Anodic Breakdown of Passive Films on Metals

The electrocapillary stability of passive films on metals has been studied in terms of energetics. It is predicted that there is a critical potential above which breakthrough pores are formed in the film. This critical film breakdown potential is less noble, as the surface tension at the metal‐electrolyte interface is smaller and hence as the anion adsorption is stronger. It is also predicted from the dissolution kinetics of semiconducting surface films that above a certain critical potential the passive film is electrochemically unstable, undergoing potential‐dependent transpassive dissolution. Anion adsorption, which introduces electron acceptor levels in the film, will lower the transpassivation potential at the adsorption sites. The passive film of n‐type oxide appears to be more stable against anodic polarization than that of p‐type oxide.