Porous anodic alumina (PAA) and anodic TiO 2 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 NH 4 F 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 H 3 PO 4 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 − , PO 4 3− ) 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 (J e ) and the formation of oxygen bubble mold. Regular ATNTs were obtained as titanium was anodized in NH 4 F electrolyte. The ACL forms and appropriate J e generates because the dissolution from F − on TiO 2 is limited. However, PAA with special morphology was obtained as aluminum was anodized in NH 4 F electrolyte. Thicker ACL results in the impulse peak of the J e because the dissolution and corrosion from F − on alumina is severe.
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