Anodic Films Grown on Magnesium and Magnesium Alloys in Fluoride Solutions


Sachiko Ono, and Noboru Masuko*
Department of Applied Chemistry, Kogakuin University,
1-24-2 Nishi-shinjuku, Shinjuku-ku, Tokyo 163-8677, Japan
* Department of Metallurgical Engineering, Faculty of Engineering,
Chiba Institute of Technology Tsudanuma, Narashino, Chiba 275-8588, Japan

As described in “Principles of Magnesium Technology” by Emely, anodizing of magnesium is commercially used for years to provide a thick porous oxide layer and to improve corrosion resistance. Previously, the authors reported that the anodic films as well as chemical conversion coating films formed on pure magnesium and AZ die-casting alloys have cylindrical porous structure comparable to the Keller's model of anodic alumina. However, the film growth mechanism by anodizing of magnesium in Dow17, which is a most commonly used commercial solution, was rather complicated in comparison with the well-known behavior of anodic film growth on aluminum. The commercial electrolyte such as Dow17 is an acidic solution and includes fluoride and chromium. Anodic film formed in such electrolyte consists of substantial amount of crystalline fluoride. According to the Pourbaix diagram, however, oxide film is formed on magnesium only in the alkaline region. The anodic film growth in acidic fluoride solution must be brought about the characteristic affinity between fluorine and magnesium. Therefore, in the present study, the behavior of anodic film growth on magnesium and magnesium alloys in fluoride electrolytes was investigated with focusing on the effects of formation voltage, substrate composition and pH of electrolytes.

Formation behavior of anodic oxide films on magnesium in fluoride electrolytes was investigated with attention to the effects of anodizing voltage and aluminum content. Chemical composition (mass%) of magnesium specimens was shown in Table 1. In the range of voltage between 2V and 100V, porous film was formed in alkaline fluoride solution associated with high current density at around 5V and at breakdown voltage. The critical voltage of breakdown to allow maximum current flow was approximately 60V and relatively independent on substrate purity. Effects of anodizing voltage and substrate impurity on film appearance formed in KOHKFNa3PO4 solution was summarized in Table 2. The films formed at breakdown voltage showed a lava like porous structure similar to those obtained on aluminum and other valve metals. Barrier films or semi-barrier films, which were composed of hydrated outer layer and relatively dense inner layer, were formed at the other voltages. In the case of AZ91D, the critical voltage increased to 70V and peculiar phenomenon at 5V was not observed, so that only barrier films were formed at less than the breakdown voltage (Fig.1). The high current density at around 5V, which may be caused by trans-passive state, was not observed for anodizing in acidic fluoride solutions such as Dow17 and ammonium fluoride (Fig.2). These phenomena can be explained by the effects of aluminum incorporation into the film to prevent dissolution and to promote passivation of magnesium. The depth profiles of constituent elements showed that aluminum distributed in whole depth of the film.

[Published in Materials Science Forum, 419-422, 897-902 (2003)]

Fig.1 Voltage - Current characteristics of anodizing of 99.95% Mg and AZ31 in 0.5moldm-3 NH4FHF solution. Anodizing was performed for 10min at 298K.

Fig.2  Voltage - Current characteristics of anodizing of 99.95% Mg in Dow17 solution. Anodizing was performed for 10min at 348K.