I am much delighted with your question because that was one hell of an application which bothered me much during starting of my career which I solved by taking help of Nickel Development Institute (NIDI). I had an application where 99% anhydrous caustic being fused at 300 deg. C. Stainless Steel is prone to caustic embrittlement and faced lot of problems with SS reactors. Ultimately we had gone for Monel coating inside.
Regards,
Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.
Pure nickel, (Alloy 200) is about the best choice that you can make for caustic soda above 50% and above 250F. I have had some experience with materials selection for bulk caustic soda production equipment and, although it is very expensive, ......pure nickel is the best choice.
Try the following websites and search for "caustic soda":