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This project seeks to develop and validate material systems and protective conditions that increase the lifetime of heat transfer fluids (HTF) and thermal energy storage (TES) containing materials at the SunShot-relevant temperatures of 550°C to 720°C. High temperature fluid candidates for concentrating solar power (CSP) plants such as molten salts (MS) and supercritical carbon dioxide (sCO2) are potentially corrosive to conventional alloys. We investigated material’s protection conditions for corrosion in MS, and effect of CO2(g) impurities in sCO2.

Approach

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The National Renewable Energy Laboratory (NREL) and its partner The University of Wisconsin, Madison investigated corrosion mitigation approaches for commercial alloys. In molten chlorides and carbonates, different surface mitigation approaches were evaluated such as pre-oxidized Ni-base with Al content coatings, and pre-oxidized alumina forming alloys (AFAs). Open circuit potential (OCP), potentiodynamic polarization sweep (PPS), electrochemical impedance spectroscopy (EIS) and conventional mass-change immersion techniques were employed to determine corrosion rates and mechanisms. For supercritical CO2 (sCO2) degradation, autoclave tests at different temperatures using different CO2(g) chemistries were used focusing on characterizing the oxidation and carburization resistance of alloys Haynes 230 and Haynes 625 in different sCO2 environments. The overall objective was to produce material systems and conditions that result in lower corrosion rates.

Innovation

Current state-of-the-art materials and protection methods are non-existent for CSP applications working in harsh environments at temperatures higher than 600°C. The innovation of this project are the alloy’s surface protection approaches, fluid’s composition, and working conditions for materials in contact with aggressive high temperature TES fluids and HTFs such as molten salts and supercritical CO2. We are determining if the presence of oxygen can be allowed during operation since it will be a cost effective approach for CSP applications. 

Utilization of the sCO2–Brayton cycle, both increases cycle efficiency and decreases component size compared to the Rankine steam cycle. For this reason, there is large interest in understanding the material demands in high temperature sCO2.

Conclusions

Surface passivation of AFAs produced by pre-oxidation treatments, demonstrated that these pre-oxidized AFAs (p-AFAs) formed stable alumina layers that protect the alloys from corrosion in molten chlorides. The p-AFAs can stand thermal cycles (550 to 700°C) in the presence of oxygen containing atmospheres. The alumina scales remained stable and were able to grow under these harsh and extreme conditions.

Pre-oxidized Al-containing Ni-base coatings, sprayed with atmospheric thermal plasma spray (APS) or high velocity oxifuel (HVOF), lowered the corrosion rates (CRs) of substrate alloys SS310 and In800H from few mm/year to 192 µm/year when exposed to molten chlorides at 700°C. Similar pre-oxidized coating compositions exposed to molten carbonates in CO2 at 700°C had CRs of 34 and 46 µm/year. This corrosion rate reduction is associated with the formation of a uniform protective layer constituted by Al- rich oxide with some Cr, and Y content. 

In chloride MS we determined that silica-base castable cements with BN top films are capable of protecting containment metallic alloys from the attack of molten chlorides at high temperatures (650°C)

In sCO2, significant differences in the oxidation of the alloys was observed when exposed to research grade (RG) (99.999%) and industrial grade (IG) (99.95%) CO2 prompting further investigation of the effects that trace concentrations of oxygen can have on corrosion rates and mechanisms. The oxidation rate of H230 and H625 follows a protective power law fit for all conditions except for the oxygen doped system. In the non-oxygen doped conditions, the degradation rate was < 5 µm/year in IG-CO2 at 750°C. Doping of CO2 with O2 was used to determine if carburization could be controlled. Carbon was observed throughout the oxide, and along grain boundaries. While the overall magnitude that O had on C ingress is still uncertain, the carburization appears to be significantly less than what has been observed in the past for ferritic alloys at lower temperatures. 

Publications, Patents, and Awards

Learn about other DOE competitive awards for concentrating solar power research that are in progress.