Recent investigations
have shown the importance of the dealloying of
S-phase (Al2CuMg) particles on the corrosion
of aluminum aircraft alloys, specifically aluminum
alloy 2024-T3 (Ref 9, 10). In 2024-T3, the S-phase particles represent approximately 60%
of the particle population. These particles are of
the order of 1 mm in diameter, with a separation
of the order of 5 mm representing an area surface
fraction of 3% (Ref 10). The selective removal of
aluminum and magnesium from these particles
leaves behind a porous copper particle that
becomes the preferential site for oxygen reduction
(Ref 9). Because the copper particles are
small in size and separated over fairly large distances,
they behave as microelectrodes and can
support much higher oxygen reduction rates than
might be expected (as high as 100 mA/cm2 or
645 mA/in.2) (Ref 9). These sites serve as the
cathodes to drive the corrosion of the aluminum
matrix, thus releasing more copper on the surface
to act as cathodic sites, further driving the corrosion
reaction.