I recently noticed some rusty deposits on the top of my water heater, so I looked up and discovered the galvanized pipes are connected directly to newer copper pipes. I understand this is not a good idea unless a dielectric union is used because corrosion occurs quickly. My question is, how "quickly" does it occur, and should I make it a priority to replace these fittings ASAP? Should I replace with dielectric unions, or should I replace ALL of the old galvanized pipes?
The galvanized plumbing is from 1951, so it's going on 60 years old. Most of it is exposed in the unfinished basement. I found about 4 places where the old galvanized pipe is fitted with newer copper piping, and attached some photos here. They all look like they are corroding. I'd rather not wait for a leak to develop, but I also don't want to be hasty in incurring the expense of replacing the galvanized pipes all the way back to the street if I don't need to. Is it better to replace with copper or PEX?