The Location of Events Producing Potential Transients During Pitting of Freely Corroding Al and Its Alloys

The behavior of a number of Al alloys have been studied using open circuit potential transients in conjunction with current density mapping, imaging, and time-delay of arrival of potential changes. Generally, potential transients are analyzed as noise relating to initiation and repassivation of metastable pits, a behavior demonstrated for stainless steels. However, measurements on pure Al, using segmented electrodes and imaging methods, show that most transients emanate from growing pits. Comparing behaviors of alloys shows that AA1100 and AA2024 are similar to pure Al. AA7075 shows similar transients but only a thin highly deformed surface layer is locally attacked along the abrasion direction. AA5083 in contrast first shows transients dominated by metastable pitting and only after hours of exposure do events within growing pits dominate the production of transients.