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We compare methods of analyzing electrochemical current (ECN) and potential (EPN) noise data associated with metastable pitting and the transition from metastable to stable pitting. Various analysis methods were applied to electrochemical noise data associated with metastable pit events on aluminum, aged Al-2%Cu, and AA 2024-T3 ST. Two experimental approaches were used. High-purity Al, roughly simulating copper-depleted grain boundary zones in aged Al-Cu alloys, was potentiostatically polarized so that current spikes associated with individual pitting events could be analyzed. Second, the coupling current between nominally identical galvanically coupled Al, aged Al-2%Cu, and AA 2024-T3 ST electrodes was recorded in conjunction with couple potential using a saturated calomel reference electrode. Pit stabilization occurred when individual pits exceeded a threshold of Ipit/rpit > 10-2 A/cm at all times during pit growth as established from potentiostatic measurements. The magnitude of this ratio is linked directly to the concentration of the aggressive solution within pits. Two related statistical pit stabilization factors (Irms/rpit total from ECN data and the mean of (Ipeak-Iox)/rpit values from each pit current spike) were obtained from galvanic ECN data containing a large number of pit current spikes. These parameters provided a better indication of the transition to stable pitting than the pitting index or noise resistance but also had shortcomings. Spectral analysis using current and potential spectral power density (SPD) data provided qualitative information on pit susceptibility. However, the transition to stable pitting could not be accurately defined because of a lack of information on pit sizes in spectral data.
AA 2024-T3, aluminum, constituent particles, copper-depleted zones, maximum entropy method, metastable pitting, noise resistance, pitting index, pitting potential, spectral power density, pit stabilization factor, repassivation, S' particles, stable pitting
Ph.D. candidatemember of technical staff, University of VirginiaRohm and Haas Chemical Co., Philadelphia, PA
Associate professor, Materials Science and Engineering, University of Virginia, School of Engineering and Applied Science, Charlottesville, VA
Full professor, Chemical Engineering, University of Virginia, School of Engineering and Applied Science, Charlottesville, VA