ABSTRACT

The corrosion behavior of two Fe-Cr-Ni alloys in eutectic nitrate (NaNO3/KNO3) salt and its vapor were investigated at 565˚C for 28 days. Results show higher corrosion rates in salt vapor than in liquid salt. In addition, the surfaces of samples exposed to vapor were severely corroded with spallation of the outer oxide layer. The oxide layer of the corroded alloys in molten salt exhibited lower Cr depletion in comparison with the surface exposed to the vapor salt. The alloy formed a loose double layer of Cr, Fe and Na oxide in molten salt vapor. The oxidation of material in the molten salt vapor contributed the most to the overall corrosion of the sample, and its Cr-depleted depth was nearly twice that of a sample immersed in molten salt. Alloy (chromium) dissolution from the metal into salt and selective oxidation at the salt/metal interface dominates the corrosion mechanism in liquid and vapor salt. This confirms the need to take into consideration the corrosion of alloy by salt vapor during practical material selection process for such applications.

INTRODUCTION

The serious challenges for harvesting solar energy from solar power tower is the inherent intermittence caused by altering of day and night [1] and by the weather event changes, posing a significant threat to the continuous supply of solar electricity. The most promising solution explored by the researchers during the last few decades would be integrating the thermal storage system with the Concentrated Solar Power Plant (CSP) to stabilise and provide the high-quality electrical energy at night or anytime without sufficient solar irradiance. Typical thermal energy storage system enables the CSP plants to store solar energy by heating the sensible storage medium, like molten nitrate salt in a hot tank.

High-temperature corrosion of materials in molten salt is therefore of great interest to renewable energy production industry, especially concentrated solar power plants (CSP) where molten salts are used as thermal energy transfer fluid and storage media. Considering the hydrodynamics and static storage behavior of molten salt in the flow lines and storage tanks, it is easy to imagine that the metallic parts are either fully or partially immersed in the molten salt with some parts also exposed to the molten salt vapor. Recently published literature [2] revealed that researchers designed an experimental corrosion vessel with samples half-immersed in liquid salt. This enables researchers to compare not only the impact of salt and the oxygen from the environment, but also the gases from decomposed carbonate salt, which are expected to worsen the corrosiveness of system in this atmosphere. They found that oxidation was the primary attack to alloys in molten salt or vapor environment and confirmed that the availability of oxygen could control the degree of oxidation/corrosion. They also found evidence to confirm that the molten salts can slow down the oxidation on the submerged samples.

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