Corrosion behaviour of Al-containing alloys in Cl-based molten salt environment

The corrosion behaviour of Al-containing alloys in KCl-MgCl₂ at 700 °C was investigated and compared with that of commercial alloys. The developed Al-containing alloys showed less or similar weight loss compared to Hastelloy N owing to Al-rich oxide formation. While 316 SS showed both uniform and localised corrosion, a high Al-containing alloy was almost unattacked by any form of corrosion. The SEM map in this study innovatively showed and confirmed the high Cr depletion at the top matrix layer of the exposed 316 SS alloy. It was found that the primary controlling mechanism of corrosion is the outward diffusion of metal ions promoted by the volatile metal chlorides, while Cr diffusion was detrimental, Al diffusion was beneficial in mitigating corrosion via the formation of Al oxides. Noteworthily, this study revealed that the KCl-MgCl₂ salt was very corrosive to the 316 SS, and its interaction led to an obvious intergranular corrosion arising from Cr-depletion of the grain boundaries due to the high Cr content of the 316 SS. While for the high Al-containing alloys, the ACES with lower Al content and higher Ni content exhibited a higher corrosion rate when in contact with KCl-MgCl₂salt than the ADSS with higher Al content and lower Ni content. The mechanism of electrochemical interaction between the KCl-MgCl₂ salt and the high Al-containing alloys was that, the increased Al content and decreased Ni content enabled the formation of α – Al₂O₃, which provided excellent protective barrier against corrosion attack, while, the lower Al and higher Ni contents lead to the formation of Mg and Ni aluminate oxides. These oxides were less protective against corrosion attack in the molten salt environment.