Corrosion Behaviour of AISI409 Stainless Steel with Al Slurry Coating in Molten Salt

Abstract

A concentrated solar power plant produces electricity by collecting thermal energy from sunlight. Then the thermal energy is stored in the heat transfer fluid (HTF). The most widely used and studied HTF is solar salt (60wt%NaNO₃–40wt%KNO₃) that achieves the most requirements of HTF. However, at high temperatures, the corrosion is still violent for most materials, thus the material selection is important. This research aims to investigate the feasibility for application of AISI 409 ferritic stainless steel as containers of the molten salt. AISI 409 is suitable for high temperature usage, and more economical than other grades which have higher chromium. To investigate the corrosion resistance in molten salt, AISI 409 samples were coated by dipping in Al slurry. Then they were heated up to 400°C to remove binders. The coatings with Fe-Al intermetallic and alumina layers were formed after binder removal and subsequently annealing at 700°C for 3 h in the air atmosphere. This research reported the corrosion behaviour of Al coated and uncoated samples which had been treated by immersion in molten salt at 500°C for 100 h with analysis results. The results demonstrated that weight and thickness of uncoated samples were increased due to formation of the oxide layers such as Fe₂O₃ and Fe₃O₄ on the surfaces. For the Al coated samples, the weight and thickness of the coat layers were massively decreased after being immersed in molten salt at the first 1 h due to removal of Al-O, slag or impurity in the coating. Nevertheless, the weight increased gradually until 100 h. The oxidation of the Fe-Al intermetallic layer exhibited about 35% slower oxidation than that of uncoated samples, thus the coating technique has potential to be applied against molten salt.