Conventional austenitic stainless steels (SSs) have been widely used as structural materials due to their advantages of heat resistance, corrosion resistance and formability. However, austenitic stainless steels are relatively expensive because of the high price of Ni. Recently high Mn-N SSs, developed by replacing the expensive Ni with Mn and N, cheap austenite stabilizing elements, have received attention, and hence are actively being studied.
In high Mn-N SSs, Mn increases nitrogen solubility, which is a strong austenite stabilizer and improves localized corrosion resistance, but decreases the corrosion resistance of the alloys. Hence we examined the effects of Mn on the passivation behavior and repassivation kinetics of Fe - 18Cr - xMn (x = 0, 6, 12 wt.%) using polarization test, electrochemical noise analysis(ENA) and scratch electrode test. In addition, XPS analysis was performed to investigate chemical composition of the passive film formed on Fe-Cr alloys containing Mn. And effects of Mn on the semiconducting properties of passive film of the alloys were also investigated to characterize the structure of the film by analyzing the photo-electrochemical and Mott-Schottky responses for the film.
Polarization test was conducted in 30℃, deaerated 0.1 M NaCl + 0.02 M $H_{2}SO_{4}$ solution. ENA test and scratch electrode test were performed in 30℃, deaerated 0.1M NaCl solution at -0.1 $V_{SCE}$. XPS analysis and photocurrent measurement were performed for the passive film formed potentiostatically on Fe-18Cr-6Mn in 25℃, pH 8.5 buffer solution. Mott-Schottky test of passive film formed on Fe-18Cr-xMn (x = 0, 6, 12 wt.%) in 25℃, pH 8.5 buffer solution was examined.
As Mn content increases, the passivity of the Fe-18Cr alloys was gradually degraded as confirmed by the decrease in the pitting potential and the increase in the passive current density in polarization curves. Degradation in the passivity of the Fe-18Cr alloys by Mn was confirmed again by the increase in number of current peaks caused by metastable pitting, and also decrease in repassivation rate, as demonstrated by the scratch test.
From the photocurrent measurement, it was found that the base structure of passive film formed on Fe-18Cr-6Mn in pH 8.5 buffer solution is a Cr-substituted γ-Fe_{2}O_{3}$ involving d-d and p-d electron transition, and Mn forms its own oxides in the passive film. Bandgap energy of d-d and p-d transitions of γ-Fe_{2}O_{3}$ were 2.8 eV and 3.4 eV. And that of Mn oxide in passive film was 4.0 eV. Flatband potential of γ-Fe_{2}O_{3}$ was -0.35 eV, and that of Mn oxide was 0.1 eV. Results of Mott-Schottky analysis showed that passive films formed on Fe-18Cr-xMn (x = 0, 6, 12 wt.%) exhibited a n-type semiconductor with donor ($V_{o}^{2+}$) density of the film increased slightly with Mn content.
