Road salt has changed from years gone by. It is now mixed with magnesium chloride, this will pull moisture out of the air even in the summer if not washed off. Road crews are also using liquid deicer that is way worse than salt.

My wintertime 84 Bronco 2 of 17 years would keep going with yearly spray cans of paint on the frame / under body. In the past 5 years what used to be solid dark rust has been replaced with light brown fuzzy rust and is also appearing in places that never rusted before.