Soda blasting is fine with mag cases. A new blaster will cost about 40 bucks at HF. Following the blasting I use the hose to rinse really well then give it a dunk in the bathtub for a bit to make double sure any leftover soda dissolves.

CO2 blasting is great, but can be abrasive if pressure too high. They use it in museums for cleaning old books at very low pressure but it can also remove rust at high pressure. Not to mention the price of a CO2 blaster is $$$$$.