I have the MBRP cat back exhaust. I origonally had the axle back piece that connects to the muffler sent to me seperately because I didn't buy their muffler. Well, after about 3 months of use or so, I changed it out for the HKS muffler that already has the proper axle back piece on it. I inspected my used last set up and discovered heavy corrosion aroun the flange. It was covered in rust. This worries me because it could wear away at the gasket. Is this normal wear and tear?

 

More than likely that the mid pipe and the muffler flange was not tighten down properly, causing exhaust to leak. The further down the exhaust system the less heat there is (water vapour will not evapourate), thats why the muffler usually rust out before the mid pipe or front pipe. And since the performance exhaust are usually lowder than stock you may have not heared it.