Change in Oxygen Reduction Reactivity of Intermetallics: A Mechanism of the Difference in Trenching around Al–Fe and Al–Fe–Si Particles on AA1050 in NaCl

Trenching around intermetallic particles on AA1050 aluminum in 0.1 M NaCl at pH 6.0 was analyzed by in situ observations. Deep trenches, which become the initiation site for pitting, were formed around the Al–Fe–Si particles but not around the Al–Fe particles. The open-circuit potentials of the bulk intermetallic compounds and Al-matrix of AA1050 without intermetallic particles were measured. It was determined that the Al–Fe–Si and Al–Fe particles were cathodically polarized by the Al-matrix of AA1050 under natural immersion conditions. This cathodic polarization was found to change the oxygen reduction reactivity of the intermetallic particles. The cathodic reactivity on bulk Al–Fe was higher than that on bulk Al–Fe–Si under as-polished condition. However, after potentiostatic cathodic polarization, the cathodic reactivity on bulk Al–Fe decreased, whereas that on bulk Al–Fe–Si increased. Micro-electrochemical measurements and surface analyses clarified that the change in the cathodic reactivity of the intermetallic particles plays a critical role in trenching on AA1050 aluminum.