Rusting metal flashing details can often be painted with a high quality, exterior-rated metal paint to extend serviceable life and prevent further deterioration.  If you see rusting flashing details atop your roof or around your chimney, I highly recommend painting to prevent further damage; replacement of deteriorated flashing details- especially around masonry fireplaces- can be costly.  These pictures show rust presenting at one flashing detail, corrosion- and due for replacement- at another.  This was discovered at a recent home inspection on Camano Island.