Oxide Scale Growth Rate Research Articles

Effects of CeO2 applied to preformed oxide scales on subsequent oxidation of Fe–20Cr at 1000°C

In order to explore the relationships between rare earth elements existing in oxides and growth rate and adhesion of oxide scales, CeO2 thin films were prepared on Fe–20Cr alloys after pre-oxidation at 1000°C; subsequent isothermal and cyclic oxidation was carried out, and the oxide adhesion was determined by using the tensile pull test. The results demonstrated that similar to the effects of CeO2 on the as polished Fe–20Cr alloy, the application of CeO2 to the preformed oxides could decrease the subsequent oxidation rate, improve the cyclic oxidation resistance of the alloy and raise the adhesion strength of the oxide scales. These beneficial effects of the applied CeO2 decreased with increasing pre-oxidation time. The fact that virtually none of CeO2 applied on the preoxidised preformed layer reached the metal/oxide interface suggests that the 'sulphur trapping effect', through which CeO2 may act, is minimal in the present system.

The Effect of Gas Flow Rate on the Oxide Scale Morphology of a 10%Cr-Ferritic Steels in Ar-H<sub>2</sub>O and Ar-H<sub>2</sub>-H<sub>2</sub>O Mixtures

The oxidation behaviour of a Ferritic 10%Cr steel in Ar-H2O mixtures was investigated at 650°C. The studies aimed at elucidating the effect of water vapour content as well as the gas flow rate on the mechanisms of oxide scale formation. An important observation of the present investigation is, that H2 produced by the reaction of water vapour with the steel, can play a significant role in the oxidation process. It affects the possibility to form an external haematite layer and may alter the oxide scale growth rate. The extend by which the H2 affects the oxidation behaviour depends on the gas flow conditions, the water vapour content and the exposure time. To confirm these observations a number of specimens were oxidized in Ar-H2-H2O mixtures. This atmosphere guarantees a very low equilibrium oxygen partial pressure, in which H2 formed by reaction of the gas with the metal, does not substantially alter the thermodynamic properties of the gas.

Effects of silicon concentration in SOFC alloy interconnects on the formation of oxide scales in hydrocarbon fuels

Oxide scale formations on Fe Cr alloy interconnects were investigated in anode gas (mixtures of CH 4 and H 2O) atmospheres for solid oxide fuel cells. The silicon concentration in Fe Cr alloy changed the microstructures of oxide scales, elemental distribution and oxide scale growth rates. Oxide scale is composed of the following phases from surface to inner oxides: Fe Mn spinel, Cr 2O 3, oxide scale/alloy interface and internal oxides of Si and Al. With decreasing the Si concentration from 0.4 to 0.01 mass%, formation of thin Si and Mn layer was observed inside the Fe Cr alloy. Oxide scale growth rate constants decreased by lowering the Si concentration in Fe Cr alloy from 4.2 × 10 −18 to 2.1 × 10 −18 m 2 s −1 at 1073 K. Diffusivity of Fe and Cr was changed by the concentration of Si in Fe Cr alloy, which affects the growth rates of oxide scale. The electrical conductivity of oxidized Fe Cr alloy shows almost same level regardless the Si concentration (in the orders of 10 S cm −2 at 1073 K).

A model for the effect of creep deformation and intrinsic growth stress on oxide/nitride scale growth rates with application to the nitridation of γ-TiAl

The growth rate of oxide and nitride scales can be considerably affected by the application of stress to the material when the underlying substrate is able to creep. This occurs even though the applied stress may be quite small. In this paper, we present a model to calculate the residual stress induced in growing scales during this process. It is shown that the applied stress causes the metallic substrate and the scale to creep at different rates leading to the accumulation of a large stress in the thin scale layer. This stress influences the diffusional flux of anions towards the metal/nitride interface, which controls the scale growth kinetics. The model is applied to the growth of TiN scale on γ-TiAl upon exposure at high temperature in a pure N 2 atmosphere. As a consequence, the nitride scale growth rate is dependent on the sign and magnitude of the applied stress wherein a tensile stress accelerates the growth process and a compressive stress slows it down. The model predictions are in good agreement with experimental data for scale thickness on the tensile and compressive faces of TiAl bars following flexural creep tests in N 2.

The Effect of Water-Vapor Content and Gas Flow Rate on the Oxidation Mechanism of a 10%Cr-Ferritic Steel in Ar-H2O Mixtures

The oxidation behavior of a Ferritic 10%Cr steel in Ar-H2O mixtures was investigated at temperatures between 550 and 650°C. The studies aimed at elucidating the effect of water-vapor content as well as the gas flow rate on the mechanisms of oxide-scale formation. It was found that, the oxide types and morphologies depend on time and exposure conditions. An important observation is that H2 produced by the reaction of water vapor with the steel can play a significant role in the oxidation process. First, it affects the possibility to form an external hematite layer and if the latter is not present, it tends to decrease the growth rate of the magnetite-base oxide scale. The extend by which the H2 affects the oxidation behavior depends on the gas-flow conditions, the water-vapor content and the exposure time.

Effects of specimen treatment and surface preparation on the isothermal oxidation behaviour of the HVOF-sprayed MCrAlY coatings

Alloys and coatings designed to resist oxidizing environments at high temperatures should be able to develop a surface oxide layer, which is thermodynamically stable, slowly growing and adherent. During oxidation in the temperature range 850–1300 °C, MCrAlY coatings should form Al 2O 3-rich scales, which are reasonably effective for long-term applications under isothermal or thermal cyclic oxidation conditions. This study is concerned with the isothermal oxidation behaviour of high-velocity oxygen fuel (HVOF)-sprayed CoNiCrAlY coatings containing 12 wt.% aluminium. Specimens treated in different ways (ground, polished and EB-remelted) were oxidized at 950 °C in synthetic air for various periods of time. Oxidation studies include thermogravimetric weight gain measurements under isothermal conditions in order to quantify the parabolic oxidation rate constant. The microstructure and morphology of as-sprayed and of oxidized coatings were characterised by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The ground and polished samples show internal oxidation as well as relatively high values of the oxidation rate constants. In the case of EB-remelted coatings, the oxide-scale growth rate is substantially smaller and no internal oxidation was observed.

Evaluation of Ferritic Stainless Steel for Use as Metal Interconnects for Solid Oxide Fuel Cells

Interconnect development for planar solid oxide fuel cells is considered a vital technical area requiring focused research to meet the performance and cost goals. A commercial ferritic stainless steel composition for oxidation resistance properties was investigated by measuring the weight gain due to air exposure at fuel cell operating temperature. A surface treatment process was found to produce a dense, adherent scale and to reduce the oxide scale growth rate significantly. A process was also identified for coating the surface of the alloy to reduce the in-plane resistance and potentially to inhibit chromium oxide evaporation. The combination of treatments provided a very low resistance through the scale. The resistance measured was as low as 10 mΩ-cm2 air. The resistance value was stable over several thermal cycles. The treated samples were exposed to a variety of atmospheres that were relevant in fuel cell operation to evaluate changes in scale morphology. Analysis of the scale after such exposure showed the presence of a stable composition. When exposed to a dual atmosphere (air and hydrogen on opposite sides of the metal sheet), however, the scale composition contained a mixture of phases. Additional process modifications are planned to reduce the effect of dual-atmosphere exposure.

ToF-SIMS depth profiling of alumina scales formed on a FeCrAl high-temperature alloy

The thickness and composition of thin oxide scales formed on a powder metallurgical FeCrAl alloy isothermally oxidised between 0.5 and 192 h at 900 °C have been investigated using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The results show that the oxide scale grows according to a parabolic law during oxidation times up to 192 h. The oxide scales mainly consist of alumina, with the accumulation of Fe and Cr oxides both within the scale and at the oxide scale/metal substrate interface. Furthermore, elements such as Mg, Si, K, Ca, Ti, Mn, Ni and Zr are also found within the oxide scale which indicates that these elements may also influence the oxide scale growth rate and growth mechanisms. The results demonstrate the usefulness of ToF-SIMS analysis for characterisation of the initial stages of oxidation of FeCrAl alloys at elevated temperatures.

Growth Rate and Phase Composition of Oxide Scales during Hot Rolling of Low Carbon Steel

The rate of scale growth on low carbon steel in air over the temperature range 600-1200°C and the phase composition changes that occur between 750-1 200°C were investigated. The low carbon steel was oxidized with the air velocity of 4.2 cm/s in order to approximate the formation of secondary and tertiary scales in hot rolling. In addition, some experiments were performed with a lower air velocity of 0.14cm/s. Above 1 000°C, with the air velocity of 4.2 cm/s, a transition from a parabolic rate of oxidation to a linear rate of oxidation was observed as the temperature increased. This transition in oxidizing mechanisms was related to the porosity of the oxide scale. The phase composition of the oxide scales changed with temperature and time. For the initial 30s of oxidation, wustite was the predominant phase in the temperature range 800-1 200°C and as oxidation proceeded, the percentages of magnetite and hematite increased. The homogeneity of the oxide decreased as the oxidation temperature increased. At 850°C, with the air velocity of 4.2 cm/s, the oxide was homogeneous, and for the first 120s of oxidation, the oxide had a high percentage of wustite and a low percentage of hematite. This indicates that 850°C is the ideal temperature for the finishing strip mill in order to reduce work roll wear and surface defects.

The effect of microstructure on the oxidation behaviour of TiAl-based intermetallics

The oxidation behaviour of three TiAl-based intermetallics, Ti47Al2Cr, Ti48Al5Cr and Ti48Al5Ni, in Ar20%O 2 at 900°C was investigated. The main emphasis was placed on the effect of alloy microstructure on the oxidation behaviour. It was found, that, depending on the exact composition, the oxidation resistance of TiAl-based intermetallics is strongly affected by the amount and distribution of the various phases which are present in the alloy. The oxide scale growth rates depends on the actual manufacturing process and subsequent heat treatment procedures. The differences imparted by these factors can be much larger than the effects imparted by ternary or quaternary alloying additions.

Surface analytical investigations on the oxidation behaviour of TiAl-base intermetallics

The oxidation behaviour of TiAl-base intermetallics has been investigated at 800°C in air. The main emphasis was placed on the mechanism by which niobium additions decrease the oxidation rate of titanium aluminides. For this purpose specimens of Ti50Al and Ti45Al10Nb were oxidized in a two-stage oxidation technique using an 18O-tracer. The scale formed during this oxidation process was analyzed by SNMS. It was found that niobium is mainly incorporated in the titanium dioxide and the nitride scale which forms beneath the initially formed alumina layer. The effect of this behaviour of the niobium on the oxide scale growth rates is discussed.

The reactive element effect on the growth rate of nickel oxide scales at high temperature

The restriction of oxide-scale growth by an incorporated reactive element has been investigated for the oxidation at 900°C of CeO2-coated and Ce-alloyed Ni. Analytical electron microscopy of scales in transverse section revealed that significant inhibition of diffusion along a network of grain-boundary pathways in NiO was associated with segregation of Ce at a high concentration. The development of this form of Ce distribution depended critically on the provision of sufficiently small and closely spaced CeO2 source particles within the scales.

The Effect of Surface Additives on the Oxidation of Chromia-Forming Alloys

Abstract The influence of the surface application of active elements on the composition, morphology, adherence, and growth rate of oxide scales formed during high temperature exposure has been investigated. The active elements were applied as aqueous solutions of nitrate salts that were subsequently transformed into oxide. The active elements used were: Y, Ce, La, Hf, Ca, and Zr. The chromia-forming substrates used were AISI 304 and 310 stainless steels and IN 738, a nickel base alloy. To determine the effect of the minor alloying elements in stainless steels on the surface doping effect, Y was applied to three alloy modifications of AISI 304. The effect of the presence or absence of Mn and Si in the alloy on high temperature corrosion behavior was determined. Several different application techniques were used to determine which technique was most beneficial to the behavior of the oxide barrier scale. The application of Y, Ce, and La was found to enhance the oxidation resistance of the commercial stainles...