I took off my braided line to replace it with fluorocarbon after fishing one day, and I noticed how moist it still was, even after several hours later. My question is that since braided lines retain water, is there a potential for rust to form on a spool?

Very short answer.....NO. 

Spools are made of aluminum, so no.

   moment... thank you guys haha.

Corrosion yes. Especially if used in a saltwater environment.

Corrosion yes. Especially if used in a saltwater environment.

This is very true. I assumed it was a freshwater outfit.

Rust is iron oxide, I don't believe braided lines contain any iron or any ferrous alloy.  Aluminum is fairly resistant to corrosion, but can occur.  Generally speaking the line on the spool reduces the amount of oxygen coming in contact with bare aluminum, oxygen is the catalyst for rust and corrosion.  With minimal maintenance a saltwater reel will not corrode.

If one were to be overly concerned, simply taking the line off after each outing then wiping dry or using hair blower would eliminate any problem.  I think that's quite a bit over the top.

 

My final step in maintenance is spraying the reel with silicone, some people use a protective wax.  I have not seen any difference from my unsprayed reels compared to the sprayed ones, I don't it's all that necessary. 

I think it might beat up your spool bearing a little more because it hold water. Reels that I use with braid and just generally more wet overall. Some of that has to get in to places somewhere.