Surface Oxide Scale Research Articles

Oxide scale behaviour and surface roughness under hot rolling conditions of Fe-Ni36 Invar alloy

In this study, hot rolling tests of Fe-Ni36 invar alloy were carried out on a 2-high mill at different temperatures and various reductions. The specimens of the hot rolling tests were designed in the shape of stairs to investigate the relationship between oxide scale, surface roughness and reduction. The surface characteristics and the cross section morphology of the oxide scale were observed under an optical microscope and with Scanning Electron Microscopy (SEM). The chemical compositions of the oxide scales were investigated using Energy Dispersive Spectroscopy (EDS). The thickness of the oxide scale and surface roughness were measured. It was found that with an increase of reduction, the oxide scale thickness and the surface roughness decrease obviously.

The effects of atmosphere condition on tensile behaviour of surface oxide scale on Type 310 stainless steel at high temperature

The effects of atmosphere condition on the mechanical properties at high temperature and on the scale structure of Cr2O3 are discussed. Tensile tests of preoxidized Type 310 stainless steel were conducted at 1000°C. Pre-oxidation was done at 1100°C for 90–300 minutes before the tensile tests in 20%O2–80%N2 simulating dry air and in 2%O2–10%CO2–16%H2O–Bal.N2 simulating the liquefied natural gas (LNG) combustion gas condition. The cracking behaviour by high-temperature tensile tests was investigated and the detailed observations of the crystal structure of the surface oxides were examined using TEM, EPMA and EDAX, etc.The length of the cracks created by the tensile test at 1000°C, found in the oxide scale formed in H2O-containing gas atmosphere, was greater than those formed in dry air. The crack density of the oxide scale formed in the H2O-containing gas atmosphere was lower than that formed in dry air. Cracking of the Cr2O3 oxide scale formed in dry air is more likely to occur compared with that formed in H2O-containing gas atmosphere.TEM observation showed that a Cr2O3 oxide scale formed in H2O-containing gas atmosphere which contained some MnCr2O4 spinel crystals inside. No MnCr2O4 spinel was found in the Cr2O3 scale generated in dry air. This MnCr2O4 spinel is considered to alleviate stresses resulting from the tensile deformation of the corundum Cr2O3 scale.Furthermore, the possibility that the oxide growth in H2O-containing gas atmosphere took place by inward diffusion of anions has been suggested. That is, the compression stress generated by the inward growth of the oxide must be accumulated and the tensile stress by tensile test can be alleviated by the accumulated compression stress in the H2O-containing gas atmospheres.

Enhanced Internal Oxidation as Reason for Breakdown of Protective Chromia Scales on FeCr-Alloys in Water Vapour Containing Gases

The oxidation behaviour of binary Fe-Cr alloys containing 10 and 20 mass % Cr, respectively, was studied in Ar-20%O2, Ar-7%H2O and in Ar-4%H2-7%H2O at temperatures between 800 and 1050°C. Thermogravimetric analyses in combination with analytical studies using SEM/EDX and Raman Spectroscopy revealed, that in atmospheres in which water vapor is the source of oxygen, Cr exhibits a higher tendency to become internally oxidized than in the Ar-O2 gas. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the surface oxide scale, the present results reveal that the presence of water vapor also affects the transport processes in the alloy. The enhanced internal oxidation, which is likely the result of water vapor increasing the solubility and/or the diffusivity of oxygen in the alloy, explains the frequently observed effect that Fe(Ni)Cr alloys with intermediate Cr contents (e.g. 10-20%, depending on temperature) exhibit protective oxidation in dry gases but breakaway type oxidation in steam. The temperature dependence of the change from protective to non-protective behaviour in Ar-H2O differs quantitatively, but not qualitatively from that in Ar-O2.

Direct Measurements of the Chromium Activity in Complex Nickel Base Alloys by High Temperature Mass Spectrometry

Chromium rich, nickel based alloys Haynes 230 and Inconel 617 are candidate materials for the primary circuit and intermediate heat exchangers (IHX) of (Very)-High Temperature Reactors. The corrosion resistance of these alloys is strongly related to the reactivity of chromium in the reactor specific environment (high temperature, impure helium). At intermediate temperature – 900°C for Haynes 230 and 850°C for Inconel 617 – the alloys under investigation are likely to develop a chromium-rich surface oxide scale. This layer protects from the exchanges with the surrounding medium and thus prevents against intensive corrosion processes. However at higher temperatures, it was shown that the surface chromia can be reduced by reaction with the carbon from the alloy [1] and the bare material can quickly corrode. Chromium appears to be a key element in this surface scale reactivity. Then, quantitative assessment of the surface requires an accurate knowledge of the chromium activity in the temperature range close to the operating conditions (T ≈ 1273 K). High temperature mass spectrometry (HTMS) coupled to multiple effusion Knudsen cells was successfully used to measure the chromium activity in Inconel 617 and Haynes 230 in the 1423- 1548 K temperature range. Appropriate adjustments of the experimental parameters and in-situ calibration toward pure chromium allow to reach accuracy better than ± 5%. For both alloys, the chromium activities are determined. Our experimental results on Inconel 617 are in disagreement with the data published by Hilpert [2]. Possible explanations for the significant discrepancy are discussed.

Protective coatings for very high temperature reactor applications

The future very high temperature reactors (VHTR) are nuclear systems that shall operate at a maximum temperature of about 950 °C. Primary circuit materials thus require good creep and corrosion resistance on very long time. Use of high-strength alloys with protective coatings could significantly improve the service life of high temperature reactor components. However, coating systems are mainly designed for shorter term purposes, often under extremely aggressive atmospheres, that cannot be extrapolated to the VHTR environment. We present our first investigations on the environmental resistance of Alloy 800H coated with two different protective systems under VHTR representative conditions: NiAl(Pt)/EBPVD ZrO2(Y) and NiCrAl(Y)/CVD ZrO2(Y). Isothermal exposures were carried out up to 1000 h at 950 °C in impure helium. This specific atmosphere was shown to induce formation of a surface oxide scale together with carburisation of the bare Alloy 800H. After high temperature exposure to impure helium, the microstructure of the coated specimens has changed due to both thermal ageing and corrosion. Performances of the two coating systems are compared regarding the VHTR application.

Effect of grain boundaries on the formation of oxide scale in Fe–Cr alloy for SOFCs

The effect of grain boundaries on the formation of oxide scale was investigated for two different Fe–Cr alloys at 1073 K in H 2–H 2O atmosphere: typical stainless steel (SUS430) and Laves-phase forming Fe–Cr alloy. Identified oxide scale phases were Cr–Mn–(Fe) spinel and Cr 2O 3 on both surfaces of the Fe–Cr alloys. A relatively fast diffusion of elements was observed at the grain boundaries of SUS430 with a formation of ridges on the oxide scale surface. On the other hand, the diffusion of elements was controlled by precipitating a Laves-phase (Fe 2(Nb, Mo) phase) at the alloy grain boundaries in the Laves-phase forming alloy. The oxide scale growth rates of the Laves-phase forming alloy was 9.8 × 10 − 15 cm 2 s − 1 , which was much smaller than that of SUS430 (4.5 × 10 − 14 cm 2 s − 1 ) in the same condition. This was due to the lower diffusivity of elements at the grain boundaries of alloy. The control of elemental diffusivity at the alloy grain boundary (Laves-phase forming alloy) is effective to form stable and thin oxide scales on the Fe–Cr alloy surface in H 2–H 2O.

A STUDY ON THE MICROSTRUCTURAL CHANGES OF Cr-MODIFIED ALUMINIDE COATINGS ON A NICKEL-BASED SUPERALLOY DURING HOT CORROSION

Microstructural changes of chromium-modified aluminide coating on IN-738LC superalloy under hot corrosion conditions were studied in this research. Samples were coated using two separate pack cementation processes, i.e. chromizing and aluminizing. Hot corrosion resistance of the coatings was evaluated using the furnace test method. For this purpose the specimens were coated with a thin layer of Na2SO4-5 wt% NaCl and then oxidized in air for a period of 1000 hours at 850 °C. The results of the hot corrosion test indicated that lower chromium content in the chromizing pack powder (C10A coating) resulted in the formation of a uniform adhesive surface oxide scale, containing mainly Al2O3. However, the C20A coating with higher chromium content exhibited Cr2O3 as a major surface oxide. The maximum depth of sulphur penetration in the C10A coating was 60 μm, which was very low in comparison with that of the C20A coating (i.e. 350 μm). In addition, the distribution of elements and precipitates within the coatings and the substrate before and after the hot corrosion tests was analyzed. From this investigation, some suggestions were made for better hot corrosion resistance of the coatings. Dans cette recherche, on a étudié les changements de microstructure d’un revêtement d’aluminide modifié au chrome sur le superalliage IN-738LC sous des conditions de corrosion à chaud. On a revêtu les échantillons en utilisant deux procédés distincts de cémentation solide, soit la chromisation et l’aluminisation. On a évalué la résistance à la corrosion à chaud des revêtements en utilisant la méthode d’essai au four. Dans ce but, on a revêtu les échantillons avec une couche mince de Na2SO4 – 5% en poids de NaCl et on les a ensuite oxydés à l’air sur une période de 1000 heures à 850 °C. Les résultats de cet essai de corrosion à chaud ont indiqué que la plus faible teneur en chrome de la poudre de chromisation solide (revêtement C10A) avait pour conséquence la formation d’une écaille de surface adhésive et uniforme contenant principalement du Al2O3. Cependant, le revêtement C20A, avec une plus haute teneur en chrome, exhibait du Cr2O3 comme principal oxyde de surface. La profondeur maximale de pénétration du soufre dans le revêtement C10A était de 60 μm, ce qui était très faible en comparaison avec celle du revêtement C20A (i.e. 350 μm). De plus, on a analysé la distribution des éléments et des précipités dans les revêtements et dans le substrat avant et après les essais de corrosion à chaud. À partir de cette investigation, on fait quelques suggestions pour une meilleure résistance à la corrosion à chaud des revêtements.

Development of high strength ferritic steel for interconnect application in SOFCs

High-Cr ferritic model steels containing various additions of the refractory elements Nb and/or W were studied with respect to oxidation behaviour (hot) tensile properties, creep behaviour and high-temperature electrical conductivity of the surface oxide scales. Whereas W additions of around 2 wt.% had hardly any effect on the oxidation rates at 800 and 900 °C, Nb additions of 1% led to a substantially enhanced growth rate of the protective surface oxide scale. It was found that this adverse effect can be alleviated by suitable Si additions. This is related to the incorporation of Si and Nb into Laves phase precipitates which also contribute to increased creep and hot tensile strength. The dispersion of Laves phase precipitates was greatly refined by combined additions of Nb and W. The high-temperature electrical conductivity of the surface oxide scales was similar to that of the Nb/W-free alloys. Thus the combined additions of Nb, W and Si resulted in an alloy with oxidation resistance, ASR contribution and thermal expansion comparable to the commercial alloy Crofer 22 APU, but with creep strength far greater than that of Crofer 22 APU.

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.

High temperature corrosion of a nickel base alloy by helium impurities

High temperature corrosion properties of Haynes 230 were investigated in a purposely-designed facility under a typical very high temperature reactor (VHTR) impure helium medium. The study was focused on the surface oxide scale formation and its stability at about 1223K. The alloy developed a Mn/Cr rich oxide layer on its surface under impure helium at 1173K. Nevertheless, a deleterious reaction destructing the chromium oxide was evidenced above a critical temperature, TA. Reagents and products of this last reaction were investigated.

Stability of Fe-Cr Alloys in SOFC Fuel Atmosphere

The chemical stability of Fe–Cr alloys (ZMG232 and SUS430) was examined in fuel atmosphere of SOFC, especially in H2–H2O atmosphere. Oxide scales were formed on the alloy surface with a thickness of 3–5 μm at 1073 K for 1000 h by the diffusion of cations through the scale. The microstructures and elemental distribution of oxide scale/alloy interfaces were analyzed by the depth and surface analysis techniques: Mn–Cr–(Fe) spinel, Cr2O3, and internal Si oxides were identified from surface to inner oxides. An anomalous thick oxide scale formation was found on SUS430 surface in H2–H2O atmospheres. A thick oxide scale was composed of Fe, Cr, and Mn oxides. A relatively high concentration of Fe was observed at top surface of oxide scale. The distribution of Si was found inside the oxide scale, suggesting an increase of oxygen potential and fast diffusion of Si inside the oxides. The formation of anomalous thick oxide scale can affect the stability of Fe–Cr alloy interconnects by changing the cell connections at scale/alloy/cell interfaces.

Surface roughness transformation and deformation of the scale in hot rolling

In hot strip rolling, the oxide scale surface significantly affects the roughness of the strip which, in turn, affects the product quality through subsequent downstream processing via the pickle line and the five stand cold mill. In this paper, we focus on the modelling of the hot rolled strip with oxide scale and quantify the effects of various rolling parameters on the final surface roughness. A Finite Element Model (FEM) is used to simulate the surface roughness transfer of the oxide scale and the steel surface roughness in hot rolling.

Effect of surface condition on the oxidation behaviour of MCrAlY coatings

Coatings of the MCrAlY type (M = Ni, Co) are commonly used as overlay coatings and as bond coats (BC's) for ceramic thermal barrier coatings (TBC's) in industrial gas turbines. During high temperature service the MCrAlY coatings form aluminium based surface oxide scales. The technologically most relevant properties of the oxide scales, growth rate and adherence do not only depend on the exact MCrAlY composition but also on the surface condition after coating manufacturing. Depending on the coating manufacturing process, the MCrAlY surfaces may substantially differ in roughness. In addition the coatings may be subjected to surface treatments (e.g. shot peening, grinding or smoothening prior to deposition of TBC). In the present work the effect of the surface roughness commonly prevailing in the case of vacuum plasma sprayed MCrAlY coatings on the morphology and composition of the alumina scales formed during high-temperature oxidation was studied. For this purpose free standing coatings with rough and polished surfaces were oxidised in the temperature range from 800 to 1100 °C with exposure times up to 1000 h. The surface scale composition and morphology were analysed by optical metallography and SEM. Fluorescence spectroscopy was used for stress measurements in the oxide scales. It has been found that the oxide scales formed on rough surfaces of MCrAlY coatings have an intrinsically different morphology and growth rate compared to those formed on the flat ground surfaces.

Characterisation of high temperature oxide scales for plasma sprayed NiCrAlY coated Ni‐ and Fe‐based superalloys

PurposeTo characterise the high temperature oxide scales for some plasma sprayed NiCrAlY coated Ni‐ and Fe‐based superalloys.Design/methodology/approachNi‐22Cr‐10Al‐1Y metallic coatings were deposited on two Ni‐based superalloys; Superni 601 and Superni 718 and one Fe‐based superalloy; Superfer 800H by the shrouded plasma spray process. Oxidation studies were conducted on uncoated as well as plasma spray coated superalloys in air at 900°C under cyclic conditions for 50 cycles. Each cycle consisted of 1 h heating followed by 20 min of cooling in air. The thermogravimetric technique was used to approximate the kinetics of oxidation. X‐ray diffraction, SEM/EDAX and EPMA techniques were used to analyse the oxide scales.FindingsAll of the coated, as well as the uncoated, superalloys followed an alnost‐parabolic rate of oxidation. The NiCrAlY coating was found to be successful in maintaining its continuous contact with the superalloy substrates in all the cases. The oxide scales formed on the exposed NiCrAlY coated superalloys were found to be intact and spallation‐free. The main phases analysed for the coated superalloys were oxides of nickel, chromium and aluminium and spinel of nickel and chromium, which are expected to be useful for developing oxidation resistance at high temperatures.Practical implicationsThe coated superalloys showed remarkable cyclic oxidation resistance under simulated laboratory conditions. However, it is suggested that these coated superalloys also should be tested in actual industrial environments of boilers and gas turbines, etc. so as to obtain more practical and reliable oxidation data.Originality/valueThe knowledge of the reaction kinetics and the nature of the surface oxide scales formed during oxidation is important for evaluating the alloys for their use and degradation characteristics in high temperature applications such as steam boilers, furnace equipment, heat exchangers and piping in chemical industry, reformer, baffle plates/tubes in fertilizer plants, jet engines, pump bodies and parts.

Characterisation of thin oxide scale and its surface roughness in hot metal rolling

The deformation of the thin oxide scale formed in hot metal rolling and its surface roughness characterisation are very important for the quality of the finished product. Finite element (FE) simulation of the thin oxide scale deformation and surface roughness transfer has been carried out. The authors focus on the surface asperity deformation of the thin oxide scale and strip. Surface characterisation and the mechanics of the thin oxide scale deformation are obtained from the FE simulation and experimental measurements. Simulation results are close to the measured values. The forming features of surface roughness transfer during hot metal rolling with lubrication are also discussed.

Deformation and Fracture Behavior of Surface Oxide Scale on Fe-13Cr Alloy in Hot-Rolling Process

The behavior of the surface oxide scale on steel products during hot rolling process influences the surface properties of final products. To investigate the deformation and the fracture behavior of surface oxide scale of Fe-13Cr alloy, a hot rolling test was carried out. The oxide scale rolled out was observed in detail by using TEM. The specimen was hot-rolled after oxidation at 1100 for 90 minutes in air. The hot rolling tests with two conditions (. The hot rolling test of the outer scale {=whole layer scale} , . The hot rolling test of the inner scale that removed the outer scale) were carried out. The rolling reduction rate was 25, 44, 58, and 68%. The outer scale was composed of Fe2O3 and F3O4, and the inner scale was composed of Fe3O4, FeCr2O4, and a small amount of Fe2SiO4. Fe2SiO4 formed along the grain boundaries of the other oxides (Fe3O4, FeCr2O4) was observed by TEM. In the test , Fe2O3 of the outer scale was pulverized to fine particle that looks like red powder, and Fe3O4 of the outer scale was cracked by hot rolling. A ductility-like behavior was observed in the inner scale (Test ). That is, it was found by the SEM observation that porosity and micro cracks of the surface oxide disappeared gradually according to the increase in the rolling reduction. It was thought that the porosity and the micro cracks eased the compression stress caused by hot rolling. In the case of high reduction rate, FeSi2O4 ,which is a low melting point oxide, formed on grain boundary caused grain boundary slipping. When the rolling reduction is very high, plastic deformation by dislocation occurred in Fe3O4 and FeCr2O4. However, these oxides were broken, when their plasticity would not be able to accept considerably high rolling reduction.

Modelling of oxide scale surface roughness in hot metal forming

Oxide scale formed in hot metal forming and its surface roughness characteristics are very important for the quality of the finished product. In this paper, the oxide scale and steel surface roughness changes in hot strip rolling are analysed using the finite element software MSC-MARC package. A model generating a surface roughness profile which reproduces the random characteristics of surface roughness and asperity shape was applied in this study. Surface roughness deformation characteristics of the oxide scale are obtained from the finite element simulation. The features of the oxide scale surfaces with and without oil lubrication in the roll bite are investigated. Simulation results are compared to the measured values. The rolling temperature effect is also discussed.

Oxide scale behaviour on aluminium and steel under hot working conditions

Elevated temperature exposure to air of aluminium alloys and steel during thermomechanical processing inevitably results in oxide scales forming on the surface. With carbon and low alloy steels, the oxide scale is distinct and relatively easily removed from the surface in between hot rolling passes or when cooling down after rolling. The scale can behave in a brittle or ductile manner. The oxide scale that forms on aluminium alloys is much more tenacious. There is also strong evidence of the plastic deformation which results in intermixing of the surface metal and oxide scale, leading to a complex subsurface layer. This paper contrasts these two metals’ behaviour, based on laboratory measurements and computer-based simulations

Surface analyses and depth profiles of oxide scales formed on alloy surface

AbstractThe distributions of elements were analyzed at oxide scale surface and around the oxide scale/alloy interfaces. Glow discharge optical emission spectrometry (GDOES) depth profiles and secondary ion mass spectrometry (SIMS) imaging have been applied to analyze the oxide scales formed on FeCr alloys in CH4H2O (thickness from 100 nm to 4 µm, at 1073 K for 3 to 1050 h). The oxide scale is identified as the following phases from surface to inner oxide: MnFe oxide spinel, Cr2O3 oxide, SiO2, Al2O3 inner oxide and FeCr alloy. The concentration of Si was estimated from the quantitative analysis of Si with the GDOES depth analysis. The Si concentration increases with the oxidation duration and the highest concentration after 1050 h treatment is estimated to be 4.9 mass% from the quantitative analysis of Si. Copyright © 2006 John Wiley & Sons, Ltd.

Oxidation Behavior of Fe–Cr- and Ni–Cr-Based Alloy Interconnects in CH4 – H2O for Solid Oxide Fuel Cells

Oxide scale formations in atmospheres were compared for different alloys as interconnects in solid oxide fuel cells (Fe–Cr and Ni–Cr alloys). Oxidation with anode gases (, , CO, and ) caused a relatively thick oxide scale formation on the alloy surfaces even in the low oxygen partial pressures at . The distribution of elements in the oxide scale and the growth rates of oxide scales were precisely analyzed among the examined alloys by glow discharge optical emission spectrometry. Mn spinels formed on the top surface of oxide scales for both Fe–Cr and Ni–Cr alloys. Oxide scale thickness grew with annealing time by a parabolic relationship, and the growth rates were in the orders of at . Ni–Cr alloy shows relatively low growth rate compared with the Fe–Cr alloy. The electrical conductivity after forming oxide scales was different depending on the oxide scale phases and thickness, which was on the order of at .