Microelectrochemical studies of pit initiation at single inclusions in Al 2024-T3

The onset of pitting at single inclusions on aluminum 2024-T3 has been observed in the micrometer range by using a microelectrochemical cell in order to study the mechanisms by which intermetallic particles, particularly AlCuMg and AlCuFeMn, induce localized corrosion. The results showed that pitting started at only few AlCuMg inclusions within 200 mV (saturated calomel electrode, SCE) of the pitting potential of large scale measurements. Pit initiation at lower potentials was found to occur at the adjacent matrix of AlCuMg inclusions. Measurements stopped which were made during pit initiation (maximum current = 10 nA) did not indicate the presence of adsorbed copper around inclusions, but measurements which were made during pit propagation (maximum current = 1 mu A) showed adsorbed copper. Compared to the AlCuMg inclusions the onset of pitting at AlCuFeMn inclusions occurred at potentials 200 to 600 mV more positive. Microcracks in the bulk material, presumably formed by hydrogen evolution, were found to provide starting points for pitting. The onset of pitting at areas without inclusions occurred at potentials higher than +500 mV (SCE). If copper was homogeneously dissolved in the host matrix the resistance against pitting was strongly improved.