Study of formation mechanism of corrosion resistant oxide films and surface oxidation phenomena of magnesium alloys

The practical importance of magnesium has remarkably increasedespecially in ecological and environmental points of view. However，no enough information on the microstructure and fundamental growth mechanism of oxide films onmagnesium available. In the present researchformation behavior of anodic films grown on pure magnesium and magnesium alloys was investigated byelectrochemical analysis， TEMSEM and surface analysis such as XPS and GD-OES. Electropolishing of magnesium can be performedsuccessfully to obtain bright surface following the similar mechanism to that of aluminum. Whenanodic voltage increases to around 5v in alkaline solutionsa current density extensively increases. Further increase in voltage decreases the current until thevoltage reaches to a critical voltage of electric breakdown. The critical voltage is dependent onmagnesium substrate purity and aluminum content. It increases in the order；99.6%, az 31,AZ91 and 99.95%. Including impurities decrease breakdown voltage while aluminum decreases breakdownvoltage. When aluminum ions are added in electrolytes the critical voltage also increases. thepresence of小amount of aluminum in either substrate or electrolyte improves the film propertyby selective deposition of spinel (MgAl - D22 - D2O - D24 - D2) in the oxide film. the cylindrical porestructure and the barrier layer, which are similar to the Keller's model of anodic alumina，are also confirmed by direct cross-sectional observation After the sealing of anodic电影in aboiling solution containing sodium silicate, platelet-like hydroxides in poreswhich are similar to sealed anodic films on aluminum, is indicated。

The practical importance of magnesium has remarkably increased, especially in ecological and environmental points of view. However, no enough information on the microstructure and fundamental growth mechanism of oxide films on magnesium was available. In the present research, formation behavior of anodic films grown on pure magnesium and magnesium alloys was investigated by electrochemical analysis, TEM, SEM and surface analysis such as XPS and GD-OES. Electropolishing of magnesium can be performed successfully to obtain bright surface following the similar mechanism to that of aluminum. When anodic voltage increases to around 5V in alkaline solutions, a current density extensively increases. Further increase in voltage decreases the current until the voltage reaches to a critical voltage of electric breakdown. The critical voltage is dependent on magnesium substrate purity and aluminum content. It increases in the order; 99.6%, AZ31, AZ91 and 99.95%. Including impurities decrease breakdown voltage while aluminum decreases breakdown voltage. When aluminum ions are added in electrolytes the critical voltage also increases. The presence of small amount of aluminum in either substrate or electrolyte improves the film property by selective deposition of spinel (MgAlィイD22ィエD2OィイD24ィエD2) in the oxide film. The cylindrical pore structure and the barrier layer, which are similar to the Keller's model of anodic alumina, are also confirmed by direct cross-sectional observation. After the sealing of anodic film in a boiling solution containing sodium silicate, platelet-like hydroxides in pores, which are similar to sealed anodic films on aluminum, is indicated.