Influence of temperature and the role of chromium on the kinetics of sulfidation of 310 stainless steel

The sulfidation of 310 stainless steel was studied over the temperature range from 910 to 1285° K. By adjusting the ratio of hydrogen to hydrogen sulfide, variations in sulfur potential were obtained. The effect of temperature on sulfidation was determined at three different sulfur potentials: 39 N·m^–2, 1.4×10^–2 N·m^–2, and 1.5×10^–4 N·m^–2. All sulfide scales contained one or two surface layers in addition to a subscale. The second outer layer (OL-II), furthest from the alloy, contained primarily Fe-Ni-S. The first outer layer0 (OL-I), nearest the subscale, contained Fe-Cr-S. The subscale consisted of sulfide inclusions in the metal matrix. Two different phases were observed in OL-II depending on the temperature and sulfur potential. Below 1065°K OL-II is composed of a mixture of monosulfides of iron and nickel (Fe Ni)_1–xS and pentlandite (Fe_4.5Ni_4.5S₈) with the pentlandite phase exsolved as lamellae upon cooling. At temperatures higher than 1065°K only the pentlandite phase was formed, which melted above 1145°K at sulfur potentials greater than 10^–2 N·m^–2, yielding metal-rich iron-nickel-sulfur. Above 1145°K, and at sulfur potentials less than 10^–2 N·m^–2, OL-II ceased to exist (this temperature is termed transition temperature). Below the transition temperature, where OL-II exists, OL-I could be represented by the general composition (Fe, Cr)_1–xS. This phase on cooling transformed into an array of structures differing in FeCr ratio. These substructures, however, were not observed in quenched samples. Above the transition temperature OL-I changed to a chromium-rich sulfide composition and was associated with a sudden decrease in reaction rate. Subscale formation was found to be due to the dissociation of OL-I at the scale-metal interface, and the extent of subscale growth was found to depend on the temperature and the sulfur potential, as well as the composition of OL-I. At a given temperature and sulfur potential the weight-gain data obeyed the parabolic rate law after an initial transient period. The parabolic rate constants obtained at the sulfur potential of 39 N·m^–2 did not show a break when the logarithm of the rate constant was plotted as a function of the inverse of absolute temperature. Sulfidation carried out at a sulfur potential below 2 × 10^–2 N·m^–2, however, did show a break at 1145°K. This break was found to be associated with the changes which had occurred in the FeCr ratio of OL-I. Below the transition temperature the activation energy was found to be approximately 125 kJ · mole^–1. Above the transition temperature the rate of sulfidation decreased with temperature but depended on the FeCr ratio in the ironchromium-sulfide layers of the OL-I. A reaction mechanism consistent with the experimental results has been proposed in which the diffusion of cations through OL-I is the rate-controlling step. Below the transition temperature the diffusion of Fe and Ni through OL-I contributes to the scale formation, whereas above the transition temperature the diffusion of Cr through OL-I controls the scale formation. Existing literature on the Fe-Ni-S system is compared with the present results.     

Kinetics of thermal decomposition of niobium hydride

High-purity niobium powders can be obtained from the well-known hydride-dehydride (HDH) process. The aim of this work was the investigation of the structural phase transition of the niobium hydride to niobium metal as function of temperature, heating rate and time. The niobium powder used in this work was obtained by high-temperature hydriding of niobium machining chips followed by conventional ball milling and sieving. X-ray diffraction measurements were carried out in vacuum using a high-temperature chamber coupled to an X-ray diffractometer. During the dehydriding process, it is possible to follow the phase transition from niobium hydride to niobium metal starting at about 380 °C for a heating rate of 20 °C/min. The heating rate was found to be an important parameter, since complete dehydriding was obtained at 490 °C for a heating rate of 20 °C/min. The higher dehydriding rate was found at 500 °C. Results contribute to a better understanding of the kinetics of thermal decomposition of niobium hydride to niobium metal.     

Optimization of temperature and rate regimes of plastic deformation by the criterion of dissipation of mechanical energy

Conclusions  

Prediction of hardening of binary iron alloys due to nitriding on the basis of calculation models

Conclusions  

Study of the phase transformation of cobalt

The allotropic transformation of cobalt was studied with a differential scanning calorimeter. The temperatures of the fcc to hep transformation, A_s, and fcc to hcp transformation, M_s, exhibit a linear dependence on the heating and cooling rates: A_s(K) = 723.2 + 0.28R and M_s(K) = 671.4-0.28R whereR is the rate in K/min. A_s, M_s, and the associated enthalpies of the phase changes were observed to increase with increasing number of cycles through the transformation. The M_s reaction was observed to proceed slowly isothermally while exhibiting athermal behavior when cooled rapidly.     

High-temperature oxidation of tantalum of different purity

The kinetics, structural aspects, and phase morphologies were studied for tantalum oxidation in air from 600 to 1000°C for samples of different purity (99.15%, 99.76%, and 99.95% Ta). Regardless of purity, tantalum oxidation in the temperature range of 600–800°C as a rule is governed by a linear rate law. From 900 to 1000°C the initial-stage oxidation is governed by the parabolic rate law, which changes to the linear rate law with time. TGA, XRD, SEM, and AES methods were used. The, effect of purity on tantalum oxidation was shown to be determined by the mechanism of intermediate-oxide formation. They are TaO _z (Ta₂O) at 600–800°C and TaO at 900–1000°C. The final product of oxidation was -Ta₂O₅.     

Stepwise Depletion of Coating Elements as a Result of Hot Corrosion of NiCrAlY Coatings

Present investigation deals with the hot corrosion behaviour of the NiCrAlY coatings deposited by HVOF technique on Superni76 under cyclic conditions at 900  °C in the presence of Na₂SO₄ + 60% V₂O₅ salt. The weight change behaviour of the coatings was followed with time up to 200 cycles and K _p value was calculated for the hot corrosion process. Surface and cross-section of the corroded samples were examined by FESEM/EDS and XRD to follow the progress of corrosion up to 200 cycles. In earlier cycles, the corrosive species oxidised top surface of the coatings. With increasing number of cycles, oxidation of the coatings occurred up to 40-μm depth. A Cr-depleted band was seen below the oxide scale. Further increase in number of cycles led to migration and oxidation of Al to form Al₂O₃ sublayer at coating/scale interface, thereby leading to formation of Al-depleted zone in the coating below the Al₂O₃ sublayer. The corrosion resistance of the NiCrAlY coatings is attributed to the formation of the continuous and dense Al₂O₃ sublayer at the coating/scale interface, which acts as barrier to the migration of Cr to the surface. The appearance of Al₃Y after 100 and 200 cycles also contributes to the increased corrosion resistance of coatings after 100 and 200 cycles.     

Formation, retention, and discharge of products of atmospheric corrosion of metals. 4. Model: Corrosion-discharge of products

An empirical mathematical model K = K ₀ + km _los (relating atmospheric corrosion to the weight loss of its products) for incubation + transient and steady-state stages of the process, is considered. To develop the model, we used the results of long-term (from 3 month to 1–17 years) field tests of metals according to the Russian, International ISOCORRAG, and Russia-Vietnam-Cuba programs. The K ₀ and k factors in the model were determined according to the corrosion of Ct3, copper, brass, zinc, aluminum, Δ16 alloy, and AMц alloy in the zones of cold, moderate, subtropical, and tropical climates. The model quantitatively relates the corrosion losses of a metal to those of the products discharged to the environment. It may be useful when it is necessary to estimating the amount of detrimental metals, e.g., unprotected copper and zinc dispersed to the environment, but there is no data on the weight of products that had been retained by the metal in the air. Original Russian Text ? Yu.M. Panchenko, P.V. Strekalov, T.V. Nikulina, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 2, pp. 167–191.     

The mechanism of the action of inorganic inhibitors

Abstract

The studies on the inhibition of the corrosion of iron and steel made in the Oak Ridge National Laboratory originated in the discovery of the inhibitory action of the pertechnetate ion, TcO₄^-. The properties of technetium compounds relevant to inhibition are presented, together with an outline of the hypothesis of electrostatic polarisation by the XO^4n- ions which led to the discovery.

Results of several studies related to mechanism are given. These demonstrated the extreme effectiveness of the pertechnetate ion, in comparison with other inhibiting anions, even in the absence of oxygen or other oxidising agents. It was shown that TcO₄^-, CrO₄^2- and MnO₄^- all exert an effect on the metal that does not depend upon reduction of the inhibitor. The result of this action is that the metal may be passivated anodically by much lower current densities than are requisite in non-inhibiting electrolytes, and with passage of no more charge than is necessary to form the passive film.

The nature of the interface between metal and electrolyte in both active and passive conditions is briefly discussed in relation to the alternative reaction paths which lead to active dissolution or to passivation, respectively. The various ways in which the inhibitor may enter into these processes are presented from the point of view of electrode kinetics, as modified by the specific interactions in which the structure of the inhibitor may playa part. In effect, the inhibitor reduces the ‘activity’ of the metal, as represented by its exchange current, so that it is then capable of being passivated as easily as a stainless alloy.     

Effect of Misfit on Preload Maintenance of Retention Screws of Implant-Supported Prostheses

The aim of this study was to evaluate the effect of unilateral misfit at different levels on a crown-implant-retention screw system of implant-supported crowns. Hexagon castable UCLA abutments were cast in Co-Cr alloy to fabricate 48 metallic crowns divided into four groups (n = 12). Group A: crowns did not present misfit; Groups B, C and D: crowns were fabricated with unilateral misfit of 50, 100, and 200 μm, respectively. The crowns were attached by titanium retention screw with 30 N/cm to external hexagonal osseointegrated implants embedded in acrylic resin. After 2 min, the retention screw of each replica was submitted to detorque evaluation by an analogic torque gauge. Three retention screws were used to perform detorque evaluation at each replica and the procedure was repeated twice with each screw. Each group was submitted to 72 detorque measurements. Data were evaluated by ANOVA and Tukey test (P < 0.05). All groups exhibited significant decrease (P < 0.05) in preload and the lowest decrease occurred in Group A. Groups B, C, and D were statistically significant different from Group A (P < 0.05), but there was no statistically significant difference between Groups B and D (P > 0.05). Crowns with unilateral misfit presented higher preload decrease than crowns completely fitted to osseointegrated implants.     

In situ optical dilatometric measurements of the initial stages of sintering of alumina

The initial stage of shrinkage of a series of alumina powder compacts containing different proportions of small and large particles was measured using an in situ optical dilatometry technique. The effect of sample radiance, leading to overestimation of sample dimensions, was determined by comparing the volume change of a fully dense alumina body with that predicted by thermal expansion data. Using this technique, it was possible to produce a calibration curve to eliminate the effect of sample radiance at elevated temperatures. Results showed that with increased levels of large particles the degree of densification decreased. Shrinkage data were then compared with a model that predicted parallel straight lines for plots of ln[1−(ρ_i/ρ)^1/3] against ln[t/R⁴_L] where ρ and ρ_i are current and initial density, t is time and R_L is the size of the large particles. The experimental results obtained were shown to be parallel with changes in the slopes of the lines occurring at identical times irrespective of compact composition. This behaviour was attributed to the range of the particle sizes present—beyond the bounds of the model—giving rise to changes in the densification rate with time. The analysis has shown that it is necessary to consider the range of particle sizes that occur in real powder systems when attempting to model the sintering behaviour of powders.     

Cyclic-oxidation behavior of TiAl and of TiAl alloys

The cyclic-oxidation behavior of (in w/o) Ti-36Al, Ti-35Al-0.1C, Ti-35Al-1.4V-0.1C and Ti-35Al-5Nb-0.1C was studied between 800 and 1000° C in air. A few experiments were also performed in oxygen. Scale spallation after oxidation in air occurs during cooling on TiAl, TiAl-C, and TiAl-V at or close to the metal/scale interface when a critical scale thickness has been achieved. This process repeats and can lead to a stratified scale. These three materials form scales composed of an inward-growing fine-grain mixture of TiO₂-Al₂O₃ and an outward-growing coarse-grain TiO₂ layer or TiO₂+Al₂O₃ mixture. The TiAl-Nb alloy had a significantly different behavior. The scale on this material grew very slowly because a protective Al₂O₃ layer formed at the metal/scale interface. This behavior resulted in much better resistance to spallation because the critical scale thickness was reached only after a much longer time, and is different from the behavior of the other three alloys. Oxidation in air leads to slight nitridation of the subsurface zone beneath the scale. In comparison to oxidation in air, oxidation in oxygen improves the cyclicoxidation behavior. Whereas the scale formed in air was uniformly thick over the entire surface, the scale grown in oxygen varied locally in structure and thickness. A large fraction of the surface was covered with a thin Al₂O₃ layer, while the remaining part formed a two-layer scale similar to that formed in air. The results are discussed briefly in the light of a recently published model for scale spallation under compressive stress, however, quantitative estimations are not possible due to a lack of relevant data.     

The Inhibitive Effect of Traces of SO2 on the Oxidation of Iron

The oxidation of Fe was investigated at 500–700°C in the presence of O₂ with 0–1000 ppm SO₂. The exposures were carried out in a thermobalance and lasted for 24 h. The oxidized samples were investigated by grazing-angle XRD, SEM/EDX, GDOES and XPS. The rate of oxidation of pure iron is slowed down by traces of O₂ in O₂ below 600°C while SO₂ has no effect on oxidation rate at higher temperatures. Exposure to SO₂<600°C resulted in the formation of small amounts of sulfate at the gas/oxide interface. In addition, sulfur, probably sulfide, accumulated at the metal/oxide interface. The influence of SO₂ on oxidation rate is attributed to surface sulfate. The sulfur distribution in the scale is rationalized in terms of the thermodynamic stability of compounds in the Fe–O–S system. Exposure to SO₂ caused the formation of hematite whiskers.     

Peculiarities of electrochemical behavior of copper in alkaline solutions in the presence of alanine

Anodic oxidation of copper in alkaline solution in the presence of alanine is studied by electrochemical and ellipsometric methods. Thickness of films formed on the metal in the working solutions, are determined. Small additives of β-alanine (10⁻⁴–10⁻³ mol/l) cause additional stabilization of copper passivity due to an increase in the film thickness. The adsorption kinetics of α-and β-alanine is studied. It is shown experimentally that β-alanine adsorbs on copper according to the Frumkin isotherm. By contrast, no definite type isotherm can be obtained for α-alanine due to its higher activity and ability to form soluble complexes with copper ions. Original Russian Text ? E.V. Nikitchenko, S.A. Kaluzhina, N.P. Andreeva, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 3, pp. 279–284.     

Modeling of solid-state electrotransport for purification of gadolinium

Modeling of solid state electrotransport (SSE) was formulated to understand the transport phenomena of the interstitial impurities in gadolinium, are of the rare earth metals. Through numerical analysis, the optimum conditions for SSE are theoretically predicted. The processing parameters such as pressure, temperature, and reaction time can be determined to attain maximum effectiveness. The concentration profiles of interstitial impurities such as oxygen, nitrogen, and carbon were calculated based on this model. The electromigration of oxygen is faster than nitrogen and carbon because of its greater diffusivity and mobility. When the reaction time was 10⁵ seconds, the oxygen and nitrogen were refined up to 92%–95% at 10⁻⁵Pa while carbon was refined to 98% at 10⁻⁶Pa. The increase in temperature gives rise to a shorter reaction time, but extremely low pressure is required.     

添加剂对锌电极性能的影响(英文)

为了改善镍锌电池的性能,同时研究电极中添加剂对锌电极性能的影响,选用正交设计对影响锌电极的4个因素(乙炔黑、Bi2O3、PbO、包覆La(OH)3的Ca[Zn(OH)3]2·2H2O)选择3个水平进行测试。对没有任何添加剂的氧化锌在20%KOH电解液中进行充放电循环实验,研究添加剂的作用。为了更好地解释锌电极放电容量衰减的原因,对正交实验的最差样品和最优样品放电产物所含的Zn和Ca的比例进行分析。通过表征,得出了在该实验条件下的电极添加剂的最优配比方案为:在5g样品中,乙炔黑0.02g,Bi2O3 0.5g,PbO 0.3 g,包覆La(OH)3的Ca[Zn(OH)3]2·2H2O)0.2g。     

CuS在针状铁素体形核过程中的作用

设计出了研究夹杂物在针状铁素体形核过程中的作用的物理模拟试验方法。研究了CuS、Al2 O3 和Al2 O3 ·CuS在针状铁素体形核过程中的作用 ,用数字式金相显微镜观察了夹杂物界面附近区域的金相显微组织变化 ,用电子探针分析仪对夹杂物界面附近微区的化学成分进行了分析。研究结果表明 ,在热循环作用下 ,CuS和Al2 O3 ·CuS能使夹杂物与金属交界面附近出现贫Mn区 ,CuS与Al2 O3 ·CuS具有诱导针状铁素体形核的能力 ;而Al2 O3 不能使夹杂物与金属交界面附近区域的化学成分发生变化 ,Al2 O3不具有诱导针状铁素体形核的能力。夹杂物表面的CuS在针状铁素体形核过程中起着重要作用     

Assessment of the effect of fillets and special features of technology on the mechanical properties of brazed joints

The factors determining the contribution of fillets to the mechanical properties of brazed joints are systematically analysed. A model is proposed for evaluating the contribution of fillets in the tensile test. The mechanical properties of the joints are influenced by: the geometrical factor α, which determines the ratio of the fracture areas S₁ (fillet) and S₂ (joint) α = S₁/S₂; structural and technological factors which determine the chemical composition of the joint, fillets, and also their dimensions and structure. The contribution of the fillets to the mechanical properties of the joints may be equal to 10–70%. In brazing heat exchangers at α = 0.5–0.8, the contribution of the fillets to the mechanical properties does not exceed 10%, and the special features of the technology have a controlling effect.