The Effect of Pre-Oxidation on the Corrosion Performance of Haynes 214 in Molten Chloride Salt Mixtures

Abstract

Molten chloride salts are finding renewed focus as heat transfer fluids in molten salt reactors, concentrated solar power plants and thermal energy storage systems. The main issue in molten chloride salt systems is impurity driven corrosion which can cause significant damage to structural alloys and significantly reduce the lifespans of these systems. Pre-oxidation of Al2O3-forming alloys has been proposed as a strategy to mitigate corrosion. In this work, the corrosion resistance of Haynes 214, an Al2O3-forming alloy, is evaluated by comparing samples with a uniform pre-oxidized Al₂O₃ layer to as-received specimens after exposure to 55 mol% LiCl – 40 mol% KCl – 5 mol% MgCl₂ at 700 °C under Ar for 50, 100, 150, and 200 h, to assess whether the pre-oxidized layer delays or prevents corrosion. Post-corrosion characterization was primarily conducted by SEM-EDS, XRD and mass loss. Post-corrosion SEM-EDS characterization revealed that the uniform Al2O3 layer after exposure transformed into a multi-layer oxide consisting of an MgO outer layer, MgAl2O4 inner layer and a Al2O3 layer embedded within the substrate. The kinetics, as measured by EDS depletion of Cr and Al, with the pre-oxidized samples were found to be parabolic. The corrosion protection was attributed to the MgAl2O4 forming a denser layer preventing, ingress of impurities. The as-received samples also formed MgAl2O4. This layer was found to be unprotective due to its defective nature as it was formed by corrosion products, experiencing progressive corrosion. Therefore, it was found that at temperature pre-oxidized Al2O3 can effectively delay impurity driven corrosion in molten chloride salt mixtures. Spallation of both pre-oxidized and as-received coupons was attributed post-testing cooldown, caused by CTE mismatch and residual stresses from volume expansion with the incorporation of MgO