Carbon steel is the most common material of construction for < 51% sodium hydroxide solutions up to 50 degrees C. A cathodic protection system is not normally needed. A coating or lining is not necessary unless iron contamination is a problem. Many plastics and rubbers have chemical resistance to caustic solutions, up to their temperature limits. According to Pamphlet 94 from the Chlorine Institute, PVC, CPVC, polypropylene, polysulfone, PTFE, PFA, FEP, and ECTFE will normally not be degraded by 50% caustic solutions. Plastics liners can fail if maximum temperatures are exceeded. Epoxy resins and some vinyl resins have been successfully used for tank linings. Selected coating systems have proven successful for use as a barrier when iron contamination is a concern. In all cases, the coating or lining manufacturer should be consulted for a recommendation for specific applications as well as qualified applicators. I suggest you consult Pamphlet 94 "Sodium Hydroxide Solution and Potassium Hydroxide Solution (Caustic) Storage Equipment and Piping System" from the Chlorine Institute for additional information. This pamphlet is available at no charge and can be downloaded from their web site.