Effects of formation voltage and electrolyte ions concentration on the structure and passivity of anodic films on magnesium

Abstract

Structure and passivity of the films formed on magnesium in alkaline solutions containing aluminate ions were evaluated by using various methods including electrochemical measurements and a field emission scanning electron microscope. When concentration of sodium ions was kept constant, increase in aluminate ions effectively depressed the peak current at around 5 V and increased the breakdown voltage. This is caused by the improvement of surface passivity of magnesium and oxide protectively. Aluminum content incorporated in the film lineally increased with log of increasing concentration of aluminate ions added in the solution. However, besides the concentration of aluminate ions in the solution, the corrosion resistance evaluated by salts spray test was highest in the thick films formed at around 5V with extremely high current density. Further, it is clarified that all films formed at 5V exhibit vertical cylindrical cellular structure similar to the Keller model of porous anodic alumina.