Materials Science Forum Vols. 488-489

Abstract: Arc-aided glow discharge plasma penetrating technique is a new surface coating method. With the help of vacuum arc discharge, a cold cathode arc source continually emits ion beams of coating elements with high currency density and high ionizing ratio. As the ion bombard working on, nitrogen is let in the chamber simultaneously. Then the surface of the parts formed ceramic coatings. Under lab condition, a commercial magnesium alloy Az91 had been coated with TiN film layer with the aim of improving the corrosion resistance. This paper mainly summarizes the testing and analyzing of the coating layer. The composition and microstructure of the coating layer had been analyzed by means of X-ray diffraction (XRD) and glow discharge spectrum (GDS), and the surface appearance had been surveyed by scanning electronic microscope (SEM). The adhesion strength between film and matrix had been evaluated by experiments of sticking-tearing. The results indicated that the coated layer on magnesium alloy were homogeneous, dense and robustly adherent.

Abstract: The synergistic fluoropolymer self-lubrication coatings on magnesium alloys were studied. Magnesium alloy was firstly treated by electroless nickel plating. Then micropores of electroless nickel plating layer was enlarged. Then the fluoropolymer particles were infused into micropores, precise vacuum heat treatment ensured thorough infusion with fluoropolymer, finally the synergistic coating was formed. The hardness of the coating reached 600HV, the dynamic coefficient of friction of the coating was less than 0.15, and the low coefficient of friction could be kept during a long duration wear to realize the permanent dry lubrication. The corrosion resistance was also excellent to endure 5% neutral salt spray test for 500 hours. Also the structure and composition of the coating were observed and studied.

Abstract: The article introduces the ceramic membrane technology by micro arc oxidation on Mg-Alloy surface and its properties. The important factors of technical parameters on the properties of ceramic membrane are analyzed. The formation and prevention of defects during the process are also discussed.

Abstract: Effects of nickel coating on electrochemical properties of Mg2Ni hydrogen storage alloys were presented in this paper. X-ray diffraction ( XRD) and scanning electron microscope (SEM) were employed to examine the crystal structure and surface morphologies of the as-obtained bare and Ni-coated Mg2Ni alloys and their electrochemical properties were characterized by cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS). Results showed that Ni coating not only decreased charge transfer resistance, but also decreased H atom diffusion resistance for Mg2Ni alloys. It was also found that Ni coating effectively improved the discharge capacity.

Abstract: Aluminum arc spray process was employed to form an aluminum coating on AZ31 magnesium alloy substrate. Anti-corrosion property of the Al-coated specimens was evaluated by salt immersion tests and electrochemical tests. There was no apparent inter-layer diffusion or compound layer formation between the aluminum coating and the substrate under as-sprayed condition. To improve the density of the coating and the bonding strength between the coating and the substrate, a heat treatment process was conducted after spraying to form an interlayer by metallurgic diffusion. The interlayer consisted of large primary crystal Mg17Al12 phase and minute lamella structure that consisted of Mg17Al12 and α-Mg. The hardness of both the aluminum coating and the interlayer are higher than that of the substrate. In electrochemical experiments, the corrosion current density decreased from 2.4×10-1mA/cm2 of AZ31 to 7.1×10-3mA/cm2 of heat-treated specimen. It suggests that the corrosion resistance of AZ31 can be greatly improved by aluminum arc spray with a following heat treatment process.

Abstract: A powder material of Mg-Ni-Cu-Y was synthesized by mechanical alloying method. Considerable amounts of amorphous and nanocrystalline phases formed after a mixture of elemental powder with the overall composition of Mg78Ni9Cu11Y2 was mechanical alloyed for 200 h. The phase formed in the synthesized powder materials was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The electrochemical properties were examined with a solution of 6M KOH.

Abstract: The coating formed on AM50 magnesium alloy by microarc oxidation (MAO), using DC pulse power in Na2SiO3 solution. The oxide coating contains a loose layer and a compact layer, the loose layer consisting of MgO, Mg2SiO4 and MgSiO3 phases and the compact layer mainly of the MgO phase. The curves of hardness (H) and elastic modulus (E) across the coating are similar, from the surface to the interior of the coating, H and E increase, and both of them exhibit a maximum value at the same depth of the coating. After MAO treatment, the mechanical properties, such as hardness and elastic modulus, can be improved, and the microstructure of the alloy substrate has not been changed.

Abstract: Thixocasting is a new semi-solid processing route for magnesium alloys; it is claimed that finer microstructures can be produced and as a consequence, better corrosion resistance can be achieved. Therefore, it is of great interest to compare the corrosion properties of two Mg-Zn-RE alloys produced by standard squeeze casting and new semi-solid casting technique. The influence of the two different processing routes and the replacement of rare earth elements by Ca additions on the corrosion behavior were studied in NaCl aqueous solutions by (a) analyzing the corrosion morphology, (b) measuring electrochemical polarization curves, and (c) carrying immersion tests at constant pH-value. Using light microscopy, scanning electron microscopy and X-ray diffraction, the corrosion results were related to the microstructures on the specific alloys. The results indicate that Ca cannot replace rare earth elements under corrosion aspects, but they also showed that the thixocasting process resulted in better corrosion resistance.

Abstract: This study is concerned with the effect of high energy ion beam irradiation on surface properties of AZ91 magnesium alloy. The study included a characterization of ion beam surface modification zone in terms of microstructure, and mechanical properties like nanohardness, wear, and corrosion resistance of the surface layer were studied in details. Nanohardness of the modification layer was improved about 2 times as that of the as-received AZ91. The corrosion resistance of the modified layer was significantly improved in NaCl solution because of refined grains. The wear resistance of the modification layer was also improved as compared to as-received AZ91.

Abstract: The corrosion of a magnesium-based MMC, comprising a AZ91D alloy matrix and 5 vol% titanium carbide particles has been studied in saline environments, using immersion and electrochemical tests. Corrosion was localized initially and developed subsequently into general corrosion , the local corrosion rate of AZ91D/TiCp being faster than diecast AZ91D due to the formation of less protective corrosion products. Galvanic corrosion for AZ91D/TiCp, due to coupling of the matrix and reinforcement, was more severe than diecast AZ91D.