I've played around with tube flies for salt, in fact because of the way coho hit the fly the hook rates are radically higher with a tube than a standard fly. Long shank hooks are better than standard, double snells like the moochers use are the highest hook rate but the tube is a close second.
I use the nylon tubes for salt, no corrosion at all. The only problem I encountered was when the plastic got compressed by over-zealous amount of materials and thread (for other's sake coho flies are large!). I had a hard time getting them off the mandrel then. Eventually did, of course.
Metal tubes do not have this problem.
Also at the hobby store is aluminum tubing suitable for use in SW tube flies. Although it won't rust I suppose the pitting could damage line if not washed after every use.
I had this really crazy tube fly tied with rabbit fur, flashabou, grizzly hackles, etc - boy did it ever catch hooknoses! I landed several in the mid-teens that trip. We each averaged over 20 fish per day at Sekiu, Washington in early September's fall coho run. When the fly would run up the leader from the fight, another coho would often appear in pursuit of the fly now dangling up the line. Wild times!