Oxide Microstructural Changes Accompanying Pore Formation During Anodic Oxidation of Aluminum

Porous anodic oxide formation results from a morphological instability of uniform barrier oxide growth leading to the establishment of pores. To gain insight into this process, evidence for microstructural changes in the oxide accompanying pore initiation in anodic alumina was sought through two typ...

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Bibliographic Details
Published in:Electrochimica acta 2017-04, Vol.232, p.303-309
Main Authors: Ide, Shinsuke, Capraz, Ömer Özgur, Shrotriya, Pranav, Hebert, Kurt R.
Format: Article
Language:English
Subjects:
Aluminum oxide
Anodic dissolution
Anodization
Anodizing
Burning
Burning rate
Conduction
Current density
Density
Electric currents
Lattice vacancies
Metal oxides
Oxidation
Plastic flow
Pore formation
Porosity
Porous anodic oxide
Residual stress
Self-organization
Stability
Studies
Sulfuric acid
Voltammetry
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Summary:Porous anodic oxide formation results from a morphological instability of uniform barrier oxide growth leading to the establishment of pores. To gain insight into this process, evidence for microstructural changes in the oxide accompanying pore initiation in anodic alumina was sought through two types of experiments: potentiodynamic anodizing and stress measurements during dissolution of the anodic films. Cyclic voltammetry during anodizing in sulfuric acid shows that pore formation coincides with the appearance of localized ohmic- conducting regions close to the oxide-solution interface of individual pores. These defects induce large current increases which are apparently responsible for growth of concave scalloped features on the metal-oxide interface at the pore base. It is argued that interface defects are generated by surface forces in the oxide that accompany flow contributing to pore initiation. Stress measurements reveal that pore initiation introduces tensile residual stress into the oxide at the pore base, suggesting the formation of vacancy-type defects in the film. Such defects are explained by increases of local conduction current density due to the concentration of anodizing current in pores. Both sets of experiments reveal that pore formation generates locally defective oxide, which may help explain the phenomenon of burning during high-rate anodizing.
ISSN:0013-4686
1873-3859
DOI:10.1016/j.electacta.2017.02.113