Originally Posted by RA40

The galvanized pipes on many stock systems are pretty durable. Have aftermarket exhaust done on those stock pipes and the re-welding ruins the coating in that area. That leads to it rusting over time. As well, various environmental factors can also have a say. Stainless is stain/corrosion resistant and even my days in the machine shop, these steels can rust under the circumstances that allow it, quiet easily at times. Once it starts, it will often grow larger and larger.

Some of the very old cars, getting the slip fit pipes off is a pain. Even mine, the rear mufflers are stuck pretty good. I got one side off but when the other didn't want to budge, I said "forget it".