In this instance we did not use anything to prevent the pipes from rusting. The client wanted it to rust, that was part of the design. FYI they do have rust preventative products that can be used both to prevent steel from rusting and to stop the process once the steel gets to the look you want. There are also special steels (Corten http://www.corten.com/frequently-asked-questions.html ) that rust to a certain point but then slows the process.