(Invited) Formation of Self-Organized Porous Anodic Films on Iron and Stainless Steels
Authors: Habazaki Hiroki , Shahzad Khurram , Hiraga Takuya , Tsuji Etsushi , Aoki Yoshitaka 
Published: February 2020

Porous anodic films formed on a range of metals and alloys have attracted much attention because of fundamental interest in self-ordering process of nanopore and nanotube arrays as well as their potential applications. Anodizing of aluminum have been studied since 1920s, but the intensive fundamental and applied research on anodizing of metals have been conducted in the last two decades. It has been believed that thick anodic films could not be formed on iron and iron base alloys because of dissolution and gas generation in aqueous electrolytes. However, nanoporous and nanotubular anodic films on iron were first reported in 2007 and their formation was achieved by using organic electrolytes containing fluoride and a small amount of water.(1) Here, our recent studies on formation and characterization of the porous anodic films on iron and stainless steels will be summarized.(2-4)
When iron was anodized in ethylene glycol (EG) electrolyte containing 0.3 mol dm-3 or less water and 0.1 mol dm-3ammonium fluoride, only nanoporous anodic films were formed. The film growth appeared to be different from the usual porous anodic alumina; the interpore distance of nanoporous anodic films on iron formed under this condition is much smaller than the thickness of the barrier layer located between the porous layer and metal substrate, while the ratio of the interpore distance to the thickness of barrier layer for aluminum is ~2.5. The latter porous film growth is recently explained by field-assisted flow of film materials from pore bottom to cell wall. This model might not be applicable to the anodic films on iron at the low water concentrations from geometrical point of view.
When water concentration was increased to 1.5 mol dm-­3in the EG electrolyte, a scalloped metal/film interface, which was typical of porous anodic alumina growth, was developed on iron. In addition, the film morphology changed apparently from nanoporous to nanotube by increasing the anodizing voltages. Thus, we can control the film morphology by anodizing voltage and electrolyte composition.
For the formation of relatively thick porous anodic films on stainless steel, the EG electrolytes with lower water concentrations were needed in comparison with the film formation on iron. When austenitic Type 304 stainless steel was anodized, nanoporous anodic films with scalloped alloy/film interface were developed even at a low water concentration of 0.1 mol dm-3. In contrast, the ferritic Type 430 stainless steel, free from nickel, formed anodic films with more irregular and highly branched pore morphology. The alloy/film interface was also flat. The steel dependent growth behavior may arise from the presence of nickel in the alloy, since the anodizing behavior of magnetron-sputtered Type 304 stainless steel, which had a bcc structure, was similar to that of bulk Type 304 stainless steel.
References
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4. H. Habazaki, Y. Konno, Y. Aoki, P. Skeldon and G. E. Thompson, J Phys Chem C, 114, 18853 (2010).