Flash rust question

[Chris_Hamilton]

This may be a stupid questions but here goes, on a freshly blasted steel surface why does heating up the metal, (my specific case a propane torch) cause flash rust to appear? Low humidity day, temps in the 60's. I'm using a propane torch to remove seam sealer and it is flash rusting in the places heat was applied. Seen it do this in the past as well, just trying to understand why this happens.

 

[sprint_9]

My best guess, condensation from the burning of the propane getting into the pores of the exposed metal causing the flash rust. Or the change from hot to cold causing some form of condensation.

 

[Chris_Hamilton]

condensation from the burning of the propane

So the propane is causing this? Or simply condensation from the metal being heated then cooled?

 

[dhutton01]

When you burn propane one of the resulting products is water vapor. It is probably condensing into the pores of the freshly blasted metal. Try a heat gun.

Don

 

[Big Dave]

I don’t know. But maybe when the metal cools, it draws moisture into it. Like heating a bolt so it will draw oil into it to get it loose.

 

[Big Dave]

Actually, Don’s point about vapor makes sense.

 

[Chris_Hamilton]

When you burn propane one of the resulting products is water vapor. It is probably condensing into the pores of the freshly blasted metal. Try a heat gun.

Don

Ahhh that makes sense. Of course I was asleep the day they taught that in chemistry class.

 

[dhutton01]

Let us know how it goes with a heat gun. I am curious…

Its also possible that burning the seam sealer is releasing some sort of corrosive gas.

Don

 

[Chris_Hamilton]

I started with the heat gun and found it to be too slow going. I have a good heat gun too, the big Master one. Trips a breaker if I use it in a 15 amp circuit. There is a mile of seam sealer in this car. I'll probably use the oxy-acetylene torch and see how that goes. Gotta re-blast all the seam sealer areas after it's out so no big deal that it flashed, just couldn't understand why it was doing it.

 

[Dean Jenkins]

I started with the heat gun and found it to be too slow going. I have a good heat gun too, the big Master one. Trips a breaker if I use it in a 15 amp circuit. There is a mile of seam sealer in this car. I'll probably use the oxy-acetylene torch and see how that goes. Gotta re-blast all the seam sealer areas after it's out so no big deal that it flashed, just couldn't understand why it was doing it.

Propane exhaust has a LOT of moisture. Propane furnace flues have to account for the condensate which is not only plentiful but caustic.
If the heat gun doesn't provide the needed BTUs, oxygen acetylene is a better choice.

 

[chevman]

This Eastwood defusor works great, but you'll probably be done before you could get it.
https://www.eastwood.com/ew-body-soldering-diffuser-tip.html