Oxide Scale Thickness Research Articles

Oxidation/reduction studies on nanoporous platinum films by electrical resistance measurements

Mechanisms and kinetics of surface reactions in nanoporous platinum films were investigated. Nanoporous films of platinum of ∼250 nm thickness were deposited on glass slides by co-sputtering Pt and carbon followed by subsequently burning off carbon in air at 450 °C. Electrical resistance was measured in air and in 10% H2 + nitrogen at 80 °C as a function of time. The change in electrical resistance was extremely fast when switched to the H2 + N2 atmosphere. When switched to air, the film resistance increased with time at a much slower rate. The increase in resistance in air was attributed to the formation of Pt-oxide on the internal surfaces of the nanoporous films. The kinetics of oxidation was described by a model which includes two surface kinetic steps and a diffusional step. The use of nanoporous films makes it possible to investigate mechanisms and kinetics of surface reactions by ensuring a large surface to volume ratio. Oxide scale thickness at 80 °C in air after several hours of oxidation was only sub-monolayer. Oxide scale thickness after 3 h at 450 °C was about 1 nm. Implications of the results for proton exchange membrane fuel cell (PEMFC) Pt catalyst degradation are discussed.

A combinatorial investigation of palladium and platinum additions to β-NiAl overlay coatings

A combinatorial approach was used to investigate the effects of higher-order additions to NiAl on the oxidation, phase stability and interdiffusion behavior of the coatings. Overlay coatings with compositions (in at.%) in the range Ni–38Al–5Cr–(0.1–0.5)Hf–(2–8)Pd and Ni–36Al–5Cr–(0.1–0.5)Hf–(2–8)Pt were deposited on René N5 single crystal substrates via ion plasma deposition (IPD). Cyclic oxidation experiments on ten different Pd+Hf- or Pt+Hf-modified NiAlCr coatings at 1100°C revealed similar weight gain and oxide scale thickness. While the thickness of alumina scales formed on Pd- and Pt-modified coatings were comparable, a higher density of oxide spalls were observed in the former. High-temperature transformations from β→γ′→γ+γ′ occurred during extended exposure in both Pd- and Pt-modified coatings owing to Al depletion to substrate and surface oxide. On cooling, Al-depleted β-NiAl transformed to two martensite phases, 3R and fine-twinned 7R. Rumpling or surface roughening occurred for some coating compositions. Microprobe measurements showed that the difference in penetration depths of Pd into the substrate was not as large as expected from the higher interdiffusion coefficient of Pd in β-NiAl and γ-Ni.

Air oxidation behaviour of standard PVD and novel HIPIMS coatings at 750°C for 1000 h

High temperature oxidation resistance of Al2Au and TiAlCr standard PVD coatings and novel nano-structured CrAlYN/CrN coatings developed by HIPIMS technology was studied at 750°C for 1000 h. The results showed better high temperature oxidation resistance offered by HIPIMS coatings where protective oxide scales formed rich in Al and Cr, with lack of spallation and low mass gain during whole test duration was observed. In contrast Al2Au and TiAlCr coated materials showed thicker oxide scales, the samples coated by Al2Au and TiAlCr TiAl–8Nb alloy showed poorer corrosion resistance in air than that offered by the uncoated alloy. Post exposure analyses were performed by means of scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS).

Comparison of oxidation performance of two nickel base superalloys for turbine applications

The oxidation performance of two nickel base superalloys, Inconel 738 and Rene 80, was determined by isothermal static oxidation test at 900°C. The metrics used to evaluate performance were weight gain per unit area as a function of time, oxide scale thickness, qualitative elemental mapping and calculation of the parabolic rate constant for each. It was found that Inconel 738 performed notably better than Rene 80, with the rate constants being KP = 2·26×10−12 g2 cm−4 s−1 for Inconel 738 and KP = 3·85×10−12 g2 cm−4 s−1 for Rene 80. This poorer performance of Rene 80 is attributed to lower amounts of chromium and aluminium, which have been shown to have a positive effect on oxidation performance, and higher amounts of titanium, which has shown to be detrimental. The oxidation results were then used in a new method of evaluating the efficacy of individual elements on overall oxidation behaviour. This method is based on the thermodynamic Gibbs free energy of the oxide and the amounts of the various oxide forming elements present in the alloys. Both alloys were found to exhibit scale growth consisting of a dense outer scale of chromia and a discontinuous inner subscale of alumina, matching predictions for the relative amounts of chromium and aluminium in each.On a déterminé le comportement à l’oxydation de deux superalliages à base de nickel, Inconel 738 et René 80, par un essai d’oxydation statique isotherme à 900°C. Les mesures utilisées dans l’évaluation du comportement incluaient le gain de poids par unité de surface en fonction du temps, l’épaisseur de l’écaille d’oxyde, la cartographie qualitative des éléments et le calcul de la constante de vitesse parabolique pour chacun. On a trouvé que l’Inconel 738 avait un rendement particulièrement meilleur que celui de René 80, les constantes de vitesse étant de Kp = 2·26×10−12 g2 cm−4 s−1 pour l’Inconel 738 et de Kp = 3·85×10−12 g2 cm−4 s−1 pour René 80. On attribue le rendement moins bon de René 80 aux plus petites quantités en chrome et en aluminium, qui ont un effet positif sur le comportement d’oxydation, et aux quantités plus élevées de titane, qui est nuisible. On a ensuite utilisé les résultats d’oxydation dans une nouvelle méthode d’évaluation de l’efficacité d’éléments individuels sur le comportement global d’oxydation. Cette méthode est basée sur l’énergie libre thermodynamique de Gibbs de l’oxyde et sur les quantités des divers éléments favorisant la formation d’oxyde, qui sont présents dans les alliages. On a trouvé que les deux alliages exhibaient une croissance d’écaille qui consistait en une écaille extérieure dense d’oxyde de chrome et en une sous-écaille interne discontinue d’oxyde d’aluminium, coïncidant avec les prédictions des quantités relatives en chrome et en aluminium dans chacun des alliages.

Surface Roughness and Friction in Hot Rolling of Stainless Steels

An experimental method was developed to examine oxidations of austenitic and martensitic stainless steels. The results show that the surface roughness along both rolling and transverse directions decreases with an increase of reduction. When the reheating time is increased, the average thickness of oxide scales of stainless steels increases, which results in relatively rough surface after hot rolling. The effects of oxide scale on the friction condition and surface roughness transfer in hot rolling depend on the oxide scale generated during reheating. The calculated surface roughness is close to the experimental results, which verifies the developed FEM model.

Corrosion Resistance of Oxide Scales Formed in High-Temperature Oxidation of Al-Bearing Ferritic Stainless Steels

Electrochemical polarization measurements of were performed on Al-bearing stainless steels covered with oxide scales formed at 973, 1173, and 1373 K, and the pinhole defect density and the reductive dissolution behaviors of the scales were investigated. The increase of oxidation temperature and the content of Al led to the growth of a thick Al-rich oxide scale. The Al-rich oxide scales had less pinhole defects and the high corrosion resistance in acidic condition. In contrast, Fe-rich oxide layers formed at lower oxidation temperatures dissolved cathodically. The Al addition was effective to decrease the pinhole defects and to enhance the corrosion resistance of the oxide scales due to the formation of the Al-rich oxide scale.

Microstructure and high-temperature oxidation behavior of wire-arc sprayed Fe-based coatings

A series of Fe–xCr cored wires (x=15, 20, 25, 30, 35 and 40% in weight) were used to produce protective coatings for components used in high-temperature environments by wire-arc spraying. The microstructure, morphology and high-temperature oxidation behavior of the Fe–xCr coatings were investigated. The Fe–xCr coatings presented a homogeneous structure with thin oxide inclusions and low porosity. Thermogravimetric analysis was used to evaluate the high-temperature oxidation behavior of the coatings at a temperature of 650°C under cyclic oxidation conditions. The results showed that all the Fe–xCr coatings exhibited significantly lower weight gain than the uncoated substrate. As Cr content of the coatings increased, the thickness of oxide scales decreased and oxidation resistance improved.

Failure evaluation of steam‐side oxide scales in superheater tubes during unsteady thermal processes: A probabilistic method

Issues resulting from failure of steam‐side oxide scales in boiler superheater or reheater tubes seriously influence the safety of coal‐fired power plants. In this work, an analysis model for the failure of steam‐side oxide scales during unsteady thermal processes is established. This model employs the advanced oxide scale failure diagram as the failure criterion, and also takes into account the influence of temperature on the coefficients of thermal expansion of both metal substrate and oxide scale. Since the stochastic nature of related geometrical, mechanical, and thermophysical parameters may hinder the usual deterministic investigation from obtaining a reliable evaluation, probabilistic failure evaluation which is not based on bivalent logic is thus performed. Focusing on the unsteady thermal processes induced by the variation of steam temperature, effects of oxide scale thickness as well as different steam temperature variation processes on the failure probability of steam‐side oxide scales are analyzed. It is found that the failure probability of thicker steam‐side oxide scales is higher, while through reasonably selecting steam temperature variation mode and appropriately lengthening variation duration, lower failure probability can be achieved. Feasible measures are presented with the aspiration to solve the steam‐side oxide scale failure issues.

Investigation into Cause of High Temperature Failure of Boiler Superheater Tube

AbstractThe failure of the boiler tubes occur due to various reasons like creep, fatigue, corrosion and erosion. This paper highlights a case study of typical premature failure of a final superheater tube of 210 MW thermal power plant boiler. Visual examination, dimensional measurement, chemical analysis, oxide scale thickness measurement, microstructural examination are conducted as part of the investigations. Apart from these investigations, sulfur print, Energy Dispersive spectroscopy (EDS) and X ray diffraction analysis (XRD) are also conducted to ascertain the probable cause of failure of final super heater tube. Finally it has been concluded that the premature failure of the super heater tube can be attributed to the combination of localized high tube metal temperature and loss of metal from the outer surface due to high temperature corrosion. The corrective actions have also been suggested to avoid this type of failure in near future.

Evaluation of Deformation Behavior of Oxide Scale in Hot Rolling Process by Vacuum Hot Rolling Mill

A method for reproduce the oxide scale in hot rolling is proposed using the vacuum hot rolling mill. This method is possible to control of scale thickness and chemical composition in actual hot rolling process.From results, it is possible to clear the deformation behavior of state generated the oxide scale, oxide scale thickness is thick enough amount of Fe2O3 billets. The oxide scale thickness is proportional to the amount of Fe2O3. Some cracks appeared before the roll bite during rolling the oxide scale. Then, peeling or destruction occurred in roll bite for roll exit.

Study of Thermal Diffusion on Oxidation Resistance of ZG40Cr24

Based on heat resistant steel ZG40Cr24, test alloys were cast by intermediate frequency induction furnace with non-oxidation method by alloying of aluminium and silicon. The oxidation resistance at 1100°C for 500 hours of test alloys was carried out according to oxidation weight gain method. The thermal diffusion were tested by Laser Heat Conductivity. The thickness of oxide scale was detected by Coating Thickness Meter. Experimental results showed that the thermal diffusion of oxide scale affected its oxidation resistance exactly, the lower thermal diffusion coefficient matched the higher oxidation resistance. The oxide scale thermal diffusion coefficient of ZG40Cr24+2%wt.Si+4%wt.Al was only 0.00092cm2.s-1, endowing itself 0.0633g.m-2.h-1 oxidation weight gain rate, reaching the complete oxidation resistance. The mechanism of the effect of thermal diffusion on oxidation resistance lay in that the lower thermal diffusion represented the inert inner particles of materials, the few quantity of diffusion particles, and lower transporting and moving rate. So the oxidation rate slowed down, realizing higher oxidation resistant property for oxide scales.

Effects of oxide scale on hot rolling of an austenitic stainless steel

Oxide scale on stainless steels in hot rolling, which affects the coefficient of friction, thermal conductivity and surface quality, is more complicated than that on carbon steels. In this study, an austenitic stainless steel 301 was selected. The humid air with 12% water vapour content was adopted for the formation of uniform oxide scale at 1,100°C. Oxidation kinetics study was carried out by using the thermogravimetric analyser (TGA). Specimens were isothermally heated for 25 and 35 minutes to obtain different thickness of scales. Hot rolling was performed on a 2-high Hille 100 experimental rolling mill at various reductions with dry and water cooling conditions. Oxide scale thickness and compositions were analysed with SEM and XRD. Surface roughness was measured after rolling. Inverse calculation of coefficient of friction was employed to analyse and discuss the friction condition between rolls and the strip under different rolling parameters.

Formation mechanism of surface scale defects in hot rolling process

Thick oxide scale on steel may result in serious surface defects on hot-worked products. Yield efficiency and productivity of processes are considerably deteriorated by formation of defects. It is highly demanded to establish a way to produce hot-worked steels free from surface defects by controlling oxide scale. The oxide scale shows various behaviors in hot rolling; (a) uniform deformation with matrix steel, (b) cracking, (c) fragmentation, (d) indentation to matrix steel, etc. Through observations using glass coating, it is found that the behavior strongly depends on the rolling temperature as well as the scale thickness before rolling. Temperature drop due to contact with cold rolls is found to cause the cracking and a major reason for the thickness dependence. It is found that the scale cracking is predictable using the estimated scale temperature and the ductile-brittle transition temperature of Wustite (FeO). Then, methodology to produce hot rolled steels without surface defects in industrial processes is presented.

Modeling of Heat Transfer Coefficient of Oxide Scale in Hot Forging

In hot forging processes, an accurate prediction of temperature distribution is important because it affects not only the forging load but also the material flow itself and the distribution of material properties in forged parts. An oxide scale will form at the surface of steel when heated to high temperature. The heat flows through this scale layer from the high temperature material to the dies or air, thus for an accurate prediction of temperature distribution, a precise evaluation of the heat transfer coefficient of scales is required. In actual hot forging processes, the thickness and composition of oxide scales will vary with the composition of steel, heating temperature and time, or the atmosphere. In this study, the heat transfer coefficients of oxide scales, formed on the surface of medium carbon steel with various heating temperature and heating time, are measured. A model of heat transfer coefficients is obtained as a function of the thickness of each oxide contents layer and the contact pressure.

Experimental results of the QUENCH-16 bundle test on air ingress

The out-of-pile bundle experiment QUENCH-16 on air ingress was conducted in the electrically heated 21-rod QUENCH facility at KIT in July 2011. It was performed in the frame of the EC supported LACOMECO program. The test scenario included the oxidation of the Zircaloy-4 claddings in air following a limited pre-oxidation in steam, and involved a long period of oxygen starvation to promote interaction with the nitrogen. The primary aim was to examine the influence of the formed oxide layer structure on bundle coolability and hydrogen release during the terminal flooding phase. QUENCH-16 was thus a companion test to the earlier air ingress experiment, QUENCH-10, which was performed with strongly pre-oxidized bundle. Unlike QUENCH-10, significant temperature escalation and intensive hydrogen release were observed during the reflood phase. Post-test investigations of bundle cross sections reveal residual nitride traces at various elevations. The nitrides were formed at upper bundle elevations characterized by steam starvation conditions. The external part of the oxide scale is of porous structure due to re-oxidation of nitrides during reflood. Relative thick internal oxide scales underneath this porous layer and residual nitrides were formed during reflood. At lower bundle elevations frozen partially oxidized melt was detected, relocated from upper elevations. Three contributors for the high hydrogen production during the reflood were recognized: re-oxidation of nitrides, secondary oxidation of residual cladding metal due to massive steam penetration through the porous oxide/nitride layer and melt oxidation.

Failure of a Pendent Reheater Tube in a 110 MW Thermal Power Plant

Abstract This investigation was conducted to examine the probable cause of failure of one pendent reheater tube of a 110 MW thermal power plant. Preliminary macroscopic examinations along with visual examination, dimensional measurement, chemical analysis, evaluation of mechanical properties, oxide scale thickness measurement were carried out to predict the probable cause/causes of failure. In addition, optical microscopy was a necessary supplement to understand the reason for the failure. It was concluded that due to progressive erosion caused by the high velocity flue gas streams on the fire side wall, the tube had thinned down and could not cope the prevailing working stress. As a result, the affected areas of the tube suffered plastic deformation. Apart from that, formation of micro creep cavities due to rise in tube metal temperature (on the steam side) at localized places led to initiation of minor cracks before culminating to final rupture. The final failure is due to both these mechanisms occurring simultaneously.

Steam oxidation behaviour of plasma nitrided Fe-based alloys

Purpose – It is well known that alloys, based on iron, were exposed to steam oxidation environment producing thick and non-protective oxide scale. More expensive stainless steels contain more Cr and are able to form more protective scales. The purpose of this research was to show ability to employ nitride coating on different alloys (T23, T91, E1250, 347HFG and HR3C) in order to enhance steam oxidation resistance. Design/methodology/approach – The alloys were exposed to steam oxidation rig. Before the test, furnace was purged by nitrogen in order to remove moisture and oxygen. Di-ionised water was pumped from the reservoir using a peristaltic pump into the furnace. System was kept in the closed circle. To reduce solubility of oxygen, di-ionised water was constantly purged by nitrogen. The total exposure time was 2,000 h at 650°C under 1 bar pressure. Findings – Due to the research, it was found that plasma nitriding process is detrimental for the protection of high-temperature structured materials; the high concentration and high activity of Cr produced a CrN phase. This phase is not stable in steam environment and underwent oxidation to Cr2O3 and further into volatile phase (CrO2(OH)2). Therefore, austenitic steels (E1250, 347HFG and HR3C) coated with nitride coating deposited by plasma nitriding process suffered similar degradation as the uncoated low Cr ferritic steel. Research limitations/implications – The main limitation of the research conducted in this study was corrosion resistance of the exposed materials. Originality/value – To the best of the authors' knowledge, this report is the first of its kind to present nitrided alloys (ferritic and austenitic) exposed in steam oxidation.

Cyclic Oxidation Behavior of the Ti–6Al–4V Alloy

The cyclic oxidation behavior of the Ti–6Al–4V alloy has been studied under heating and cooling conditions within a temperature range from 550 to 850 °C in air for up to 12 cycles. The mass changes, phase, surface morphologies, cross-sectional morphologies and element distribution of the oxide scales after cyclic oxidation were investigated using electronic microbalance, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rate of oxidation was close to zero at 550 °C, obeyed parabolic and linear law at 650 and 850 °C, respectively, while at 750 °C, parabolic—linear law dominated. The double oxide scales formed on surface of the Ti–6Al–4V alloy consisted of an inner layer of TiO2 and an outer layer of Al2O3, and the thickness of oxide scales increased with an increasing oxidation temperature. At 750 and 850 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids, cracks and the spallation of the oxide scales.

Oxidation protection of Ti–6Al–4V alloy using a novel glass–amorphous silica composite coating

A novel glass–amorphous silica composite coating was prepared by the slurry method in order to improve the oxidation resistance of Ti–6Al–4V alloy at high temperatures. The microstructure of the as-prepared composite coating was analyzed by SEM, XRD and EDS techniques. The oxidation resistance and the microstructure evolution of the composite coating at 800°C for 50h were also studied. The results show that mass gains of the specimens coated with the composite coating were far less than that of the uncoated ones after oxidation of 50h. Thick oxide scales composed of plate-like rutile TiO2 and some granular α-Al2O3 formed on bare Ti–6Al–4V alloy, while quartz, cristobalite and diopside were observed in the composite coating, which are useful crystals for slowing the inward diffusion of oxygen to the substrate. The cross-sectional EDS line scanning images show that inward diffusion of oxygen and outward diffusion of Ti through the composite coating were insignificant. The microhardness profile reveals that the solid solution oxygen in Ti–6Al–4V alloy with composite coating was limited.

Anodic behavior of mechanically alloyed Cu–Ni–Fe and Cu–Ni–Fe–O electrodes for aluminum electrolysis in low-temperature KF-AlF3 electrolyte

A comparative study on the anodic behavior of Cu65Ni20Fe15 and (Cu65Ni20Fe15)98.6O1.4 materials during the electrolysis of aluminum was conducted. Both materials were prepared in powder form by ball milling and subsequently consolidated to form dense pellets that were used as anodes. The electrochemical characterization was performed at 700°C in a potassium cryolite-based electrolyte, and the composition-morphology of the oxide scales formed on both anodes were determined by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction measurements. On Cu65Ni20Fe15, a thick (170μm) and porous oxide scale is formed after 15min of electrolysis that readily dissolves (or spalls) before a denser oxide layer is formed after a longer electrolysis time (1 and 5h). In comparison, a thin (2μm) and dense oxide layer mainly composed of NiFe2O4 is observed on a (Cu65Ni20Fe15)98.6O1.4 electrode after 15min of electrolysis. The thickness of this oxide layer increases to 10 and 30μm after 1h and 5h of electrolysis. However, the outward diffusion of Cu to form CuOx at the surface of the electrode is not totally hampered by the presence of NiFe2O4 and a porous Cu-depleted region is formed at the oxide/alloy interface. As a result, electrolyte penetration occurs in the scale, which favors the progressive formation of an iron fluoride layer at the oxide/alloy interface.