Corrosion monitoring of 321H in contact with a quaternary molten salt for parabolic trough CSP plants

Abstract

In this study, the corrosive behaviour of stainless alloy 321H when in contact with a Hitec-based quaternary salt mixture is assessed. Electrochemical impedance measurements were performed on the substrate-salt system, which was tested under static conditions at 500 ◦ C for 1000 h. Results showed that corrosion in 321H is controlled by the ion transport through the oxide scale, and points to the presence of a protective layer mechanism. The fitting parameters showed that Rox diminishes sharply, from ~84 Ω to 25 Ω, after the initial stages of the test. This indicates a reduction in the protective character of the oxide layer formed when the test runs. In addition, after 168 h of testing, Rt μm⋅year 1 increased slightly, which indicates an increase in the number of species arisen from the substrate to the molten salt. In conclusion, the electrochemical results led to an estimated corrosion rate of 8.9 μm.year-1 for 321H steel. The results obtained by the corrosion monitoring system were in line with those obtained by Scanning Electron Microscopy and X-Ray diffraction. This study confirms the suitability of 321H stainless steel as a structural material for parabolic trough CSP technology and the enormous potential of the corrosion monitoring system for O&M processes. The next step is to scale up the proposed system for testing on a demonstration parabolic trough plant.