This has appeared under a kitchen sink... hot water inline feed only. It appears to have started after I put a quarter turn ball shutoff valve in a couple years ago. You'll notice the new shutoff valve is also looking strange, corroded or encrusted with something. Its mostly red (not as bright red as the photos, they really bring it out) and also white crust. While the pipe joint is moist, its not dripping at all, and there is no problem with water flow or any discoloration of the hot water.
Galvanic corrosion? But why here, there are galvanized pipes all over this older building with stop valves put on, this is the only one rusting like this. The location and age of pipes would make this a nightmare to replace, and I fear the pipe threads are rusted together so no removing.
Is there a danger of bursting? It doesn't seem so but that is my worst fear. Its not even dripping.
What if I wrapped the whole thing in Flex Tape?
I've made Flex Tape repairs on hot water lines that have lasted decades. What if I sprayed Rust Neutralizer on it first, would the tape still stick? Epoxy? Naval jelly to remove rust & repaint?
It seems there would be some cost effective smart solution along the lines of 'if it aint broke don't fix it'. Anyone can tell you to rip out and replace. Yes if I were made of money. I am sadly not.