Origins of Electrochemical Noise during Pitting Corrosion of Aluminum

The pitting corrosion of aluminum was investigated by measuring electrochemical noise using interconnected electrodes, together with an artificial-pit technique and a corrosion-visualizing method employing an agar gel containing a pH indicator. The electrochemical noise in chloride solution at open circuit shows distinct transitions at which vigorous and meandering fluctuations in current and potential commence. We demonstrated that the transients occur when the pits continue to remain active, and also when more than one pit is present. The noise, lasting up to thousands of seconds, is not due to the superposition of multiple metastable pitting events but reflects electrochemical processes occurring within active pits. The electrochemical transient during propagation of a single pit can be modeled in terms of cathodic oxygen reduction and discharging of capacitance at the passive surface. When more than one active pit is present, the pits interfere with each other because the impedance of the active pits is lower than that of the passive film, and the pits can generate cathodic currents. The complex, slow meandering patterns observed in the electrochemical noise are due to electrochemical communication between the pits.