metal talk

[luke91]

1. Aluminized Steel piping from a muffler shop - This is cheap, readily available, and comes in a variety of sizes (for NA applications, any intake piping should be no bigger than the diameter of the throttle body).  It resists rust fairly well, and can be welded with mild steel wire.  The seams and welds will rust in time, though, and should be painted to prevent this.  
As far as rusting on the inside - if used for an intake, use an oiled-cotton filter, and KEEP OUT OF WATER to prevent rust.  

2. Stainless Steel - Probably the best, imo.  It's fairly easy to find, easy to weld (ONLY with stainless steel wire and straight Argon gas), never rusts, and shows the best.  

3. Aluminum piping - Hard to find, and expensive.  Aluminum will never rust, but it still corrodes.  Should NEVER be used on the exhaust side, because it will deform and smell bad at the same time.  It's easy to weld, but different, and should ONLY be welded with aluminum wire and straight Argon gas.  
Aluminum does not conduct heat as well as steel, making it ideal for intake and intercooler piping.  Can be polished to resemble stainless steel.  It's also lighter than steel.  

--luke