Magnaflow uses 409 grade automotive stainless steel. This particular grade of stainless has the tendency to form a protective layer of surface rust in wet/humid/snowy climates or when exposed to salted roads. This layer is superficial and will not penetrate the metal itself. It can be removed using soap and water and a steel Brillo or S.O.S pad. In the rare event that 409 stainless steel pipes become rusted through, Magnaflow's lifetime warranty will cover replacement.
Additional Information On MagnaFlow Performance Mufflers:
You can find additional information regarding the characteristics of 409 in the link below: