Corrosion resistance of alumina-forming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests

Abstract

Advanced components in next-generation concentrating solar power (CSP) applications will require advanced heat-transfer fluids and thermal-storage materials that work from about 550°C to at least 720°C, for integration with advanced power-conversion systems. To reach the cost target, less-expensive salts such as molten chlorides have been identified as high-temperature fluid candidates.High-strength alloys need to be identified and their mechanical and chemical degradation must be minimized to be used in CSP applications. Approaches for corrosion mitigation need to be investigated and optimized to drive down corrosion rates to acceptable levels-in the order of tens of micrometers per year-for achieving a long system lifetime of at least 30 years.Surface passivationis agood corrosion mitigation approach because the alloy could then be exposed to both the liquid and the vapor phases of the salt mixture. In this investigation,we pre-oxidized the alumina-forming alloys Inconel 702, Haynes 224, and Kanthal APMT at different temperatures, dwelling times, and atmospheres to produce the passivation by forming protective oxides at the surface. The pretreated alloys were latercorrodedin molten MgCl 2-64.41 wt% KClat 700°C in a flowing Ar atmosphere. We performed electrochemical techniques such as open-circuit potential followed by a potentiodynamic polarization sweep and conventional long-term weight-change tests to down-select the best-performing alloy and pre-oxidation conditions. The best corrosion results were obtained for In702 pre-oxidized in zero air at 1050°C for 4 h. Metallographic characterization of the pre-oxidized alloys and of the corroded surfaces showed that the formation of dense and uniform alumina scale during the pre-oxidation appears to protect the alloy from attack by molten chloride.

Re‐establishing the paradigm for evaluating halide salt compatibility to study commercial chloride salts at 600°C–800°C

“…While a variety of thermal storage concepts can be considered, one strategy is to select a salt with higher temperature capabilities, such as chloride or fluoride salts, with chloride salts being favored due to lower costs . However, numerous recent isothermal assessments have indicated that chloride salts can be extremely corrosive to structural alloys at ≥600°C . But these results need to be considered in the context of how fluoride salts (e.g., FLiBe) were evaluated for and deployed in the molten salt reactor experiment conducted at Oak Ridge National Laboratory (ORNL) from 1965 to 1969 .…”

Review of supercritical CO2power cycles integrated with CSP

“…Chloride and carbonate salt blends have been proposed and tested, but coming after new challenges . The corrosion mechanisms of candidate salts and corresponding mitigation approaches are still an area of ongoing which are badly needed for component designers . Nickel alloys are decided to be adopted in the receiver and TES, as shown in Table .…”

“…[124][125][126] The corrosion mechanisms of candidate salts and corresponding mitigation approaches are still an area of ongoing which are badly needed for component designers. [127][128][129][130][131] Nickel alloys are decided to be adopted in the receiver and TES, as shown in Table 7. Compared with molten salt, particle as heat carrier is cheaper and more stable at high temperatures.…”

Corrosion resistance of MCrAlX coatings in a molten chloride for thermal storage in concentrating solar power applications

“…We are investigating corrosion in molten salts at high temperatures for solar applications and its mitigation to obtain degradation of containment materials around 20 µm/yr or lower, thus providing a lifetime for CSP plants of 30 years or more. [3][4][5][6][7][8][9] Corrosion behavior of steel containers in latent TES molten eutectic KCl-LiCl from 310 to 420°C in air and argon has been evaluated with the long-term weight-loss technique for up to 4000 h of immersion. The effect of water and other impurities in the salt were also studied, with pitting corrosion detected and the addition of 2 wt% H 2 O causing a severe increase in corrosion in these environments.…”

“…13 Several alloys have been tested in molten chlorides systems for high-temperature solar thermal applications. [3][4][5][6][7] Stainless steels AISI 347 (347SS) and AISI 310 (310SS), Incoloy 800 H (In800H), and Inconel 625 (IN625) were tested in molten eutectic 34.42 wt% NaCl -65.58 wt% LiCl at 650 and 700°C in nitrogen atmosphere. 3 At 650°C, the lowest corrosion rate of 2800 ± 380 µm/yr was obtained for IN625.…”