Structure Of Oxide Scale Research Articles

Effect of Temperature on Structure and Corrosion Resistance of Oxide Scale on Hot Rolled Strip

Various structure scales at the surface of SS400 hot rolled strip were fabricated by heat treatment processes involving different temperatures. A simulation about the effect of various temperatures on the oxide scale structure during the coiling process was carried out. The structure and corrosion behavior of different oxide scales formed at the surface of hot rolled strip were investigated in sodium bisulfite (NaHSO3) solution by scanning electron microscope (SEM), X-ray diffraction (XRD), polarization curves and electrochemical impedance spectroscopy (EIS). The scale prepared at 550 °C is mainly composed of one layer of Fe3O4 phase. The scales prepared at 600 °C and 700 °C consist of the outer thin Fe2O3 layer and the inner (Fe3O4+Fe particles) layer. The scale prepared at 650 °C is mainly composed of Fe3O4 phase as well as a spot of Fe2O3 phase. The thickness of scale prepared at 650°C is observed to be more homogeneous than that of other scales and the bonding between the scale and substrate is found to be very strong. The experimental results clearly reveal that the hot rolled strip with scale prepared at 650 °C exhibits the most excellent corrosion resisting property in 0.01 mol/L NaHSO3 solution.

Effect of Compressive Stresses on Microstructure of Scales Formed on Pure Iron During Continuous Air Cooling

The microstructure development of oxide scale on pure iron under the mutual action of compressive stress and cooling conditions was investigated. Oxide scale structure was examined by optical microscopy (OM) and scanning electron microscopy (SEM). It was found that oxide scale formed under normal cooling conditions had a structure mainly consisting of an outer magnetite and an inner wustite layer. When a compressive stress was applied, numerous magnetite precipitates formed within wustite layer homogeneously at starting cooling temperature of 900 °C, and the wustite layer in the scale was transformed into a mixture of mostly magnetite/iron eutectoid and magnetite layer at starting cooling temperature of 700 °C. The wustite decomposition and precipitation of magnetite in wustite under compressive stress were discussed.

Oxidation of Low-Carbon Steel in 17H2O-N2 at 900 °C

The oxidation kinetics of two low-carbon steels in a flowing 17H2O-N2 gas mixture at 900 °C and the scale structures developed are examined. Similar linear and parabolic oxidation kinetics are observed for the two steels, although some differences are observed within the first 5 minutes of oxidation and in the linear-to-parabolic transition period. The oxidation behaviors observed in the linear kinetics stage are more consistent with published results, exhibiting typical surface-reaction-controlled patterns. However, the observed parabolic oxidation rates are two orders of magnitude smaller than those of iron and steel oxidation in air and oxygen as well as that predicted using Wagner’s parabolic oxidation theory. Similar oxide scale structures are observed on the two steels for the samples oxidized for more than 15 minutes. The surfaces of the scales exhibit pyramidal, faceted grain structures with growth ledges developed on some crystal faces and growth pits at the peaks of the pyramidal grains. In their cross sections, the scales have a columnar structure and appear two layered, with a thin, outer magnetite layer and an inner, growing wustite layer. The wustite grains coarsen with increased oxidation time and develop a growth texture with preferred (111) and (110) orientations in parallel to the sample surface after oxidation for longer than 60 minutes. Conventional oxidation theories cannot provide a satisfactory explanation of the apparently conflicting results observed during the parabolic oxidation stage.

Steam Oxidation Behaviour of Ni-Based Alloys

The steam oxidation behavior and the oxide scale structure of Ni-based alloys containing 20 to 25 mass% Cr content were studied. The oxidation test was carried out from 550 to 700°C at 50°C intervals for 100 hours in steam. All Ni-based alloys used showed good steam oxidation resistance; higher Cr containing alloys have a little better oxidation resistance than lower Cr containing alloys. The kinetics of the oxidation rate of all Ni-based alloys used was estimated as the temperature parameter in a 100 hour test and as the temperature and time parameters, respectively. These equations could be useful for industrial applications. The oxide scale of Alloy 625 was composed of two layers. The outer layer was composed of needle-like oxides and the inner layer was composed of isometric oxides. The oxide scale was composed of Cr2O3 type. The Cr/Ni ratio in the oxide scale at 700°C was by one order larger than that at 650°C. The oxidation resistance of Ni alloys is maintained by a uniform Cr2O3 layer which is composed of high Cr content.

Separate-effect tests on zirconium cladding degradation in air ingress situations

In the event of air ingress during a reactor or spent fuel pond low probability accident, the fuel rods will be exposed to air-containing atmospheres at high temperatures. In comparison with steam, the presence of air is expected to result in a more rapid escalation of the accident. A state-of-the-art review performed before SARNET started showed that the existing data on zirconium alloy oxidation in air were scarce. Moreover, the exact role of zirconium nitride on the cladding degradation process was poorly understood. Regarding the cladding behaviour in air + steam or nitrogen-enriched atmospheres (encountered in oxygen-starved conditions), almost no data were available. New experimental programmes comprising small-scale tests have therefore been launched at FZK, IRSN (MOZART programme in the frame of the International Source Term Program—ISTP) and INR. Zircaloy-4 cladding in PWR (FZK, IRSN) and in CANDU (INR) geometry are investigated. On-line kinetic data are obtained on centimetre size tube segments, by thermogravimetry (FZK, IRSN and INR) or by mass spectrometry (FZK). Plugged tubes 15 cm long (FZK) are also investigated. The samples are air-oxidised either in the “as-received” state, or after pre-oxidation in steam. “Analytical” tests at constant temperature and gas composition provide basic kinetic data, while more prototypical temperature transients and sequential gas compositions are also investigated. The temperature domains extend from 600 °C up to 1500 °C. Systematic post-test metallographic inspections are performed. The paper gives a synthesis of the results obtained, comparing them in terms of kinetics and oxide scale structure and composition. A comparative analysis is performed with results of the QUENCH-10 (Q-10) bundle test, which included an air ingress phase. It is shown how the data contribute to a better understanding of the cladding degradation process, especially regarding the role of nitrogen. For modelling of the oxide scale degradation under air exposure, important features that have to be taken into account are highlighted.

Effect of morphology of oxide scale on oxidation wear in hot working die steels

Wear tests under elevated-temperature air at 400 °C were conducted for H13 steel, H21 steel and the cast die steels. Structure and morphology of oxide scale were investigated by using XRD and SEM. XRD patterns for the worn surface of these steels exhibit that typical oxidation wear prevails. Two types of oxide scales were found to be single-layer and double-layer oxide scale, respectively. The single-layer oxide scale is characteristic of mild oxidation wear with lower wear rate. The appearance of double-layer oxide scale is attributed to obvious plastic deformation of oxide-underlying matrix. Double-layer oxide scale indicates an occurrence of the transition of mild wear to severe wear in oxidation wear.

Cyclic oxidation of Mn–Co spinel coated SUS 430 alloy in the cathodic atmosphere of solid oxide fuel cells

In order to improve oxidation resistance and long-term stability of the metallic interconnects and prevent the cathode of solid oxide fuel cells (SOFCs) from Cr-poisoning, an effective, relatively dense and well adherent Mn–Co spinel protection coating with a nominal composition of MnCo 2O 4 is applied onto the surfaces of the SUS 430 ferritic stainless steel by a cost-effective sol–gel process. The long-term thermally cyclic oxidation kinetics and oxide scale structures as well as the composition of the coated SUS 430 alloy are investigated. The Mn–Co spinel protection layer demonstrates an excellent structural and thermomechanical stability, and effectively acts as a mass barrier to the outward diffusion of cations, especially Cr, and a lowered parabolic rate constant of k p = 1.951 × 10 −15 g 2 cm −4 s −1 is obtained.

Influence of Yttrium Ion-Implantation on the Growth Kinetics and Micro-Structure of NiO Oxide Film

Isothermal and cyclic oxidation behaviours of pure and yttrium-implanted nickel were studied at 1000°C in air. Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) were used to examine the micro-morphology and structure of oxide scales formed on the nickel substrate. It was found that Y-implantation significantly improved the anti-oxidation ability of nickel in both isothermal and cyclic oxidizing experiments. Laser Raman microscopy was also used to study the stress status of oxide scales formed on nickel with and without yttrium. The main reason for the improvement in anti-oxidation of nickel was that Y-implantation greatly reduced the growing speed and grain size of NiO. This fine-grained NiO oxide film might have better high temperature plasticity and could relieve parts of compressive stress by means of creeping, and maintained a ridge character and a relatively low internal stress level. Hence yttrium ion-implantation remarkably enhanced the adhesion of protective NiO oxide scale formed on the nickel substrate.

Improving Fe 3Al alloy resistance against high temperature oxidation by pack cementation process

The paper presents the results of oxidation tests of Fe 3Al-based alloys containing additions of Cr, Zr, B, and C, with and without an aluminide coating. The coating was formed by a pack cementation process in which the surface of material got enriched in aluminum. The Al-rich layer was intended to enhance the tendency of Al 2O 3 formation. The slow-growing Al 2O 3 scale provides the best corrosion protection for structural materials at high temperatures. The cyclic oxidation tests were performed in laboratory air at 1373 K. The structure and composition of oxide scales as well as their adherence were evaluated and compared for the materials with and without aluminide coatings. Surface enrichment in aluminum and effect minor addition of Zr on oxidation behavior was discussed.

Use of EBSD to characterise high temperature oxides formed on low alloy and stainless steels

Exposure of steel to high temperatures in air leads to the formation of an oxide scale, the composition and structure of which depends sensitively on the oxidation conditions and the alloying elements contained within the steel. In this paper, the oxide scale structures formed on low alloy and stainless steels are characterised using electron backscatter diffraction (EBSD). In low alloy steels, this crystallographic information obtained using EBSD enables both the phases within the scale (i.e. haematite, magnetite and wüstite) and orientation relationships between them to be established. This showed that both strong preferred growth within the phase layers and orientational relationships between phase layers, can occur depending on the composition and oxidation conditions. For the scales on stainless steels, the technique enabled the two crystallographic structures, corundum and spinel to be isolated. These structures can be easily differentiated using the EBSD data alone, but the individual phases within them can only be distinguished using the chemical data, collected simultaneously with the EBSD data, because of their crystallographic similarity. This technique revealed two discrete phases for each structure within the oxide scales. For the spinel structure, this consisted of a predominantly chromium and iron containing layer beside the substrate below a coarse grained phase composed of nickel and iron. Meanwhile, an iron rich (haematite) layer at the upper scale surface and a thin chromium rich phase that exists within the fine grained lower scale both possessed the corundum structure.

Influence of cooling rate on the structure and formation of oxide scale in low carbon steel wire rods during hot rolling

In the present work, oxide scale formed on low carbon steel during conventional wire rod rolling (∼ 1.4 °C s − 1 cooling rate after laying head temperature, LHT, at 900 °C) was investigated. The γ-αFe transformation temperature of the grade was determined using thermomechanical simulator. Subsequently, the formation of FeO type scale was engineered through modification in cooling rate (∼ 1.4 °C s − 1 at 900 °C LHT till 750 °C and subsequently 8 °C s − 1 till reformer tub). The oxide scale formed after hot rolling was characterized using Raman spectroscopy and optical metallography. It was observed that the scale on wire rods produced through conventional cooling practices contained magnetite (Fe 3O 4) and hematite (Fe 2O 3) predominantly between the steel substrate and wüstite layer, whereas a uniform wüstite (FeO) layer with less transformed magnetite and proeutectoid magnetite was found on the wire rods with modified cooling practices. The modified cooling strategy at Stelmor conveyor showed a reduction in scale by 15% over conventional cooled wire rods. The increase in the tensile strength was minor (∼ 4 MPa) with the enhanced cooling rate.

Influence of 2 at.% Ruthenium Addition on the Oxidation of -TiAl at 900°C in Air

The structure of oxide scales formed on Ti–55Al and Ti–2Ru–54Al (at. %) by oxidation at 900°C in air was investigated by X-Ray powder diffraction, optical microscopy, scanning-electron microscopy and electron-microprobe analysis. Alloying with ruthenium increases the oxidation resistance during isothermal oxidation, but the opposite trend is observed when samples are oxidized during temperature cycling. This behavior is discussed in terms of the micro-structure of the samples and the oxide scales formed during oxidation.

The oxidation behavior of the new nickel-based superalloy Inconel 740 with and without Na 2SO 4 deposit

The new nickel-based superalloy, Inconel 740, with and without Na 2SO 4 deposit were oxidized in still air between 850 and 1000 °C to investigate the oxidation and hot corrosion behavior by means of XRD, SEM and EDS. The kinetic curves of the alloy oxidized at 950 °C without Na 2SO 4 deposit and at 850 °C with Na 2SO 4 deposit obeyed the parabolic law, whereas, the uniform parabolic growth behavior of oxide was not followed at 1000 °C without Na 2SO 4 deposit and at 950 °C with Na 2SO 4 deposit due to oxide spallation at 1000 °C and the evaporation of Na 2CrO 4 melt at 950 °C, respectively. The oxide scales consisted of Cr 2O 3, (Ni,Co)Cr 2O 4, TiO 2, Al 2O 3 and internal oxides at all temperatures. The internal sulfidation took place due to the existence of Na 2SO 4 deposit. The complex layered structure of oxide scales was in favor of the resistance to oxidation. The deposition of Na 2SO 4 accelerated the oxidation of the alloy and the degradation of corrosion resistance of alloy with Na 2SO 4 deposit was attributed to the dissolution of Cr 2O 3 induced by basic fluxing in molten Na 2SO 4.

Effects of yttrium on microstructure, mechanical properties and high-temperature wear behavior of cast Stellite 6 alloy

Effects of alloying yttrium on the wear behavior of a commercial wear-resistant alloy, Stellite 6, at elevated temperatures were investigated. Samples of Stellite 6 alloyed with various amounts of yttrium were made using an arc-melting furnace. Structure of the modified Stellite 6 was analyzed using the X-ray diffraction technique (XRD). The wear behavior of the samples at various temperatures from the room temperature up to 650 °C was evaluated using a high temperature pin-on-disc tribometer. It was demonstrated that the alloyed yttrium considerably improved the alloy with improved wear resistance and mechanical behavior. In order to better understand the effect of yttrium on high-temperature wear behavior of the alloy, the structure and mechanical properties of oxide scales developed on Y-containing and Y-free samples were investigated, respectively, employing the grazing incidence XRD and nano-indentation techniques. The resistance of the oxide scales to scratch failure was also investigated using a micro-scratcher with in situ monitoring changes in the contact electrical resistance. It was demonstrated that the oxide scale was considerably enhanced when yttrium was added to the alloy, which could contribute to the improved performance of the alloy at elevated temperatures.

Effects of Chloride Ion Addition on the Pickling of Hot-Rolled Steels in Fluoboric Acid Solution

Abstract Effects of chloride ion addition to fluoboric acid (HBF4) solution on the pickling of a high-strength, hot-rolled steel were investigated by using open-circuit potential transient technique and alternating current (AC) impedance spectroscopy. The occurrence of an open-circuit potential plateau indicated dominant dissolution of one oxide phase in the scale. In contrast, the open-circuit potential decay was caused by the simultaneous dissolution of two oxide phases in the scale. Thus, the shape of the open-circuit potential transient was determined crucially by the oxide scale structure. Time to reach a steady-state value of open-circuit potential decreased with increasing chloride ion concentration, indicating accelerated descaling. The decrease in the electrical resistance of the oxide scale estimated from AC impedance spectra also suggested that dissolved chloride ions accelerate descaling. From the experimental findings, it was concluded that the addition of chloride ions to HBF4 solution accel...

Oxidation of orthorhombic Ti2AlNb alloys in the temperature range 550–1000°C in air

Orthorhombic titanium aluminides offer the potential to substitute the current titanium alloys in aero engines, gas turbines and automotive turbo-chargers. The advantages are a wider range of thermo mechanical processing possibilities, a higher room temperature ductility, increased high temperature strength and better oxidation as well as titanium fire resistance. Nevertheless, uncertainties with regard to their oxidation behaviour presently impede their use in practical applications. In the present study a very detailed investigation of the oxide scales formed and of the resulting metal subsurface structures after oxidation for up to 1000 h was performed. The results show that both the oxide scale and the subsurface zone develop very complex structures which are responsible whether protective or non-protective behaviour is observed. In principle the oxidation behaviour of the three alloys investigated can be characterised by three types designated as type I, II or III. Each of these types has a characteristic scale structure and a different degree of protection and their occurrence depends on alloy composition and oxidation time. From a technical point of view the interstitial dissolution of oxygen in the metal subsurface zone plays an important role as this process can lead to severe embrittlement. The extent of the IAZ (interstitial affected zone) depends on the oxide scale structure. The paper presents the results from detailed metallographic and microprobe investigations which throw further light on the complex oxidation processes of this group of materials.

Role of growth stresses on the structure of oxide scales on nickel at 800 and 900°C

The development and relief of intrinsic growth stresses in oxide scales on nickel of different purity have been investigated by combining the deflection test in monofacial oxidation (DTMO) with acoustic-emission analysis (AE). Parallel metallographic analysis gave information about the development of the physical- defect structure and all other structural features. The investigations were performed for 100 hr at 800 as well as 900°C in air. The assumption of elastic behavior led to the best correlation between models, literature data, and results of the present investigations. Microcracks are responsible for the relief of growth stress and inward oxygen penetration leads to the typical NiO duplex scale. The growth of the microcracks is initiated at large pore populations at the oxide–metal interface that most probably form due to outward cation-diffusion and vacancy condensation. The pore formation is increased by the presence of impurities. An equilibrium of microcracking and crack healing is finally reached, leading to a continuous growth of the inner and outer NiO layers of the duplex scale. The scale growth stresses are mainly compressive and can reach maximum values of −560 MPa at 900°C. An estimation of the fracture toughness of the oxide–metal interface assuming a wavy interface led to a KIc value of about 2 MNm-3/2 at 900°C

Oxide-Scale Structures Formed on Commercial Hot-Rolled Steel Strip and Their Formation Mechanisms

The oxide-scale structure developed on commercial hot-rolled steel strip at the mid-coil position was examined. The initial oxide scale after rolling and cooling on the run-out table had a three-layer (hematite, magnetite, and wustite) structure; the thickness was found to be a function of the finishing temperature. From this initial structure, various final scale structures developed after coiling, depending on the coiling temperature, oxygen availability, and cooling rate. For relatively low coiling temperatures (e.g., at 520°C), the final scale structure comprised an inner magnetite/iron mixture layer, an outer magnetite layer, and, at regions away from the center, a very thin outermost hematite layer. For higher coiling temperatures (e.g., in the range of 610 to 720°C), a two-layer hematite/magnetite structure was observed at the edge regions, whereas at the center regions, these two layers were absent and the entire scale layer comprised a mixture of the wustite-transformation products, i.e., a mixture of proeutectoid magnetite, magnetite+iron eutectoid, and a certain amount of retained wustite. At regions between the edges and the center, the oxide structures were similar to those developed at low coiling temperatures (<570°C), i.e., an inner layer comprising a mixture of the wustite-transformation products, an intermediate magnetite layer and at regions near the edges, an outermost hematite layer. In addition, two distinct structures were observed on strips with a coiling temperature of 720°C. One structure comprised a very thick hematite layer (3–5 μm) formed near the edges (within 10–20 mm from the edges), while the other structure comprised a substantial amount of retained wustite formed at the center regions. The formation mechanisms of various oxide scale structures are discussed.

Morphologies of coke deposited on surfaces of pure Ni and Fe-Cr-Ni-Mn alloys during pyrolysis of propane

Morphologies of coke deposited on pure Ni and Fe-Cr-Ni-Mn alloy surfaces during pyrolysis of propane at 750–1000°C have been investigated in detail. It is found that surface scales developed initially on pure Ni and Fe-Cr-Ni-Mn alloy surfaces have no catalytic effect on deposition of filamentary coke. But metal or alloy substrates under cracked scales strongly catalyze nucleation and growth of filamentary coke along the cracks. Ni is more efficient to catalyze the growth of filamentary coke than Fe-Cr-Ni-Mn alloys. The structure of oxide scales has marked influence on distribution and size of filamentary coke deposited on alloy surfaces. Coking morphology is closely dependent of coking temperature and time. Either increasing coking temperature or prolonging coking time results in coking morphology changes from filamentary to spherical. Both dissolution/precipitation mechanism and direct nucleation and growth mechanism may make a contribution to the development of graphitic film coke.

A study of the scale structure of hot-rolled steel strip by simulated coiling and cooling

The morphological development of oxide scale on hot-rolled steel strip under various simulated coiling and cooling conditions was investigated. Oxide-scale structures developed were classified into several categories, and the conditions under which each category formed were identified and mapped. It was found that oxide scale formed under normal coiling and cooling conditions had a structure that was difficult to pickle. Increased cooling rate after coiling, or coiling at a temperature below 350°C, improved this structure. The conditions under which a magnetite layer at the wustite–steel interface was formed during continuous cooling were also identified and the mechanisms of its formation discussed.