Hot corrosion behavior of refractory high entropy alloys in molten chloride salt for concentrating solar power systems

Abstract

Refractory high entropy alloys have recently attracted widespread attention due to their outstanding mechanical properties at elevated temperatures, making them appealing for concentrating solar power and nuclear energy applications. However, their molten salt corrosion behavior has not been reported, which is critical in evaluating their application merit. Here, the corrosion behavior of two recently developed refractory high entropy alloys, namely TaTiVWZr and HfTaTiVZr, was studied in molten 33NaCl–22KCl–45MgCl2 (wt. %) eutectic salt at 450 °C and 650 °C, using potentiodynamic polarization technique. The results were compared with benchmark alloys, namely 304 stainless steel (SS304) and Inconel 718 (IN718). TaTiVWZr refractory high entropy alloy exhibited an order of magnitude lower corrosion current density (Icorr = 0.7× 10-3 A cm-2) compared to SS304 (Icorr = 9.2 × 10-3 A cm-2) at the higher temperature of 650 °C. The corrosion rate of all the alloys increased with increase in temperature from 450 °C to 650 °C with the exception of TaTiVWZr. The TaTiVWZr alloy showed a corrosion rate of ~ 5 mm/year at 650 °C compared to ~ 110 mm/year for SS304. HfTaTiVZr and IN718 showed comparable corrosion rates in the range of ~ 40 mm/year at 650 °C. The high corrosion resistance of the two refractory high entropy alloys was attributed to a combination of three factors: (i) slower chlorination rate of refractory elements in the molten chloride salt environment driven by thermodynamics, (ii) formation of stable Ta–V and Ta–V–W based complex oxides on their surface, and (iii) Ti/TiCl2 and Zr/ZrCl2 redox couple formation which retarded the depletion of refractory elements. In contrast, the Cr-, Fe-, and Ni-based surface passivation oxides for SS304 and IN718 were less protective in the molten salt environment, particularly at the higher temperature of 650 °C.