I plan to use a galvanized steel patch on this ductwork. Will it react with this ductwork? I'm not sure what it is made out of. Is this ductwork galvanized steel? It's ductwork from a house built in 1967.
Yes. What you have there is also galvanized steel. The older galvanization process resulted in a larger color pattern than modern electroplating sometimes does, but it's the same beast. Patch away.