Damage analysis of 601 nickel superalloy in eutectic Na2CO3/NaCl molten salt under isothermal and thermal cycling conditions

Establishing compatibility of high-temperature nickel alloys with molten salt phase-change materials is critical for the development of thermal energy storage for future concentrating solar power systems. This paper investigates the behaviour of alloy 601 in a 59.5 %wt Na2CO3/40.5 %wt NaCl eutectic salt mixture under thermal cycling and isothermal conditions. Under both conditions it was found that chromium was depleted from the alloy at a linear rate, inducing porosity in the metal. The corrosion scale consisted primarily of iron oxide, although chromium and aluminium are present in the inner layers. The scale was found not to be protective and does not form a barrier to salt ingress into the alloy's porous region, enabling depletion to continue unimpeded. Thermal cycling tests show scale depletion and thickness reduction compared to isothermal tests, implying that depletion and scale growth is related to the time which the salt is molten in the system. Given solar thermal energy using phase-change salt storage is cyclic, this work provides significant insight into corrosion under inservice conditions, which is critical for successful plant design.