rust on one wire

[Brian Duff]

Do you mean one strand of a wire or one whole cable ?

If you have flaking rust(or grainy textured surface to dark brown rust area) on stainless steel it is cracked, crevice corroding and otherwise failing.

Perhaps a rigger may find on actual inspection that this is not the case, but based on your brief description this is my opinion -if it has flaking rust its gotta go, its failing-