I believe 400 series stainless is magnetic and 300 series is not, unless certain machining is perforned on it such as threads and such.
400 series has little to no nickel in it which makes it magnetic and is also cheaper in price to produce. 300 series has more nickel and is in turn non magnetic and cost is increased due to the nickel content. I would say the exhaust is 400 series stainless. 400 will rust; or oxidize, if not cared for over time.