The Effect of Temperature on the Corrosion Behavior of 625 Superalloy in PbSO₄-Pb₃O₅-PbCl-ZnO Molten Salt System with 10 wt. % CdO

Corrosion behavior and degradation mechanism of 625 supperalloy under 47.288 PbSO₄ - 12.776 Pb₃O₅ - 6.844 PbCl - 23.108 ZnO - 10 CdO (wt. %) molten salt mixture and dynamic air atmosphere were studied at 600, 700 and 800 °C. Electrochemical impedance spectroscopy (EIS), open circuit potential measurements and potentiodynamic polarization technique were used to evaluate the degradation mechanisms and characterize the corrosion behavior of the alloy. Morphology, chemical composition and phase structure of the corrosion products and surface layer of the corroded specimens were studied by SEM/EDX and x-ray map analyses. Results confirmed that during the exposure of the alloy to the molten salt, Cr was mainly dissolved through an active oxidation process as CrO₃, Cr₂O₃ and CrNbCO₄ while Ni dissolved only as NiO in the system. Formation of a porous and non-protective oxide layer with decreased resistance should be responsible for the weak protective properties of the barrier layer at high temperatures of 700 and 800 °C. There were two types of attack for Inconel 625 alloy including general surface corrosion and pitting. Pitting corrosion occurred due to the breakdown of the initial oxide layer by molten salt dissolution of the oxide or oxide cracking.