White stuff on exhaust pipe

Hi everyone.
Can someone tell me what this stuff is. It is inside the furnace and it is on the exhaust pipe. Looks like some deterioation but not sure from what. I’m thinking something to do with condensation maybe.
Any help is apprdciated.

It’s a deposit left from poor fuel combustion. Connections may be loose and allow condensation to leak through.


Thanks Martin. Appreciate the quick reply.
Have a great day

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