Influence of Fe-rich intermetallic inclusions on pit initiation on aluminum alloys in aerated NaCl

The role that Al sub 3 Fe intermetallic inclusions play during initiation of pitting corrosion on aluminum alloys was investigated in 0.6 M NaCl. In aerated solutions microscopic observations showed the growth of cavities in the host metal adjacent to inclusions. The rest potential of synthetic Al sub 3 Fe was measured in aerated and deaerated NaCl solutions over a range of bulk pH values between 2 and 12 and was found to act as a cathode. In aerated solutions rotating disk electrode experiments on synthetic Al sub 3 Fe electrodes verified that the cathodic reaction corresponded to the reduction of dissolved oxygen. With scanning pH microelectrodes, measurements were carried out near synthetic Al sub 3 Fe electrodes which were coupled in a galvanic cell with Al-6061 in NaCl solution. In addition, the pH measured over Al-6061 at a distance of 25-30 mu m from the surface was observed to fluctuate between pH 4 and 8.5 for the first 2 h of immersion in NaCl solutions. Atomic force microscopy images of Al-6061 immersed in buffered aerated 0.6 M NaCl solution (pH 5.5) did not show any evidence of dissolution of the Al around intermetallics. These measurements support the view that Al sub 3 Fe particles in Al-6061 serve as local cathodes, that a high pH develops around the intermetallic panicles and creates cavities in the host metal, and that interactions among a cluster of intermetallic particles causes a large number of slowly dissolving alkaline cavities to evolve into a small number of rapidly dissolving acidic pits.