Leaking furnace exhaust pipe

I have a Ruud furnace exhaust pipe that has a leaking joint inside the furnace case (see the red arrows in the picture). The joint appears to be sealed with some kind of black silicon. A few questions I have about this situation:

- Was this joint sealed at the factory or by the installer?
- Why is silicon used instead of cement?
- What's the best way to reseal this pipe? Should I cut out a section of the pipe on the right and refit this connection?

Thanks

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source https://hvac-talk.com/vbb/threads/2244042-Leaking-furnace-exhaust-pipe?goto=newpost