Ah, yes. Dielectric Union. That's what I was forgetting about.

@HOHLfmly - the only way I know to prevent this is to use a PVC unions (or better yet - a dielectric union, as the other commenter mentioned) in-between the galvanized pipe to the brass valve. Usually you can get all the couplings to be the same type of metal, but specifically for check valves (one-way valves) , I've never seen them made of galvanized - only brass or pvc or stainless steel. If I were to do this now, I'd use PVC.