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>>

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

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>>

Thanks unotec, that's a great link.

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
 

Standard Recommended Practice
Avoiding Caustic Stress Corrosion Cracking of Carbon Steel Refinery Equipment and Piping

http://www.nace.org/nacestore/assets/freestandardsreports/RP040303.pdf

S

Corrosion Protection & Corrosion Control
 

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