300 series stainless and urea.

[TugboatEng]

We may be adding urea tanks into the engine rooms of our tug boats in the near future. While polyethylene is a great choice for in terms of chemical compatibility, I don't like the idea of using plastic tanks in a space that has a high fire potential. Are there any major corrosion issues using 304L or 316L stainless steels in a 30% aqueous solution of urea?

 

[EdStainless]

According to published tables 316L would handle all concentrations of Urea up to the boiling point.
However the risk of contamination is serious. A small amount of salt water in the tank would cause serious corrosion.
Perhaps you should look at using 2205. This is a duplex stainless and it is very strong, which allows for a much thinner tank.
This alloy would stand up much better if exposed to chlorides.

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P.E. Metallurgy, Plymouth Tube

 

[TugboatEng]

Does the urea solution make the stainless steel more sensitive to chlorides? The risk of contamination is small as the tank will be located and filled in the interior of the vessel. The tank will also be drained and refilled every 2-3 weeks so any contamination will not be persistent. 2205 looks like it straight doubles the material cost. The walls have to be thick enough to withstand vibration from the vessel. Stiffness is of greater concern than strength. It's likely going to be 3/16 wall. The tank will be roughly 700-1000 gallons.

 

[EdStainless]

Yes, the Urea will make it more sensitive to chlorides.
You could build a silo out of 3/16" 2205, that is way thicker than is needed.
Stiffness is a design issue, it isn't uncommon to see SS tanks built external stiffeners.

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P.E. Metallurgy, Plymouth Tube

 

[kingnero]

http://www.eng-tips.com/viewthread.cfm?qid=383113

Might be worth a read.

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http://be.linkedin.com/in/fusionpoint

 

[TugboatEng]

I've had bad experiences with 1/8th-ish wall tanks even in the 100 gallon region on tugs. That was cold rolled carbon steel filled with AW46 oil. The vibration is intense. I don't want to repeat that.