Steel corrosion in near-neutral pH electrolytes with bicarbonate, chloride, and sulfate ions

• Steel can passivate in pH 8.0 and 9.5 electrolytes, with or without HCO 3 - . • Steel corrodes in pH 8.0 electrolytes with Cl - and SO 4 2- , but passivates at pH 9.5. • Green rust, magnetite, lepidocrocite, and goethite precipitate. • Corrosion product layers evolve as a function of O 2 content. • Catalyzed dissolution, competitive adsorption, and iron speciation control corrosion. Steel corrosion in near-neutral pH environments, such as carbonated concrete and soil, is a major durability challenge. Despite its importance, the role of electrolyte composition in such corrosion processes remains poorly understood. Here, we systematically investigate the influence of 0-10 mM HCO 3 - , Cl - , SO 4 2- on the corrosion kinetics of steel at pH 8.0 and 9.5 buffered electrolytes under aerobic conditions, employing a comprehensive electrochemical, surface characterization, and thermodynamic modeling approach. At pH 8.0, steel passivated in electrolytes with and without HCO 3 - , though higher HCO 3 - concentrations slightly increased iron dissolution by forming iron-carbonate complexes. In contrast, Cl - promoted localized corrosion, temporarily inhibited by competitive adsorption of HCO 3 - when both ions had equimolar concentrations. Irrespective of the presence of HCO 3 - and Cl - , SO 4 2- ions were particularly aggressive, catalyzing iron dissolution and forming FeSO 4 (aq). Conversely, steel passivated at pH 9.5 in electrolytes even with Cl - and SO 4 2- , attributed to the higher OH - concentration prevailing over these ions. The corrosion products were identified as layers containing green rust, magnetite, lepidocrocite, and goethite, according to in-situ Raman spectroscopy and scanning electron microscopy. Thermodynamic modeling confirmed the stability of these phases, and their stratified structure was validated as a function of O 2 content, with increasing oxidation states in outer layers. These findings show that a low pH alone does not necessarily lead to steel corrosion. Active corrosion stems from the combination of the iron dissolution rate, competitive adsorption, and speciation resulting from the electrolyte composition, with emphasis on the aggressive role of Cl - and SO 4 2- .