According to the Chlorine Institute's Pamphlet 94 - Sodium Hydroxide and Potassium Hydroxide Solution (Caustic) Storage Equipment and Piping System, type 304 stainless steel (or better) is the recommended pipe MOC for < 50% caustic up to 95 °C and 175 psig. I recommend you download a free copy of this pamphlet from the Chlorine Institute website (http://www.chlorineinstitute.org), as it contains detailed material of construction recommendations for valves, gaskets, pumps, and storage tanks in addition to piping.