Underbody Corrosion?

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I have a 2016 M6 GC, and it's been my daily driver in Chicago since October 2015. I recently moved to Washington state, and the dealer said lots of the bolts in the suspension assemblies are badly corroded. Like he is not sure they could even get a wrench on them, he said. It is true that I drove it on Chicago's salty winter streets for 8 1/2 years, so I am sure they are corroded. Has anybody else had this problem? Have you done anything about it. The dealer says he will let me know what they would do. I fear they will want to charge me like $200 per bolt to replace them, all to prevent some kind of future catastrophic bolt failure.