Well I know what your talking about with the humidity.Im a Tennessee man too.
Ive used FP-10 for several years as well.Its good stuff.
I have not noticed any corrosion or pitting on my AR.But I tend to clean the bolt and carrier after every use.If not Ive always coated it lightly with oil.If I do not field strip and clean the bolt I put a drop of oil in each of the gas ports on the side of the carrier as well as a drop in the carrier key,and work the bolt a few times in the carrier.That way things do not get dry.Like GregoryK said,the carbon fouling can attract and hold moisture if there is no oil to protect the metal.Plus a little oil helps break the fouling down and keep it soft between cleanings.
That said I did neglect the muzzle brake on my Armalite A2 for sometime.I finally noticed alot of carbon bulid up as it started to flake off the ports.Once removed there was some light pitting in the ports.So dont over look the FH or MB either.