Anything other than minor rust or staining of the rods or nuts is cause for concern. The nuts should be removed from rusting fasteners, one at a time, and the threads of each inspected for signs of crevice corrosion. Deformed, pitted, or galled threads mean that a fastener is compromised. Options in this case are few. If the stud is long enough, its nut may be shimmed to move it to an area where the threads are unaffected. In a worst-case scenario, studs may have to be replaced, a substantial and costly undertaking.