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Burning off nitric acid
3

Burning off nitric acid

Burning off nitric acid

(OP)
We have a 17-4 PH cylinder that has welded structures attached.  We will be passivating the assembly in a dilute nitric acid bath and then rinsing.  We are concerned that trace amounts of nitric acid may be trapped in crevices.  We are considering heating up the assembly to about 400F to burn off any trace amounts of nitric acid.  Does anyone have any expierence with this?  Will heating trace amounts of nitric acid oxidize it and burn it off the surfaces?

Thanks for your help.

Paul

RE: Burning off nitric acid

Paul,
< Will heating trace amounts of nitric acid oxidize it and burn it off the surfaces?>

Yes, heat will evaporate the acid but it will also make it much more corrosive.  An old rule of thumb is that for every 10°C rise in temp you can expect a chemical reaction to double in speed.
If you could pull a vacuum on the container, you wouldn't need temps as high to drive the acid off.  If you can't do that. Fill the container with water and bring it to a boil a couple times.  This will drive vapors out of microscopic crevases and dilute any acid with the water.  Having a wetting agent present will help.
Nitric acid boils between 83 and 121°C (depending on water content) so you wouldn't need to go up to 400°F.

RE: Burning off nitric acid

2
Since the nitric isn't corrosive to the stainless, you needn't be too concerned. In the production process bright annealed stainless is commonly run through dilute nitric as a final cleansing. Some is certainly left, as it always is with field passivation, also.
 

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