Caustic soda and aluminum do not like each other either. The muffler will not be glossy smooth when you get the grease off. Slightly acidic rinses with like a PH of 5.5-6.5 leaves the aluminum more shiney.

Maybe the acid afterwards fixes it back up?

Caustic oven cleaners will turn your muffler a dull medium gray color in short order.

Enjoy,

Jim