Oxidation kinetics of high strength low alloy steels at elevated temperatures

High strength low alloy (HSLA) steels are candidate Rockbolt materials for use as underground roof supports at Yucca Mountain nuclear waste repository. Oxidation kinetics of International Rollforms Split Set Friction Rock Stabilizers (SS46), and Swellex Mn24 steels have been determined by temperature modulated thermogravimetry at temperatures ranging between 600 and 900 °C in pure oxygen atmosphere for 100 hr. The imposed sinusoidal temperature modulations (±5 °C for a period of 1 cycle per 200 s) on the isothermal temperature did not have any noticeable effect on the weight gain characteristics during oxidation. Weight gain data on the steels indicate two distinct regions with different oxidation profiles, where a definite change in rates of oxidation is observed: a first oxidation regime where the steels followed a rate law y = kt^0.40-0.63 (changing index of rate law depending upon steel and temperature) and a second stage oxidation regime that follows the parabolic law. The results of characterization of the oxide films using SEM/EDAX, X-ray diffraction and Synchrotron white beam X-ray microdiffraction are presented. The oxidation data of the steels presented here is expected to be useful for characterizing those steels for use in underground rock bolt system and as roof support for the DOE proposed Yucca Mountain Nuclear Waste Repository. To the best of our knowledge this is the first time thermogravimetric studies of this kind have been done on these steels.     

Stochastic approach to the pit growth kinetics of Inconel alloy 600 in Cl ion-containing thiosulphate solution at temperatures 25-150 °C by analysis of the potentiostatic current transients

The pit growth kinetics of Inconel alloy 600 in aqueous 0.1 M Na₂S₂O₃ + 0.1 M NaCl solutions was investigated as a function of solution temperature by analysis of the potentiostatic current transient, based upon a stochastic theory. Potentiostatic current transients revealed that time necessary for pit embryo formation, t_pit,form decreased with increasing solution temperature. Analysis of the statistical distribution of t_pit,form demonstrated that the pit embryo formation rate function at higher solution temperature exhibits a weaker dependence on exposure time compared with that at lower solution temperature. This is attributable to the increase in the active sites necessary for pit embryo formation due to the increase of the defects in oxide film grown at higher solution temperature. Moreover, it was found that the rate of pit growth decreased with increasing solution temperature over the whole applied potential range. Based upon the experimental findings, the kinetics of pit growth with respect to solution temperature is discussed in terms of the changes of the shape parameter derived from the stochastic theory, the pit growth rate parameter determined from the logarithmic potentiostatic current transient and fractal dimension of the pits formed on the electrode surface.     

AZ71E镁合金的高温摩擦学行为(英文)

用销-盘摩擦磨损试验机考察 Z71E压铸镁合金在载荷为10~50 N时的高温摩擦学行为,利用光学显微镜(OM)和扫描电镜(SEM)对磨损表面和亚表面进行分析,通过光学显微镜(OM)、X射线衍射仪(XRD)、差示热扫描(DSC)等对AZ71E合金的高温微观结构、热稳定性和力学性能进行研究。结果表明:随着载荷和滑动距离的增加磨损率增大,而摩擦系数则随着载荷的增加而减少。在低载荷时,AZ71E镁合金的磨损机制主要是磨粒磨损;在150℃和高载荷下,粘着磨损和轻微的剥层磨损是主要的磨损机制;而在200℃及高载荷下,镁合金的主要磨损机制是严重的剥层磨损和熔融磨损。AZ71E镁合金的高温摩擦学性能提高的内在机制是AZ71E镁合金中第二相Al11Ce3使镁合金的高温拉伸和延展性能显著提高。     

Oxidation mechanism of the Inconel 601 alloy at high temperatures

The Inconel 601 alloy oxidation was performed in air, in the temperature range 1000–1150 °C, during 90 h. Kinetic results show that the parabolic behavior is always followed in this temperature range. The Arrhenius plot of the k_p values shows two different activation energies. Between 1000 and 1050 °C the activation energy is E_a1 = 160 ± 10 kJ/mol. In the 1050–1150 °C temperature range a higher value is calculated E_a2 = 252 ± 20 kJ/mol. The E_a2 value and the X‐ray diffraction (XRD) results and scanning electron microscope (SEM) energy dispersive X‐ray spectroscopy (EDS) examinations are in accordance with a scale growth mechanism limited by a growing Cr₂O₃ scale acting as a diffusion barrier. In the 1000–1050 °C temperature range the activation energy is lower and the structural analyses show that the oxide scale is not only composed of Cr₂O₃. Then, the oxide scale is composed of titanium oxides (TiO₂ and Ti₂Cr₇O₁₇) and chromia mixed together. A doping effect of the chromia scale by titanium can be envisaged. Our results also show the presence of some Mn_1.5Cr_1.5O₄ at the external interface. This external subscale spalls off easily during cooling after the highest temperature oxidation tests. Nevertheless, XRD results and SEM–EDS observations show that the Cr₂O₃ scale remains very adherent on the substrate and can give a good oxidation protection. This good adherence can be related to the presence of a low amount of aluminum in the Inconel 601 alloy composition.     

AerMet100超高强度钢热变形行为

在热模拟机上对AerMet100超高强度钢进行了恒温和恒应变速率的热压缩实验,温度范围是900℃～1100℃,应变速率范围是0.01s-1～10s-1。实测了高温下应力-应变关系曲线,观察了变形后的显微组织,计算了材料的激活能,并建立了峰值应力与变形温度和应变速率的关系。结果表明,材料的流动应力随着变形温度的升高而降低,随应变速率的增大而增大;材料在不同变形条件下其软化机制分别受动态回复和动态再结晶控制;在实验条件范围内,AerMet100超高强度钢的再结晶温度在1000℃～1050℃之间,材料的热变形激活能为261.2kJ/mol。     

Mg-13Gd-4Y-2Zn-0.6Zr合金高温本构方程及加工图

采用Gleeble-3500热模拟试验机,研究Mg-13Gd-4Y-2Zn-0.6Zr合金在300℃~500℃、应变速率0.001s-1~1s-1时的高温流变行为,获得了合金的真应力-应变曲线。实验结果表明,随着温度上升、应变速率下降,合金的流变应力、峰值应力和峰值应变均减小。利用真应力-应变数据,进行数值拟合、回归计算,求得合金的热变形激活能Q为273.4kJ·mol-1,并建立该合金的流变应力本构模型,该模型结果与实验数据的最大误差小于5%。同时,根据动态材料模型,计算并得到了该合金在不同真应变下的热加工图,分析了其变化规律。并以真应变为0.7的热加工图为依据,结合材料的微观组织,确定了该合金的最佳变形工艺为480℃/0.01s-1。利用金相图解释了各失稳区的组织演变特点。     

The oxidation behaviour of an AZ91D magnesium alloy at high temperatures

As-cast AZ91D magnesium alloy was exposed to air in the temperature range from 470 to 800 K for time intervals up to 10 h. Thermogravimetric measurements revealed three distinct stages of the reaction where an initial formation of protective oxide was followed by an incubation period with a subsequent transient to non-protective oxidation, at a rate either constant or sharply increasing over time. The approximate temperature and time frames for an onset of each stage were identified. A strong link was found between the oxidation kinetics and the scale morphology. The initial protective film, grown anisotropically over the microstructural features of the substrate, was transformed to oxide ridges. The non-protective oxidation was associated with a formation of oxide nodules and their further coalescence into a fine-grained scale of a loose structure. All the morphologies were comprised of a randomly oriented magnesium oxide MgO with traces of MgAl₂O₄ spinel. The oxidation mechanism represented a complex reaction where morphological and phase transformations within the alloy substrate were accompanied by magnesium sublimation/evaporation and subsequent condensation within the scale pores or cracks, and superimposed on the reaction with oxygen. Some implications for the high temperature processing of magnesium alloys are discussed.     

Pit formation and growth of alloy 600 in Cl ion-containing thiosulphate solution at temperatures 298-573 K using fractal geometry

Pit formation and growth of alloy 600 has been investigated in aqueous 0.1 M Na₂S₂O₃+0.1 M NaCl solution at elevated solution temperatures 298-573 K and at pressures 0.1-8 MPa in terms of fractal geometry using potentiodynamic polarisation experiment, potentiostatic current transient technique, scanning electron microscopy (SEM), image analysis method and ac-impedance spectroscopy. From SEM observation, it was realised that pit morphology changed from cylindrical shape developed at 60 °C to highly branched shape formed at 150 °C. Furthermore, corrosion pits formed and further grew without any morphological change during the whole pitting process below 200 °C. On the other hand, above 200 °C, the morphology of the pits changed from highly branched shape in the early stage of the pitting process to widely grooved shape in the later stage. After SEM observation of the pits, the fractal dimension of pits was determined as a function of solution temperature 60-150 °C using perimeter-area method. The value of the fractal dimension of the pits increased with increasing solution temperature. This is caused by the increase in the ratio of perimeter to area at higher solution temperature, indicating the formation of pits with highly branched shape. The appearance of specific shape and fractal dimension value of the pits at each solution temperature implies that the formation and growth of pits proceed with the typical fractal geometry throughout the whole pitting process irrespective of pit size. In addition, constant phase element (CPE) behaviour observed from the impedance spectra is discussed in terms of the fractal dimension of pits.     

Sulfidation of iron at high temperatures and diffusion kinetics in ferrous sulfide

The kinetics and mechanism of iron sulfidation have been studied as a function of temperature (950–1200 K) and sulfur pressure (10^–3-0.065 atm). It has been stated that a compact Fe_1–yS scale on iron grows according to the parabolic rate law as a result of outward lattice diffusion of metal ions through cation vacancies. The activation energy of sulfidation increases with sulfur pressure and the 1/n exponent increases with temperature. This nontypical dependence of iron sulfidation kinetics on temperature and pressure results from the analogous effect of both these parameters on defect concentration in ferrous sulfide. The chemical diffusion coefficients,D_FeS, and diffusion coefficients of defects, D_d, in ferrous sulfide have been calculated on the basis of parabolic rate contacts of iron sulfidation and deviations from stoichiometry in ferrous sulfide. It has been shown thatD_FeS is practically independent of cation vacancy concentration whereas the diffusion coefficient of defects depends strongly on that parameter. A comparison of self-diffusion coefficients of iron in Fe_1–yS calculated from the kinetics of iron sulfidation to those obtained from radioisotopic studies indicates that within the range studied of temperatures and sulfur vapor pressures the outward diffusion of iron across the scale occurs preferentially along the c axis of columnar ferrous sulfide crystals.     

Oxidation kinetics of an Fe-31.8Mn-6.09Al-1.60Si-0.40C alloy at temperatures from 600 to 900 °C

An Fe-31.8Mn-6.09Al-1.60Si-0.40C alloy was subjected to thermogravimetric tests, in atmospheres of pure oxygen and synthetic air. The alloy showed good oxidation resistance, especially at 600 and 700 °C in air and at 600 °C in oxygen. Owing to autocorrelation identified in the results, kinetic data were analyzed by the statistical method of Cochrane and Orcutt. Parabolic kinetic behaviour was observed for the Fe-Mn-Al-Si-C alloy oxidation process in air at 600, 700 and 800 °C and in pure oxygen at 900 °C.     

The corrosion behaviour of iron in hydrogen chloride gas and gas mixtures of hydrogen chloride and oxygen at high temperatures

Catastrophic corrosion of iron by oxychlorination in hydrogen chloride gas containing 0–75 vol. % oxygen at 300–800°C has been investigated mainly by means of measurements of weight loss and the amount of sublimates. In pure hydrogen chloride gas, the corrosion behaviour was determined by the formation and sublimation of ferrous chloride. Addition of oxygen to hydrogen chloride gas greatly accelerated corrosion of iron due to the formation and sublimation of a low melting point volatile ferric chloride by oxychlorination reactions.     

The corrosion behaviour of chromium in hydrogen chloride gas and gas mixtures of hydrogen chloride and oxygen at high temperatures

A kinetic investigation of the CrHClO₂ system at 1 atm was carried out varying the oxygen content from 0 to 75 vol% at 400–800°C by the thermogravimetric method in stagnant gases and by measurements of weight loss and weight of sublimates after corrosion tests in flowing gases. In hydrogen chloride gas the corrosion rate is determined by the rates of formation and evaporation of a CrCl₂ scale: the scale was protective, to some extent, up to 600°C, but rapidly evaporated at still higher temperatures. The addition of oxygen led to suppression of corrosion loss up to about 500°C but to catastrophic corrosion at higher temperatures. The scale formed in the gas mixtures consisted mainly of Cr₂O₃ but vigorous vaporization of CrCl₂ and water occurred at the higher temperatures due to oxy-chlorination.     

The corrosion behaviour of nickel in hydrogen chloride gas and gas mixtures of hydrogen chloride and oxygen at high temperatures

A kinetic investigation of corrosion of nickel in hydrogen chloride gases containing 0–75% oxygen at 400–700°C was carried out by the thermogravimetric method. There was no great difference in the corrosion behaviour of nickel in the hydrogen chloride gas and its mixtures with oxygen because of exclusive formation of NiCl₂ scale, regardless of the gas compositions. The corrosion behaviour showed the mixed reaction mode; that is, NiCl₂ scale formation according to parabolic kinetics and NiCl₂ scale evaporation according to linear kinetics. With increasing temperature above 500°C, linear kinetics became predominant.     

合金钢锭铬元素高温均质化过程的动力学分析

采用光学显微镜,扫描电镜及能谱仪分析了含铬合金钢锭铸态及均质化后的显微组织和元素偏析规律,建立了均质化过程中元素扩散的动力学模型,旨在奠定大型钢锭均质化制度优化的理论基础。研究结果表明,经过不同时间和温度下的均质化处理后,含铬合金钢锭的元素偏析得到不同程度的改善。本文建立的动力学扩散模型较好地反映了均质化过程中元素的变化规律。     

Oxidation kinetics of cobalt and cobalt—iron alloys at high temperatures

The oxidation kinetics for cobalt and two cobalt—iron alloys alloys, CoFe and CoFe₃, were determined between 1073 and 1280 K in 1 atm. of oxygen. The oxidation of cobalt and CoFe follows a parabolic law, while the oxidation of CoFe₃ is better described by a direct logarithmic law. The oxidation rates for the cobalt—iron alloys alloys are lower than those expected from combining oxidation kinetics of pure iron and pure cobalt, thus suggesting a significant cobalt enrichment occurs at the oxide surface. This cobalt enrichment is observed using energy dispersive analyses. The logarithmic kinetics of CoFe₃ during oxidation is discussed, and a model is proposed to explain this behavior.     

The kinetics of the anodic dissolution of copper in acid chloride solutions

The anodic dissolution of copper has been studied in different acid chloride solutions under potentiostatic conditions. A rotating disk electrode technique was employed.The results showed that the slow cuprous chloride complexes transferred towards the bulk is the r.d.s. all over the polarization range. Anodic limiting currents have been also observed and explained.     

Steady state flow of the FeCoNiCrMn high entropy alloy at elevated temperatures

Steady state flow behavior of the FeCoNiCrMn high-entropy alloy at temperatures ranging from 1023 to 1123 K was systematically characterized. It was found that the stress exponent (i.e., the reciprocal of strain-rate sensitivity) was dependent on the applied strain rate, and specifically the stress exponent is high (∼5) in the high strain rate regime, but decreases with decreasing strain rate. Microstructural examinations of the samples before and after deformation were performed to understand the interplay of the microstructures with the corresponding properties. Based on the observations, it was proposed that, at high strain rates, the deformation of the current high-entropy alloy was controlled by dislocation climb and the rate limiting process was the diffusion of Ni. At low strain rates, however, the deformation appeared to be controlled by the viscous glide of dislocations. Moreover, at the slowest strain rate (i.e., the longest thermal exposure time), new phases evolved, which caused elemental redistribution and weakening of the material.     

Ti600高温钛合金的力学性能

研究Ti600高温钛合金经过不同退火制度处理后的显微组织和力学性能。结果表明:Ti600合金具有良好的热稳定性,片层组织可以获得强度与塑性的良好匹配,蠕变性能优异。随着退火温度的升高,合金的强度升高,合金经过双重退火后的强度高于一次退火的强度。