Its aluminized steel. I'm not sure how its done but it seems like they put a thin aluminum coating on the surface of the steel. This helps protect the steel since aluminum is a stronger oxide former than iron, so it forms a dense protective oxide layer over the pipe that keeps the steel from rusting (anodic protection). The problem is that these exhaust systems are welded after this process, so the first area to form rust is the weld joints which are unprotected.
I have a BBK catted (goddamn emmissions) x-pipe that I put on 1.5 years ago, and its aluminized, and its still clean and kinda shiny, except for the welded areas where its starting to rust. A shot of exhaust paint prior to installation could be the answer though. So these systems can last you a long time with some added extra care, but if you want something that will last for a very long time and not worry about it get stainless. If I had to do it again I'd go with a completely stainless system.
