Research On Corrosion And Anodizing Of Magnesium

Magnesium alloys, the lightest metal structural material in use, have been widely applied in automotive, electronic and aeronautical industries because of a number of desirable features, including high strength/weight, high hardness/weight, damping characteristic and recycled easily. However, their applications are limited severely for their low corrosion resistance. The corrosion properties of Mg-Al-Zn alloys and the measures to improve their corrosion resistance are researched in this paper. Static and dynamic state corrosion behaviors of magnesium alloy AZ91 in sodium chloride solution are studied, and effects of the concentration of sodium chloride solution and temperature on the corrosion rate are also investigated. The results show that corrosion rate of the alloy increases with the concentrations and temperature as well as stirring speed of sodium chloride solution; the corrosion rate wavily alters with corrosion time. The XRD results show that new phases after corrosion are identified as Mg(OH)₂ and MgAl₂O₄. SEM results show that the corrosion route of the alloy is inter-granular in sodium chloride solution.Films in different anodizing processes are prepared through changing process parameters such as electrolyte, voltage, temperature and time. Corrosion products and morphology are determined by X-ray diffraction and Scanning Electron Microscopy, respectively. The electrolyte of newly and environment-friendly for anodizing of magnesium alloys were developed by orthogonal layout,which main contents are KOH,Na₂SiO₃ and NaAlO₂. Results by salt-spray-test corrosion and SEM show that the corrosion resistance of anodizing films is improved with voltage by observation of surface morphology. Thickness of anodizing films increases with increasing voltage, but decrease with temperature. The function of K₂SiO₃ in anodic oxidation is to change the composition of anodizing film and form by XRD. KF accelerates anodic oxidation. The surface of anodizing film have homogeneous distributions of small holes and good combination between the film and matrix alloy is found by SEM. NH₄HF₂ is used as additive of restraining sparking. The relults show that NH₄HF₂ has remarkable essect on anodizing of magnesium alloys. The experiments of immersion in 5 wt.%NaCl solution and electrochemical impedance spectroscopy show that NH₄HF₂ has improved the...