Anodizations of Al and Ti in NH4F or H3PO4 Solutions and Formation of Porous Anodic Alumina with Special Morphology

Abstract

Porous anodic alumina (PAA) and anodic TiO2 nanotubes (ATNTs) have been widely investigated for decades. However, their formation mechanism and growth kinetics remain unclear. Here two unconventional and two conventional anodizations of aluminum and titanium are contrasted to overcome this challenge. PAA with special morphology was fabricated in NH4F electrolyte for the first time, which cannot be explained by the popular field-assisted theory. Combining the oxygen bubble mold and the oxide flow model with the dissolution model, a new explanation for the special morphology is presented. In addition, anodic titanium oxide films with plenty of cavities were obtained in H3PO4 electrolyte, which absolutely differ from the general compact films. The cavities in anodic titanium oxide films also result from the oxygen bubbles within the oxide films. The anions in electrolyte (e.g., F–, OH–, PO43–) accumulate, and the anion-contaminated layer (ACL) forms due to the electric field. The ACL plays a decisive role in the generation of the electronic current (Je) and the formation of oxygen bubble mold. Regular ATNTs were obtained as titanium was anodized in NH4F electrolyte. The ACL forms and appropriate Je generates because the dissolution from F– on TiO2 is limited. However, PAA with special morphology was obtained as aluminum was anodized in NH4F electrolyte. Thicker ACL results in the impulse peak of the Je because the dissolution and corrosion from F– on alumina is severe.