An examination has been made of the mechanism of breakdown of passive films on iron in borate buffer solution false(normalpH=8.4false) caused by chloride ions. Various electrochemical kinetic criteria were measured. XPS, SIMS, and ISS studies were made of the systems used in the electrochemical work. The rate of breakdown was found to be proportional to cnormalCl− and cH+ and exponentially dependent on the electrode breakdown potential and field drop in the oxide film. XPS data showed that when chloride ions caused breakdown, the Onormaltotal/normalFe and H2O/normalFe ratios changed from 2 to 1.5 and 0.5 to 0.1, respectively. SIMS data revealed that heating passive films up to 200°C drove out water from the films and that chloride ions penetrated the whole film thickness on breakdown. ISS data indicated that on changing from a passive to a depassivated film, the O/normalFe ratio changed from 2.07 to 1.5. Discussion of the electrochemical kinetic data shows that it is inconsistent with adsorption‐displacement models, pore models, and chemico‐mechanical models, but is not inconsistent with ion‐exchange processes, point‐defect models, and hydrated polymeric oxide models. Confrontation of the spectroscopic data with the expectations of the latter three models shows some points of agreement with all these models, but the data taken together is most consistent with the hydrated polymeric oxide model.

Models of the passive layer on iron are of two kinds--hydrated amorphous (I) or anhydrous polycrystalline (2).

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