A387 Gr 91 Cr-Mo alloy corrosion compatibility with LiOH-KOH mixture was evaluated at 315 °C (Test#1) and 360 °C (Test#2). This alloy is studied for a techno-economical optimization of a steam-PCM heat exchanger proposed in the thermal energy storage (TES) system of a direct steam generation (DSG) facility. The corrosion damage was analyzed with visual inspection, optical microscopy, SEM, EDS and XRD, and corrosion rates were also calculated. A387 Gr 91 alloy generates protective oxide layers over base metal. Localized corrosion damages were not detected with these test conditions, while XRD and EDS profiles showed CrO and LiFeO2 as main corrosion products generated by this alloy. On the other hand, corrosion rates decrease with exposure time showing the passivation of the alloy. A387 Gr 91 corrosion performance was compared with A316L stainless steel, which was preliminary proposed by the authors in previous studies. In addition to corrosion performance, parameters such as cost, thermal properties, and mechanical properties are discussed. In conclusion, the use of A387 Gr 91 instead to A316L alloy for the construction of the steam-PCM heat exchanger involves the techno-economical optimization of the equipment.