If you get stainless steel exhaust, everything is stainless except the welds. The welds begin to rust first as in your photos and then spreads to the exhaust which makes a hole. This has always been a problem for me anyways. Use a circular wirebrush on the end of a drill. That rust will be gone in no time. Maybe use hi temperature paint like the kind they sell for painting calipers. The stock pipe between the cat and the exhaust usually has a splash guard welded to the pipe which protects it better.