In reply to Rob Kennard:
Just to put my point of view across, given that I work on ships and the pipe I use is 316 SS and lasts for years getting splashed by the sea and they have 0 rust on them.
The 316 part is the chromium oxide in the steel making this highly rust resistance by keeping the oxygen reacting with the steel part of the metal alloy.
If you ever see rust on stainless steel pipe it will usually be a low grade stainless or it will be by a weld where the metal structure will change and make the surface reduce its chromium structure and causing rust.
This is only what I learnt at collage and what I know from pipework.......