Bushmaster bolt and carrier. Generally kept fairly well maintained except a lot of the time I don't take the bolt out of the carrier and scrape the carbon off the bolt tail. Also don't worry too much about the carbon inside the carrier. Yesterday I noticed pretty definite pitting on the shoulder running up to the gas rings! Anybody else experienced this? It can get humid here in TN but it's not that extreme(I thought). Anybody?
ETA : lubed previously with Breakfree CLP but  have been using FP10 for the past 3 years(great stuff). And I generally like things sloppy wet, but that lube gets blasted pretty hard inside an AR bolt as we all know.

This is the only reason why I remove the carbon off the back of the bolt. Carbon asbords mostuiure and will cause pitting.  I have seen it on an oly arms bolt as well as a Colt. It takes years but does happen.

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.

Blank - where in TN? I'm near Springfield.

Im in the Springhill area...south of Nashville.

Hot water + brush + simple green or carbon killer should do the trick.

but be sure to rinse the simple green off VERY WELL and dry your parts with a hair dryer.  I've also found dishsoap and a toothbrush to work very well for gun cleaning.  It even removes ALL the old dirty oil.  It's like starting new again.

You don't need to remove the carbon..............it's self limiting, remember!