1991 Volume 31 Issue 2 Pages 177-183
The layer structure and composition of passive films formed on iron in borate and phosphate solutions at pH 8.42 and 11.50 have been investigated by means of cathodic reduction combined with ellipsometry and chemical analysis. Breakdown of the passive films has also been examined in 0.5 kmol/m3 NaCl solution at a constant potential higher than a critical pitting potential in the same solution. It is found that the composition and breakdown of the passive films change with the potential of film formation, anion species and solution pH, while the film is always of two-layer structure consisting of a barrier layer in contact with the metal and a hydrated deposit layer next to the solution. The depedence of anion species and pH on the composition and breakdown of the films may be systematically explained in terms of an ion selective property of the passive films. Therefore it is suggested that the ion transport in the layers plays an important role in determining the passive film composition and the breakdown.