Electrochemical current noise on aluminum microelectrodes

Aluminum disk microelectrodes were used to investigate electrochemical curr ent noise in pH 8.8 borate buffer. The current noise spectra, expressed In terms of the current spectral density, had a characteristic two-plateau str ucture in the experimental bandwidth of 0.05-50 Hz, were potential-independ ent, and increased Proportionally to electrode area. Injection of NaCl solu tion near the electrode surface, at potentials below that of the onset of p itting corrosion, caused 0.1-1 Hz current fluctuations to appear. From the frequency and area dependence of the current spectral density in the chlori de-free solution, it was concluded that The noise arose from a number of di screte, approximately evenly distributed voltage noise sources positioned e lectrically in series with the inner barrier layer of the oxide film. A mat hematical model for the current noise was developed which described a physi cal mechanism for noise production based on fluctuations in the widths of c racks or pores in the outer part of the surface mm. The model was consisten t with the observed area and frequency dependence of the current spectral d ensity, suggesting that the physical process it described is a possible mec hanism of noise generation. It could not be determined whether the noise so urces were isolated defects or flaws, or pores in an outer precipitated por tion of the oxide film. (C) 1999 The Electrochemical Society, S0013-4651-(9 8)01-075-1. All rights reserved.