If you're keeping it in a leather holster - that may be the problem. Leather will pick up humidity which in turn will be translated into moisture against the metal. You also don't state how often you clean and wipe the pistol down? Over the last 50 years, I've found that keeping a firearm wiped down and oiled works wonders at preventing rust and damage. The same goes for any handgun carried in a leather holster for CCW - if you're in an area with high humidity or a warm climate where you sweat a lot - you'd best be keeping the handgun wiped down and make sure that your holster isn't retaining moisture - it doesn't take much to cause rust overnight.
If a pair of '51 Navies were good enough for Billy Hickok, then a single Navy on my right hip is good enough for me . . . besides . . . I'm probably only half as good as he was anyways. Hiram's Rangers Badge #63