Differential Composition Profiles in Depth of Passive Films on Iron-base Alloys

Abstract

A differential method for measuring the number of component atoms in surface atomic layers by use of AES and argon sputter-etching was applied to the analyses of thin anodic oxide films formed on Fe-30Cr alloy, 304- and 316-stainless steels in deaerated 0.5M sulfuric acid solution. From the measured composition profiles in depth, the surface excess, Γ_Cr, Γ_Ni, Γ_O and Γ_S on the dividing surface with Γ_Fe≈O was estimated as a function of anodic potential.
In the passive potential region, the surface excess changes depending on the species of alloys as follows: Γ_Cr=1-3×10¹⁵ atoms/cm², Γ_Ni=4-8×10¹⁴ atoms/cm², Γ_O=3-6×10¹⁵ atoms/cm² and Γ_S=2-8×10¹⁴ atoms/cm². The surface excesses of chromium and oxygen, Γ_Cr and Γ_O of 316 stainless steel are smaller than that of 304 stainless steel, indicating that molybdenum affects Γ_Cr and Γ_O. The effect of molybdenum on Γ_Cr and Γ_O was discussed in connection with the electrochemical characteristics of molybdenum during passivation.