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In this investigation the oxidation behavior of AISI 441 stainless steel (SS) in the range from 850 °C to 950 °C was determined during 50 h in two different atmospheres: (a) synthetic air in a tubular oven; (b) argon with 1 ppm of O 2 in a thermal balance. The oxidation kinetics was determined from the measured mass change as a function of oxidation time. Examination of the microstructure of the oxides and determination of their chemical composition were performed by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and glancing angle X-ray diffraction (GAXRD). Chemical analyses showed that films formed on the AISI 441 steel surface consisted mostly of chromium oxide but manganese, iron, titanium and silicon oxides were found in the oxidized layer. In synthetic air, the steel oxidation rate increased gradually as the temperature increased, but in the argon atmosphere with 1 ppm of oxygen, the highest oxidation rate was observed at 900 °C and the lowest at 950 °C.

Long term isothermal and cyclic oxidation behavior of yttrium dioxide (Y2O3) coated Fe-20Cr alloy was studied. Oxidation tests were carried out in air for periods of up to 200 hours at 1000°C and the oxidation behavior of the alloy was evaluated gravimetrically. The Y2O3 coating reduced markedly the mass gain of the alloy. Energy dispersive spectroscopic analysis of scale sections on oxidized Y2O3 coated specimens revealed a Cr rich layer close to the metal/oxide interface and the presence of Y and Cr near the oxide/air interface, indicating diffusion of Cr through the Y2O3 layer. Further evidence of this diffusion was seen as change in colour of the Y2O3 coating from white to green after prolonged oxidation. X-ray diffraction analysis of the scale revealed perovskite-type YCrO3.This study attributes the increased oxidation resistance of the Y2O3 coated Fe-20Cr alloy to increased oxygen ion diffusion through the Cr2O3 scale due to YCrO3 formation and grain boundary segregated Y ion blocking alloy cation diffusion.

This study reports the high temperature erosion-oxidation (E-O) behavior of conventional and nanostructured Cr3C2-25(Ni-20Cr) coatings prepared by high velocity oxygen fuel (HVOF) spraying. As-received and nanostructured Cr3C2-25(Ni-20Cr) powders with mean crystallite sizes of 145 nm and 50 nm respectively, were used to prepare 120 - 200 µm thick coatings on AISI 310 samples. The E-O behavior of the coatings prepared with the as-received (AR) and nanostructured (NS) powders was determined as weight change in a custom designed rig at room temperature, 450, 700 and 800 ºC. The Vickers microhardness, Young's Modulus and fracture toughness of the AR and NS coatings were determined, and the NS coatings exhibited higher values compared with the AR coatings. The E-O resistance of the NS coating was higher than that of AR coating at all temperatures, and particularly at 800 ºC. The increase in E-O resistance of the NS coatings is due to its superior mechanical properties as well as to the presence of some heterogeneities in the AR coatings. The E-O mechanisms of both types of the coatings are discussed, with special attention to that at high temperatures. The results suggest that at 800 ºC the E-O process is controlled by erosion of the oxide.

Nanostructured coatings have been used to protect components exposed to severe service conditions. High energy milling is widely used to produce nanocrystalline feedstock of coating materials such as chromium carbide and tungsten carbide. This paper presents the structural and thermodynamic properties of Cr3C2–25(Ni20Cr) powders that were high energy milled for different times. During the high energy milling of Cr3C2–25(Ni20Cr) powder, severe plastic deformation takes place. A small part of the energy spent in this process is stored in the crystal lattice as deformation energy. The crystallite size and microstrain in nanocrystalline Cr3C2–25(Ni20Cr) powders milled for different times were determined by X-ray diffraction measurements. Differential scanning calorimetric (DSC) studies of the milled powders revealed a broad transformation, characteristic of a large exothermic reaction in the nanostructured powder. The enthalpy variation measured by DSC permitted determination of the deformation energy stored in the Cr3C2–25(Ni20Cr) powders milled for different times. These measurements also enabled calculation of the specific heat variation of the milled powders.

Materials used as solid oxide fuel cell (SOFC) interconnects should have high electrical and thermal conductivities, high corrosion resistance and adequate mechanical strength at room as well as at high temperatures. In the light of this, the aim of this study is to determine the mechanical behavior of FeCr alloys without and with Y and/or Al additions at room and at high temperatures. Ingots of four alloys, FeCr, FeCrY, FeCrAl and FeCrAlY were prepared by induction vacuum melting and casting. These ingots were forged and rolled. Tensile test specimens were machined from the rolled sheets. The tensile tests were carried out at room temperature, 800°C and 900°C. Scanning electron microscopy was used to examine the microstructure of the alloys and the fractured surfaces after the tensile tests. The microstructure of all the alloys revealed a large number of inclusions. Among the alloys, FeCrAlY has the best mechanical behavior. The fracture surfaces of all the alloys tensile tested at the different temperatures revealed coalescence of micro-cavities (dimples) indicating ductile fracture.

Cr3C2-NiCr coatings have been used for corrosion and wear resistant applications. In the last decade, thermal sprayed coatings using nanostructured feed stock of other materials has exhibited higher hardness, strength and corrosion resistance. Hence, it is anticipated that nanostructured Cr3C2-NiCr coatings will also exhibit these properties and therefore posses improved performance characteristics. Preparation of nanostructured feed stock powders is the first step in the synthesis of nanostructured coatings and mechanical milling is an effective process to obtain the powders. Preliminary studies in which commercial Cr3C2-NiCr powders were milled in hexane and gaseous nitrogen revealed that particle and crystallite size were significantly smaller in powders milled in nitrogen. This paper presents the effect of hexane content and milling parameters on Cr3C2-Ni20Cr powder characteristics. Use of just sufficient hexane as the milling media reduced significantly the particle and crystallite sizes.

Rare earths (RE) have been used to increase high temperature oxidation resistance of chromia and alumina forming alloys. The RE can be added as elements (or oxides) to the alloys or applied as oxide coatings to the alloy surface. This paper presents the effect of different RE oxide coatings and lanthanum chromite coatings on the high temperature oxidation behavior of Fe20Cr and Fe20Cr4Al alloys. The oxidation resistance of the Fe20Cr alloy increased with increase in ionic radius of the RE element in the coating. The RE oxides decreased chromia growth rate more than alumina growth rate. In extended cyclic oxidation tests that were carried out from peak temperatures of 900 °C, 1,000 °C and 1,100 °C to room temperature at cooling rates of 300 °C/s and 1,000 °C/s, the La2O3 coating increased cyclic oxidation resistance of the Fe20Cr alloy significantly more than the Pr2O3 coating. The role of RE in increasing overall oxidation resistance of chromia forming alloys is discussed.

Pitting corrosion of the aluminium cladding of spent nuclear fuels stored in light water pools has been observed. To prevent this, coating of the Al cladding with hydrotalcite (HTC) was proposed. This paper presents the effect of chemical bath and processing parameters on microstructure and corrosion behavior of HTC coatings on alloy AA 6061 specimens. The HTC coating from the high temperature nitrate bath was homogeneous, thicker and consisted of well-defined intersecting platelets than that formed from the room temperature carbonate bath. Electrochemical polarization tests carried out with HTC coated AA 6061 specimens in 0.01 M NaCl revealed that specimens coated with HTC from the nitrate bath and further treated in a cerium salt solution were the most resistant to corrosion. Field tests in which un-coated and coated AA 6061 alloy coupons as well as full-size plates were exposed to the IEA-R1 reactor's spent fuel basin for duration of up to 14 months further corroborated the high corrosion resistance imparted by the high temperature HTC + Ce coating. The mechanism by which the HTC coating and cerium protect the Al alloy is discussed.

AISI 304L stainless steel powder compacts containing 2 vol% high purity rare earth oxides were prepared by mixing the different powders in a vibratory mill followed by pressing. The compacts thus obtained were sintered in a vacuum furnace and isothermal oxidation measurements were carried out in a muffle furnace, in air, up to 200 hours at 900 °C. The oxidized surfaces were examined in a scanning electron microscope and micro regions of the reaction products were studied using energy dispersive analysis. The addition of rare earth oxides decreased the oxidation rate of the stainless steel. Further evidence of predominant oxygen ion diffusion controlling the overall oxidation process in rare earth containing chromium oxide forming alloys has been observed.

The erosion-oxidation (E-O) behavior of high velocity oxy fuel (HVOF) sprayed Ni20Cr alloy as well as WC and Cr3C2 cermet coatings on a steel substrate were studied. The E-O tests were carried out in a rig with specimen assemblies that were rotated through a fluidized bed of erodent particles in the temperature range 500-850°C and with erodent impact velocities of 2.5-19.5 ms-1. Alumina powder (~200Am) was used as the erodent. The E-O resistance of the coatings was determined as wastage, as a function of temperature. The three coatings did not exhibit any significant change in E-O at temperatures up to 500-600°C. At higher temperatures, wastage increased with temperature, reached a maximum at 700°C and then decreased with further increase in temperature. Different E-O regimes were identified. The specimen surfaces were examined in a scanning electron microscope and their roughness determined. E-O maps have been drawn that define conditions under which the coatings undergo low, moderate and severe wastage.

Particle reinforced metal matrix composite (MMC) coatings have been developed due to property combinations such as increased hardness, high creep/fatigue resistance as well as superior wear and oxidation resistance. MMC coatings can be produced by various techniques, which include powder metallurgy (plasma spray and HVOF processes), liquid metal processes and electrodeposition. This paper presents the development of the electrodeposition process to obtain nickel-alumina composites. The process conditions were optimized and based on these gradient nickel-alumina composites were prepared. Microstructural characterization and hardness determinations were carried out. The effect of process parameters on the amount and distribution of alumina particles as well as morphology of the deposits with and without alumina gradients are presented and discussed.

Rare earth elements are often added to chromium dioxide forming alloys to improve its high temperature oxidation resistance. The rare earths can be also added as oxide dispersions. Significant cost reductions are possible if rare earth oxide concentrates can be used instead of pure rare earth oxides, the former being the precursor to obtaining pure rare earth oxide. In this study the effect of adding pure and concentrates of rare earth oxides to AISI 304L on its oxidation behavior has been evaluated. AISI 304L stainless steel powder compacts containing 2 vol% of pure lanthanum and yttrium oxides or their concentrate were prepared by milling followed by pressing. The compacts were vacuum sintered and isothermally oxidized in air for up to 200h at 900°C. The parabolic rate constants were determined and the reaction products examined using a scanning electron microscope. X-ray diffraction analysis of the reaction products was also carried out. The compacts with pure rare earth oxides and the concentrates exhibited similar oxidation behavior.

The oxidation and erosion–oxidation (E–O) behaviour of high velocity oxy fuel sprayed Ni–20Cr alloy as well as WC and Cr3C2 cermet coatings on a steel substrate were studied. The E–O tests were carried out in a rig with specimen assemblies that were rotated through a fluidized bed of erodent particles in the temperature range 500–850°C and with erodent impact velocities of 2·5– 19·5 m s−1. Alumina powder (∼200 μm) was used as the erodent. The oxidation behaviour of the coatings was determined gravimetrically. The Ni–20Cr and Cr3C2–25(Ni–20Cr) coatings showed significantly higher oxidation resistance than the WC–20Cr–7Ni coating, especially at temperatures above 800°C. The E–O resistance of the coatings was determined by wastage, as a function of temperature. The three coatings did not show any significant change in E–O at temperatures up to 500–600°C. At higher temperatures, wastage increased with temperature, reached a maximum at 700°C and then decreased with further increase in temperature. Diff...

High temperature corrosion of structural alloys in sulphur bearing environments is many orders of magnitude higher than in oxidising environments. Efforts to increase sulphidation resistance of these alloys include addition of alloying elements. This paper reports the effect of yttrium, aluminium and yttrium plus aluminium addition on the sulphidation behaviour of an Fe–20Cr alloy in H2–2%H2S at 700 and 800°C. The effect of preoxidation of these alloys on sulphidation behaviour was also investigated. The initial stages of sulphidation of these alloys were observed and overall sulphidation kinetics was determined. In the presence of yttrium, the sulphidation rate of the alloy decreased by an order of magnitude, and the sulphide layer on the FeCrY alloy was significantly thinner than that formed on the FeCr alloy. Aluminium addition increased sulphidation resistance even more. The FeCrAl and FeCrAlY alloys exhibited parabolic sulphidation kinetics at the two temperatures. On both alloys, initial chr...

Ni20Cr+WC composites with a gradient in the volume fraction of hard WC particles are ideal materials for applications where high temperature oxidation and erosion resistance are desired properties. The WC particles impart erosion resistant and the Cr content of the alloy, oxidation resistance. This paper presents the influence of WC particle content and consequently the extent of densification, on the oxidation behavior of hot pressed Ni20Cr+WC composites. Oxidation measurements in the range 700-1000°C revealed overall parabolic behavior. The increase in oxidation rate with WC content has been attributed to formation of islands of WO 3 globules protruding through the Cr 2 O 3 .

The sulfidation behavior of Fe20Cr and Fe20Cr0.7Y alloys in H2–H2S atmospheres at 700 and 800°C was determined by thermogravimetry. Isothermal measurements were carried out and the sulfidation kinetics were evaluated from the mass gain vs. time curves. The reaction products were examined in a scanning electron microscope and the compositions of micro-regions were determined using energy dispersive spectroscopy and X-ray diffraction. Yttrium addition increased the sulfidation resistance of the FeCr alloy.

This paper discusses the influence of coatings of praseodymium, dysprosium, and yttrium oxide gels on the oxidation behaviour of Fe–20Cr alloys. The morphology of the coatings was found to vary with the rare earth (RE) element used. The oxidation rates of Fe–20Cr specimens with all three RE oxide coatings were significantly lower than that of the uncoated alloy. The extent of influence on oxidation rate decreased in the following order: Pr, Dy, Y. The scale formed in the presence of RE oxides was a very thin, adherent chromia scale. Micrographic evidence of RE oxides in the chromia scales was found, further confirming the mechanism of scale growth in the presence of REs. Chromium ion diffusion is hindered by RE oxide coatings, making oxygen ion diffusion dominant. A direct correlation between RE ion radius and the extent of influence on chromia growth at 1000°C has been found.

The main characteristic of amorphous materials is the lack of long range crystallographic order in their structure that confers chemical and physical properties different from their crystalline counterparts. One such property is the superior corrosion resistance of certain Cr containing Fe based amorphous alloys. Conventional electrodeposition from aqueous solutions is one of the techniques that has been used to obtain these alloys. This paper represents a study of the influence of various process parameters on the composition, morphology, and properties of electrodeposited Fe– Cr–P amorphous alloy coatings. The effect of ion selective membrane, formic acid (a Cr3+ complexant), sodium lauryl sulphate (a surfactant), and aging time of the formic acid containing bath were investigated. The results show that it is possible to obtain these coatings under certain experimental conditions. Electrochemical tests, carried out in sulphuric acid, revealed corrosion behaviour intermediate between the carbon s...

Swelling was performed on dried membranes, normal ones and dried gel to unravel the role of crosslinking and degradation on polymeric structure during drying and hydration process. The comparison of the swelling results suggested that the network were formed only by PVP molecules. The complex mixture of macromolecules showed a irreversible behavior upon drying and hydration, probably as a function of PEG and/or agar entangling in effective physical crosslink. The best network regularity and useful properties was found at 20–30 kGy.

Particle-reinforced, aluminum-based metal-matrix composites (MMCs) are being considered for a range of applications. Their mechanical properties have been investigated in detail, but more ...

The influence of niobium content (0.5–2.0 wt.%) on the oxidation behaviour of Fe-15% Cr-15% Ni austenitic stainless steels has been studied by cyclic oxidation tests in air at different temperatures in the range 800–1200°C. The isothermal oxidation rate, the scaling index and the cumulative oxide loss decreased with increasing niobium content. X-ray diffraction and fluorescence analysis of detached oxide scales revealed that the niobium in the alloy matrix facilitated the formation of chromium rich (Fe, Cr)2O3 and niobium containing mixed oxide near the metal/metal oxide boundary, which decreased further oxidation. SEM/EDX investigations revealed that the external oxide changed from being predominantly iron rich iron-chromium oxide to being chromium rich iron-chromium oxide. Einflus von Niob auf das Oxidationsverhalten vollaustenitischer rostfreier Fe-15% Cr-15%Ni-Stahle Der Einflus des Niobgehaltes (0,5–2,0 Gew.-%) auf das Oxidationsverhalten von austenitischen rostfreien Fe-15% Cr-15%Ni-Stahlen wurde untersucht mit Hilfe von isothermen und zyklischen Oxidationstests in Luft bei unterschiedlichen Temperaturen zwischen 800 und 1200°C. Die isotherme Oxidationsrate, die Verzunderungsrate und der Gesamtverlust an Deckschichtoxid verringerten sich mit ansteigendem Niobgehalt. Rontgendiagramme und eine Fluoreszenzanalyse der abgelosten Oxidschichten zeigten, das das Niob in der Legierungsmatrix die Bildung eines chromreichen (Fe, Cr)2O3 und eines niobhaltigen Mischoxids nahe der M/MO-Grenze ermoglichte, was die weitere Oxidation verlangsamte. REM- und EDX-Analysen zeigten, das die ausere Oxidschicht von uberwiegend eisenreichem Eisenchromoxid zu chromreichem Eisenchromoxid wechselte.

The effect of niobium additions to completely austenitic Fe-15%Cr-15%Ni stainless steels on the microstructure in the as cast and as forged state, and after heat treatments such as solubilization and aging have been studied. Four alloys with nominal Nb contents of 0, 0.5, 1.0 and 2.0% and containing less than 0.03% C have been studied. In the as cast condition, most of the Nb in the alloys remained concentrated in the Laves phase (Fe,Ni,Cr) 2 (Nb,Si) particles. The microstructure of the as cast specimens remained unchanged upon solubilization. Hot forging resulted in fragmentation and redistribution of the Laves phase particles

A variety of metallic components rely on properties that are specific to the alloy and its surface. Coatings have been extensively used to protect metallic surfaces from the aggressive effects of the environment to which it is exposed. In this investigation, the high temperature oxidation behavior of a FeCr and a FeCrY alloy coated with an aluminium based paint has been studied. The objective was to form the more resistant alumina surface layer on an aluminium free alloy. Aluminium based paint coated and uncoated specimens of the two alloys were oxidized for up to 200 hours at 1000 °C in air. The oxidized specimens were examined in a scanning electron microscope coupled to an energy dispersive system and the surfaces were analyzed by X ray diffraction analysis. The aluminium based paint coating increased the oxidation resistance of the alloys, mainly over extended periods. The FeCrY alloy coated with the Al based paint exhibited the highest oxidation resistance.

The high temperature oxidation and erosion-oxidation (E-O) behavior of steels AISI 1020, 304, 310, and 410 were determined. These steels were selected to evaluate the effect of chromium content on its E-O resistance. The oxidation behavior was determined in a thermogravimetric analyzer. A test rig in which a specimen assembly was rotated through a fluidized bed of erodent particles was used to determine the E-O behavior. Alumina powder (200 µm) was used as the erodent. The E-O tests were carried out in the temperature range 25-600 °C, with average particle impact velocities of 3.5 and 15 ms-1 and impact angle of 90°. The oxidation resistance of the steels increased with chromium content. The E-O behavior of the steels was determined as wastage. The E-O wastage of the steels exposed to particle impact at low velocity was low but increased with temperature above 300 °C. The E-O wastage of the different steels exposed to particle impact at high velocity was quite similar. The wastage increased with increase in temperature above 500 °C. The increases in E-O wastage of the steels observed at temperatures above 300, 400 or 500 °C, depending on the steel, were due mainly to a transition in the dominant wastage process, from 'erosion' to 'erosion-oxidation'.

Doped lanthanum chromite has been used in solid oxide fuel cell (SOFC) interconnects. The high costs involved in obtaining dense lanthanum chromite have increased efforts to find suitable metallic materials for interconnects. In this context, the oxidation behavior of lanthanum chromite coated Fe-20Cr and Fe-20Cr-4Al alloys at SOFC operation temperature was studied. Isothermal oxidation tests were carried out at 1000 °C for 20, 50 and 200 hours. Cyclic oxidation tests were also carried out and each oxidation cycle consisted of 7 hours at 1000/°C followed by cooling to room temperature. The oxidation measurements and the results of SEM/EDS as well as XRD analyses indicated that lanthanum chromite coated Fe-20Cr and Fe-20Cr-4Al alloys were significantly more resistant to oxidation compared with the uncoated alloys.

This paper gives an overview of activities carried out at IPEN, Brazil, in the IAEA-sponsored Coordinated Research Project (CRP) “Corrosion of Research Reactor Al-clad Spent Fuel in Water” and in the Latin American regional project on “Management of Spent Research Reactor Fuel”. In both projects Al alloy coupons in various configurations were exposed to the spent fuel storage pool of the IEA-R1 reactor for various times and then examined. Pitting was the main form of corrosion and pit distribution on coupons revealed the marked influence of settled solids. The AA6061 and AA1050 alloy coupons withdrawn after 2 years were stained with a layer of Bayerite caused by an increase in the average pool water temperature. This observation—part of a normal sequence of activities within the project––enabled reactor operations to explain the perplexing change in color of most in-reactor Al alloy components. It exemplified advantages of conducting a corrosion surveillance program. Analysis of sediments obtained from the pool revealed that the main constituents were oxides of aluminum, iron, silicon, and calcium. Coupons of the Russian alloy SZAV, pitted more inside crevices compared to the alloy AA6061. The former pitted more when coupled with stainless steel. Alloy grain orientation also influenced pitting. Coupons of rolled AA6061 alloy pitted more than coupons made from extruded AA6061 alloy. Details about the spent fuel storage facilities in IPEN and the water purification systems of the IEA-R1 research reactor (RR) are also presented.

The addition of small quantities of reactive elements such as rare earths (RE) to chromia or alumina forming alloys improves the high temperature oxidation resistance. Traditionally, these elements are alloying additions or are added as oxides to form a dispersion. The alloys can also be coated with RE oxides. Several methods can be used to coat alloy substrates with RE oxides and the sol-gel process is considered to be quite efficient, as it generates the very small oxide particles. This paper presents the influence of surface coatings of Ce, La, Pr, and Y oxide gels on the oxidation behavior of an Fe-20Cr alloy at 1000 °C. The morphology of the rare earth (RE) oxide coatings varied with the nature of RE. The oxidation rate of RE oxide coated Fe-20Cr was significantly less than that of the uncoated alloy. The extent of influence the RE oxide coating exercised on the oxidation rate decreased in the following order: La, Ce, Pr, Y. The scale formed in the presence of RE oxide was very thin, fine grained and adherent chromia. A direct correlation between rare earth ion radius and the extent of influence on chromia growth rate at 1000 °C was observed.

High temperature corrosion of structural alloys in sulfur bearing environments is many orders of magnitude higher than in oxidizing environments. Efforts to increase sulfidation resistance of these alloys include addition of alloying elements. Aluminum additions to iron-chromium alloys bring about increase in sulfidation resistance. This paper reports the effect of pre-oxidation on the sulfidation behavior of Fe-20Cr and Fe-20Cr-5Al alloys in H2-2% H2S environment at 800 °C. The surfaces of sulfidized specimens were also examined. Pre-oxidation of the two alloys results in an incubation period during subsequent sulfidation. After this incubation period, the Fe-20Cr alloy showed sulfidation behavior similar to that when the alloy was not pre-oxidized. The incubation period during sulfidation of the Fe-20Cr-5Al alloy was significantly longer, over 45 h, compared to 2 h for the Al free alloy. Based on the microscopic and gravimetric data a mechanism for sulfidation of these alloys with pre-oxidation has been proposed.