Nucleation and growth of anodic oxide films

The kinetics of formation of anodic oxide films can be derived from measurements of current density i_ox, thickness d and electrode capacity C in dependence on the polarization potential ε and time t. Examples of simultaneous and multistep experiments are discussed in connection with advantages of special combinations.From plots of lg i vs lg t, lg C vs lg Q or d vs ε, characteristic growth laws and properties of autocatalytic and self-inhibiting systems can be derived. Deposition layers of NiOOH and FeOOH and passive films on Au, Pt and Hf are described as examples. All systems with rate determining ionic migration or diffusion within the solid phase belong to the group of self-inhibiting systems. Due to the potential drop within the film they form anodically almost homogeneous films, but during reduction island formation is enhanced. Nucleation phenomena and special growth forms can be studied only in systems of crystallizing oxides with a high conductivity, ie negligible potential drop. Monolayer formation and the influence of surface structure can be distinguished only on single crystal planes of noble metals.