Wash with straight white vinegar (only need a few ounces) and a copper brush (like steel wool) or aggressive weave Scotchbrite type pad. Rinse with water well. Let dry, then paint with rattle-can zinc chromate self-etching primer made for aluminum.
What probably happened here is that the area was full of foam and well sealed off. Metals like aluminum and SS alloys retain their protection of the surface by the layer of oxide from the atmosphere. When this is cut-off - by stuff like foam bearing right up against it, or where a compartment is closed off, or stagnant, and there is no air flow - the pitting effect is actually called 'crevice corrosion'.
Cleaning, prepping and priming will prevent this from being a problem.