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

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.