MgCl2-KCl-NaCl molten chloride salt is a promising candidate for thermal energy storage medium and heat transfer fluid for next-generation Concentrating Solar Power (CSP) plants (Gen-3 CSP). The main challenge has yet been the selection of economical yet corrosion-resistant structural materials to be used. Previous work by the authors has demonstrated that simple corrosion control strategies can mitigate corrosion effects, thereby allowing the use of classical stainless steels as structural materials in the hot part (e.g., ≥700 °C) of the CSP system. This study addresses the selection of cold tank materials, which have to withstand corrosion effects up to temperatures of 500 °C. Two cost-effective commercial types of steels, P91 and SS 304, were examined as candidates for structural materials, and controlled corrosion experiments were performed in molten MgCl2-KCl-NaCl salt at 500 °C for 1400 h. Before the exposure tests, the chloride salt was purified using a simple yet effective Mg-doping technique. The results show that the corrosion rates (CRs) of P91 samples are consistently low (<15 μm/year) for both macroscopic and microscopic analysis. The cheaper P91 steel outperforms the more expensive SS 304 in terms of corrosion resistance and also may prove to be beneficial in terms of mechanical properties and economics. Overall, the use of P91 as a cold tank structural material allows for significant cost reduction of the cold tank for chloride-TES system and enhances its competitiveness compared with commercial nitrate-TES.