Anodic oxidation under constant voltage conditions

2 Anodic oxidation under constant voltage conditions 

The case of anodic oxidation is almost the antithesis of the case discussed in Sect. 1.13 for which growth of the oxide was presumed to be due predominantly to the diffusion of uncharged atoms or molecules. In anodic oxidation, it is the electric field, without question, which plays the dominant role with the concentration gradient producing secondary, albeit important, effects. 

The effects of space charge will be discussed briefly. Good developments of this topic are rather scarce. 

In the higher voltage domain, the concentration-gradient effects are less pronounced due to the large effect of the electric field. In this case, the very-high-field forms of the hopping transport equation can be used with 

---

Thick white and thin transparent oxide films can be grown anodically on aluminum and its alloys in nitrate melts at the lower temperatures. Claims that the films contain a-, y-, 17-, and hydrated alumina have variously been made, but the techniques used to characterize these (e.g., eddy current, weight loss, double replication) have not always been entirely suitable. Films may be formed both at constant voltage or under constant current conditions. The latter seems less desirable, since morphological changes may occur as the voltage rises the application of reverse cathodic current pulses " is positively... 

---