AlCu solid-solution alloys were dealloyed by immersion in alkaline solution to produce a high surface density of copper nanoparticles, simulating the region around a cathodic intermetallic phase in an industrial AlCuMg alloy. The cathodic behavior of the copper-enriched surface in neutral chloride or sulfate solutions showed an abnormally low limiting current density for oxygen reduction, less than 10% of the value shown by a planar copper electrode, but if the solution was substituted by a borate buffer, the limiting current increased sharply. In the unbuffered solution there is corrosion of the aluminum by cathodically generated alkali, and the surface pH rises to the value where the net or apparent cathodic limiting current density maintains the surface pH at about 9 [for a polarization potential around -800 mV (SCE)]. This process greatly reduces the available cathodic current to drive remote pits or crevice corrosion sites. The implications for corrosion in real environments of varying buffer capacity are discussed.