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Title: 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 passivation is a good 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 later corroded in molten MgCl2 – 64.41 wt% KCl at 700 °C in a flowing Ar atmosphere. We performed electrochemical techniques such as open-circuit potential followed bymore » 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. Finally, 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.« less

Authors:
 [1];  [2];  [1];  [1];  [1]
  1. National Renewable Energy Lab. (NREL), Golden, CO (United States)
  2. National Renewable Energy Lab. (NREL), Golden, CO (United States); Univ. of Antofagasta, Antofagasta (Chile)
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE), Solar Energy Technologies Office (EE-4S)
OSTI Identifier:
1353426
Alternate Identifier(s):
OSTI ID: 1396893
Report Number(s):
NREL/JA-5500-66778
Journal ID: ISSN 0927-0248
Grant/Contract Number:
AC36-08GO28308; AC36-08-GO28308
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Solar Energy Materials and Solar Cells
Additional Journal Information:
Journal Volume: 166; Journal Issue: C; Journal ID: ISSN 0927-0248
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; molten salts; corrosion; chlorides; alumina forming alloys; alumina; oxidation

Citation Formats

Gomez-Vidal, J. C., Fernandez, A. G., Tirawat, R., Turchi, C., and Huddleston, W. Corrosion resistance of alumina-forming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests. United States: N. p., 2017. Web. doi:10.1016/j.solmat.2017.02.019.
Gomez-Vidal, J. C., Fernandez, A. G., Tirawat, R., Turchi, C., & Huddleston, W. Corrosion resistance of alumina-forming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests. United States. doi:10.1016/j.solmat.2017.02.019.
Gomez-Vidal, J. C., Fernandez, A. G., Tirawat, R., Turchi, C., and Huddleston, W. Fri . "Corrosion resistance of alumina-forming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests". United States. doi:10.1016/j.solmat.2017.02.019. https://www.osti.gov/servlets/purl/1353426.
@article{osti_1353426,
title = {Corrosion resistance of alumina-forming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests},
author = {Gomez-Vidal, J. C. and Fernandez, A. G. and Tirawat, R. and Turchi, C. and Huddleston, W.},
abstractNote = {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 passivation is a good 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 later corroded in molten MgCl2 – 64.41 wt% KCl at 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. Finally, 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.},
doi = {10.1016/j.solmat.2017.02.019},
journal = {Solar Energy Materials and Solar Cells},
number = C,
volume = 166,
place = {United States},
year = {Fri Feb 24 00:00:00 EST 2017},
month = {Fri Feb 24 00:00:00 EST 2017}
}

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  • Next-generation power systems require higher temperatures to increase the efficiency of electricity production in the power block. Concentrating solar power (CSP) technology is looking for high temperature thermal fluids able to work in the range of 550–750 °C. Molten chlorides containing NaCl, KCl, MgCl 2, and/or ZnCl 2 are being considered for solar receivers and/or sensible- or latent- thermal energy storage systems. Vapor pressures of chlorides are high enough that in combination with oxygen gaseous compounds will produce a harsh atmosphere that is generally very aggressive to common chromia forming alloys. Corrosion mitigations must consider a solution in which bothmore » zones (immersed in fluid and exposed to vapor phase) will be protected. This could easily be obtained using alloy surface modification approaches. Surface passivation, produced after pre-oxidation treatments, of alumina forming alloys (Inconel 702, Haynes 224 and Kanthal APMT) was evaluated in molten 35.59 wt% MgCl2 – 64.41 wt% KCl thermally cycled from 550 °C to 700 °C in flowing Ar and static zero air (ZA) atmospheres. Electrochemical impedance spectroscopy tests and metallographic characterization showed that the best performing alloy was pre-oxidized In702 in ZA at 1050 °C for 4 h due to the formation of protective, dense and continuous alumina layers. The alumina layers were unstable when flowing Ar was used as the inert atmosphere during corrosion evaluations. Corrosion results in static ZA are promising for next-generation CSP applications using molten chlorides because alumina scales were stable after 185 h of immersion in the oxygen-containing atmosphere. Alumina layers in pre-oxidized Al-FA In702 grew from 5 µm (before immersion) to 13 µm (after 185 h of immersion). As a result, the use of these alloys could be commercial feasibility and cost-effective because of the possibility of using oxygen-containing atmospheres instead of keeping enclosed systems with inert atmospheres to protect alloys from corrosion in molten chlorides.« less
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