Dissolution of sputter-deposited iron oxide films used as a model for the passive film on iron
This paper reports results from XANES (X-ray absorption near edge spectroscopy) studies during polarization of thin sputter-deposited Fe-oxide films in acidic solutions. The dissolution rate of Fe-oxides in acidic solutions was found to be strongly increased by the presence of Fe(2+) in the oxide. During anodic polarization in acidic solutions, a deleterious effect of chloride anions is found compared with sulfates. In HCl solutions of increasing concentration, not only the pH decrease, but also the increasing anion concentration accelerates dissolution. On the other hand, the dissolution rate in sulfuric acid does not depend on the sulfate concentration. During cathodic polarization, the dissolution rate is not affected by the presence of chloride ions. This could be due to the negative surface charge of n-type oxides at potentials lower than the flat-band potential, retarding anion adsorption on the surface. These results suggest that the detrimental role of chloride anions on the stability of Fe oxide films is due to a surface complexation effect. The dissolution rate is fairly independent of the potential in the anodic range, except at very high anodic potentials. The XANES spectra reveal no changes in the average oxide valency during anodic polarization. Thus in the passive range, the dissolution that takes place is mostly chemical rather than electrochemical. The findings and their relevance to the stability of natural passive films are discussed.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States)
- Sponsoring Organization:
- USDOE, Washington, DC (United States); Swiss National Science Foundation, Bern (Switzerland)
- DOE Contract Number:
- AC02-76CH00016
- OSTI ID:
- 170589
- Report Number(s):
- BNL-62351; CONF-951007-12; ON: DE96003319
- Resource Relation:
- Conference: 188. meeting of the Electrochemical Society, Chicago, IL (United States), 8-13 Oct 1995; Other Information: PBD: [1995]
- Country of Publication:
- United States
- Language:
- English
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