Abstract
The passive films on 17 Cr-13 Ni steels containing 0∼3 wt%Mo and 20 Cr-25 Ni steels containing 0∼5 wt%Mo in 0.5 mol/l H2SO4 and in 1.0 mol/l-HCl have been measured by ellipsometry under potentiostatic control.Optical constants for the film-free surfaces of these steels were ellipsometrically determined in dehydrated methanol immediately after the film removal treatment in 0.5% bromine-methanol solution. The values were used for the analysis of ellipsometric data obtained for passive films on the steels. It was found that the optical constants of the passive films formed in 0.5 mol/l-H2SO4 were virtually independent of the Mo content of the steels; N2=2.3−0.3i for 17 Cr-13 Ni steels containing 0∼3 wt%Mo, N2=2.3−0.2i for 20 Cr-25 Ni steels containing 0∼5 wt%Mo. The film thickness and its growth rate with potential were both increased with incresing Mo content.The optical constant of the passive film on 20 Cr-25 Ni-5 Mo steel in 1.0 mol/l-HCl was 2.0−0.2i. The film thickness and its growth rate with potential were smaller than those for the passive film formed in 0.5 mol/l-H2SO4. The difference in optical properties between the film formed in 0.5 mol/l-H2SO4 and that in 1.0 mol/l-HCl may be attributed to different chemical compositions of the films.
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