Corrosion in gas nozzle of ammonia/water 180¦C boiler

[virk]

Hello!

I would like to get some advice of the following corrosion problem:

In a boiler (more or less directly gas fired steam boiler) a mixture of ammonia and water boils at 175°C and 12 bara. The boiler is provided with several level gauges, which are examined periodically. After approx. 2 years of operation the gaseous nozzle (tube) for the level gauge is completely closed by thick magnetic material with density of about 3.000 kg/m3. The liquid nozzle remains nice and open.

There is a corrosion inhibitor (Na2Cr2O7) in the ammonia/water solution.

1) What chemical reaction has taken place?
2) How can we prevent this corrosion in the future?
3) Any more experiences helpful....

Thanks in advance for any suggestions....

VIRK

 

[dkdk]

If the tube for the level gauge is of a copper material it will corrode in the hi Ph of the ammonia solution.Steel is corrosion resistant at higher Ph's.

 

[virk]

I forgot to write that the boiler is completely made with carbon steel. The gas nozzles are made of 1.4571 (316) stainless steel.

Greetings Virk

 

[EdStainless]

So this gas nozzle is in continious exposure to saturated steam/ammonia vapor?
I would guess that there is no inhibitor in the vapor phase.
The corrosion product is magnetite, but is it from the SS? Has this tube corroded itself? Or is it carry over from corrosion in other locations? It might be 'mud' that just has a chance to get caught and harden in this location.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[virk]

>So this gas nozzle is in continious exposure to saturated steam/ammonia vapor?<

Yes, I think you are right.

>I would guess that there is no inhibitor in the vapor phase.<

I think so too, and I am also thinking in this direction.

>The corrosion product is magnetite, but is it from the SS?<  

We do not see any corrosion at all, with the exception of this "accumulated magnetic material".

>Has this tube corroded itself? <

No, as far as we could see.

>Or is it carry over from corrosion in other locations?  It might be 'mud' that just has a chance to get caught and harden in this location.<

What chemical reaction has taken place?

Just one more thing...
There are four similar gas nozzles coming out of the boiler and are "going" to stand pipes for level gauges. Three of these gas pipes have slope to the standpipe. These tubes are not affected anywhere. The fourth tube has slope from the standpipe to the boiler. This one is the tube which is hardly affected. Perhaps: when condensation takes place, condensate will run into the standpipes at the three tubes. Condensate in the fourth tube will run back into the boiler. Perhaps the problem is founded herein somehow.

What do you (or others) think of it?

Kind regards

virk