显微组织对Ni-Cr合金热腐蚀行为的影响

采用机械合金化法和真空熔炼法制备了不同显微组织的Ni-20Cr（质量分数，％，下同）合金，并研究其在950℃，75％Na2SO4＋25％K2SO4（质量分数，下同）盐膜下的热腐蚀行为。结果表明：细晶的机械合金化Ni-20Cr合金热腐蚀后表面形成了单一连续的Cr2O3外氧化膜，有效地阻止了硫的向内扩散，延长了热腐蚀过程的孕育期，呈现出较普通晶粒尺寸的熔炼Ni-20Cr合金更好的抗热腐蚀性能。     

Effect of NaCl vapor on the oxidation of Ni-Cr alloys

Ni-Cr alloys are known for their resistance to high temperature oxidation. The kinetics of scale formation and the nature of the scale in these alloys are affected by NaCl liquid or vapor. There have been a few investigations dealing with the influence of NaCl on long-time exposure. But the nature of reaction at short times can provide information on the initiation of such attack. In this investigation, Ni-Cr alloys with Cr varying from 0 to 25 wt% were exposed to NaCl vapor at 850°C for a few minutes. The surface chemistry of these alloys along with the unattached ones was analyzed by Auger electron spectroscopy. The nature of scale and the distribution of chlorine was found to vary with the Cr content in the alloys, which has a direct bearing on the rate of oxidation of these alloys in NaCl vapor.     

Compositional changes in the underlying alloy produced by the oxidation of Ni-Cr alloys

Compositional changes in the alloy beneath scales have been examined for the oxidation of Ni-27.4%Cr and Ni-40.2% Cr in 1 atm oxygen in the temperature range 1073–1473°K. Calculations of the rate of approach of the interfacial alloy composition to a constant value are compared with experimental data. Theoretical chromium depletion profiles obtained using both a finite difference analysis and an analytical expression are shown to be essentially equivalent and in good agreement with experimental measurements. The consequences of alloy depletion for the scaling behavior, when the protective scale is ruptured, are discussed.     

Investigations of the aqueous corrosion of pretreated magnesium alloys by means of electrochemical noise measurements

An important possibility to improve the corrosion behaviour of magnesium alloys is the application of protective coatings. The quality of such coatings depends mainly on the pretreatment and the exposure conditions after pretreatment. Since magnesium surfaces change much faster under atmospheric conditions than those of almost any other technical material, it is necessary to pay special attention to this particular feature. The activity of acid‐pickled surfaces of the magnesium alloys AZ31 and AZ91 in dependence on the exposure time and the humidity conditions was investigated with electrochemical noise (EN) measurements. In addition to pickling, plasma chemical vapour deposition processes open new possibilities for an economical, as well as ecologically quite safe, pretreatment. The results of EN investigations after acid pickling as well as specific plasma oxidation treatments of the two magnesium alloys after exposure to air with different humidities are presented.     

The attack of Co-Cr alloys by Ar-SO2 atmospheres

Cobalt alloys containing up to 25% chromium have been exposed to Ar-10% SO₂ atmospheres at temperatures between 600 and 1000° C. The results show that, although an increase in chromium content leads to a reduction in the reaction rate, even to negligible rates in the cases of the higher chromium contents, all of the alloys are eventually subjected to rapid attack at more or less longer times, depending on the chromium content. The mechanism of the reaction appears to involve the formation of a more or less protective oxide layer which is eventually penetrated by sulfur. The sulfur forms chromium sulfides at the metal-scale interface, removing the chromium from solution and causing an expansion that cracks the protective scale, allowing both the ingress of gas and the formation of rapidly growing cobalt compounds. The process occurs rapidly with Co-5% Cr alloys, whereas, only the initial sulfur penetration is observed with Co-25% Cr alloys during the time scale of the investigation. The penetration of sulfur is thought to occur as a molecular gas species permeating through the scale down physical defects.     

Electrochemical behavior of Co-Cr and Ni-Cr dental cast alloys

The cast structures influencing the electrochemical corrosion behavior of Co-Cr and Ni-Cr dental alloys were studied using potentiodynamic polarization and AC impedance in 0.9% (mass fraction) NaCl solution at (37±1) °C. The phase and microstructure of the alloys that were fabricated using two different casting methods viz. centrifugal casting and high frequency induction casting, were examined using X-ray diffraction analysis, scanning electron microscopy and energy dispersive spectroscopy. The roles of alloying elements and the passive film homogeneity on the corrosion resistance of Co-Cr-Mo and Ni-Cr-Mo dental cast alloys were reviewed. The results of electrochemical study show that the dependence of corrosion resistance on the microstructure associated with the casting methods is marginal. The Co-Cr alloy exhibits more desirable corrosion resistance properties than the Ni-Cr alloy. There is severe preferential dissolution of Ni-rich, Cr and Mo depleted zones in the Ni-Cr alloy.     

Oxidation of high-chromium Ni-Cr alloys

The oxidation of binary Ni-Cr alloys containing 44 and 50 wt. % Cr has been studied over a range of oxygen partial pressures at temperatures between 800 and 1100°C. The effects of cold work, surface preparation, and distribution of the Cr-rich second phase have been studied. The oxidation behavior is complex and cannot be described by a single model. The oxide grows by short-circuit diffusion as well as bulk transport through Cr ₂ O ₃ scales. The scale-growth mechanism includes extensive metal-oxide separation requiring Cr vapor transport to the scale, compressive stresses within the oxide which result in scale bulging and cracking, and the formation of a second oxide layer which results in voids being incorporated into the scale. Any factor which reduces the oxide grain size, such as cold work, finer distribution of the Cr-rich phase or reduced oxygen pressure, results in an increased oxidation rate of binary alloys because of an increased number of grain-boundary short-circuit diffusion paths.This work is based on a portion of a thesis by G. M. Ecer submitted to the University of Pittsburgh in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Metallurgical and Materials Engineering.Formerly graduate student. Department of Metallurgical and Materials Engineering. University of Pittsburgh.     

Evaluating electrochemical behaviour of recrystallized titanium alloys in Ringer's solution

By anodic potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques, the effect of recrystallization on the corrosion and electrochemical behaviour of Ti12Mo and Ti12Mo5Ta alloys was studied. Aerated Ringer's solution was employed as electrolyte solution at 37 °C. The pH was 6.9 and adjusted to 2.5 simulating the disease state. The results show excellent corrosion resistance for recrystallized Ti12Mo and Ti12Mo5Ta alloys, corroborated by high values of polarization resistance and low values of passive current density. Recrystallized structure rendered the Ti12Mo and Ti12Mo5Ta alloys extremely corrosion resistant.     

Electrochemical behaviour of recasting Ni-Cr and Co-Cr non-precious dental alloys

Corrosion behaviour of dental alloys in artificial saliva was studied using different chemical and electrochemical techniques. The order of corrosion rate for the three alloys in artificial saliva is: wironit < wirolloy < wiron99. This order agrees with the results of chemical studies for determining the cumulative ion concentration using ICP/MS. The open-circuit potential of wirolloy is more positive than wiron99. The higher corrosion rate of wiron99 compared to wirolloy is due to presence of high concentration of Mo. Increasing casting number leads to decrease R_ct value and increasing C_dl.     

Ni-Cr/BN烧结工艺过程探讨

研究了烧结温度对Ni-Cr/BN组织和性能的影响。结果表明BN的加入降低了材料的烧结温度;随着烧结温度的提高材料的强度、硬度和密度都有所提高;该材料的合适烧结温度在1000℃-1100℃之间。     

The hot-corrosion behavior of novel CO-deposited chromium-modified aluminide coatings

This paper reports the successful co-deposition of inclusion-free chromiummodified aluminide coatings using a pack-cementation process. The substrate used was the nickel-base superalloy, René 80H. The coatings were of the outward-diffusion type; however, unlike the usual outward-diffusion coatings, the present coatings were relatively free of pack inclusions. The coatings consisted of -Cr precipitates in a matrix of -NiAl. The morphology and distribution of the -Cr precipitates could be adjusted to the extent that two types of coating structures could be obtained. The Type I coating structure contained lamellar -Cr precipitates situated in the surface region of the coating, whereas the Type II coating structure contained small, spheroidal -Cr precipitates distributed throughout the outer of a two-layered coating. Both coating types exhibited significantly improved hot-corrosion resistance in a 0.1% SO₂-O₂ environment at 900°C compared to a commercial aluminide coalting. A study of the corrosion behavior of Type I coatings containing pack inclusions showed that the inclusions were deleterious to the corrosion resistance of the coatings. The corrosion behavior of chromium-aluminide coatings was dependent on both the distribution and amount of -Cr precipitates in the coating.     

FeCo基大块非晶合金盐雾与电化学腐蚀研究

通过电化学测试方法与中性盐雾腐蚀实验研究Fe24 xCo24-xCr15Mo14C15B6Y2 (X=0, 2, 4, 6, 17)块体非晶合金的耐蚀性。合金在1 mol/L HCl中出现了宽的钝化区,其EIS图谱均由单一容抗弧组成,显示出良好耐蚀性;结合极化曲线与交流阻抗结果分析可得合金耐蚀性随着Co含量的增加先增大后减小,当Co含量为20%时合金耐蚀性最好。利用能谱分析(EDS)与原子力显微镜(AFM)对合金的中性盐雾腐蚀结果进行分析,可知它们的腐蚀产物主要由铁和钴的氧化物以及它们的氯化物组成。同时FeCo基大块非晶合金腐蚀程度随着Co含量的变化而变化,其盐雾腐蚀耐蚀性规律与电化学腐蚀耐蚀性规律一致。     

The hot corrosion of directionally solidified Ni-Cr-Nb-Al (γ/γ′-δ) eutectic alloys

The hot-corrosion resistance of two /- eutectic alloys, Ni-23.1 Nb-4.4 Al and Ni-19.7Nb-6Cr-2.5 Al, has been studied using three techniques: (1) a salt-coated test, in which the specimens are coated with a thin layer of Na₂SO₄ and oxidized; (2) the Dean rig test, in which salt is evaporated into the gas stream and allowed to condense continuously on the specimen; and (3) a sulfidation/oxidation test, in which the specimens were presulfidized for a short time in a H₂/H₂S mixture and then oxidized. The salt-coated test suggested that these alloys are generally quite resistant to sodium-sulfateinduced corrosion, although localized severe attack was sometimes encountered. The morphology of the corrosion products suggested that the mechanism of the attack was sulfidation/oxidation, and that salt fluxing mechanisms were relatively unimportant. In the Dean rig test it was shown that sodium chloride greatly increased the severity of attack, as has been shown earlier for alumina-forming superalloys.     

Corrosion behavior of Ti‐xNb‐13Zr alloys in Ringer's solution

Ti‐6Al‐4V alloy has been widely used in restorative surgery due to its high corrosion resistance and biocompatibility. Nevertheless, some studies showed that V and Al release in the organism might induce cytotoxic effects and neurological disorders, which led to the development of V‐free alloys and both V‐ and Al‐free alloys containing Nb, Zr, Ta, or Mo. Among these alloys, Ti‐13Nb‐13Zr alloy is promising due to its better biomechanical compatibility than Ti‐6Al‐4V. In this work, the corrosion behavior of Ti, Ti‐6Al‐4V, and Ti‐xNb‐13Zr alloys (x = 5, 13, and 20) was evaluated in Ringer's solution (pH 7.5) at 37 °C through open‐circuit potential measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy. Spontaneous passivity was observed for all materials in this medium. Low corrosion current densities (in the order of 10^?7 A/cm²) and high impedance values (in the order of 10⁵ Ωcm² at low frequencies) indicated their high corrosion resistance. EIS results showed that the passivating films were constituted of an outer porous layer (very low resistance) and an inner compact layer (high resistance), the latter providing the corrosion resistance of the materials. There was evidence that the Ti‐xNb‐13Zr alloys were more corrosion resistant than both Ti and Ti‐6Al‐4V in Ringer's solution.     

Effect of commercial AB5 alloy addition on the electrochemical properties of Ml-Mg-Ni-based hydrogen storage alloys

A commercial AB5 hydrogen storage alloy was used as an additive to improve the electrochemical properties of Ml-Mg-Ni-based hydrogen storage alloys.The effect of AB5 alloy addition on the phase structure,charge/discharge characteristics,and electrochemical kinetics of Ml0.90Mg0.10Ni3.08Mn0.13Co0.63Al0.14 alloy was investigated.The maximum discharge capacity of Ml0.90Mg0.10Ni3.08Mn0.13Co0.63Al0.14 + 4 wt.% AB5 electrode reaches 406 mAh/g.The anodic polarization,cyclic voltammetry,and potential step discharge experiments show that the electrochemical kinetics of the electrode with additives was promoted,with the LaNi5 phase of AB5 alloy acting as electro-catalytic sites in the electrode alloy.The high-rate dischargeability of Ml0.90Mg0.10Ni3.08Mn0.13Co0.63Al0.14 + 4 wt.% AB5 alloy electrode at 1100 mA/g reaches 60.9%,which is 9.4% higher than that of Ml0.90Mg0.10Ni3.08Mn0.13Co0.63Al0.14 alloy electrode.The cycling stability of the electrode with 4 wt.% AB5 alloy has also been improved.     

Corrosion performance of magnesium alloys MEZ and AZ91

The corrosion performance of sand cast MEZ_S, zirconium-grain-refined MEZ_R, sand cast AZ91_S, and high pressure diecast AZ91_D magnesium alloys were evaluated by means of salt spray testing, optical metallography, hydrogen evolution, polarisation curve measurement and AC impedance spectroscopy. The results show that the corrosion resistance of the four alloys can be ranked in decreasing order as AZ91_D > AZ91_S ≈ MEZ_R > MEZ_S and that the intergranular phases and chemical composition of the matrix phase have a significant influence on the corrosion performance. Alloys with a finer grain size and higher aluminum or zirconium contents exhibit better corrosion resistance.     

Electrochemical monitoring of high-temperature molten-salt corrosion

Hot molten-salt corrosion can cause serious metal degradation in boiler plant, incinerators, and furnaces. In this research, electrochemical-impedance and electrochemical-noise techniques have been evaluated for the monitoring of hot-corrosion processes in such plants. Tests have been carried out on Ni-1 % Co and Alloy 800, a commercial material of interest to operators of industrial plants, utilizing a bulk molten-salt environment and also a simulated combustion test, where thin films of molten salt were established on the alloy surfaces. Electrochemical-impedance and electrochemical-noise data were compared with the results of metallographic examination of the test alloys and showed reasonable correlation between the electrochemical data and the actual degradation processes. Current-noise analysis gives valuable information on the initiation stages of hot corrosion, while impedance measurements can detect propagation or rapid corrosion of the base metal. This preliminary work indicated that the electrochemical techniques show considerable promise as instruments for the monitoring of high-temperature corrosion processes.     

Preliminary electrochemical testing of some Zr–Ti alloys in 0.9% NaCl solution

The aim of this study is to investigate the corrosion behaviour of three ZrTi alloys (denoted with Zr5Ti, Zr25Ti, and Zr45Ti) in 0.9% NaCl solution. For comparison, cp‐Ti was also investigated. In order to study the localized corrosion resistance and corrosion behavior at open circuit potential versus time, the open circuit potential (E_OC) was recorded, and the cyclic potentiodynamic polarization (CPP) and electrochemical impedance spectroscopy (EIS) were performed. Scanning electron microscopy (SEM) observations were made following the CPP tests. The Zr5Ti alloy was the most susceptible to localized corrosion. The Zr25Ti alloy presents a dangerous breakdown potential but have a sufficiently negative zero corrosion potential that the difference between them is sufficiently to provide a higher localized corrosion resistance in comparison with Zr5Ti. Among ZrTi alloys subjected to investigation, the Zr45Ti alloy had a much larger passive range in the polarization curve and was the most resistant to localized corrosion. For used test conditions, the localized corrosion was not found for the cp‐Ti. The EIS tests show that both investigated ZrTi alloys and cp‐Ti exhibit passivity after 168 h immersion in 0.9% NaCl solution, at open circuit potential.     

含锆镁基储氢合金的合成及其电化学性能

用固相扩散法合成了Mg2-xZrxNi (x=0, 0.2, 0.3, 0.4, 0.5, 0.6)系列合金. XRD结构分析表明 主相仍为Mg2Ni; Zr替代Mg使合金的微晶结构发生了改变, 趋于非晶化; 添加Zr的合金的容量比由扩散法合成的合金的容量有所提高; 合金中添加Zr, 提高了电极的放电容量, 增大了放电平台; 特别是Zr 对六方晶系Mg2Ni合金结构中Mg的部分取代大幅度提高了电极的循环寿命, 未经任何处理的 Mg1.4Zr0.6Ni合金电极经25个循环后, 放电容量仅衰减4.11%, 表明添加Zr对提高合金的循环寿命具有重要的作用.