Preparation of micro-arc oxidation coatings on magnesium alloy and its thermal shock resistance property

1. Introduction Due to their high specific strength, good electro-magnetic shielding characteristics, high damping characteristics, good cast ability, and excellent pol-ishing capability, magnesium alloys are extensively used in aeronautical, automobile, and electro- communication industries [1-3]. But magnesium has some disadvantages, such as low chemical stability, high negative electric potential, and low hardness, so it is necessary to use surface disposal to accommo-date the demand for re…     

Facile synthesis,morphology and structure of Dy<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles through electrochemical precipitation

The aim of this research was to introduce a new, facile and simple method for synthesis of Dy₂O₃ nanostructures at room temperature. For the first time, galvanostatic electrodeposition was used to synthesize Dy₂O₃ particles, and the influence of the current density on the structure and morphology of the product was studied. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET). The results show that the current density has little effect on the chemical composition but great effect on the structure and morphology of the samples. The average size of the particles decreases as the applied current density increases. The grain size of as-prepared samples decreases from 500 to 70 nm when the current density increases from 0.5 to 6.0 mA·cm^?2. To obtain oxide product, the as-prepared samples were heat-treated at 1,000 °C. The results show that the heat-treated samples have smaller particles. The XRD results show that the similar patterns are observed in the samples synthesized at different current densities, and the only difference from the JCPDS card is the ratio of peak intensities. With the increase in the current density, a decrease in the current efficiency is observed.     

The pitting of mild steel in phosphate-borate solutions in the presence of sodium sulphate

The localized corrosion of mild steel in neutral buffered solutions containing Na₂SO₁ was studied using potentiostatic and potentiodynamic techniques complemented by scanning electron microscopy. The breakdown potential changes linearly with the logarithm of Na₂SO₄ concentration. The logarithm of the induction time for pit initiation decreases linearly with the reciprocal of the applied potential, until the inhibition potential region is reached. Open circuit potential decay measured shows that the apparent thickness of the passive oxide film decreases at potential values more negative than the breakdown potential. The pitting current density is close to the diffusion limiting current of iron dissolution through a FeSO₄ film. Results suggest that two-dimensional FeSO₄ salt islands are formed before pitting, as it has been postulated for iron in halide-containing solutions.     

Influence of electrodeposition parameters on the deposition rate and microhardness of nanocrystalline Ni coatings

In this paper, nanocrystalline nickel (nc-Ni) coatings were prepared by a direct current electrodeposition technique. Their microstructure and microhardness were investigated by a high-resolution transmission electron microscopy and a microhardness tester. It is found that the electrodeposition parameters, including content of C₇H₄NO₃SNa?2H₂O, temperature and current density, have significant influences on the electrodeposition rate and microhardness of nc-Ni coatings. The electrodeposition rate increases with the current density stepwise. The largest electrodeposition rate is achieved at 60 °C. It decrease when the temperature is larger than 60 °C. The electrodeposition rate decreases with the increased content of C₇H₄NO₃SNa?2H₂O. The microhardnesses of the nc-Ni coatings are higher on the condition of the larger current density, lower temperature or higher content of C₇H₄NO₃SNa?2H₂O. But, it remains stable when the current density is in the range of 700-1000 A m^− 2. The relationship between the mean grain sizes and microhardness fits for the Hall-Petch function, approximately.     

电镀锌铁合金工艺及光亮剂的研究

介绍了一种硫酸盐电镀锌铁合金工艺及新型光亮剂。研究了镀液中FeSO4.7H2O浓度、锌铁总离子浓度、光亮剂用量、阴极电流密度、pH值、温度等因素对镀层含铁量的影响。结果表明:随着镀液中锌铁总离子浓度和温度的升高,镀层的含铁量降低;随着阴极电流密度和pH值的增大,镀层的含铁量先增加,后降低;随着FeSO4.7H2O浓度的升高,镀层的含铁量增加。探讨了新型光亮剂对镀层的影响,获得了最佳电镀方案。     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Content on Electrical Breakdown Properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Cu Composite

Al₂O₃/Cu composites were prepared by external addition of Al₂O₃, and the effect of Al₂O₃ content on microstructure, density, hardness, electrical conductivity and vacuum electrical breakdown properties was studied. The results show that with increasing Al₂O₃ addition, the density of Al₂O₃/Cu composite significantly decreases, the hardness sharply increases and then slowly decreases, but the electrical conductivity invariably decreases. The vacuum breakdown test shows that with increasing Al₂O₃ addition, the breakdown strength first sharply increases and then decreases when the Al₂O₃ content exceeds 1.2 wt.%; the chopping current always exhibits a decreasing trend and the arc life first increases and then decreases. According to the morphology of arc erosion and analysis, the arc erosion resistance increases and then decreases sharply. In the range of experiments, the optimal arc erosion resistance of Al₂O₃/Cu composite can be obtained with the addition of 1.2 wt.% Al₂O₃.     

Oxide contributions on arc plasma in tungsten inert gas welding of magnesium alloy

Abstract

To study the effects of oxide activating flux on the arc plasma, the investigation is carried out in tungsten inert gas (TIG) welding on magnesium alloy. In this study, five oxides, ZnO, MnO₂, Cr₂O₃, CeO₂ and CdO, are selected to study the roles of activating flux on the plasma characteristics (shape, electron temperature and electron density) and the arc voltage. The mechanism is also discussed by comparisons between single TIG welding processes with and without fluxes. Results reveal that the electron temperature of arc plasma decreases sharply under the influence of oxide; however, the electron density and the arc voltage get enhanced. The reasons for the increase in arc voltage are the greater electricity resistances that result from the improved welding current density and the constriction of conduction channel.     

Role of ClO4 in breakdown of tin passivity in NaOH solutions

The anodic behaviour of a tin electrode in NaOH solutions containing different concentrations of NaClO₄ was studied by employing potentiodynamic, potential transient under constant current density methods and complemented with scanning electron microscopy (SEM). In perchlorate-free NaOH solutions, the E/i response exhibits active/passive transition. The active region involves two anodic peaks corresponding to the formation of Sn(II) and Sn(IV) species respectively. The permanent passive layer is duplex and consists of SnO and SnO₂. Additions of NaClO₄ to the alkali solution, accelerates the active dissolution of tin and tends to breakdown the duplex passive layer at a certain breakdown potential. SEM examination confirms the occurrence of film breakdown. The breakdown potential decreases with an increase in ClO₄⁻ concentration, but increases with increasing both OH⁻ concentration and scan rate. The potential-time transients display that the incubation time for pit initiation decreases with increasing both ClO₄⁻ concentration and anodic current density.     

Current efficiency during the electrochemical machining of iron and nickel

Current efficiency determinations from weight-loss measurements were made on pure iron and pure nickel anodes in 4M NaClO₃ solution in a flow cell at flow rates between 500 and 3000 cm/s in a current range from 5 to 50 A/cm². The current efficiency for metal removal was virtually independent of current density and flow rate on iron anodes. On nickel anodes the current efficiency increased strongly with current density. In the high current density region, the current efficiency decreases with flow rate up to 2000 cm/s and then increases with higher flow rates. This behavior was accounted for by differences in the nature and properties of the anodic films formed on iron and nickel anodes.     

Surface damage during transient thermal load of 50% thickness reduced W-2% (Vol.) Y2O3 sheet with different recrystallization volume fraction

Thermal shock damage of tungsten as a plasma facing material (PFM) depends on thermal shock power density level, duration and repeated time, and microstructure of the sample. The recrystallization process will degrade the mechanical property of material and thus change the its thermal shock resistance. The effects of recrystallization volume fraction on thermal shock response of W-Y₂O₃ under different power density levels (0.22–0.44 GW/m²) has been systematically studied. Electron beam pulse of duration of 1 ms with 100 recycles was used to simulate the transient thermal load of fusion device. The changes of morphology, distance, depth, width of crack and surface roughness on the rolling direction-normal direction (RD-ND) surfaces of W-Y₂O₃ samples with different recrystallization volume fraction were investigated. The results showed that recrystallization process have significant influence on the thermal shock resistance of W-Y₂O₃ samples. For the rolled sample, crack depth, width and surface roughness increased with the increased of power density level while crack distance decreased. The partially and fully recrystallized samples showed significant wider crack networks and severe surface modification.     

The influence of pulse parameters on Zn-Ni alloy coatings plated on AZ91 magnesium alloy

The influence of pulse parameters on zinc-nickel coatings plated on AZ91 magnesium alloy is investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The particles of zinc-nickel alloy plated on AZ91 magnesium alloy become smaller and the surface roughness decreases with the increase of current density and frequency. However, long plating times and a high ratio of t_on/t_off have decremental effects on the particles and surface roughness. The contents of crystal phases of zinc-nickel alloy coatings are higher under pulse current deposition than under direct current. The frequency, current density, plating time and ratio of t_on/t_off have different impacts on the thickness and Ni content of zinc-nickel coatings.     

Influence of high-pressure deformation and annealing on the structure and properties of a bulk MgB<Subscript>2</Subscript> superconductor

A synthesized MgB₂ superconductor has been investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and by the measurements of the superconducting characteristics and microhardness after cold high-pressure deformation in a Toroid chamber and in Bridgman anvils and subsequent high-temperature annealing. A nanocrystalline structure is formed in the superconductor after high-pressure treatment, but internal cracks appear, and the critical current density decreases strongly. The annealing leads to a coarsening of the structure and to an increase in the critical current density up to 5.8–6.7 × 10⁴ А/сm², which is more than three times greater than that in the initial state.     

Die instationäre Korrosion des passiven Eisens in saurer Lösung

The instationary corrosion of passive iron in acid solution The corrosion rate of the passive iron, i.e. the dissolution rate of the passivating layer, in 1 n H₂SO₄ is investigated, with the layer thickness being increased and decreased, respectively (non-stationary conditions). The corrosion rate is determined chemical-analytically as well as coulometrically on the basis of galvanostatic measurements. Both methods yield the same result. The corrosion current density, i_K, increases as the formation current density, i_S, of the layer increases, and decreases the more below the stationary corrosion value, i_K, the higher the layer reduction current density (i_S <0) is. i_K and i_S, are independent of the electrode Potential of the passive iron electrode, similar to the behaviour of the stationary value, i_K,0. This independence is explained by the semiconductor properties of the passivating oxide (γ-Fe₂O₃). There is an unique correlation between i_K and i_Ss which can be explained also theoretically by a variable overvoltage at the phase boundary oxidelectrolyte. The kinetics of the corrosion and of the layer formation is evaluated from the values of the apparent charge transfer coefficients by referring to the p_H dependence of the stationary corrosion.     

Shock-Induced Microstructures in Explosively Fabricated Superconductors

Scanning and transmission electron microscopy (including electron diffraction) have been applied to examine the consolidation, bonding, and characterization of orthorhombic, superconducting YBa₂Cu₃O₇ powder particles in explosive (shock-wave) fabricated aluminum matrix monoliths. The shock wave front (at pressures of approximately 4 GPa) roughly creates an order of magnitude greater density of microtwins and other defects in the orthorhombic structure, which is unaltered by the shock-wave compression. Some evidence for interparticle melting and rapid solidification at large interparticle voids is apparent in small, scattered amorphous regions in the dynamically consolidated superconducting powder regimes. Observations of large d.c. current carrying capacities and current densities estimated to be > 10³ A/cm² in zero applied magnetic field at 77K in practical electrical configurations suggest prospects for very high cirtical current densities resulting from strong flux pinning effects in the YBa₂Cu₃O₇ superconductor at shockwave induced defects.     

The role of corrosion-resistant alloying elements in passivity

Passivity of alloys containing corrosion-resistant elements were reviewed. Chromium and valve metals except aluminum form stable oxyhydroxide films even in aggressive hydrochloric acids. Molybdenum forms a passive MoO₂ film in the active region of stainless steels and hence decreases the active dissolution current. In the passive region of transition metals and valve metals, molybdenum is generally in the transpassive state and dissolved. However, if the outer oxyhydroxide film is stable the inner MoO₂ film is protected by the outer oxyhydroxide film and the MoO₂ film acts as the effective barrier against diffusion of matters through the film. Thus the passive current density of 30Cr-2Mo ferritic stainless steel is more than two orders of magnitude lower than that of 30Cr steel without molybdenum in 1 M HCl.     

The influence of sodium tungstate concentration and anodizing conditions on microarc oxidation (MAO) coatings for aluminum alloy

Microarc oxidation (MAO) coatings on 5052 aluminum alloy are prepared in silicate–hypophosphite electrolytes with sodium tungstate. The effects of sodium tungstate concentrations and current density on the surface morphology, phase composition and properties of the coatings are investigated. With the addition of sodium tungstate in the electrolyte and increase of current density, the final voltage at the microarc discharge process increases. The results also show that the MAO coatings are composed mainly of α-Al₂O₃ and γ-Al₂O₃ and the proportion of α-Al₂O₃ and γ-Al₂O₃, pore size, surface roughness as well as thickness of the coatings strongly depend on the sodium tungstate concentration and current density. Thus, the hardness, friction coefficient and corrosion resistance of the coatings are significantly influenced by the magnitude of the current density and sodium tungstate concentration. These oxide films on aluminum were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thickness gage, and polarization curves, respectively.     

Effect of Al2O3 content on the synthesized products by the laser ignited self-propagating high-temperature synthesis reaction of Al2O3–Ti–C system

The effect of Al₂O₃ content on ignition temperature and combustion temperature, the phase composition, the density of the products and the grain size of TiC was investigated by self-propagating high-temperature synthesis reaction of Al₂O₃–Ti–C system. The results show ignition temperature increases and combustion temperature decreases with the increasing of Al₂O₃ content; the density of the products varies with Al₂O₃ content, TiC and Al₂O₃ are the two stable phases after SHS, TiC particle size decreases with the increasing of Al₂O₃ content, furthermore, the fracture type of the sintered specimens is a nearly completely intergranular mode.     

Effect of spherically converging stress waves on the phase composition,structure, and physicochemical transformations of the mixture of aluminum and quartz powders

The phase composition and structure of a mixture of aluminum and quartz powders taken in a ratio of 1:1 have been studied after loading by spherical converging shock waves. A number of concentric layers (zones) have been observed in a meridian section of the sample after shock-wave loading. The pressures in the converging shock wave and in the diverging shock wave reflected from the center of the sample have been estimated as a function of the radial position of Lagrangian particles on the basis of the calculation of pressure profiles P(R, t). The data on the phase composition of different zones are considered in respect to the pressures induced by the converging and diverging shock waves at their boundaries. It has been established that pressures below ～45 GPa cause only additional compacting of the material and deformation of aluminum and quartz. In this case, the quartz grain size substantially decreases up to the transition into the X-ray amorphous state. The attainment of a pressure of ～45 GPa initiates the solid-state reaction of SiO₂ decomposition, which leads to the precipitation of pure silicon and the evolution of oxygen. The beginning of the silicon precipitation and the chemical reaction of Al₂O₃ formation are separated over the pressure scale. The critical pressure, which is necessary for the solid-state chemical reaction of the Al₂O₃ formation is about 50 GPa.     

植酸修饰改性对TC4钛合金微弧氧化膜层性能的影响

通过向电解液中添加有机酸植酸,提升了TC4钛合金微弧氧化涂层的耐腐蚀性能和热稳定性。通过扫描电子显微镜、X射线衍射仪、X射线光电子能谱仪和热冲击实验等技术手段,分析了植酸对涂层形成、形貌和性能的影响。结果显示,植酸的添加使放电微孔更加细小,提高了涂层的形成效率并优化了相结构。通过动电位极化测试,发现添加植酸显著提高了微弧氧化涂层的耐腐蚀性能。将电解液中的植酸浓度调整为12 mL/L（最佳植酸浓度）后,腐蚀电流密度由8.406×10^-5 A·cm^-2降低至2.580×10^-6 A·cm^-2。循环高温氧化试验结果表明,TC4钛合金的耐热冲击性能和高温抗氧化性能得到了改善。     

Effect of metallic phase content on mechanical properties of （85Cu- 15Ni）/（10NiO-NiFe2O4） cermet inert anode for aluminum electrolysis

（85Cu-15Ni）/（10NiO-NiFe2O4） cermets were prepared with Cu-Ni mixed powders as toughening metallic phase and 10NiO-NiFe2O4 as ceramic matrix. The phase composition, microstructure of composite and the effect of metallic phase content on bending strength, hardness, fracture toughness and thermal shock resistance were studied. X-ray diffraction analysis indicates the coexistence of （Cu-Ni）, NiO and NiFe2O4 phases in the cermets. Within the content range of metallic phase from 0% to 20% （mass fraction）, the maximal bending strength （176.4 MPa） and the minimal porosity （3.9%） of composite appear at the metallic phase content of 5%. The fracture toughness increases and Vickers＇ hardness decreases with increasing metal content. When the thermal shock temperature difference （At） is below 200 ℃, the loss rate of residual strength for 10NiO-NiFe2O4 ceramic is only 8%, but about 40% for （85Cu-15Ni）/（10NiO-NiFe2O4）cermets. As At is above 200 ℃, the residual strength sharply decreases for sample CN0 and falls slowly for samples CN5-CN20.