Fabrication and formation mechanisms of ultra-thick porous anodic oxides film with controllable morphology on type-304 stainless steel

Abstract

In this work, pore initialization and further growth of metal-oxides-film (MOsF) with nanoporous structure on stainless steel (SS) substrate by electrochemical anodization have been investigated for controlling morphology of the porous MOsF. The competition between electric field assisted dissolution of the oxidized metals and anodization of SS substrate are analyzed by current-time response combined with the thickness variation of the barrier layer. The current density-time (i-t) curves show a drastic increase-a drastic drop-a gradual increase-stable shape while the barrier layer thickness presents a drastic increase-a gradual decrease-stable trend. Furthermore, the morphology (pore size, pore shape and film thickness) of the porous MOsF on SS substrate is strongly dependent on voltage drop across the barrier layer, anodizing duration and chemical constituents of the SS substrate. An electric field-assisted selective chemical dissolution model has been proposed to explain the formation of porous MOsF with cross-connected nanotube-shaped pores. An anodic etching model and an equivalent circuit of anodization system are built to theoretically analyze the formation mechanisms of pore shrinking (pore size varies from 53 ± 7 nm at the top to 34 ± 4 nm at the bottom) across the porous MOsF. Based on the achieved results, we are able to design an anodization scheme by step-potential tailoring to fabricate thick, porous MOsF with uniform nanostructures. By applying the step-potential tailoring anodization to keep a quasi-stable electric field across the barrier layer, ultra-thick (>17 μm), uniform, porous nanostructured MOsF on the SS substrates have been reported.