Determination of ohmic/voltage drop and factors influencing anodic overvoltage of carbon anodes in Na_3AlF_6-Al_2O_3 based melts

1　INTRODUCTIONAlthoughtheanodicprocessandmeasurementofanodicovervoltageincryolite aluminameltshavebeenthesubjectsofnumerousinvestigations ,problemssuchasthereactionmechanismandtherate deter miningstepoftheanodereactionarestillcontrover sialandneedclarification .Theexperimentalinforma tionischaracterisedbymajordivergenceregardingthemagnitudeandthetypeofanodicovervoltage ,whichismostlikelycausedbythefollowingfac tors[16 ] :1)Theanodicovervoltagediffersamongtheva rietyofcarbonmaterials,sucha…     

Electrocatalysis of carbon anode in aluminium electrolysis

The anodic overvoltage of the carbon anode in aluminum electrolysis is of the order of 0.6 V at normal currem densities. However, it can be reduced somewhat by doping the anode carbon with various inorganic compounds. A new apparatus was designed to improve the precision of overvoltage measurements. Anodes were doped with MgAl2O4 and AlF3both by impregnation of the coke and by adding powder, and the measured overvoltage was compared with that of undoped samples. For prebake type anodes baked at around 1150℃, the anodic overvoltage was reduced by 40-60 mV,and for Soderberg type anodes, baked at 950 ℃, by 60-80 mV.     

Electrochemical dissolution behaviour of iron in dilute sodium hydroxide solution

The anodic dissolution and passivation behaviour of pure iron in 0.1N sodium hydroxide solution has been investigated using a galvanostatic pulse technique. Analysis of the resulting chronopotentiograms has yielded information on the transfer overvoltage/dissolution current relationship and the passivation behaviour of iron in this environment. These results have been used to identify a possible reaction mechanism for the dissolution and have been related to current theories of stress corrosion cracking in this system.     

STUDY OF ANODIC OVERVOLTAGE IN NEODYMIUM ELECTROLYSIS

The anodic overvoltage of neodymium electrolysis was determined by slow scanning oscillograrn. The effects of some factors, i.e. the temperature, the anodic current density, the concentration of Nd2O3 and the components of the electrolyte were investigated and the approaches to decrease the anodic overvoltage were also discussed. The results show that the anodic overvoltage increases with the anodic current density and decreases with the increasing temperature. The linear relation between the anodic overvoltage and the current density corresponding to Tafel equation is determined to some extent. The anodic overvoltage decreases with the increasing concentrations of LiF and NdF3. It also decreases by controlling the anodic current density properly, increasing the temperature or the concentrations of LiF and NdF3 and the reducing polar distance.     

Ring-disc studies of titanium-palladium alloy corrosion

Ring-disc measurements have been made over the potential range 90 to ?500 mV in sulphuric acid solution. The titanium corrosion current for TiPd (0·2%) is similar to that of pure Ti initially but decreases markedly with time over a 24-h period. The decrease is explained by an accumulation of loosely adherent palladium particles (size ～ 0·025 μm) on the alloy surface which have the effect of lowering the hydrogen overvoltage and inhibiting the dissolution.     

Hydrogen overvoltage at manganese in ammonium-bromide-perchloric solution

Kinetic characteristics of electrochemical hydrogen evolution at manganese rotating disc electrode in ammonium-bromide-perchloric solution at 18, 40, and 60°C are determined (the overvoltage, Tafel slopes, exchange currents, and activation energy). The zero-charge potential is found. Mechanism of the electrode process is suggested. It is shown that S-shaped bends in the polarization curves of hydrogen evolution can be entailed with changes in the electrode surface charge. The overvoltage and current density corresponding to beginning of the activationless range of electrochemical hydrogen evolution are determined.     

Wear and corrosion resistance and electroplating characteristics of electrodeposited Cr–SiC nano-composite coatings

Cr–SiC nanocomposite coatings with various contents of SiC nanoparticles were prepared by electrodeposition in optimized Cr plating bath containing different concentrations of SiC nanoparticles. Direct current electrocodeposition technique was used to deposit chromium layers with and without SiC nanoparticles on mild carbon steel. The effects of current density, stirring rate and concentration of nanoparticles in the plating bath were investigated. Scanning electron microscopy was used to study surface morphology. Energy dispersive analysis technique was used to verify the presence of SiC nanoparticles in the coated layers. The corrosion behaviors of coatings were investigated by potentiodynamic polarization and electrochemical impedance spectroscopy methods in 0.05 mol/L HCl, 1 mol/L NaOH and 3.5% NaCl (mass fraction), respectively. Microhardness measurements and pin-on-disc tribometer technique were used to investigate the wear behavior of the coatings.     

脉冲激励远场涡流管道检测技术的有限元仿真

研究了脉冲激励下的远场涡流管道检测技术,采用有限元仿真的方法对检测系统参数对检测结果的影响做了详细的分析。表明该技术将远场涡流和脉冲激励的优势有效结合,能提取较多检测信息,可同时测量管道内径和内、外壁缺陷信息,且具有信号幅值高,功耗低的优点。     

逆变弧焊机中IGBT的防护

本文介绍了ＩＧＢＴ逆变弧焊机中的过电压、过电流保护电路，包括外部过电压、内部过电压、输出过电流和ＩＧＢＴ器件过电流的防护措施。文中还就驱动电路的应用进行了讨论，以提高ＩＧＢＴ器件的使用可靠性。     

Current deflection NDE for the inspection and monitoring of pipes

Routine inspection of oil and gas pipes for time dependent degradation is essential. Pipelines are most commonly inspected using In-Line Inspection (ILI), however restrictions from pipe geometry, features or flow rate can prevent its use. Facility pipework rarely facilitates ILI, and external inspection often warrants the undesirable removal of the pipe insulation and cladding. This work investigates the applicability of a current deflection non-destructive evaluation technique for both the detection and growth monitoring of defects, particularly focusing on corrosion. Magnetic sensors are used to monitor variations in the spatial distribution of the induced magnetic flux density outside a pipe that arise from deflection of an injected electric current around inner or outer wall defects. An array of orthogonal magnetoresistive sensors has been used to measure the magnetic flux density surrounding six-inch schedule 40 seamless and welded carbon steel and austenitic steel pipes. The measurements were stable and repeatable to the order of 100 pT which suggests that the defect detection or growth monitoring of corrosion-type defects may be possible with a few amps of injected current when measurements are taken at around 50 mm lift-off. The sensitivity of the technique is dependent on factors including defect geometry, sensor lift-off, bends, variations in nominal pipe geometry or material properties, and the presence of ferromagnetic objects, each of which were investigated using either experiment or a validated finite element model.     

电沉积Ni-S合金作碱液电解活性阴极的研究进展

电沉积Ni-S合金作为碱液电解阴极材料,具有很高的析氢反应活性,能够降低析氢反应的过电位.基于文献调研并结合作者的工作,归纳了Ni-S合金的制备、晶体结构、析氢反应活性及其析氢反应机理等方面的研究状况.     

Detection of SCC initiation in austenitic stainless steel by electrochemical noise measurements

The electrochemical noise (EN) measurement technique is one of the most promising tools for continuous in situ corrosion monitoring in technical systems with a certain potential to be used for the detection of stress corrosion cracking (SCC). To evaluate the suitability of the EN technique for the detection of SCC initiation, a small but systematic test programme was started, performing EN measurements on type 304 austenitic stainless steel during constant extension rate tensile tests in aqueous thiosulphate solution at room temperature. SCC could be detected by EN measurements, which was verified by interruptions of the experiments at different stages, by testing steel with different degrees of sensitisation and by post‐test fractography in the scanning electron microscope. Conclusions on the cracking mechanism could be drawn based on the current noise signal pattern.     

Wavelet analysis of electrochemical noise measurements during corrosion of austenitic and superduplex stainless steels in chloride media

A wavelet variance analysis procedure has been used for the characterisation of electrochemical noise measurements (ENM) made during corrosion of three grades of austenitic and superduplex stainless steels, with varying sulphur contents, when exposed to chloride media. The resulting electrochemical noise has been interpreted in terms of coupled current and free corrosion potential measurements. The wavelet variance exponent has been used to characterise the behaviour of the signals. It is shown that the wavelet analysis technique is able to discriminate between various corrosion processes covering a wide range of ENM.     

Rapid Assessment of Oxidation Behavior in Co-Based γ/γ′ Alloys

A high-throughput, non-destructive photostimulated luminescence spectroscopy (PSLS) technique was used to analyze fifty oxidized Co-based γ/γ′ alloy samples for the presence of α-Al₂O₃. Alloys were produced by combinatorial ion-plasma deposition, and oxidation was performed at 1100 °C for 1 h in air. PSLS measurements are compared with microscopy of oxides in cross-section to relate the presence of the luminescence signal of α-Al₂O₃ with the thickness of the oxide scale. Analysis of the current dataset validates the use of PSLS as a rapid screening technique of oxidation behavior for the present materials system.     

Magnetic field techniques for the inspection of steel under concrete cover

The results of a study comparing residual magnetic field measurements to magnetic flux leakage (MFL) measurements as methods to detect broken pre-stressing steel are presented. Analysis of two- and three-dimensional magnetic field plots shows that the residual magnetic field technique has strong potential in this application, with detectable signals from a single broken wire on a seven strand cable being found up to 70 mm from the cable surface. MFL measurements with the same yoke produced detectable signals only when the cable was completely severed, indicating that the yoke was not driving the cable into saturation during the magnetisation process. However, the residual field technique was also found to be very sensitive to the technique used to magnetise the object being inspected, indicating that considerable care would be necessary to use the technique successfully in the field.     

An Experimental Study of Microstructure-Property Relationships in Thermal Barrier Coatings

The thermal-mechanical properties of thermal barrier coatings are highly influenced by the defects present in coating microstructure. The aim of this study was to meet the future needs of the gas turbine industry by further development of zirconia coatings through the assessment of microstructure-property relationships. A design of experiments was conducted for this purpose with current, spray distance, and powder feed rate as the varied parameters. Microstructure was assessed with SEM and image analysis was used to characterize porosity content. Evaluations were carried out using laser flash technique to measure thermal properties. A bi-layer beam curvature technique in conjunction with controlled thermal cycling was used to assess the mechanical properties, in particular their nonlinear elastic response. Coating lifetime was evaluated by thermo-cyclic fatigue testing. Relationships between microstructure and coating properties are discussed. Dense vertically cracked microstructure and highly porous microstructure with large globular pores were also fabricated. Correlations between parameters obtained from nonlinear measurements and lifetime based on a priori established microstructural analysis were attempted in an effort to develop and identify a simplified strategy to assess coating durability following sustained long-term exposure to high temperature thermal cycling.     

Application of the electrochemical microcapillary technique to study intergranular stress corrosion cracking of austenitic stainless steel on the micrometre scale

The electrochemical microcapillary technique was applied for the first time to study the electrochemical dissolution signals from single growing cracks due to stress corrosion cracking of thermally sensitised AISI 304 stainless steel in potassium tetrathionate solution. Potentiostatic current measurements on initiating cracks along grain boundaries were performed. Typical current signals of potentiostatic and potentiodynamic measurements consisted of a series of current peaks showing fast rise and exponential decay, sometimes interrupted by passive phases. The results indicate that stress corrosion crack growth is a discontinuous process of passive and active phases, which might be explained by the film rupture model.     

Pulsed remote field technique used for nondestructive inspection of ferromagnetic tube

Remote field eddy current (RFEC) technique is an effective method for measurement of ferromagnetic tube. However, traditional RFEC is unable to differentiate the internal and external defect and the probe has a long length. Pulsed eddy current techniques excite the induction coil with a pulsed waveform and have the richness of frequency harmonics. The wideband excitation is thought to be a potential in providing more information about the flaw. In this paper, pulsed RFEC technique is used to inspect ferromagnetic tube. The finite element analysis and experiment method is used to give a thorough analysis of the influence effect with the variations of the system parameters. Results show that this technique effectively combines the advantages of RFEC and pulsed excitation, which not only acquires more inspection information, including measurements of inner diameter of tubes, internal and external defects, but also reduces the length of probe and power consumption. The agreement between simulation and experiment shows that the present method is correct.     

EQUIVALENT CIRCUIT OF NONLINEAR EFFECT OF SPECTRAL IP

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