Corrosion resistance of alumina forming alloys against molten chlorides for energy production. II: Electrochemical impedance spectroscopy under thermal cycling conditions

Abstract

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, MgCl2, and/or ZnCl2 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 both 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 35.59wt% MgCl2 – 64.41wt% 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 4h 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 185h of immersion in the oxygen-containing atmosphere. Alumina layers in pre-oxidized Al-FA In702 grew from 5µm (before immersion) to 13µm (after 185h of immersion). 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.