2019 Subaru forester, four days old, wife noticed dark rust around the end of the muffler where piece of pipe is welded on. this is supposed to be a new SUV. why
A fairly common cause of "rust" on stainless is contamination with carbon steel ; such as a steel brush used the prepare or clean the stainless weld, also cleaning the stainless with steel/cast iron shot as a Wheelabrator. And normal high temperature oxides on the stainless from production or welding ; with time these usually black oxides become brownish as routine rust. These are only cosmetic conditions . Modern exhaust systems are 13% chrome ( ferritic ) stainless.