A two-step anodic method to fabricate self-organised nanopore arrays on stainless steel

Abstract

We report a simple two-step anodic method to fabricate self-organized nanopore arrays on 316L stainless steel surface, with the initial anodization in perchloric acid-based electrolyte followed by the second anodization in sodium dihydrogen phosphate-based electrolyte. The morphology and chemical analysis of anodic overlayers after the first and second anodizations were explored by AFM, SEM and XPS. The influence of various applied voltages, current density, various anodization times and temperatures on nanoscaled morphology was investigated in the process of the second anodizing step. As a result, the dimension of the nanopores depends linearly on the anodization time and was controlled by the applied voltage and current density in a certain range. At the premise of no appreciable expansion in the mean pore size after the second step in contrast to the initial pores, the depth of nanopores could reach seven times of the previous depth of 10nm. The analysis of XPS spectrum performed on the nanostructure, the energy shifts of Cr 2p3/2, Fe 2p3/2, O 1s, and Ni 2p3/2 levels showed that principal constituents of the anodic overlayer are chromium and iron components. No metallic Fe, Cr, and Ni peaks were detected on the surface after the second anodization. The depth of nanopores was roughly in agreement with the thickness estimations obtained from depth profiling data and different chemical compositions that exist at different depths of even a thin anodic overlayer.