEM/EDAX techniques are used to characterize the oxide scale formed. Coated alloys showed a lower corrosion rate as compared to uncoated alloys. The coatings subjected to oxidation and hot corrosion showed slow scale growth kinetics. Preferential oxidation of Co, Cr, W and its spinel blocks the transport of oxygen and corrosive species into the coating by providing a barrier, thereby making the oxidation rate to reach steady state. As compared to the substrate alloys, coatings show better hot corrosion resistance.

Spallation resistance of oxide scales on Alloy 617 enhanced by boron addition

We report on the effects of B additions to Alloy 617 on its high-temperature oxidation behavior and on the spallation of oxide scales. Alloy 617 and a B-added derivative (Alloy 617B) were oxidized at 1050 °C in air and comparatively studied. Cr2O3 scales formed on both alloys with a spinel phase and a Ti-rich oxide on top. While both alloys showed similar scale growth kinetics, Alloy 617B showed substantially enhanced resistance to oxide scale spallation. B segregation at Cr2O3 grain boundaries, detected by atom probe tomography, was found to be the reason for the enhanced spallation resistance of Alloy 617B.

Early stages of scale formation during oxidation of γ/γ′ strengthened single crystal ternary Co-base superalloy at 900 °C

A study on the individual role of γ and γ′-phase during the scale formation on Co-9Al-9W (at.%) at 900 °C was conducted. High-accuracy thermogravimetric analysis was used to locate changes in the kinetics of scale growth. Compositions of three separated layers within the scale have been elucidated in detail. The results demonstrate the substantial impact of the two-phase microstructure on the initiation of oxidation for the appearance of the scale. Examination of STEM-EDX elemental mappings of the internal oxidation front during the early stages of scale growth can explain the origin of isolated Al2O3 precipitates instead of a closed protective layer.

Mechanical reliability and life prediction of coated metallic interconnects within solid oxide fuel cells

Metallic cell interconnects (IC) made of ferritic stainless steels, i.e., iron-based alloys, have been increasingly favored in the recent development of planar solid oxide fuel cells (SOFCs) because of their advantages in excellent imperviousness, low electrical resistance, ease in fabrication, and cost effectiveness. Typical SOFC operating conditions inevitably lead to the formation of oxide scales on the surface of ferritic stainless steel, which could cause delamination, buckling, and spallation resulting from the mismatch of the coefficient of thermal expansion and eventually reduce the lifetime of the interconnect components. Various protective coating techniques have been applied to alleviate these drawbacks. In the present work, a fracture-mechanics-based quantitative modeling framework has been established to predict the mechanical reliability and lifetime of the spinel-coated, surface-modified specimens under an isothermal cooling cycle. Analytical solutions have been formulated to evaluate the scale/substrate interfacial strength and determine the critical oxide thickness in terms of a variety of design factors, such as coating thickness, material properties, and uncertainties. The findings then are correlated with the experimentally measured oxide scale growth kinetics to quantify the predicted lifetime of the metallic interconnects.

Effect of Nitrogen on the Kinetics of Oxide Scale Growth and of Oxygen Dissolution in the Ti6242S Titanium-Based Alloy

Titanium-based alloys are increasingly used in aircraft industry due to their good mechanical properties and low density. To reach better efficiencies, the engine design is constantly evolving and the temperature of the combustion chamber is always increasing. Then, surrounding parts are exposed to higher working temperatures and consequently have to cope with more severe oxidation phenomena. For Ti-based alloys, this leads to the formation of an external oxide scale and a large oxygen dissolution in the metal, which may be detrimental for ductility. As nitrogen is the major element of the atmosphere, it seems relevant to study its effect on the oxidation behavior of Ti-6Al-2Sn-4Zr-2Mo-0.1Si titanium-based alloy used for these applications. Thermogravimetric analyses were carried out at 650 °C for 100 h in synthetic air (20%O2–80%N2) and in a mixture of 20%O2–80%Ar. Results showed that nitrogen decreases the oxidation kinetics by slowing down the oxide scale growth but also by limiting the oxygen dissolution in the alloy.

Influence of Gaseous Media Flow in the Dual Ar-H2-H2O/air Atmosphere Setup on the Scale Growth Kinetics of Crofer 22APU Ferritic Stainless Steel

The problem of gaseous media distribution within the metallic interconnects in solid oxide fuel cells (SOFCs) and its influence on the oxidation resistance of the applied materials is currently of great interest. In the presented work, an influence of gas flow within the dual Ar-H2-H2O/air atmosphere experimental setup on the oxidation behavior of the Crofer 22APU ferritic stainless steel was investigated. Examination of the sample oxidized for 1000 h in temperature of 800 °C revealed the presence of coaxial regions on the scale surface, with the differences in scale’s thicknesses in those regions being clearly visible. Additionally, the morphology of the surface changed significantly in a function of the radial distance from the sample’s center. To further examine the phenomena of uneven gas distribution, a model of the dual-atmosphere setup was created, using Ansys Workbench software. Obtained results suggest that the correlation between scale morphology and distribution of temperature and pressure on the sample’s surface, created by gas flow in the system, can be justified.

Oxidation behavior during prolonged service of boiler tubes made of 2.25Cr1Mo and 12Cr1Mo0.3V heat resistance steels

During service of thermal power plant (TPP) units, different components are exposed to high temperature due to technological cycle of TPP unit. Service lifetimes of these components, especially boiler heating surfaces, may be limited due to creep, fatigue or oxidation, but materials designed for use at high temperatures have been developed primarily for their creep properties and microstructural stability during long term exposures at elevated temperatures. Having in mind that oxidation in steam environment on the inner surface of boiler tubes and in flue gass on the outer surface of boler tubes could lead to a different consequences regarding service life of tubes, either directly through metal wastage or indirectly through raising local temperatures due to the lower thermal conductivity of the oxide scale, the oxidation behaviour of a different heat resistant steels become very important characteristics. In this paper are presented some data about the oxidation behavior of boiler tubes made of 2.25Cr1Mo and 12Cr1Mo0.3V steel after service of approximately 130.000 and 200.000h in two 620MW TPP units. Characterization of oxide scales on the inner side on tubes made of two steels with different chromium content, after two different prolonged periods of service, were compared and also their influence on the service life of tubes and kinetics of oxide scale growth were analyzed.

Comparative Iinvestigation of the High Temperature Corrosion Resistant Model and Commercial Fe-Cr-Al-RE Alloys with Different Chromium Content

The alloys of Fe-Cr-Al-RE system are the high temperature oxidation resistant materials and they are normally are intended to apply at the temperatures up to 1200OC. Usually the composition of the mentioned system is varied by the main elements values as well as RE’s and other alloying elements amount and the variety of the latter. The high temperature behavior of the Fe-Cr-Al-RE bulk alloys and the coatings with the chromium content of up to 25wt% and the Al that of up to 5wt% is investigated widely, whereas higher chromium content alloys of the mentioned system are barely studied.The oxidation peculiarities of high (>40wt%) chromium content Fe-Cr-Al-RE model alloy with some minor additives were compared to that of the two commercial (Aluchrom YHf and PM2000) alloys of the same system. According to the experimental results the new material investigated exhibited the slowest oxidation rate (as it is shown in the Fig.1a) and the thinnest oxide scale formation comparatively to the other two alloys which makes it a quite promising high temperature oxidation resistant material [1-3].The processes taking place on the surface of model alloy exposed to high temperatures are discussed and compared to those of the two commercial alloys. The attempt to disclose the ways differentiating the oxide scale formation on the investigated low and high chromium containing alloys of Fe-Cr-Al-RE system was made and the factors influencing the slower oxide development process for the model alloy were suggested.The obtained kinetic data after the high temperature cyclic oxidation (120/15 minutes of heating up/cooling down cycling) in laboratory air at 1100OC during 2000 hours is compared for all the investigated tree alloys having either polished or rough surface conditions before the exposures at the elevated temperatures. Thus, the initial surface condition on the protective oxide scale growth kinetics was studied. Besides, the oxide scale growth rate dependence on the specimen thickness was studied as well for the alloys exposed at the same elevated temperatures during 1000 hours, where new model alloy has showed the outstanding priority comparing to the other two commercial alloys investigated (Fig. 1b). SEM analysis was applied for the evaluation of the oxidized specimens’ surfaces as well as their cross-sections. ReferencesTsurtsumia O., Kutelia E., Bulia B., Mikadze O., Materials Science Forum Vols. 595-598, 2008, (Trans Tech Publications) pp. 833-840Tsurtsumia O., Kutelia E., Bakhtiyarov S., Proceed. Of 17th Int. Corrosion Congr. USA, Las Vegas, Nevada, October, 2008Kutelia E., Tsurtsumia O., Eristavi B., Adanir H., Bakhtiyarov S., Journal on Engineering and Technology, 2006, Vol. 1, No. 4, pp. 61-71