Electrochemical Current Noise on Aluminum Microelectrodes

Irregular electrochemical current fluctuations on metals with protective oxide films, such as iron, aluminum, and stainless steel, can occur due to stochastic events associated with initiation, growth, or passivation of corrosion pits. In recent years, this electrochemical noise has been extensively studied on a number of metal and alloy systems. On stainless steel, transient bursts of current in chloride solutions have been shown to be caused by the growth and passivation of very small metastable pits at sulfide inclusions. 1,2 In addition, current bursts in chloride-free solutions have been related to the dissolution of sulfide inclusions. 3 The frequency of metastable pitting events is believed to be influenced by the passive current density 1 or surface oxide film thickness, 2 suggesting a possible relationship between pit initiation and film characteristics. Similar current fluctuations as those on steel can also be found on pure metals, but in the absence of heterogeneities such as inclusions, it has been difficult to ascribe them to features of the film or metal surface. Heusler and co-workers employed spectral analysis to study current noise on iron, both in the presence and absence of chloride ions. 4-6 The addition of chloride ions to the solution produced current fluctuations of magnitude 10-100 pA during the induction period prior to pit formation. With no chloride ions, they showed that the electrochemical noise spectrum was well described by a voltage noise source in series with the oxide film impedance, whose spectral density had a 1/f 2 dependence on frequency below about 10 Hz. The series relationship of the noise source and film impedance implied that the noise was associated with processes at the metal/film or film/ solution interface. Current bursts on pure aluminum have been found in chloride solutions at potentials within a range 100-200 mV cathodic of the pitting potential. 7-9 Typical current bursts were tens of nanoamperes in magnitude and 0.1-1 s duration. Pride et al., 9 through microscopic ob