That's because of the heat. It promotes chemical reactions. It's also because you live in the corrosion capital of the world. The humidity coupled with the salt is a lethal combination.

Just spray the trouble areas with some (at least three coats) heat-resistant rustoleum paint. Once-a-week rustoleum undercarraige treatment is a must for anybody living in the northeast (a $3 add-on at most decent car washes).