Other than steel..
I have used 3/8" copper tubing (in rolls,its softer and more flexible than rigid straight lengths that could crack),on my older plow trucks,when I got tired of replacing rotted gas line tubing about every year..back when I did that to my 74 K20 in 1999,I never had any troubles again,and it was still in good shape when I scrapped the truck last year,(had a thick green overcoat,but no salt damage!)..
Unfortunately,copper does not meet the safety requirements of brakes,even if you use the thicker "air brake rated" tubing,which is quite costly..I see many "buggies" on cranberry bogs made from defunct 4x4 trucks,cut down and made into bog sanders ,that use copper tubing for the brakes,they hardly ever fail due to corrosion,and have never seen one pop from a panic stop,but you cant trust it on the street..DMV here wont give a sticker if its not steel or stainless tubing..I'd trust copper more than a crusty steel line!.
It was only about 10 bucks for a 10' roll of 3/8" copper tubing in 1999,its probably triple that now though..got mine at Home Depot...
Some guys tried covering their fuel ,brake,and tranny lines with rubber hose,in an effort to preserve them longer and keep salt off of them--but the steel lines rotted anyway--seems salt finds its way in,and eats it out even worse!..I've thought about encasing mine in some of that milky flexible vinyl tubing they sell,and pumping some grease in there ,thats how much I hate changing rotted steel lines on vehicles here!..my tranny cooler lines will be the next ones I'll be replacing--AGAIN!!..
