Formation of a self-organized nanoporous structure with open-top morphology on 304L austenitic stainless steel.

A novel and simple method is reported for producing a self-organized nanoporous structure on austenitic stainless steel (SUS-304L) with open-top morphology. Uniform nanopores with a quasi-hexagonal arrangement were obtained on a very large scale with no crack formation by using single-step anodization. Electropolishing of SUS-304L in ethylene glycol monobutyl ether and perchloric acid electrolyte prior to anodization was the key factor to obtain self-organized and regularly ordered nanopores. Under optimized electropolishing conditions, a honeycomb-like patterned morphology of shallow nanopores was developed on the surface of SUS-304L. Anodization of the patterned morphology in ethylene glycol-based electrolyte generated self-organized and ordered nanopores. Morphology, structure and chemical analyses of the samples were carried-out using FESEM, EDAX, XRD, XPS and ToF-SIMS. FESEM images revealed the formation of hexagonal and ordered nanopores with uniform diameter. EDAX analysis confirmed that the nanoporous oxide layer is composed of iron, chromium, nickel and oxygen. A blue energy shift in the XPS spectra was observed after annealing, which is attributed to the absence of F-species. ToF-SIMS depth profile analysis confirmed the high content of chromium oxide on the surface of the nanoporous oxide layer. The hexagonal nanoporous ordered morphology is useful in anti-corrosion and decoration applications.