Are you sure it is aluminum? I can't imagine an aluminum keg standing up to the abuse kegs get for very long.
I would use some pbw and a green scrub pad a d get the whole inside shines, rinse well with water, then boil water in it, that will reform the oxide layer.
Don't take any more faith in anything I say than you would anyone else on the internet. If you listen to what I say, then hurt your self or break something it is your own fault, I am just expressing my opinion or experience.
THINK for your self!!