Stable pit formation on AA2024-T3 in a NaCl environment

Scanning vibrating electrode (SVE), particle induced X-ray emission spectroscopy (PIXE) and standard electrochemical measurements were used to study the establishment of stable pits on AA2024-T3 in neutral sodium chloride solution (0.1 M NaCl). Pits were allowed to develop until hydrogen evolution was observed. Typical current at the mouth of the pits were in the vicinity of 1 mA. PIXE maps revealed the intermetallic (IM) particle distributions in the surface as well as significant chloride buildup around the pits. A significant fraction of the small selection of pits examined here appeared to have an S-phase particle (or remnant) within 20 µm of a AlCuFeMn type IM particle suggesting a coupling between the two. The electrochemistry of the coupling between different IM particle types was further investigated using potentiodynamic scans in 0.1 M aqueous NaCl solution of macroscopic electrodes made according to the IM particle compositions. Current densities at the open circuit potential of AA2024-T3 were largest, typically (0.2 up to 1 mA/cm2) for phases that were anodic with respect to AA2024-T3. Coupling of the IM particles was examined by comparing the degree of clustering around chloride attack sites to the average particle density for each map. There were significantly higher number of IM particles surrounding pit sites than the average IM particle densities indicating that local clustering played an important role in pit initiation.