Serious corrosion of vent connector duct in gas hot water system
I would start with a couple of small 1/2” holes if you disable the damper totally you loose efficiency quickly where several smaller holes will allow a draft and the combustion moisture to dry 15 years is a fair amount of time but I have had systems last much longer in some cases when doing annual inspections I have recoated single wall with zinc rich paint it could have the last piece dipped in a batch but excess moisture is probably the cause so start small and see if that works without totally disabling the energy saving feature of your damper.