Study On New Anodizing And Electrodepositing Technologies Of Magnesium Alloy

Magnesium and its alloys are increasingly being utilized in fields such as aerospace, automotive, power tools and computer industries on account of their excellent properties including low density, high strength-to-weight ratio, excellent dimensional stability, high impact resistance and good thermal as well as electrical conductivity. However, poor corrosion resistance of magnesium restricts its engineering application. Enhancing the corrosion resistance by appropriate surface treatment is significant. But the conventional surface treatment technologies have some disadvantages, such as intense sparking during anodizing, toxic electrolyte of anodizing and complicated process of electroplating. Some new processes of magnesium surface treatment were studied in this paper and the main results obtained are presented as follows:1. A new constant voltage and arc-free anodizing process was developed. Compact anodic films with high hardness and good corrosion resistance on magnesium alloys were prepared by this process. The effects of anodizing parameters such as applied voltage and solution temperature on peak current density and growth of film were investigated. In addition, the morphologies and corrosion resistance of films were investigated by scanning electron microscopy (SEM) and potentiodynamic polarization, respectively. The results show that higher applied voltage corresponds to higher peak current density and thicker films. However, too high applied voltage may result in breakdown of films and intense sparking which may deteriorate the properties of the anodic films and bring about unsafety. The new anodic film can be prepared in a wide range of temperature.2. Anodizing of AZ91D Magnesium alloys in an environmental-friendly bath solution without chromate, fluoride, phosphate and any other injurious ingredients was investigated. Smooth film which shows good micro-hardness and corrosion resistance can be deposited on AZ91D by constant voltage in alkali electrolyte which contains Na2B4O7,Na2SiO3,organic carboxylic acid salt and amine. The influences of electrolyte components and preparation parameters on the film-forming process as well as properties of the film were discussed in detail. The results show that Na2B4O7 can accelerate the film-formation and lead to a thicker film; Na2SiO3 can make the film smooth and improve the corrosion...