The initiation of selective oxidation of a ferritic stainless steel at low temperatures and oxygen pressures

Oxidation of a ferritic stainless steel of type Fe-18Cr-2Mo has been performed in the temperature range 285–495°C and oxygen partial pressure range 10^?9-10^?8 torr. The chemical composition of thin oxide layers formed has been analysed by means of Auger Electron Spectroscopy and interpreted in terms of available chromium, iron and oxygen at the solid/gas interface. The selective oxidation of chromium is considered by different probabilities for the oxidation of available chromium and iron respectively. At oxide thicknesses below 100 Å the supplies of chromium and iron are ruled by diffusion in the steel matrix. The results have been used to predict the chemical compositions of two subsequently growing oxide layers provided the thickness of the first oxide layer is below 50 Å.     

Diffusion of transition metals in “in situ” duplex oxide grown on a 316 stainless steel

Concentration profiles of chromium, maganese and iron radioactive tracers through a duplex oxide grown on 316 stainless steel have been obtained after a diffusion anneal at 1273 K using the serial sectioning technique. The iron tracer diffused normally in both inner and outer oxides of the duplex scale. The concentration of the chromium and manganese tracers increased with increasing penetration into the outer Fe₃O₄ layer but showed a normal profile in the inner spinel. The diffusion coefficients obtained from the profiles were in the range 6 × 10^?12 – 3 × 10^?11 cm² sec^?1 for diffusion in the spinel phases.     

Oxidation Behavior of ZrB<Subscript>2</Subscript>–SiC Composites at 1650 °C

The oxidation behavior of ZrB₂–SiC composites in air was studied at 1650 °C. Diffusion-controlled oxidation kinetics were found for the composites studied. A parabolic rate constant of 1.2 × 10^?8 g² cm^?4 s^?1 was measured for ZrB₂–10 % SiC composite. A transition in the oxidation kinetics was observed for ZrB₂–30 % SiC composite with the initial parabolic rate constant being 1.3 × 10^?8 g² cm^?4 s^?1. After exposure for 60 min, the parabolic rate constant was found to be 0.3 × 10^?8 g² cm^?4 s^?1. A single ZrO₂-rich oxide layer was found in the oxide scale structure of ZrB₂–10 % SiC composite. On the other hand, three-layer oxide structures, namely SiO₂-rich top layer, followed by ZrO₂-rich oxide scale and SiC-depleted layer, were found for ZrB₂–30 % SiC composite. The outer layer in the oxide scale structure of ZrB₂–SiC composite was tapered with enhanced oxidation at the corners of the sample. Vortex formation during the viscous flow of B₂O₃–SiO₂–ZrO₂ liquid near the corners on the surface was proposed as the root cause for enhanced oxidation at the corners of the sample.     

Influence of Irradiation on the Oxide Film Formed on 316 L Stainless Steel in PWR Primary Water

Oxidation of stainless steel in PWR primary water at 325 °C was studied by investigating the influence of defects created at the alloy subsurface by proton irradiation before corrosion exposure. Corrosion experiments were conducted by sequentially exposing samples, with H ₂ ¹⁸ O used for the second exposure. The oxide layer, formed in these conditions was studied by SEM and TEM and could be divided in two parts: an external discontinuous layer composed of crystallites rich in iron and an internal continuous layer richer in chromium. Tracer experiments revealed that the growth of this protective scale was due to oxygen diffusion in the grain boundary of the oxide layer. Defects created by irradiation have an effect on the two oxide layers. They are a preferential nucleation site for the external layer and so increase the density of the crystallites. They also induce a slower diffusion of oxygen in the internal layer.     

Improving the Physicochemical Properties of Fe–25Cr Ferritic Steel for SOFC Interconnects via Y-Implantation and Y2O3-Deposition

The present study investigated the role of the reactive-element effect (REE) in improving the corrosion resistance, chromium vaporization rate, and electrical conductivity of the Fe–25Cr ferritic steel modified either by means of yttrium implantation or chemical deposition of yttrium oxide from metaloorganic compound vapors. The corrosion kinetics of the Fe–25Cr steel, both pure and modified, were determined under isothermal conditions in air and an Ar–H₂–H₂O gas mixture at 1,073 K. A significant improvement in corrosion resistance was observed after surface modification. XRD and SEM–EDS investigations showed that the protective Cr₂O₃ layer formed the main part of the scale. Measurements of Cr vaporization rate in the air–H₂O gas mixture revealed that both surface modifications of the steel significantly suppressed the formation of volatile chromium compounds to a large degree. The yttrium-implanted steels oxidized both in air and the Ar–H₂–H₂O mixture were characterized by the lowest area specific resistance and thereby did not exceed the acceptable ASR level (0.1 Ω cm²) for interconnect materials in the temperature range of 973–1,073 K, unlike pure steel and the steel coated with Y₂O₃.     

Sulfidation properties of Fe-6.1 at.% Mo alloy at 973–1273 K

Sulfidation of an Fe-6.1 at% Mo alloy was investigated in H₂S-H₂ atmospheres, 10^?4 ? P_s2 ? 10²Pa, at 973-1273 K. The reaction kinetics are parabolic except at 1273 K as liquid sulfide formation leads to catastrophic corrosion. This solid-liquid transformation between Fe₂Mo₂S₄ and Mo₂S₃ occurs at 1214 ± 9 K. At 1073 K and P_s2 = 10^?4Pa, growth of a duplex Mo₂S₃/FeMo₂S₄ scale offers high resistance to sulfidation. At 973, 1073 and 1173 K, 10^?2 ? P_s2 ? 10²Pa, parabolic sulfidation kinetics of the same magnitude as for pure iron describe growth of a duplex scale composed of an inner (FeMo₂S₄ + Mo₂S₃) layer and at an outer FeS layer. Marker measurements indicated that growth of the inner two-phase layer was supported by inward migration of sulfur and that growth of the outer FeS layer resulted from outward migration of iron.     

Characterization of the initial oxide formed on annealed and unannealed 20Cr-25Ni-Nb-stabilized steel in 50 torr CO2 at 973 K

The oxidation of annealed and unannealed 20Cr-25Ni-Nb (wt.%) steel in 50 torr CO₂ at 973 K has been studied in situ using X-ray photoelectron spectroscopy with a view to the characterization of the chemical composition and nature of the oxides formed. The oxide first formed on the annealed steel is shown to be iron rich. Analysis of the bulk oxide using a variety of different spectroscopic techniques including X-ray diffraction, energy dispersive X-ray analysis, and scanning Auger microscopy showed that virtually all of the oxide scale formed after 100hr is a spinel of the type (Fe)Fe_2–x)Cr_xO₄ and is composed of an outer, iron-rich layer covering an iron/chromium-rich layer. By contrast, the oxide first formed on the unannealed steel is chromium rich, and is shown to be patchy consisting of a mixture of different oxides. This layer changes on further oxidation to develop into a manganese-iron-chromium spinel, which is present as the major oxide phase after 100 hr. The reasons for these differences are discussed, and it is argued that a major influence on oxidation behavior is the presence of cold work in the unannealed steel enhanc ing the diffusion of chromium to the surface.     

The Effects of KCl,NaCl and K<Subscript>2</Subscript>CO<Subscript>3</Subscript> on the High-Temperature Oxidation Onset of Sanicro 28 Steel

The present study investigates the early stages in the oxidation process of Sanicro 28 (Fe31Cr27Ni) stainless steel when exposed to an alkali salt (KCl, NaCl or K₂CO₃) for 2 h at 450 and 535 °C. After the exposure, the oxidized samples were analyzed with a combinatory method (CA, XPS and SEM–EDX). It was found that all three salts were corrosive, and the overall oxidation reaction rate was much higher at 535 °C than at 450 °C. There were clear differences in terms of the impact of cations (Na⁺, K⁺) and anions (Cl^?, CO₃ ^2?) on the initial corrosion process at both temperatures. When focusing on the cations, the presence of potassium ions resulted in a higher rate of chromate formation than in the presence of sodium ions. When studying the effect of anions, the oxidation of iron and chromium occurred at higher rates in the presence of both chloride salts than in the presence of the carbonate salt, and chloride salts seemed to possess higher diffusion rate in the gas phase and along the surface than carbonate salts. Moreover, at the higher temperature of 535 °C, the formed chromate reacted further to chromium oxide, and an ongoing oxidation process of iron and chromium was identified with a significantly higher reaction rate than at 450 °C.     

Inhibition of copper sulphidation by boron implantation

Pure polycrystalline copper and copper implanted with boron ions at a dose of 3.0 × 10¹⁷ ions cm^?2 at 25–150 keV have been exposed to hydrogen sulphide in moist air. The rate of growth of a sulphide film on the boron-implanted copper is lower than on pure copper by at least a factor of four after 18 h under the exposure conditions ([H₂S] = 3.0 ppm, T = 22.5°C, RH = 85%). The resistance of the implanted copper to sulphide corrosion is ascribed to inhibition of copper diffusion through the surface oxide layer.     

Volatilization and Transport Mechanisms During Cr Oxidation at 300 °C Studied In Situ by ToF-SIMS

The oxidation of chromium at 300 °C was investigated in situ by ToF-SIMS for three different oxygen pressures (\(P_{{{\text{O}}_{2} }} = 2.0 \times 10^{ - 7}\), 6.0 × 10^?7 and 2.0 × 10^?6 mbar). Sequential exposure to the ¹⁸O isotopic tracer was performed to reveal the governing transport mechanism in the oxide film. The evolution of the oxide thickness was monitored. Volatilization of Cr₂O₃ was evidenced. A model was used to describe the kinetics resulting from the measurements. Both the parabolic and volatilization constants showed a dependence on oxygen partial pressure like \(P_{{{\text{O}}_{2} }}^{ - 1/n}\), with n = 1.9 ± 0.1, indicating a defect structure mainly consisting of oxygen vacancies. The re-oxidation in ¹⁸O₂ shows a growth of the oxide layer at the metal/oxide interface, demonstrating an oxidation process governed by anionic transport via oxygen vacancies. The diffusion coefficient of oxygen in the oxide was determined by fitting the ToF-SIMS depth profiles. It is 2.0 × 10^?18 cm² s^?1.     

A determination of the chromium concentration for ‘healing’ layer formation during the oxidation of chromium-depleted 20cr-25ni-nb stainless steel

High temperature oxidation tests have been performed at 973–1123 K in a CO₂ based gas on specimens of 20Cr-25Ni-Nb stainless steel in which the surface chromium concentration was reduced by prior vacuum annealing at 1273 K. It is shown that the rapid initial oxidation rate decreases as a result of the formation of a chromium-rich ‘healing’ layer at the oxide-metal interface. For oxidation temperatures ≥ 1073 K, a chromia-type layer readily forms whereas lower oxidation temperatures favour the formation of a chromium-rich spinel after considerably longer oxidation times. Use of electron probe micro-analysis to determine the pre-existing chromium depletion profile and measurements of the depth into the steel at which this ‘healing’ layer forms enable an assessment to be made of the chromium concentration necessary to form this layer. This concentration increased from 15.2 w/o at 1123 K to ≥ 18.5 w/o at 973 K.     

The oxidation of type 316 stainless steel in low pressure oxygen at temperatures between 873 and 1223 K

The oxidation of type 316 stainless steel in low pressure oxygen (1.8 × 10^?2 to 1.3 × 10^?5 N/m²) in the temperature range 873 to 1223K has been studied. The kinetics are determined from weight gain measurements and the surface oxide composition determined by a number of experimental techniques. In the as-received condition a thin Cr₂O₃ layer forms with some incorporation of manganese, at rates governed by lattice diffusion in the alloy. After a vacuum anneal for 24 hours at 1273K two types of behaviour are found. In the first a chromium rich M₂O₃ layer forms, with in certain cases initial formation of Fe₂O₃, the kinetics of which indicate that grain boundary diffusion in the alloy is rate determining. In the second a more rapid growth of an Fe₂O₄ oxide is found, subsequently followed by slower kinetics as a Cr₂O₃ layer again forms. No discernable relationship between oxidation rates and either pressure or temperature is found.     

Study of Reaction Mechanisms for Copper-Cobalt-Iron Sulfide Concentrates in the Presence of Lime and Carbon

The reaction mechanisms for the carbothermic reduction of complex mineral sulfide concentrates in the presence of lime were studied between 1073 K and 1323 K. The reaction mechanisms were studied by stopping the reduction experiments at different times and analyzing the reaction products by x-ray diffraction and scanning electron microscopy techniques. Magnetite (Fe₃O₄) and digenite (Cu_1.8S) were the initial phases formed during reduction of CuFeS₂ and Cu₅FeS₄ mineral particles, such that metallization of iron occurred before copper above 1173 K and at an equal stoichiometric ratio of CaO and C. The metallization of iron was found to take place via reduction of intermediate oxide phase (Fe₃O₄/FeO), whereas metallization of copper occurred via diffusion of S^2? ions away from the mineral particles or via formation of Cu-O-S liquid phase. Metallic iron and cobalt were embedded in the copper matrix due to a preferential reduction of iron and cobalt from the Cu-Fe-S and Cu-Co-S type of mineral particles. The effects of CaO/C ratio were analyzed and the rate of reactions was increasing with an increase in the CaO/C ratio. The formation of liquid phase has been discussed. The experimental results were found to be in good agreement with the thermodynamic predictions.     

Zur Korrosion des Hafnium – Oxide und Hydride

Corrosion of hafnium – oxides and hydrides Investigations concerning the growth and behaviour of hafnium as well as electrode kinetics have been performed on hafnium (Goodfellow and Material Research Company) in various electrolytes (acidic, alkaline and Cl^?-containing solutions). Conventional electrochemical methods and impedance spectroscopy were applied. Corrosion rates derived from electrochemical measurements were found to be significantly higher than the published data obtained from the loss weight observed usually over long of time. – The DK was evaluated to be 11.5. – SIMS profiles indicate that the oxide layer in not uniform, due to the nonstoichiometry of the Hf oxide at 22°C. The layer shows a very low n-conductivity – quasi insulator behaviour; the Mott-Schottky-plot leads to the flat band potential U = ? 10 mV. The initial oxide layer was determined by capacity measurements to 2.7 nm. Cl₂-evolution begins at U = 0.5 V in 0.5 M NaCl solution. In the cathodic potential region hydrogen is generated at Hf oxide at U = ? 1.69 ? 0.058 V. With respect to the hydrogen induced cracking of Hf the behaviour of hydrogen in Hf was investigated using the nuclear physics ¹⁵N-method. After cathodic polarisation hydrogen is absorbed by hafnium and the hydride HfH_1.6 is formed. The hydrogen diffusion coefficient is determined to D_20° = 3.8 × 10^?11 cm²s^?1. At 385°C hydrogen is released from HfH_1.6. The results are discussed and compared to those found for titanium and zirconium.     

Optimum conditions in the thermal passivation of aisi 430 and 304 stainless steel in controlled oxygen atmosphere

The oxidation of stainless steels AISI 304 and 430 was undertaken in the temperature range 450–600°C and oxygen partial pressure range 6.7 × 10⁷ to 1.33 × 10^?5 Pa. The composition of the oxide layer formed has been analysed by AES and ESCA. Optimum conditions were determined for the formation of a ～40 nm thick oxide layer with a high chromium content. Attention is drawn to the role of the chromium content of the bulk in the passivation process.     

熔盐电解精炼提纯金属硅(英文)

对熔盐电解质中硅的沉积过程进行电化学研究。在 973~223K,在硅氯化物熔盐中采用电解精炼提纯金属硅。结果表明,液态硅铜合金阳极有利于 CaCl2-NaCl-CaO-Si 熔盐体系的电解精炼。ICP-AES 分析结果显示,通过电解精炼可有效去除原料中大量的钛、铝、铁等金属杂质,硅中的硼和磷含量分别由 36×106和 25×106降低至 4.6×10 6和 2.8×10 6,电解能耗约为 9.3 kW·h/kg。     

Cyclic Oxidation Behavior of IN 718 Superalloy in Air at High Temperatures

Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950 °C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 ± 20 kJ mol^?1. The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750 °C. At 850 °C, a continuous and very thin oxide scale was formed. At 950 °C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO₂?CMnCr₂O₄ spinels, inner layer of Cr₂O₃, and the inner most layer composed of Ni₃Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO₂ at the oxide-gas interface as well as internally and at the oxide alloy interface. The amount of rutile (TiO₂) at the oxide surface increased with temperature. In view of Mn contents in the alloy, the manganese?Cchromium spinel oxide was inferred to have played an important role in cyclic oxidation behaviour of IN 718, where the change in oxidation kinetic was noted. The Al contents would cause internal Al-rich oxide formation at grain boundaries.     

Effect of La-Substitution on the Electrical Conductivity of Sr1−x La x MoO4+δ (x = 0-0.3) Compounds

Various compositions of Sr_1?x La _x MoO_4+δ (x = 0.05, 0.1, 0.2, and 0.3) compounds were prepared by solid state reaction route. The samples were characterized by powder-XRD, TG-DTA, and SEM-EDAX techniques. Formation of single crystalline phases of Sr_1?x La _x MoO_4+δ was confirmed from powder-XRD patterns. The thermal stability of La-doped SrMoO₄ compounds was investigated by TG-DTA. Uniform grain distribution was observed in the SEM image of 10-20 mol.% La-substituted compositions. Needle-shaped structures were observed in the SEM image of Sr_0.3La_0.1MoO_4+δ and were confirmed to be La₂Mo₂O₉ by XRD examination. The electrical conductivity of these compounds was measured by AC-impedance technique in the temperature range of 373-1073 K in air ambience and compared with that of pristine SrMoO₄. The electrical conductivity was found to decrease for La-substituted SrMoO₄ compared to pristine SrMoO₄. The diffusion coefficient calculated from the electrical conductivity was found to be in the range of 1.94 ± 0.02 × 10^?13 to 1.15 ± 0.01 × 10^?11 cm²/S at 873-1173 K for substituted composition and 3.47 ± 0.02 × 10^?13 to 2.48 ± 0.01 × 10^?10 cm²/S for pristine SrMoO₄ at 673-1073 K temperature range.     

Mild steel corrosion modelling in presence of hydrogen sulphide in aqueous solutions

Abstract

Corrosion of mild steel in aqueous solutions containing hydrogen sulphide was modelled under the condition that an iron sulphide film was formed on the steel surface. In the present model, the iron sulphide forms on the steel surface as a result of a solid state reaction between iron and hydrogen sulphide which has several steps. First a very thin film of iron sulphide nucleates on the steel surface. Then, due to further growth of the initial thin layer, a more porous layer of iron sulphide forms on the initial film. In the present model, it is assumed that mass transfer through the thin iron sulphide layer (i.e. adjacent to the steel substrate) controls the corrosion rate of steel in H₂S aqueous solutions, and as a result electrochemical reactions were not considered. The model was verified against the published experimental data and effects of some parameters such as hydrogen sulphide partial pressure were investigated. The results show that increase in partial pressure of hydrogen sulphide leads to an increase in the corrosion rate of mild steel at the primary stages of the reaction, but as a consequence of formation of iron sulphide scales on the steel surface, it drops with respect to time.     

Thermal stability of nanocrystalline Al−10Fe−5Cr bulk alloy

Thermal stability of nanocrystalline Al?10wt.%Fe?5wt.%Cr bulk alloy was investigated. The initial micro-grained mixture of powders was processed for 100 h using mechanical alloying (MA) to produce nano-grained alloy. The processed powders were sintered using high frequency induction heat sintering (HFIHS). The microstructures of the processed alloy in the form of powders and bulk samples were investigated using XRD, FESEM and HRTEM. Microhardness and compression tests were conducted on the bulk samples for evaluating their mechanical properties. To evaluate the thermal stability of the bulk samples, they were experimented at 573, 623, 673 and 723 K under compression load at strain rates of 1×10^?1 and 1×10^?2 s^?1. The annealed samples exhibited a significant increase in their microhardness value of 2.65 GPa when being annealed at 723 K, as compared to 2.25 GPa of the as-sintered alloy. The bulk alloy revealed compressive strengths of 520 MPa and 450 MPa at 300 K and 723 K, respectively, when applying a strain rate of 1×10^?1 s^?1. The microstructural stability of the bulk alloy was ascribed to the formation of iron and chromium containing phases with Al such as Al₆Fe, Al₁₃Fe₄ and Al₁₃Cr₂, in addition to the supersaturated solid solution (SSSS) of Cr and Fe in Al matrix.