Why don't you paint the black iron? The portion of my black iron propane line that is outside rusted in less than a year. It's now painted with some leftover POR-15.
I've used galvanized for air lines in the last 4 places I lived. Removed and re-used most of it each time I moved. Still looked OK on the inside after the last move. Cheaper to buy 21 ft lengths at a plumbing supply.
Many of the chemical plants I've been in use galvanized for instrument air lines and never have problems with flaking.
