Abstract
The passive oxide film on SUS 304 stainless steel (SS) was studied in a 0.1 mol dm−3 sulfuric acid solution as a function of passivation time. The passive oxide films were measured by ellipsometry and X-ray photoelectron spectroscopy (XPS). A Mott-Schottky plot of the film capacitance was employed to determine the donor density in the n-type semiconducting oxide film, and current measurements of the Fe3+/Fe2+ redox couple were employed to investigate the electronic transfer process on the passive oxide film. The passive oxide consists of Cr-Fe-Ni oxides in which enrichment of Cr to 57 mol% occurs as the potential increases. During the aging of the passive oxide at 0.6 V vs. Ag/AgCl/saturated KCl (SSC) for 43 ks, the current decreased from 30 μA cm−2 at 10 s to 0.025 μA cm−2 at 43 ks, and the Cr ratio in the oxide increased from 49 to 57 mol% with an increase in the O2– ratio. Notably, the film thickness remained nearly constant at 1.3 nm during the aging process. Enrichment of the Cr content resulted in a decrease in the donor density of the n-type semiconducting passive oxide and the inhibition of electronic charge transfer from/to the Fe3+/Fe2+ redox couple in the electrolyte.
Highlights
IntroductionA Mott-Schottky plot of the film capacitance was employed to determine the donor density in the n-type semiconducting oxide film, and current measurements of the Fe3+ /Fe2+ redox couple were employed to investigate the electronic transfer process on the passive oxide film
During the aging of the passive oxide at 0.6 V vs. Ag/AgCl/saturated KCl (SSC) for 43 ks, the current decreased from 30 μA cm−2 at 10 s to 0.025 μA cm−2 at 43 ks, and the Cr ratio in the oxide increased from 49 to 57 mol% with an increase in the O2– ratio
A small anodic peak appeared at –0.26 V, and the passive region in which the CD was constant at 0.8 μA cm−2 began at 0.0 V
Summary
A Mott-Schottky plot of the film capacitance was employed to determine the donor density in the n-type semiconducting oxide film, and current measurements of the Fe3+ /Fe2+ redox couple were employed to investigate the electronic transfer process on the passive oxide film. The passive oxide film on conventional austenitic stainless steel (SS) SUS304 has been investigated with a focus on its composition,[1,2,3,4,5,6] thickness[7,8,9] and semiconducting properties.[5,6,10,11,12,13,14,15,16,17,18,19,20] For the oxide formed in an acidic aqueous solution, the chromium oxide component was enriched to a ratio of approximately 50–60 mol % in passive oxide in an acidic aqueous solution.[1,2,3]. The Mott-Schottky plot in the potential range, in which little reduction of the film occurs, indicated that the passive oxide had an n-type semiconducting property.[6,10,13,16,19,21]
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