Modeling the Effect of Cathodic Protection on Superalloys Inside High Temperature Molten Salt Systems

Abstract

Cathodic protection is one way to mitigate corrosion of metal surfaces of concentrated solar power (CSP) systems, by shifting the potential of the alloy below its open circuit potential (OCP). The behavior of molten salt CSP systems under cathodic protection can be obtained by developing a three-dimensional (3-D) computational corrosion model. A corrosion model was designed for and benchmarked against a thermosiphon reactor. For the cathodic protection case, magnesium (Mg) was added to the salt as a sacrificial anodic species, which reduces the corrosion rate by cathodic polarization of a corroding metal surface. The model then calculated the new corrosion rate at the surface of the coupons. Results were in good agreement with experimental values for the cases with and without the cathodic protection and at isothermal and non-isothermal conditions. The results showed that by adding even small amounts of Mg into the molten salt (KCl-MgCl 2 ) can rapidly reduce the corrosion rate at the surface of the coupons for both isothermal and non-isothermal conditions. The predicted results also showed that the corrosion rate of Haynes-230 in KCl-MgCl 2 containing 1.15 mol% Mg was 35 times lower than baseline tests with no cathodic protection and met the DOE SunShot targets. Concentrating solar power (CSP) plants rely on thermal energy from the sun to generate electricity. Because they do not rely on any fossil fuel backup, CSP plants can be considered a clean energy source of the future and have the potential to replace greenhouse gas emitting fossil fuel power plants. 1 To make this system feasible, heat transfer fluids (HTFs) are required that can operate at high temperatures (above 800• C) for long periods of time without significant degradation reaction of either the HTF or the materials of construction for the system. Among different heat transfer fluids, molten halide salts are promising due to their high thermal conductivity, large specific heat, low melting point, low viscosity, and low vapor pressure.2,3 However, material corrosion has been recognized as an issue for metals in contact with molten salts, particularly at high temperatures (700-1000• C). 4 Although Superalloys have been developed for high-temperature applications, they are not able to meet both the high-temperature strength and the high temperature corrosion resistance simultaneously in some of the most promising molten salts. To increase the lifetime operation of the CSP system, it is important to prevent the corrosion of the superalloys. One of the most commonly used methods of retarding corrosion and extending the life of structures is cathodic protection, and this method was investigated herein. Cathodic protection is a well-known method for preventing alloy corrosion in aqueous solutions by shifting the cathodic alloy potential to the previous alloy corrosion potential, either by using impressed current or by using a sacrificial anode.5 However, cathodic protection has little documented experience in high-temperature molten salt systems outside ...

Hot corrosion behavior of commercial alloys in thermal energy storage material of molten MgCl2/KCl/NaCl under inert atmosphere

“…Both, experiment and simulation, have proved that the corrosion reactions of Ni-based alloys (e.g., Ha 230) were controlled by electron transfer when exposed to molten KCl-MgCl 2 at 700-1000°C [29,30]. Moreover, the corrosion rate of Ha 230 had a linear increase with corrosive species in the molten KCl-MgCl 2 [29,30]. In this work, it can be assumed for the corrosion mechanism proposed above that corrosion reactions (9)- (10) are controlled by the kinetics of the electron transfer reaction at the alloy surface.…”

“…In general, the corrosion of alloys is controlled by the kinetics of the electron transfer reaction at the alloy surface [28]. Both, experiment and simulation, have proved that the corrosion reactions of Ni-based alloys (e.g., Ha 230) were controlled by electron transfer when exposed to molten KCl-MgCl 2 at 700-1000°C [29,30]. Moreover, the corrosion rate of Ha 230 had a linear increase with corrosive species in the molten KCl-MgCl 2 [29,30].…”

Purification strategy and effect of impurities on corrosivity of dehydrated carnallite for thermal solar applications

“…The chemical purication follows the principles of using an active metal such as Mg or Zr to remove MgOHCl impurity as given by the German Aerospace Center (DLR) and Savannah River National Laboratory (SRNL). 13,16,17 Following SRNL's suggestion based on their investigation, we used 1.7 wt% of elemental Mg chips (99.98% trace metals basis, 6-35 mesh, Sigma Aldrich). 1.7 wt% of Mg was a conservative estimation to ensure Mg is in excess.…”

“…Metallic Mg is known to provide cathodic protection to metal alloys in molten chlorides where Mg serves as a sacricial anode. 16,17,19 It is generally accepted that Cr leaching is the major corrosion mechanism of Cr-containing alloys in molten chlorides. 16,17,19,20,24 Past work from SRNL 16,17 showed that without Mg, the corrosion potential of a Fe-Ni-Cr alloy is higher than that of CrCl 2 such that Cr in the alloy will leach and form CrCl 2 .…”

“…16,17,19 It is generally accepted that Cr leaching is the major corrosion mechanism of Cr-containing alloys in molten chlorides. 16,17,19,20,24 Past work from SRNL 16,17 showed that without Mg, the corrosion potential of a Fe-Ni-Cr alloy is higher than that of CrCl 2 such that Cr in the alloy will leach and form CrCl 2 . Addition of Mg shis the corrosion potential of the alloy by more than 0.5 V to be lower than that of both CrCl 2 and CrCl 3 -meaning that Cr in the alloy is thermodynamically stabilized.…”