Sealing mechanism of nanoporous alumina in fluorozirconate salt containing solutions

Abstract

In order to fix colouring pigments in the decoration domain, to trap anticorrosion inhibitors or to heal structural defects in the field of protection and durability, the clogging or sealing of the nanoporous network of aluminium anodic oxides is required. It is classically achieved by the formation of nanometric overlayers throughout chemical impregnation. The purpose of the present work is to investigate the sealing mechanism and the composition of the clogging materials by impregnation in a Cr3+/ZrF62− solution. As shown by in situ electrochemical measurements, the upper part of the anodic layer firstly dissolves by a reaction of fluoride-hydroxide exchange between ZrF62− salt and the alumina anodic layer. Due to the resulting local pH increase, measured thanks to a micro-pH electrode in the diffusion zone, Cr3+ and ZrF62− salts precipitate into Cr(OH)2.5(SO4)0.25 and Zr(OH)4-xFx respectively, as revealed by SEM/TEM analysis and XPS measurements. Finally, the ingress of the metallic elements is limited and only fluorides diffuse in-depth within the nanoporous network. A part of Cr(III) cation is oxidized into Cr(VI) in the top layer, which might explain the process efficiency in anticorrosion applications.