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NaOH and carbon steel
3

NaOH and carbon steel

NaOH and carbon steel

(OP)
I have an application where I have a pump moving NaOH from a storage tank into one of our systems.  The pump is designed for this type of application and comes with a plastic pipe.

However, we have to move the liquid about 20' and cannot use the plastic tube.  We would like to use plain old carbon steel, is this going to be a problem?  This system might be a bit above room temperature at it's maximum.

I'm told the NaOH will be about 0.5mol/L.  Is there a point where the concentration will be too high or is that not a concern?

I checked one of my texts from school and that says carbon steel should be fine.

RE: NaOH and carbon steel

2
I'd, at least, coat the carbon steel, if at all possible. Check this web page: http://www.coleparmer.com/techinfo/chemcomp.asp, hope it helps

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

RE: NaOH and carbon steel

(OP)
I would rather not coat the steel.  Is this absolutely necessary?

RE: NaOH and carbon steel

We have a few caustic application in our plants at ambient temperatures. I have come across publications stating opposite views. However, given the temperatures you are going to be using, it might not be necessary to coat at all. I do not know the particulars of your process and upsets, but hopefully the following link can shed some light and help you to make a decision: http://www.dow.com/causticsoda/safety/material.htm. It appears to be safe <50ºC.
Unfortunately my applications have not run long enough for us to be able to determine if any damage has been done to the piping. I know we made a trick out of pump impellers, but it might have been unrelated to the pH. I am still investigating.

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

RE: NaOH and carbon steel

(OP)
Thanks unotec, that's a great link.

RE: NaOH and carbon steel

It's correct that a safe limit for CS is 50°C, if you look at the caustic soda service graph you'll see that for any conc up to 50% you can use CS without Stress relieving while for T 50-75° you'll need the stress relieve to avoid any possibility of caustic SCC (CSCC).

" Corrosion in Caustic Solutions


Corrosion by caustic (sodium or potassium hydroxide) at all concentrations is easily handled at room temperature with a variety of metals and alloys, including carbon steels. It becomes increasingly more corrosive with increasing temperature and concentration. The useful safe limit of carbon steel is approximately 150oF/65oC, both with regard to caustic stress corrosion cracking (CSCC) and corrosion. Stainless steels are more resistant to general corrosion compared with carbon steel; however, they can suffer CSCC at approximately 250oF/121oC. "
http://www.hghouston.com/naoh.html

hope this help
S
 

Corrosion Protection & Corrosion Control
 

RE: NaOH and carbon steel

FRP will take the pressures/temperatures better than plastic and you won't have the weird metallurgy worries.

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