SEDL / STP / STP1188-EB / STP18074S

Evolution of Electrochemical Impedance During Sealing of Porous Anodic Films on Aluminum

Dawson, JL
Senior consultant, CAPCIS Ltd/CAPCIS-MARCH Ltd, Manchester,

Thompson, GE
Professor, Corrosion and Protection Centre, UMIST, Manchester,

Ahmadun, MBH
Scientist, Metal Industry Development Centre, Selangor,

Pages: 21    Published: Jan 1993

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Abstract

Electrochemical impedance spectrocopy (EIS) has been used to study the hydrothermal sealing of porous anodic films on aluminum. The 5-μm and 15-μm-thick films, originally formed by anodizing 99.99% wt% aluminum in sulphuric acid, were immersed in boiling distilled water and in-situ impedance measurements were made continuously during the sealing process. Comparisons of the spectra were also made after removal of the films from the boiling water bath after various times and immersion in distilled water at ambient temperature.

Analysis of the EIS data, presented as Bode plots of impedance and phase angle against frequency, showed development of hydrated films and decrease of the original barrier layer thickness. Electrical equivalent circuit models were used to interpret the detailed changes in solution resistance, hydration of the films (Rh and Ch) and the barrier layer (Rb and Cb). The major difference between the in-situ boiling water and the cold water spectra was considered to be the result of alumina gel formation at higher temperatures, with distinct precipitation at lower temperatures. Evidence of precipitation, at the outer film surface, pore mouth blocking, and eventual hydration within the pores is also presented.