Title

Electrochemical potential noise generation by aluminum in alkaline media

Date of Completion

January 1998

Keywords

Engineering, Metallurgy

Degree

Ph.D.

Abstract

Pure aluminum generates electrochemical potential noise on contact with alkaline solutions. The intensity of this noise reaches a maximum under moderately alkaline conditions, near pH 10. The phenomenon of electrochemical potential noise generation for aluminum in these electrolytes is taken to relate to the processes accompanying the dissolution of the passive film that is known to protect the surface of aluminum in neutral pH media. An understanding of the mechanism responsible for electrochemical noise generation is relevant, therefore, to the understanding and control of rapid corrosion processes of industrial importance such as pickling, etching and chemical milling, that are governed in part by the dissolution of passive films.^ Experiments have been carried out to investigate the surface film structure of high purity aluminum electrodes in situ in alkaline solution and to characterize the electrochemical and mass transport processes contributing to the generation of electrochemical potential noise. These experiments included classical techniques such as DC polarization, electrochemical impedance spectroscopy and scratch repassivation studies, as well as new approaches such as swept-pH electrochemical noise scans and a novel method for generating electrochemical contrast in the atomic force microscope.^ The generation of electrochemical potential noise by aluminum in alkaline media is shown to be caused by the obstruction of discrete cathodic sites on the aluminum surface by the crystallization of the bayerite form of hydrated aluminum oxide. This process is highly sensitive to disruption in the regime between pH 9 and 11 due to a steep pH gradient that develops across an outer diffusion-controlling layer of the passive film. Electrochemical potential noise measurements in the time and frequency domains are shown to be robust indicators of the presence of unstable passive films and to embody information that is useful in characterizing the development and behavior of these films. ^