Nanoporous and Nanotubular Anodic Films on Iron Substrates. Synthesis and Structure

Self-ordering nanostructured anodic films fabricated onto the surface of iron and ironbased alloys opened new horizons for their application in the recent energy harvesting and storage devices and catalysis. The anodic passivity of iron in the aqueous solutions is known since the beginning of the past century due to formation of extremely thin iron oxides or salt layer. Ten years ago the discovery of the formation of thick and self-ordered nanoporous and nanotubular iron anodic films in organic solutions containing some fluorides and water opened a new pathway for investigation and application of these materials and stimulated a still growing interest. Therefore, the purpose of this review paper is to provide a better insight in the processes of iron anodizing for nanoporous and nanotubular film formation, their composition, and possible application trends in the view of the latest and our advances in this field. Apart from the formation of nanostructured films in well-known ethylene glycol electrolytes, the peculiarities of iron anodizing in other organic electrolytes, such as dimethyl sulfoxide, are presented herein. Since earlier published papers are almost forgotten, but they could give the basic knowledge on nanostructured anodic film formation in other electrolytes, we briefly introduce the behavior of iron in aqueous solutions resulted from the active dissolution of iron and polarization-dependent the passive film formation. To reveal the composition of as-grown anodic films on iron, X-ray diffraction, X-ray photoelectron, X-ray energy dispersive and Mössbauer spectroscopies are used and the reactions of passive films formation are discussed.