Investigation of oxide films on ferritic stainless steel

While annealing cold-rolled strip of ferritic stainless steel in protective atmosphere oxide films may form on the surface. These parts of strip are defective material because of their deteriorated workability and appearance. For detailed investigation of the structure of the oxide layers concerning chemical composition and thickness, they were analysed by SNMS, SEM, and chemical methods. The oxide layers which look yellowish, brown, blue or grey (annealing colours) have a max. thickness of 0.4 μm. They consist of oxides of Cr, Mn, Si and Ti. Thickness, chemical composition, colour and the sequence of different oxides in the layer depend on annealing time, annealing temperature and the oxidation potential of the protective atmosphere.