We had a fire to the hot tub pump inside the house. The restoration company cleaned all of the PVC smoke particulate from all of the ducting and cold air returns. There is no moisture present. Air scrubbers are still running in the house. We now have powdery rust on 8' runs of ducting and the cold air returns. In some areas it looks like we painted it rust color! We wipe it off but it returns within days. Any idea what could be causing this?

Moisture in the air will cause duct to rust. Usually it takes time because the duct galvanized.

Burning PVC produces hydrochloric fumes. Along with the heat, it may have eaten most of the galvanizing off the duct. You might hit an auto supply and try some metal prep or rust converter.

There are some coatings that are used on Evap water coolers to keep the pan from rusting and to cover the rust if it has, check with a heating and AC dealer to get info.

It is called cold galvanizing compound in a spray can. Wear a face mask when using.