## 5 Conclusions  
The corrosion process of molten nitrate salts depends on several factors, and thus the EIS technique has proven to be useful in monitoring the corrosion process in AISI 304 and AISIS 430 stainless steels, and HR-224 nickel based alloy, for both salt solar as ternary mixture. Based on these results, monitoring of the ferritic steel VM12 was also carried out in the ternary mixture, but at different immersion times. As a result, the corrosion mechanisms and their evolution in the first hours of immersion in the molten salt have been identified through adjustments to equivalent circuit models, obtaining the formation of a protective layer in all cases. Due to the composition of each analysis material, variations in their behaviours were identified in the high and low frequency spectra in the impedance curves. For the AISI 430 immersed in solar salt (KNa) NO\({}_{3}\), it was possible to see the formation of a small semicircle at high frequency in the 96 h of immersion which is associated with the reaction of charge transfer at the layer/salt-molten interface. On the other hand, AISI 304 and AISI 430 stainless steels have a certain sustained similarity in the low frequency semicircle, due to the oxide layer already being formed.  
In addition, an analysis of the corrosion mechanisms was made from the point of view of the reactions that take place at the metal/salt-molten interface, in case the the layers of the corrosion products that occur in the metal dissolve, and therefore affect the thermophysical properties of the salt mixture. Based on the models obtained from the EIS tests of the four proposed materials, they were contrasted with the reaction mechanisms and the possible compounds that would form at the metal interfaces were found. In addition, they were compared with SEM/EDX tests in a semi-quantitative way to show the corrosion surface formed.  
Linear polarization resistance tests were performed for the AISI 304, AISI 430, and HR-224 materials after 100 h of exposure in the two proposed mixtures. Furthermore, for VM12 ferritic steel, only in the ternary mix containing lithium at 100 and 1000 h of exposure, all tests were performed at a temperature of 550 \({}^{\circ}\)C. The corrosion rates obtained showed a better performance of the materials with the ternary mixture with lithium content, compared to the solar salt, a material that is currently used as HTF in CSP plants. Furthermore, on the basis of the results obtained, the VM12 material was also analyzed. This material has good corrosion resistance properties, obtaining a corrosion rate of 0.95 mm/year after 1000 h of immersion in the ternary lithium salt.  
There is still much to investigate in the corrosion process of nitrate salts for different applications of solar technology, including thermal stress and the loss of ductility in materials, for a better understanding of corrosion phenomenon and material degradation in solar thermal applications.  
Conceptualization, A.M. and C.P.; methodology, A.G.F. and A.M.; lab testing, A.M. and M.J.K.; software, A.M.; formal analysis, A.G.F., C.P., F.G. and A.M.; investigation, A.M. and A.G.F.; writing--original draft preparation, A.M. and A.G.F.; writing--review and editing, A.M. and A.G.F.; supervision, A.M. and A.G.F.; project administration, A.M. and A.G.F.; funding acquisition, A.M. and A.G.F. All authors have read and agreed to the published version of the manuscript.  
The authors would like to acknowledge the financial support provided by CONICYT/FONDAP 15110019 \"Solar Energy Research Center\" SERC-Chile and FONDEQUIP EQM170111.  
The authors would like to thank Valloure company for providing samples.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.