Corrosion on hot water heater pipes

I haven't seen it on any of my water heaters, but then they are copper lines, soldered in. Looks like it's galvanic corrosion where different metals meet. The new union is dielectric, was the old one too? Looks like it could be, I see some red plastic at one end. If it was, then using dielectric didn't help. I've read many explanations of dielectric, how it's supposed to work, but I've never read an explanation that actually made sense. And how much of a factor this is or isn't probably depends on differences in small ground loop currents in one installation compared to another. Me, I just solder in using copper all the way to the water heater fitting, no dielectric unions, no flex pipe, no problems. And water heaters went in that way for decades. I think you'll find differences of opinion among plumbers too. Plus I guess is if it lasts the life of the water heater that's all it needs to do and if it looks bad before that, it's not hard to change out. Worse would be it failing from the inside, where you can't see it. I'd cut the old one apart and see what it looks like inside.

Another factor would be if the galvanic electrode in the tank is still there or has it been used up? That's there to protect the tank, but it likely helps protect the piping too. There are differing opinions on that too. Some say to remove it from the tank and check every few years If it's mostly gone, replace. Others say that the tanks have various failure modes and that by the time the electrode is gone the tank likely doesn't have much life left anyway, so don't bother.