It sounds like you have a severe case of caustic induced stress corrosion cracking (SCC) otherwise known as "caustic embrittlement." Caustic stress corrosion cracking occurs above threshold temperatures when stresses in the pipe exceed the yield point. Carbon steel pipe used for 50% caustic service should be stress relieved to relieve stresses caused by welding, cold forming, bending, field flaring etc. Is your line heated to keep the caustic from freezing (the freezing point of 50% sodium hydroxide is about 15 °C)? If so, heat tracing systems need to be carefully designed to avoid overheating or localized "hot spots" as elevated temperatures exacerbate caustic SCC. For more information, I suggest you consult Pamphlet 94 "Sodium Hydroxide Solution and Potassium Hydroxide Solution (Caustic) Storage Equipment and Piping System" from the Chlorine Institute. This pamphlet is available at no charge and can be downloaded from their website (http://www.chlorineinstitute.org.)