Microcharacterization of colored films formed on AISI 304 by different electrochemical methods

The colored oxide films grown on AISI 304 stainless steels by three different methods (chemical treatment, square-ware potential pulse, and triangular current scan) in electrolytes containing chromic and sulfuric acids have been studied by transmission electron microscopy, selected area diffraction, energy dispensive X-ray spectrometry, micro-Raman spectrometry, and scanning electron microscopy. The morphology and structure of films formed by all coloration processes are relatively similar. The oxide layers are constituted of elongated nanocrystalline grains with dimensions ranging from 11 X 5 to 13 X 8 nm of the spinel type (Ni, Fe) X (Fe, Cr)(2)O-4. The composition is not uniform, with the Fe content and hydration grade increasing from the inner metal/oxide interface to the outer oxide surface, while the Ni content is constant and similar to its value in the alloy substrate. A higher Cr content is obtained by the E-pulse process. This allied to the more pronounced grain boundary attack caused by the chemical process, points out to a higher chemical stability and corrosion resistance of samples produced by electrochemical methods studies here, especially the E-pulse process. (c) 2006 The Electronic Society.