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This research aims to assess and monitor the corrosion of 321H stainless steel in a quaternary salt mixture that consist of the Hitec salt, to which a 1%wt. of LiNO3 is added. Assessment and monitoring of the steel was carried out by an online two-electrode system that enables electrochemical impedance measurements in real-time. The system was initially developed for laboratory analysis and then scaled up to a demonstration parabolic-trough plant. Results obtained at lab-scale indicated that corrosion in 321H steel is controlled by ion transport in the scale, providing a corrosion rate of 8.87 µm·year-1. Lab results were validated by conventional lab-scale characterization techniques, which after 1000 h of testing showed a corrosion layer mainly constituted by Fe2O3 and FeCr2O4. Results at plant-scale provided a corrosion rate of 12.91 µm·year-1, which is in the same order that that obtained at lab-scale. The differences between lab and plant results are related to the more realistic conditions found in plant, since measurements were taken after 7000 h of operation. Results showed the strong potential of the developed corrosion monitoring system for assessing corrosion rates and corrosion mechanisms in parabolic-trough concentrated solar power plants. [ABSTRACT FROM AUTHOR]