The Effects of Mn and Si Content on the Crystal Structure of Oxide Films, and Their Protectiveness to Corrosion of Bright-annealed, Type 430, Ferritic Stainless Steel

Abstract

The crystal structures of oxide films formed on the surface of 17% Cr-Fe alloys containing various amounts of Mn and Si, when treated in H₂-H₂O atmospheres with dew points of - 25° and - 36°C, were investigated. The influence of the oxide films with different crystal structures on the corrosion resistance of alloys to dilute sulphuric acid was also studied.
The results obtained are as follows:
The alloys form (Cr. Fe)₂O₃, MnCr₂O₄ and SiO₂, depending on the content of Mn and Si, even when the oxygen potentials (dew points) of the atmospheres are those that would normally produce a Cr₂O₃ film on pure Cr-Fe alloys. These oxides show less resistance to corrosion than Cr₂O₃.
A pure 17% Cr-Fe alloy forms only Cr₂O₃ and exhibits especially excellent corrosion resistance. However, 17% Cr-Mn-Fe alloys with a small amount of Mn in the range of 0.08 to 0.25% appear to form spinel type (MnCr₂O₄ etc) oxides and have a different behavior in their resistance to corrosion. The minimal corrosion rate is attained at 0.08% Mn, beyond which the corrosion rate increases as the Mn content increases. On the contrary, the 17% Cr-Si-Fe alloy with a Si content of 0.10% forms (Cr. Fe)₂O₃ and SiO₂. When the Si content is increased beyond 0.10%, (Cr. Fe)₂O₃ disappears and Cr₂O₃ forms, improving corrosion resistance.
The corrosion resistance behavior of the alloys mentioned above can be explained by the variation of the crystal structures and thickness of the oxide films formed under varying controlled atmospheres.