I work with and around steel. Lots of steel. And salt water. Lots of salt water. On large, steel, commercial vessels, on the Delaware & Schuylkill Rivers. That, I should think, qualifies me to make the following statement.
It is not possible that your bolt "rusted through" in that time frame (5 months, you said). It may have developed some surface discoloration, or even a little surface rust between the threads. It may have become slightly corroded and bonded to the nut, and then you may have snapped it off trying to tighten or loosen it. But, since you're not storing your bike under a pier next to an ocean, it is a vast prevarication to say that it rusted to the point of failure.