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.