Before stating the obvious, lets talk about this vent for a moment.
Understanding how dryers work will help explain why this roof damper is deterirating from excessive rust. Dryers bloiw hot air into the drum, which causes water vapors to evaperate, and then it removes the water vapers through the dryer vent. On average, it can remove aproximatly 1 gallon of water per hour. If the dryer vent has lint in it, the lint will become damp during this process. If lint is left in the vent for extended periods of time, as in years, this damp lint will cause discoloration and rust in the vent.
One of the reasons we do not use the brush and vacume method that most of our competitors use is because it cannot remove lint from the bottom of the goooseneck where the vent terminates, leaving lint there, which in turn, allows for the gooseneck to rust out. Nonetheless, this level of rust is rare and if the roof wasn’t going to be replaced in the next year, we would have recomended replacing it through a roofing company. Instead, we were able to clean this vent and cover the spots where it was rusting through the side of the walls.