(Invited) Self-Organized Formation of Porous Anodic Films on Iron and Their Application
Authors: Habazaki Hiroki 
Published: February 2020

Self-organized porous anodic films with the cylindrical pores being arranged hexagonally are well-known on aluminum, and have been used for corrosion protection, decoration of aluminum. The films have attracted much attention for a wide range of applications in nanotechnology and nanodevices. The anodic aluminum oxide is an insulator, while porous semiconductive and conductive oxides formed by anodizing of other metals, may have wider applications. In fact, nanotubular anodic TiO2 films formed on titanium mostly in fluoride-containing electrolytes have been studied extensively in the last decade, because the TiO2is a chemically stable semiconductor and has many potential applications.
Iron is the practically most widely used metal and iron oxides are n-type semiconductor (alpha-Fe2O3) or conductor (Fe3O4). Thus, the porous anodic iron oxide films with high surface area are of great interest as functional materials. 1, 2In this presentation, our recent studies on the critical factors influencing the film morphology, formation mechanism of porous anodic films and the applications of anodic films on iron and iron-base alloys will be discussed.
We formed porous anodic films on iron in ethylene glycol electrolytes containing fluoride and relatively small amounts of water.3 The water concentration influenced largely on the film morphology and composition. The thickness of the barrier layer sandwiched between the porous layer and the metal substrate increased with a decrease in water concentration even at the same anodizing voltage. A scalloped metal/film interface, which is typical in porous anodic films, was developed only at relatively high water concentrations (≥1.5 mol dm-3). The higher concentration of fluorine, carbon and oxygen species were incorporated in the anodic films formed at lower water concentration. From these findings, the growth mechanism appeared to change with the water concentration.
The formation voltage is one of the critical factor controlling the formation of either nanoporous or nanotubular anodic films. At high formation voltages, such as 100 V, nanotubular anodic films were formed in the electrolyte containing 1.5 mol dm-3water, while nanoporous anodic films were formed at formation voltages ≤50 V. We found a good correlation between the film morphology and current transient; the current increased continuously at a constant voltage when nanotubular anodic films were formed. TEM observations disclosed that fluoride species were enriched at the cell boundary and at the metal/film interface. The fluoride enrichment must be associated with the faster migration of fluoride ions compared with oxide ions inwards. The nanotubular morphology is, therefore, developed by preferential dissolution of fluoride-rich layer. Even at the low formation voltages, fluoride species were enriched at the cell boundary in the inner part of the anodic film, as well as at the metal/film interface. Thus, there is a potential possibility of the nanotube formation even at the low formation voltages.
An attempt was made to use the anodized iron with nanoporous oxides for corrosion protection.4We prepared polypyrrole (PPy)/anodic oxide composite coatings on iron and carbon steel. Polypyrrole is a promising coating material for anodic protection; its high oxidizing ability ennobles the corrosion potential and keeps the substrate in the passive state. However, it was reported that PPy coating did not show sufficient adhesion to metallic substrate. PPy was formed by electropolymerization and successfully deposited also in the nanopores of the anodic oxide layer. The adhesion of the composite coating was highly improved and the durability of the corrosion protection of the composite was remarkably improved in comparison with the PPy coating on the passivated iron and carbon steel.
References
1. S. K. Mohapatra, S. E. John, S. Banerjee and M. Misra, Chem. Mater., 21, 3048 (2009).
2. H. E. Prakasam, O. K. Varghese, M. Paulose, G. K. Mor and C. A. Grimes, Nanotechnol., 17, 4285 (2006).
3. H. Habazaki, Y. Konno, Y. Aoki, P. Skeldon and G. E. Thompson, J. Phys. Chem. C, 114, 18853 (2010).
4. Y. Konno, E. Tsuji, Y. Aoki, T. Ohtsuka and H. Habazaki, Faraday Discuss, 180, 479 (2015).