Double-wall flue (bvent) connections have an aluminum inner liner which would not rust, but might corrode if subject to acidic condensation. Condensation in the chimney depends upon the amount of heat the appliance delivers to the chimney and how the chimney is constructed. A new chimney built to new code would have an inner clay liner spaced away from the surrounding brick to act as a double-wall in itself to prevent condensation. Older constructions with the tile liner tight against the outer brick makes the walls of the chimney colder to produce condensation. The only way to tell would be to have a temperature reading of the upper part of the chimney while the unit is in operation to tell if it is above the dew point. Water collecting at the base of the chimney is a sure sign that you need an additional liner.
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