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).

This paper reports SIMS analysis of passive layers. Pure Fe disks are used in borate buffers of pH = 8.4. Preelectrolysis is carried out to remove trace impurities. Potential step control of the electrode is utilized. Two types of experiments are carried out. In one, passive layers are prepared and then exposed to chloride ions ~br various times. In the other, films are heated at various temperatures for preselected times. The passive layers subjected to these treatments are exposed to SIMS analysis. The distribution of OH-with depth, from SIMS analysis, shows an exponential-like decrease away from the electrode/solution interface for passive films exposed to 0.5M buffered chloride solutions for various times or heated at a given temperature for a predetermined period of time. The OH-peak heights at a fixed depth in passive layers decrease in a similar manner with increasing exposure time to chloride ions or with increasing annealing temperature for a given time period. C1-peak heights for films exposed to chloride ion-containing borate solutions increase rapidly for times up until breakdown, decrease sharply after breakdown, and then level off at times much longer than breakdown. O-peak heights decrease exponentially with depth for samples exposed to chloride ions and decrease linearly for specimens annealed at various temperatures for a number of different times. Corresponding Fe § peak height data display a linear decrease with depth for chloride-treated films and an almost constant value for layers subjected to thermal treatments. Positive SIMS O § peak heights increase sharply for thermally treated films over a surface region of approximately 7•, remain constant in the bulk regions, and decrease rapidly in the film]metal interfacial regions. Homogeneous films are indicated with a thickness of about 25~. Detailed mechanistic discussions of the two principal models of passivity show that the results are in greater agreement with the amorphous hydrated polymeric oxide model than with the crystalline oxide model, in which water is considered only in terms of adsorbed water on the surface of the passive film.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.122.253.228 Downloaded on 2014-10-19 to IP