A Statistics-Based Approach to Studying Aluminum Pit Initiation

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Abstract
A statistical description of pit initiation in high-purity aluminum (99.99) is developed based on the statistical frameworks established by Baroux [Corros. Sci., 28, 969 (1988)] and Shibata and Takeyama [Corrosion, 33, 243 (1977) and 52, 813 (1996)]. The statistical treatment is used to examine the dependency of pitting behavior on electrode area, scan rate, and chloride concentration. It is shown that the average pitting potential is proportional to the log of the scan rate and the log of one over electrode area, implying an exponential dependence of pit germination rate on applied potential. The constants that describe pit germination rate are independent of area for electrodes of diameter 125 μm and smaller. However, the constants are area-dependent for larger electrodes. Based on these results a homogeneous pit nucleation process is proposed to account for pit initiation on 125 μm diam and smaller electrodes, while a heterogeneous process is proposed to control the behavior of 500 μm diam electrodes. The results from these studies also demonstrate the importance of correctly choosing experimental conditions when studying pitting behavior. The dependencies of pitting potential on scan rate and area are found to be sensitive to electrode size, scan rate, and aggressiveness of the electrolyte. © 2003 The Electrochemical Society. All rights reserved.