But it's an emergency system, not a general operation system. Thus it's not going to be exposed to the salty environment most of the time. You could certainly put the whole thing in an airtight box.

Look at how the military builds surface-based missiles these days: it's in a factory-sealed box. Molten salt batteries so they last for decades. (You don't see molten salt in most purposes because once it's been triggered it's lifespan is in minutes or even less. They're used in applications that only need to deliver power once.)