Non-thickness-limited growth of anodic oxide films on valve metals

A method of anodizing valve metals in glycerin-based solutions at elevated temperature to produce nonporous oxide films is presented. The thickness is not limited by the anodizing voltage, but by the amount of time at voltage or, more accurately, by the total amount of ionic current passed through the film. The importance of hydroxyl ion injection to film aging (i.e., the reduction in the level of current passing through a film held at constant voltage) during traditional anodizing is demonstrated.