Corrosion of 316 SS in chloride molten salt for thermal energy storage: Inhibitory effects of Al powder

Abstract

MgCl₂-KCl-NaCl is regarded as one of the most prospective high-temperature thermal energy storage mediums and heat transfer fluids (HTF) for 3rd generation concentrated solar power (CSP) systems. However, high corrosion to alloys limits its application. In this paper, corrosion tests were conducted on 316 SS, in MgCl₂-KCl-NaCl at 800°C with different content (0 wt.%,1 wt.%, and 10 wt.%) of Al powder addition as a corrosion inhibitor. The impact of Al powder was assessed through electrochemical methods, specifically impedance spectroscopy (EIS) and potentiodynamic polarization (PDP). Following corrosion tests, the morphologies and phase compositions of 316 SS were determined by using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD). The addition of Al powder can significantly reduce the corrosion current density of 316 SS in MgCl₂-KCl-NaCl at 800°C, which was 183.29 times higher than that with 10 wt.% without Al addition. Al and the degree increased with increasing content of Al. With the addition of 1 wt.% Al, the thickness of the diffusion layer is significantly reduced, which was 54.6 μm (100 h), 275.1 μm (200 h), 370.4 μm (300 h), and 500 μm (400 h), respectively. When the addition of Al reaches up to 10 wt.%, the inwards diffusion of Al caused the formation of Al enriched layer, which was identified as the FeAl phase, on the surface of 316 SS during the high-temperature corrosion processes. The thickness of the Al enriched layer was associated with the diffusion time of Al, and its depth was 40.4 μm (100 h), 45.3 μm (200 h), 103.5 μm (300 h), and 139.5 μm (400 h).