Debunking the effect of water content on anodizing current: Evidence against the traditional dissolution theory

Currently, most explanations of nanotube growth are limited to chemical or field-assisted dissolution, in the belief that an increase in water content will increase the corrosive effect of the electrolyte and accelerate the rate of growth of the nanotubes. Here, titanium foils were anodized at a constant voltage (50 V) in 0.5 wt% NH4F ethylene glycol electrolyte containing 2 wt% H2O or 10 wt% H2O. It was found that the growth rate of nanotubes in electrolyte with 2 wt% H2O was significantly faster than that in electrolyte with 10 wt% H2O, contradicting the traditional dissolution theory. A detailed explanation is given using electronic current theory and the oxygen bubble model. We believe that electronic currents, rather than chemical dissolution, play the key role in nanotube growth. This conclusion was validated by the matched lengths of an interesting two-layer nanotube array obtained by a novel two-step anodization process.