I recently had my upper plenum for my TPI shot blasted. It looked nice for a month or so, and now it is starting to corrode. I haven't spilled anything on it, so I am not entirely sure why it is doing this. Any info would help, thanks.

 

i'm gonna guess oxygen... lol
I think you probably should have had it clear coated as well. Aluminum corrodes in open air just like steel, it just takes longer. Also like steel, once the original surface is compromised, it will corrode much faster.