Galvanostatic Growth of Nanoporous Anodic Films on Iron in Ammonium Fluoride−Ethylene Glycol Electrolytes with Different Water Contents

Abstract

The growth of porous anodic films on iron has been examined at a constant current density of 50 A m−2 in 0.1 mol L−1 NH4F−ethylene glycol electrolytes containing 0.1−1.5 mol L−1 water. Nanoporous films are formed in all the electrolytes, with the growth rate increasing with the decrease in the water content of the electrolyte. A barrier layer, in which a high electric field is applied during anodizing, thickens in proportion to the formation voltage at a ratio of 1.9 nm V−1, regardless of the water content of the electrolyte. However, there is a transition water content between 0.3 and 0.5 mol L−1, at which growth behavior changes. Above the transition level, the formation voltage is constant after an initial voltage rise, with the constant voltage slightly rising with a decrease in water content. In contrast, the formation voltage increases continuously to more than 150 V when the water contents are below the transition level. The anodic films are poorly crystalline and contain a significant amount of fluoride species. A high enrichment of fluoride species occurs near the metal/film interface when the water content in the electrolyte is below the transition level. Such enrichment is not as significant, or possibly absent, in electrolytes of increased water content.

Factors influencing the growth behaviour of nanoporous anodic films on iron under galvanostatic anodizing

“…The film composition and phase structure may be dependent upon the H 2 O concentration in electrolyte, such that the rate of film thickening increases and the gas generation is suppressed with a decrease in the H 2 O concentration. The previous TEM observations of the anodic films disclosed that nanocrystals were present in the anodic films formed at higher H 2 O concentrations while the anodic films were fully amorphous at reduced H 2 O concentrations [8]. The nanocrystals may be probable sites of gas generation, as in the case of anodic TiO 2 [13,14], resulting in the enhanced gas generation at higher H 2 O concentrations.…”

“…GDOES depth profiles of the anodic films the high electric field during film growth [15,16]. However, the enrichment was only obvious at lower H 2 O concentrations, at which the formation voltage increased to higher than 100 V [8]. The H 2 O concentration-dependent enrichment of fluoride may be related to the mechanism of porous film growth, i.e., either field-assisted dissolution [17] or field-assisted flow [18,19].…”

“…Our previous study using relatively thin, sputter-deposited iron films revealed that the initial growth rate of the nanoporous anodic films during galvanostatic anodizing in ethylene glycol electrolyte containing NH 4 F and H 2 O increased with a decrease in the water content, due to suppression of oxygen generation. The reduction of water content also results in a marked increase of formation voltage to higher than 150 V, disordering of the porous structure and enrichment of fluoride species at the metal/film interface [8].…”

Efficient growth of anodic films on magnesium in organic electrolytes containing fluoride and water

“…Recently, fluoride-containing organic electrolytes have been utilized to form self-organized nanotubular and nanoporous anodic films on titanium [3,4], zirconium [5,6], niobium [7], tantalum [8] iron [9][10][11] and stainless steel [12]. The use of organic electrolytes enables the formation of thick porous anodic films on iron and stainless steel, and improves the uniformity of the self-ordered pore or nanotubular array as well as the thickening of the anodic films.…”

Control of morphology and surface wettability of anodic niobium oxide microcones formed in hot phosphate–glycerol electrolytes

“…Self-organized porous anodic oxides have also been formed on zirconium [18], niobium [19], tantalum [20], and even on iron [21][22][23].…”