Came across this at an inspection.
The galvanized vent pipe from the furnace has a white power on it.
My guess is the temperature of the furnace is too high. Natural Gas to the furnace was turned off.
Thanks for the help.
Lots of reasons, but for my area, it’s typically due to high humidity, humidifier attached to furnace ductwork, stand-alone humidifiers in room, etc. Colder it gets, dryer it gets, (I’m in Minnesota) peoples crank up the humidifiers here from November until March… and then they usually forget to turn them down/off when out natural humidity kicks in (10,000 lakes and corn fields thing)!!
This might help you here.
White Powder around Gas Vent - HVAC Forum - The Inspector’s Journal
Good info, but, IMO… a different issue altogether.
Well if it is, why don’t you state what it is?
I did.
Well, like you said, it’s an opinion.
Well, mho, would be, white rust caused by oxidation from moisture. 
It is an indication of a flue gas leak which can be caused by a number of factors (improperly sized flue, open joints, improper termination, etc.) Defer to a licensed HVAC contractor.
“White rust is a white powdery deposit formed from a natural corrosion process that can occur on any new zinc coated steel product. It is damaging to the appearance of the steel, not to mention to the anti-corrosion layer of zinc that protects the body of the steel.”
Its all about how much and how long for. The white powder is oxidized zinc, the zinc is a sacrificial coating that protects the steel from corrosion (aka rust). Flue gases are hot and contain large amounts of water vapor, a product of combustion. Corrosion is a normal chemical process, chemical processes are accelerated with heat. When the zinc has been consumed the pipe will rust and need to be replaced. An excessive amount of white powder indicates that something may be wrong with the venting, a small amount is acceptable, especially if it is older installation. A mid efficiency furnace typically requires a smaller vent than a natural draft furnace, a too large vent could have condensation, which will result in more corrosion. Your call, if you think the amount of white powder is excessive, recommend a qualified HVAC or gas appliance tech to repair it.
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