Abstract
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 (Na(2)SO(4)) and with 0.1 M Na(2)SO(4) 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 Na(2)SO(4), 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 Na(2)SO(4) 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/Fe(2)O(3) film at pH 10, while Fe(3)O(4)/Fe(2)O(3) 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.