During 40 years, I have never seen a seriously rusted duct system inside a home. Most residential humidifiers can not add enough moisture to make saturated air and the humidistat would shut the humidifier off to limit the humidity. A heating duct would be insulated on the outside to keep the metal duct warm enough to keep the dew-point high. A cooling duct would be insulated on the inside to keep the metal duct warm. Most ductwork is insulated so thinly with a 1-inch wrap that the metal stays above dew point in a cold attic or crawl.
An exception would be a poorly insulated duct in a cold attic or crawl where humid air from a well-sealed home would gravitate to the cold duct. This poor level of installation should not be tolerated.