What factors determine the pore growth rate of porous anodic alumina?

The formation mechanism of porous anodic alumina has traditionally been attributed to field-assisted dissolution (FAD), a theory necessitating a dynamic equilibrium between electrochemical oxide growth and FAD. However, direct experimental verification of dissolution rates matching pore growth rates remains elusive. This study quantitatively decouples the contribution of corrosive dissolution from pore propagation kinetics by anodizing aluminum in phosphoric acid electrolytes (1-4 wt%) modified with polyethylene glycol (PEG). Crucially, the experimental results contradict the fundamental prediction of FAD theory: the addition of 50 wt% PEG unexpectedly accelerated the pore growth rate from 152 nm min^-1(in pure aqueous solution) to 250 nm min^-1, despite significantly reducing the electrolyte's corrosive power. Furthermore, immersion tests without electric field reveal a maximum chemical corrosion rate of only ∼2.73 nm min^-1at up to 60 °C, representing a two-order-of-magnitude discrepancy compared to the 300 nm min^-1growth rate required by FAD models. These findings demonstrate that chemical corrosion is kinetically insufficient to drive rapid pore channel extension. Consequently, this work challenges the validity of the FAD theory and provides robust evidence supporting the oxygen bubble model, wherein pore growth is governed by the plastic flow of barrier oxide around oxygen gas molds generated by electronic current.