Study of Single Corrosion Pits on Aluminum in Chloride Media
Wong, Kai Pui
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https://hdl.handle.net/2142/69794
Description
Title
Study of Single Corrosion Pits on Aluminum in Chloride Media
Author(s)
Wong, Kai Pui
Issue Date
1988
Doctoral Committee Chair(s)
Alkire, Richard C.
Department of Study
Chemical Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Abstract
This work attempted to determine the as yet unexplained phenomena of the maintenance of the depassivated state of the metal surface of aluminum pits in aqueous chloride solutions, and the mechanism of rate control of the subsequent corrosion reaction. Novel experimental techniques, including UV and NMR spectroscopy, were used to study the pit chemistry. Results obtained by chemical analysis provided new understanding of the chemical environment within the pits.
A method was discovered for creating single corrosion pits on aluminum exposed to 1 M NaCl solutions at pH 11. Implantation of iron ions through a single 5 um mask opening was used to create a site, which, upon immersion in the electrolyte at $-0.5$ V SCE, corroded preferentially. Individual natural corrosion pits created by this method was round when viewed from above and, during early stages of growth, had shapes that corresponded to smooth spherical sections. Such morphological features are usually associated with anodic dissolution phenomena at surfaces covered with a film of precipitated dissolution products. By measuring the current emanating from single corrosion pits during growth and by determining the variation of pit surface area with time by a sequence of optical measurements, it was possible to obtain the time-dependent variation of current density during pit growth. It was found, for the system studied that the current density decreased with the square root of time and was in quantitative agreement with theoretical predictions. These results support the hypothesis that the corrosion rate was controlled by the rate of dissolution of an aluminum oxychloride film formed by precipitation of dissolution products.
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