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This study investigated the corrosion protection by electrodeposited copper layer on AZ31 Mg alloy with and without additional silver layer. The single copper layer electrodeposited on AZ31 from copper pyrophosphate bath exhibited nodular structure and crevices between the nodules which resulted in the rapid initiation of pitting corrosion. Employing an additional electrodeposited silver layer onto the copper deposited AZ31 alloy decreased the number of imperfections and changed structure of electrodeposited copper from nodular to fine particle type resulting in improved corrosion resistance. This study further revealed that triple-layer coating of Cu/Ag/Cu exhibited the most efficient corrosion protection of AZ31 Mg alloy.

We report that alloying with Y remarkably enhances the corrosion resistance of extruded Mg–Al–Ca-based alloy. The corrosion rates, measured by immersion tests in 0.6 M NaCl solution at 25 °C, were 0.31 and 1.84 mm y −1 for the Mg–6Al–1Zn–0.5Ca–0.3Mn (wt.%) alloys with and without 0.25 wt.% Y, respectively. The improved corrosion resistance is mainly attributed to a change in the type of Al-containing intermetallic particles toward reducing their electrochemical nobility, resulting in reduced H 2 evolution rate during corrosion.

In this study, the effects of processing history on corrosion behaviours of pure Mg with controlled impurity content, were analysed systematically. Results revealed that impurity Fe and Si were precipitated into the Fe-Si phase by heat treatment prior to extrusion, leading to severe micro-galvanic corrosion of as-extruded pure Mg despite the same impurity contents with as-cast pure Mg. This result suggests that control of the actual tolerance limit, considering not only the impurity content but also the processing history, is required in designing alloys.

Microstructure and corrosion properties of extruded AZ61 Mg alloys with varying Ca contents were investigated. Microstructure of the extruded AZ61 alloy consisted of α-Mg grains and Al 8 Mn 5 particles. Alloying with 0.1–0.5 wt% Ca to the AZ61 alloy resulted in the additional presence of (Mg,Al) 2 Ca particles in the extruded microstructure. In 0.6 M NaCl solution at 25 °C, the extruded AZ61 alloys with 0.1–0.5 wt% Ca showed corrosion rates over 2 times faster than that of the AZ61 alloy without Ca addition. Such deteriorated corrosion resistance due to Ca addition is discussed in terms of passivation and microgalvanic corrosion.

In this work, effect of zincate treatment of AZ91 Mg alloy on the following electrodeposition of copper was examined in a non-cyanide bath containing pyrophosphate ions in view of surface morphology and adhesion of the electrodeposited copper layer. Without zincate treatment, the electrodeposited copper layer showed very porous structure and poor adhesion. On the other hand, the copper layer electrodeposited on the zincate-treated surface showed dense structure and good adhesion. The dissolution rate of AZ91 Mg alloy after the zincate treatment appeared to decrease about 40 times in the copper pyrophosphate bath, as compared to that of the surface without zincate treatment. The porous morphology and poor adhesion of a copper layer on the AZ91 Mg alloy surface without zincate treatment were attributed to small number of nucleation sites of copper because of rapid dissolution of the magnesium substrate in the pyrophosphate bath. Based on the experimental results, it is concluded that the zincate treatment to form a conducting and protecting layer on the AZ91 Mg alloy surface is essential for successful electrodeposition of a copper layer on AZ91 Mg alloy with good adhesion and dense structure in the copper pyrophosphate bath.

Copper electrodeposition on AZ91 Mg alloy was studied in views of preferential deposition on α- or β-phases and how to achieve uniform deposition over the entire surface on α- and β-phases in a cyanide solution. The inhomogeneous microstructure of AZ91 Mg alloy, particularly α- and β-phases, was found to result in non-uniform deposition of zincate layer, preferential deposition of zincate on β-phases, which leads to non-uniform growth of copper layer during the following electrodeposition process. The preferential depositions of zincate can be attributed to higher cathodic polarizations on the β-phases. Pin-hole defects in the copper electrodeposit were observed at the center of large size β-phase particles which is ascribed to gas bubbles formed at the β-phases. The activation of AZ91 Mg alloy in hydrofluoric acid solution was used to obtain uniform growth of zincate layer on both the α- and β-phases. By choosing an optimum activation time, a uniform zincate layer was obtained on the AZ91 Mg alloy surface and thereby uniform growth of copper was obtained in a cyanide copper electroplating solution.

The corrosion behaviours of commercial AZ91D alloy and Ca–Y-containing AZ91 alloy, fabricated by casting, were evaluated, and their microstructural changes and surface properties were analysed. The Ca–Y-containing AZ91 alloy showed superior corrosion resistance compared with the AZ91D alloy mainly because of the decrease in the Volta potential difference between the matrix and the secondary phases formed in the Ca–Y-containing alloy, such that micro-galvanic corrosion was significantly suppressed. Furthermore, passivity and stability of the surface film improved with the addition of Y to the AZ91 alloy. Thus, the overall corrosion resistance was significantly improved in the Ca–Y-containing AZ91 alloy.

Corrosion behavior of extruded Mg and Mg5Sn (in wt%) alloy was investigated, and a passivated surface of the Mg5Sn was examined. Immersion test in a 0.6 M NaCl solution revealed that corrosion of the Mg was significantly faster than the Mg5Sn alloy, and vigorous H2 evolution was accompanied with the severe corrosion of Mg. Polarization tests confirmed that cathodic reaction was inhibited in the Mg5Sn alloy. It was considered that the decreased H2 evolution of the Mg5Sn alloy was attributed to the formation of Sn-accumulated layer between the passive film and matrix, which was examined by an XPS analysis.

Microstructure and corrosion properties of extruded Mg–8Sn–1Zn alloys with varying Al contents were investigated. Microstructures of the extruded alloys mainly consisted of α-Mg grains and Mg2Sn particles in common. Addition of Al to the ternary alloy resulted in the formation of different types of Al–Fe-based intermetallic particles depending on Al contents. In 0.6 M NaCl solution, the extruded alloys with 1–3 wt% Al showed over 30 times higher corrosion rates than those containing less than 0.3 wt% Al; this discrepancy was attributed to the presence of Al5Fe2 particles acting as strong cathodic sites for H2 evolution during corrosion.

We present simulations of the nucleation and equiaxed dendritic growth of the primary hexagonal close-packed $$\mathrm{\alpha} $$ -Mg phase followed by the nucleation of the $$\mathrm{\beta} $$ -phase in interdendritic regions. A zoomed-in region of a melt channel under eutectic conditions is investigated and compared with experiments. The presented simulations allow prediction of the final properties of an alloy based on process parameters. The obtained results give insight into the solidification processes governing the microstructure formation of Mg-Al alloys, allowing their targeted design for different applications.

The effects of microstructural factors, including fraction of second phase particles and solute content in a matrix, on the corrosion behaviour of Mg–5Sn–xZn (x = 1, 3 wt%) alloys were evaluated by using potentiodynamic polarisation and immersion tests. The hydrogen evolution rate and the corrosion potential increased with increasing Zn content, while the passive current density decreased. The average corrosion rate measured by immersion test increased with increasing Zn content, and was consistent with the hydrogen evolution rate. The increase of the corrosion rate with increasing Zn content resulted from increasing fraction of Mg2Sn particles and Zn content dissolved in the matrix.

The corrosion behavior of as-extruded Mg–5Sn–(1–4)Zn (in wt%) alloys in a 0.6 M NaCl solution were investigated using an immersion test, a polarization test, and a zero resistance ammeter technique, and the corrosion properties of the alloys were discussed in consideration of the microstructure. The immersion test showed that the Mg–5Sn–2Zn alloy exhibited the lowest dissolution rate among the alloys, and further addition of Zn increased the corrosion rate. The electrochemical test results revealed that the overall corrosion rate of the passivated Mg–5Sn–(1–4)Zn alloys showed a strong correlation with the H2 evolution rate rather than the stability of the passivity.

The corrosion product films formed on the surface of Mg–2Zn and Mg–5Zn alloys in NaCl solution were investigated by electrochemical measurement, scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) analysis. It is found that a compact corrosion product film is formed in the initial stage of immersion, and then the film gradually degrades due to dissolution reaction. The product film formed on Mg–2Zn alloy presents better protection property than that on Mg–5Zn alloy, which can be attributed to the different chemical composition and microstructure of the both alloys.

The corrosion behaviour of extruded Mg–(2–8 wt%)Sn alloys was investigated in aqueous chloride solutions, and the influences of the volume fraction of Mg 2 Sn intermetallics, solutionised Sn, and area fraction of grain boundaries on the corrosion behaviour were discussed. The Sn addition to α-Mg increased the solutionised Sn content in the matrix, increased the volume fraction of Mg 2 Sn intermetallics to 7.3%, and changed the grain size distribution. Regarding the corrosion behaviour, the Sn addition up to 8 wt% increased the overall dissolution rate of the alloys, although it promoted the passivity, which resulted from the increased H 2 evolution rate and increased numbers of initiation sites for pitting corrosion. The overall corrosion behaviour primarily depended on the amount of Mg 2 Sn intermetallics, which promoted passivity and notably increased the H 2 evolution rate, and functioned as pitting corrosion initiation sites. As the area fraction of grain boundaries increased, the H 2 evolution rate was also accelerated. In contrast, the solutionised Sn decreased the H 2 evolution rate.

The effects of Zn on corrosion behaviour of as-extruded Mg-(1-4)Zn alloys were investigated using an immersion test, a zero resistance ammeter technique, and a potentiodynamic polarisation test. As a result, it was revealed that the solutionised Zn enhanced protectiveness of the passive film, and accelerated the H2 evolution rate of the Mg–Zn binary alloys. The acceleration of the H2 evolution rate by addition of Zn leads to an increase in the net corrosion rate of the Mg–Zn alloy. In this research, the polarisation test was found to have some limitations for evaluating the true corrosion behaviour of passive Mg–Zn alloy.

The naturally formed oxide films on Mg–2Zn and Mg–5Zn alloys were investigated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. The oxide films on the both alloys present a similar chemical composition, consisting of surface layer of basic magnesium carbonate and MgO following with MgO and ZnO, but the oxide film on Mg–5Zn is thicker and contains more defects. The protection performance of the oxide film on Mg–5Zn is worse under open circuit potential but better in a suitable anodic potential scope compared with that on Mg–2Zn alloy.

The effect of Zn concentration on the corrosion behavior of Mg-xZn alloys is investigated. The microstructure is observed by scanning electron microscopy (SEM). The corrosion resistance is evaluated by electrochemical and immersion tests. The initiation and propagation of corrosion are analyzed based on the electrochemical measurements and corrosion morphologies observation. It is found that the MgxZny second phases are precipitated on the boundaries and their volume fractions rise with increasing Zn concentration. As a result, the corrosion resistance of Mg-xZn alloys reduces with increasing Zn concentration, which is greatly associated with the micro-cathodic effect of the MgxZny second phases. (C) 2012 Elsevier Ltd. All rights reserved.

The MgxZny second phases in Mg-5Zn alloy were adjusted by T4 and T6 treatment, and then the role of second phases in the corrosion behavior of Mg-5Zn alloy was investigated. The dimension of the MgxZny second phases is only some hundred nanometers. They present discrete distribution on the boundaries of the as-received sample and continuous distribution in the T6 sample. The T4 sample in absence of second phases exhibits the best corrosion resistance while the T6 sample with the largest volume fraction of second phases exhibits the worst one, which can be attributed to the micro-cathodic effect of second phases. (c) 2012 Elsevier Ltd. All rights reserved.

Mg–23.5 wt.%Ni–(5, 10 and 15 wt.%)La alloys were prepared by gravity casting and their hydrogen-storage properties were examined after pulverizing. The gravity cast Mg–23.5Ni–(5, 10 and 15)La alloys consist of α-Mg, Mg2Ni and Mg17La2 phases. The activated Mg–23.5Ni–10La alloy has the highest hydrogen-storage capacity of 4.96 wt.%H (from P–C–T curve) and the highest initial hydriding rate (hydrogen content 3.83 wt.%H at 10 min) with an initial hydrogen pressure in the channel of 11 bar H2 at 573 K. This is attributed to its containing the largest amount of the Mg17La2 phase, which is easily dissociable during the hydriding reaction.

ZrCl4 was added into Mg-0.3Al model alloy in order to reduce the Al content in the castings below 0.005 mass%. Amount of ZrCl4 was determined by thermodynamic calculation. Various Al-Zr intermetallic compounds were formed in the melt at 1013 K. Large intermetallic compounds would be floated to melt surface by Ar bubbling and then removed by skimming before pouring. Smaller ones remained after Ar bubbling and isothermal holding would be partially removed by filter inserted into a mold during pouring. The Al content in the castings was higher than that predicted by thermodynamic calculation due to incomplete removal of Al-Zr intermetallic compounds from the melt. [doi:10.2320/matertrans.MRP2008246]

The hydriding properties of Mg– x Ni ( x = 13.5, 23.5 and 33.5 wt%) alloys with different microstructures produced by various processing routes were evaluated in this study. The hydrogen storage capacity and kinetics of hydriding of Mg– x Ni alloys were strongly dependent on their microstructures. The capacity and kinetics of hydriding were larger and faster when the average size of the hydriding phase was smaller and the volume fraction of the phase boundary was larger.

The effect of Ca addition on the corrosion resistance of AZ31 magnesium alloys was evaluated by observation of microstructure and measurement of corrosion potential and average corrosion rate. The main mechanism of corrosion of AZ31+xCa (x ¼ 0� 5 mass%) alloys was galvanic-corrosion between � -Mg and second phase. But the propagation behavior of corrosion was different with Ca content. In the alloy containing below 0.7 mass%Ca, a micro-galvanic cell formed between matrix � -Mg and second phase formed semi-continuously at grain boundaries and the corrosion progressed in transgranular mode. The propagation of corrosion was retarded when the corrosion front met the second phase acted as corrosion barrier. But in the alloys containing above 1 mass%Ca, a micro-galvanic cell formed between eutectic � -Mg and discontinuous second phase in the eutectic region and the corrosion rapidly progressed along eutectic � -Mg formed continuously at grain boundaries because the discontinuous second phase could not prohibit the propagation of corrosion effectively.

Recently application of magnesium alloys is expanded rapidly over the whole industry, especially in automotive and electronic industries. Most of magnesium alloy parts are produced by casting process and the quality control of castings is very important to reliability and durability of parts. Various properties of magnesium alloy castings are deteriorated by defects such as non-metallic inclusion and pore. 1–5) Especially control of non-metallic inclusion remained in magnesium alloy castings is very important because the possibility of entrance of non-metallic inclusion into melt is very high due to high reactivity of magnesium alloy melt with gas species above melt surface. There are various methods for removal of inclusions in melt including addition of flux, gas bubbling and filtration. 6–8) Formerly a method of addition of flux was mainly applied to remove inclusions from magnesium alloy melt. 9–11) But part of flux was entered into melt during pouring when flux could not be entirely removed as dross and/or sludge before pouring and the flux remained in casting degraded the quality of castings as harmful defect. So a method of bubbling of inert gas into melt has been paid attention as new method for removal of inclusions from melt, especially during steelmaking. 12–15) But studies on removal of inclusions from magnesium alloy melt by gas bubbling have not been carried out actively. In this study the effect of Ar gas bubbling on the removal of inclusions from AZ91D magnesium alloy melt is evaluated indirectly by measurement of tensile elongation of gravity mold castings of AZ91D magnesium alloy and analyzed qualitatively.

In this study, the effects of Ca content and processing variables on the external surfaces and tensile properties of hot-rolled AZ31-xCa sheets were evaluated systematically. The number and length of side crack were decreased with increasing preheating temperature and decreasing reduction ratio per pass and Ca content. The UTS and YS were not strongly dependent on the Ca content but the elongation decrease with increasing Ca content. The amount of decrease in elongation due to increase in Ca content was least when the sheets were fabricated under preheating temperature of 400 � C and reduction ratio per pass of 15%. The sheets had the sound external features with little side cracks by homogenization of gravity cast AZ31-xCa alloys before hot rolling. In the cases of AZ31-xCa alloys containing under 1 mass% Ca, the annealed sheets after homogenization and hot rolling had the similar tensile properties to those of AZ31 sheet.

Semi-solid processing is an emerging technology for near net-shape production of engineering components. Although there has been significant progress in semi-solid processing of Al alloys, very limited information is available on semi-solid processing of Mg alloys, except for the thixomolding process. In semi-solid casting process, it is necessary to properly control the flow and solidification behavior of semi-solid slurries for high quality products. There are a number of parameters that affect the flow and solidification behavior of semi-solid slurries such as viscosity, casting pressure, shapes of gate and mold cavity, gate velocity, mold temperature, etc. In the present study, the effects of various thermo-mechanical treatments were investigated on the change in viscosity of the semi-solid AZ91D magnesium alloys by using a concentric cylinder type viscometer. The effects of gate velocity and thickness on mold filling behavior of the semi-solid AZ91D alloys were also investigated by using a high-speed camera and the results were compared with those obtained from computer simulations. From these results and microstructure examination, a processing map for high pressure die casting of the semi-solid AZ91D alloy was constructed in order to produce sound castings.