Factors Controlling Stress Generation during the Initial Growth of Porous Anodic Aluminum Oxide

Correlation between Defect Density and Corrosion Parameter of Electrochemically Oxidized Aluminum

It has been recognized that a connection may exist between defects of oxide coating and its corrosion protection. Such a link has not been substantiated. We prepare two coatings of anodized aluminum oxide (AAO) and plasma electrolytic oxidation (PEO), and analyze them with Mott-Schottky plots and potentiodynamic polarization scans. The as-grown and annealed AAO coatings exhibit both p-type and n-type semiconductor behaviors. Polarization resistance of the AAO coating increases from (1.8 ± 1.7) × 108 to (4.3 ± 0.5) × 108 Ω·cm2, while corrosion current decreases from (6.1 ± 3.6) × 10−7 to (2.3 ± 0.9) × 10−7 A·cm−2, as annealing temperature increases from room temperature to 400 °C. The parameter analysis on AAO indicates a positive correlation between corrosion current and donor density, a negative correlation between polarization resistance and donor density. The attempt on correlating corrosion potential gives rise to considerable deviation from a linear fit. The results suggest protection of AAO hinges on its donor density, not acceptor. On the PEO coatings, only the n-type behavior is observed. Intriguingly, the donor density of PEO coating is influenced by the annealing temperature of its pre-anodized layer. The most resistant PEO coating, with pre-anodized and 400 °C annealed AAO, exhibits polarization resistance (2.1 ± 0.4) × 109 Ω·cm2 and corrosion current (1.7 ± 0.4) × 10−8 A·cm−2.

Nonlinear evolution of a thin anodic film

The formation of pores in anodic aluminium oxide films is treated with a model equation. The model treats the oxide layer as a thin viscous liquid in two dimensions. Surface tension on the top boundary, electrostriction due to the external electric field and mass flow through the bottom boundary due to oxide formation are all included. Viscous flow is treated with the creeping flow assumption. The model equation is solved numerically using a Fourier spectral method in space and Adams–Bashforth/Adams–Moulton methods in time. Initial conditions include sinusoidal shapes as well as random shapes. The results show that pores form at the trough of the initial sinusoidal shape. Random shapes get smoothed before forming pore structures with spacing different than predicted by linear theory.

The Role of Stress in the Self-Organized Growth of Porous Anodic Alumina

Ridges and depressions were formed on the barrier layer during chemical and physical etching of porous anodic alumina (PAA) from the bottom side, indicating nonuniform etching rate around each cell. These behaviors cannot be explained solely by the well-known composition variation, but were in line with the hexagonal distribution of stress within the barrier layer of each cell. Such stress variation should be attributed to the interactions of neighboring cells undergoing volume expansion. These interactions could account for the self-organization of PAA.

Self-ordering dual-layered honeycomb nanotubular titania: a study in formation mechanisms

A model for the development of a novel self-ordering honeycomb morphology of TiO₂ is presented using a unique anodization process.