I am a civil engineer in charge of mechanical design at a large public agency in California. We use black iron (really steel with a light coating) for steam in small diameters. We also use 316 stainless. I don't know what a ground joint union is, but if it is and un-insulated union, it won't work long term to prevent corrosion and leaks when connecting SSTL to steel pipe.
We have not found (so far) steel to stainless insulated unions, so we use flanges and flange insulation kits.
Incidentally, we get better performance with paint linede and coated plain steel pipe than galvanzed pipe (salty environement)
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