Effect of magnesium hydride on the corrosion behavior of an AZ91 magnesium alloy in sodium chloride solution

The effect of magnesium hydride on the corrosion behavior of an as-cast AZ91 alloy in 3.5 wt.% NaCl solution was investigated using gas collection method and potentiostatic test. The Pourbaix diagram of Mg–H₂O system was built using thermodynamic calculation. It was possible that magnesium hydride could form in the whole pH range in theory. The experimental results showed that at cathodic region, magnesium hydride formed on surface, which was the controlling process for the corrosion behavior of AZ91 alloy; at anodic region and free corrosion potential, magnesium hydride model and partially protective film model, monovalent magnesium ion model and particle undermining model were responsible for the corrosion process of AZ91 alloy.     

Effects of potential, temperature and pH on hydrogen absorption and thermal desorption behaviors of Ni-Ti superelastic alloy in 0.9% NaCl solution

The effects of electrochemical potential, solution temperature and pH on the hydrogen absorption and desorption behaviors of Ni-Ti superelastic alloy immersed in 0.9% NaCl solution for 2 h have been investigated systematically by hydrogen thermal desorption analysis. For hydrogen cathodic charging under constant applied potential, upon increasing solution temperature, the critical potential for the hydrogen absorption very slightly shifts to a noble direction. As solution pH decreases, the critical potential for the hydrogen absorption shifts markedly to a noble direction and approaches to the corrosion potential; the critical current density for the hydrogen absorption slightly decreases. At a less noble potential than the critical potential for the hydrogen absorption, the amount of absorbed hydrogen increases markedly with decreasing applied potential. For hydrogen cathodic charging under constant current density, the amount of absorbed hydrogen increases with increasing solution temperature and decreasing solution pH. The basic hydrogen desorption behavior only slightly depends on solution temperature or pH. Nevertheless, hydrogen desorption at low temperatures for specimens subjected to cathodic charging under constant current density is observed distinctly as compared with that under constant applied potential.     

Kinetics and mechanisms of nickel metal dusting I. Kinetics and morphology

The kinetics of nickel metal dusting were investigated at different gas compositions at 680 °C. The carbon uptake rate on nickel exposed to supersaturated H₂/CO/H₂O gas mixtures (nominal a_C = 19) increased as p_CO increased from 0.31 to 0.68 atm, but decreased with further increase in p_CO. This behaviour was ascribed to the existence of parallel, independent reaction paths, and the rate was well described by

     

Volume size factor and lattice parameter in cubic intermetallics with the L12 structure

Lattice parameters of binary ordered L1₂ intermetallic compounds have been calculated using partial molar volumes of the individual elements in the disordered solid solutions. The calculations have been performed using the volume size factor of the solute atom and the molar volume of the solvent atom. It has been observed that this approach results in excellent agreement between calculated and experimental values. The approach has been successfully extended to the prediction of lattice parameters of ordered ternary L1₂ compounds.     

镧对镁吸氢和放氢动力学及热力学性能的影响

研究纳米La2O3对Mg颗粒的助磨作用以及吸氢和放氢的催化作用,对克服Mg吸氢和放氢动力学差的问题有重要意义。Mg粉中添加1.0 mol.%的La2O3于球磨后,XRD分析表明Mg晶粒为49.7 nm,而纯Mg球磨后的晶粒为51.6 nm,La2O3使球磨后Mg晶粒的更小,这有利于提高Mg的吸氢和放氢速率。SEM观测表明,球磨后La2O3纳米晶粒与Mg晶粒紧密接触,呈镶嵌结构,这有利于La2O3对Mg吸氢和放氢发挥催化作用。吸氢和放氢动力学测试表明,添加La2O3使Mg吸氢和放氢速率加快,吸氢反应活化能降低49.8 kJ/mol,放氢反应活化能降低23.1 kJ/mol,即显著地了改善Mg吸氢和放氢的动力学性能。p-c-T等温吸附平衡测试表明,添加La2O3球磨后Mg的吸氢活性更高,平衡氢压较低。DSC放氢分析表明,添加La2O3后MgH2放氢温度有所降低。硬度比Mg大的La2O3纳米颗粒在球磨过程中起助磨作用,不与Mg反应;在加氢和放氢过程中La2O3起催化作用,不与H2反应。     

Formation of hydrogen blister on AZ91 magnesium alloy during cathodic charging

The process of hydrogen blister formation on as-cast AZ91 magnesium alloy was investigated in 0.1 M NaOH solution using galvanostatic test. The results showed that cathodic charging of AZ91 alloy resulted in hydrogen blister formation, and blister rupture caused the nucleation and propagation of transgranular micro-cracks around the ruptured blister.     

Corrosion of heat treated magnesium alloy ZE41

This paper investigates the effect of heat treatment upon the corrosion morphology and mechanism of ZE41 alloy. The results of optical and scanning electron microscopy (SEM) together with potentiodynamic polarisation reveal the importance of the microstructure in the initiation and propagation of corrosion in an aqueous environment. The corrosion of the heat-treated alloy is significantly altered due to changes in the microstructure, specifically the Zr-rich regions and the grain boundary T-phase.     

Single-crystal growth of the complex metallic alloy phase β-Al–Mg

The development of a single-crystal growth route for the complex metallic alloy phase β-Al₃Mg₂ is presented. After initial probing of the phase diagram in the vicinity of the existence range of the β-phase, we performed single-crystal growth experiments employing various techniques. The Czochralski and self-flux growth turned out to be the most suitable for this phase, and with both we reproducibly achieved single crystals of several cubic centimeters in volume. While the Czochralski technique allows for the production of deliberately oriented single crystals, the self-flux technique is capable of producing very large but unoriented single grains.     

Corrosion of friction stir welded magnesium alloy AM50

The microstructure of a friction stir welded magnesium alloy AM50 was examined by means of optical light microscopy. The chemical composition, particularly the iron content, and morphology of the oxide film were analyzed and discerned via auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Corrosion behaviour of the welds and base materials were investigated by virtue of neutral salt spray tests and potentiodynamic polarization measurements in conventional cells and in a mini cell. The results demonstrate that minor increases in iron concentration as might be speculated to occur as a consequence of tool/work piece interaction during the welding process on the corrosion resistance of the weld can be ignored. The corrosion morphology was predominantly influenced by the distribution of the Mg₁₇Al₁₂ phase. Here, it was also found that the corrosion resistance of the friction stir weld varied in response to changes in the joint microstructure.     

The Al–R–Mg (R=Gd, Dy, Ho) systems. Part I: experimental investigation

Key experiments were carried out on the three Al–R–Mg (R=Gd,Dy,Ho) systems and the results obtained used for the thermodynamic optimisation reported in a separate paper in this issue [Caccasmani G, De Negri S, Saccone A, Ferro R. Intermetallics this issue.]. The samples were characterized by differential thermal analysis (DTA), X-ray powder diffraction (XRD), light optical microscopy (LOM), scanning electron microscopy (SEM) and quantitative electron probe microanalysis (EPMA). The isothermal sections at 400 °C are all characterized by extended homogeneity regions at a constant rare earth content. The extension of the (Mg,Al)R solid solution, cP2-CsCl type, varies with the R atomic number. Ternary compounds (τ) of Al₂(R,Mg) stoichiometry (hexagonal Laves phases with MgNi₂-type structure) have been found to exist at 400 °C in all the systems. Their temperatures of formation were detected by DTA measurements.     

Electrodeposition of a protective copper/nickel deposit on the magnesium alloy (AZ31)

An environmental-friendly Cu electrodeposition process was proposed for the Magnesium alloy (AZ 31). Experimental results show that a good bonding between Cu deposit and Mg alloy surface can be achieved with a pretreatment of galvanostatic etching and then copper electrodeposition in the alkaline copper-sulfate plating bath. Microstructures between Cu deposit and Mg alloy substrate were examined with scanning electron and energy-filtering transmission electron microscopes (SEM and EF-TEM). The Cu-deposited Mg alloy can be further electroplated in acidic Cu and Ni plating baths to acquire a protective Cu/Ni deposit.     

Oxidation behaviour of molten magnesium and AZ91D magnesium alloy in 1,1,1,2-tetrafluoroethane/air atmospheres

The oxidation rates, the morphologies and compositions of the surface films on molten magnesium and AZ91D alloy in covering gas of 1,1,1,2-tetrafluoroethane/air at 760 °C were investigated using thermogravimetric techniques, SEM, XRD and XPS. The oxidation rates of the two melts followed parabolic law in the atmosphere of high concentrate 1,1,1,2-tetrafluoroethane, and obeyed linear law in the atmosphere of low concentrate 1,1,1,2-tetrafluoroethane. But the oxidation rates of molten magnesium and AZ91D alloy presented a certain difference. The oxidation products on its surface films were composed of MgF₂ which played an import role to prevent the molten oxidation and other compounds.     

Hydrogen storage properties of Mg100−xNix (x = 5, 11.3, 20, 25) composites prepared by hydriding combustion synthesis followed by mechanical milling (HCS + MM)

We reported the structure and the notable hydrogen storage properties of the composites Mg_100−xNi_x (x = 5, 11.3, 20, 25) prepared from metallic powder mixtures of magnesium and nickel by the process of HCS + MM, i.e., the hydriding combustion synthesis (HCS) followed by mechanical milling (MM). X-ray diffraction (XRD) and scanning electron microscopy (SEM) results demonstrated that mechanical milling led to drastic pulverization and grain refinement of the composite produced by HCS. All the composites with different compositions showed a remarkable decline in dehydriding temperature comparing with that of the hydride mixtures prepared only by HCS. Furthermore, the hydriding rates of these composites were excellent. At 313 K the composite Mg₈₀Ni₂₀ showed the highest hydrogen capacity of 2.77 wt.% within 600 s among these four composites. The Mg₉₅Ni₅ showed maximum capacity of 4.88 wt.% at 373 K and 5.41 wt.% at 473 K within only 100 s. Some factors contributing to the improvement in hydriding rates were discussed in this paper.     

Corrosion-induced hydrogen embrittlement in aluminum alloy 2024

The present paper focuses on the observed corrosion-induced embrittlement of alloy 2024 and tries to answer the key question on whether the observed embrittlement is attributed to hydrogen uptake and trapping in the material. Hydrogen is produced during the corrosion process and is being trapped in distinct energy states, which correspond to different microstructural sites. The formation of a hydrogen-affected zone beneath the corrosion layer is supported by fractographic analysis. Removal of the corrosion layer leads to complete restoration of yield strength but only partial restoration of ductility. Additional heat treatment to release the trapped hydrogen leads only to complete restoration of ductility.     

Stannate and permanganate conversion coatings on AZ31 magnesium alloy

The formation of stannate and permanganate–phosphate conversion coatings on AZ31 magnesium alloy was investigated in situ by EIS measurements and their protective performances were studied by different electrochemical techniques in diluted (0.05 M) sodium sulphate solution.The influence that short or long treatment times exert on the performances of such conversion coatings is discussed. While permanganate–phosphate baths always built layers characterized by penetrating cracks, long stannate baths produced layers without interconnected porosity, but were defective. This accounted for the initial greater protectiveness achieved with the stannate treatment; nevertheless, the easy penetration of the electrolytic solution through such a layer quickly decreased its corrosion resistance.     

States and transport of hydrogen in the corrosion process of an AZ91 magnesium alloy in aqueous solution

Mott–Schottky measurement and secondary ion mass spectroscopy (SIMS) were used to investigate the states and transport of hydrogen during the corrosion behavior of an AZ91 magnesium alloy in 0.1 M sodium sulfate solution. The results showed that when samples were immersed or charged in solution, hydrogen atoms diffused into the film and reacted with vacancy to cause the increases of the carrier concentration (excess electron or hole carrier) and diffusion rate of hydrogen. Some hydrogen atoms diffused to interior of matrix and enriched in β phase while others resorted in the corrosive film. With the increase of immersion or charging time, magnesium hydride would be brittle fractured when the inner stress caused by hydrogen pressure and expansion stress of formation of magnesium hydride was above the fracture strength, which provided the direct experimental evidence of the hydrogen embrittlement (HE) mechanism of magnesium and its alloys. After immersion in solution, the transfer of excess electrons to the interfaces of corrosion film and solution would destroy the charge equilibrium in the film and stimulate the adsorption of , which resulted in the initiation of localized corrosion; after cathodic charging and then immersion, the enrichment of hydrogen atoms at interior of corrosion film would combine into hydrogen gas to form high pressure and result in the rupture of corrosion film, and localized corrosion initiated and developed at surface. Therefore, localized corrosion nucleated earlier on the charged samples than on the uncharged samples. Hydrogen invasion accelerated the corrosion of matrix.     

Nanocrystalline Al3Ni2 alloy with high hardness produced by mechanical alloying and high-pressure hot-pressing consolidation

An elemental powder mixture corresponding to Al₃Ni₂ phase stoichiometry was subjected to mechanical alloying. A metastable nanocrystalline AlNi intermetallic phase with the mean crystallite size of 12 nm was formed upon milling. Heating of the synthesised powder in a calorimeter up to 720 °C caused phase transformation into an equilibrium Al₃Ni₂ intermetallic phase with the mean crystallite size of 41 nm. The product of mechanical alloying was consolidated at 1000 °C under the pressure of 5 GPa and 7.7 GPa. During consolidation, a phase transformation analogous with the one observed in the course of heating in the calorimeter took place. Both bulk materials have nanocrystalline structure with mean crystallite size of 67 nm and 58 nm, the smaller one in the sample consolidated under the higher pressure. The hardness of the produced Al₃Ni₂ intermetallic is 8.81 GPa (898 HV1) and 8.72 GPa (887 HV1), while the specific yield strength, estimated using the Tabor relation, is 624 kNm/kg and 617 kNm/kg for the sample hot-pressed under 5 GPa and 7.7 GPa respectively. On the basis of the obtained results, we can assume that the quality of consolidation with preserving a nanocrystalline structure is satisfactory and the hardness as well as the estimated specific yield strength of the produced materials are relatively high.     

Plasma anodized ZE41 magnesium alloy sealed with hybrid epoxy-silane coating

Anodic coatings on magnesium ZE41 alloy were formed by DC plasma electrolytic oxidation (PEO) in spark regime in solution composed of NaOH, Na₂SiO₃ and KF. The positive effect of poly(ethylene oxide) addition into the anodizing electrolyte on PEO process, anodic film porosity and its protective performance was described. Anodic films were sealed with hybrid epoxy-silane formulation. The corrosion behavior of the coated ZE41 was studied through electrochemical impedance spectroscopy (EIS) in 0.6 M NaCl solution. Resulting duplex PEO/epoxy-silane coating provides good protective performance without significant signs of corrosion during 1 month of immersion test.     

新型钛钒系贮氢电极合金

研究了新型钛钒系贮氢电极合金Ti0.8Zr0.2V2.665Mn0.535Cr0.8Ni的相结构、微观组织及电化学性能。XRD及EDS分析表明：铸态合金主要由C14 Laves相母体和树枝晶的钒基固溶体相组成，同时由于成分偏析的缘故，合金中还存在少量的TiNi基的第三相。热处理使得合金中C14 Laves相及钒基固溶体相的晶胞参数和晶胞体积增大，促进合金成分的均匀化，同时极大地改善了合金电极的综合电化学性能。