A mechanistic study of iron passivation and transpassive behavior in sulfate solutions using thermo-kinetic diagrams

Understanding the dissolution and passivation of iron in aqueous environments is essential for enhancing its corrosion resistance and expanding its applications. We present Thermo-Kinetic (TK) diagrams for iron in deaerated solutions with no added sodium sulfate (Na₂SO₄) and with 0.1 M Na₂SO₄ over the pH range 1-14, constructed by integrating current density contours from potentiodynamic polarization with thermodynamic E-pH diagrams. TK diagrams indicate that in solutions with no added Na₂SO₄, iron passivates above pH 7, with a minimum passive current density (i_p) of 5 ×10^-6 mA·cm^-2 at pH 8. The addition of 0.1 M Na₂SO₄ delayed passivation until pH 12 and increased i_p nearly tenfold. Galvanostatic (GS) polarization and EIS validated the TK diagram results. XPS after GS polarization revealed an FeOOH/Fe₂O₃ film at pH 10, while Fe₃O₄/Fe₂O₃ dominated at pH 12 and 14. These results clarify how sulfate compromises iron passivity and highlight TK diagrams as a powerful tool for mapping corrosion behavior.