High-Temperature Corrosion of Unpassivated Carbon Steel in Simulated Boiler Water: Electrochemical Thresholds and Mechanisms for Chloride and Sulphate Contamination

Cl- and SO42- in boiler water pose a significant threat to carbon steel boiler tube integrity, yet their corrosion mechanisms at industrial temperatures remain poorly characterized. This study quantifies corrosion thresholds and mechanistic roles of these contaminants for unpassivated SA210-A1 carbon steel in simulated all-volatile treatment (AVT) boiler water at 310 °C and 10.3 MPa, using in situ linear polarization resistance measurements (LPR) supported by FactSage™ modeling. The corrosion threshold for SO42- was 1.5 – 3 times higher than for Cl-, supporting industry heuristics. Mechanistic analysis indicates that Cl- lowers solution pH, increasing iron solubility, while SO42- reduction to H2S is thermodynamically favoured under high-temperature reducing conditions and could provide an additional cathodic pathway. The initial chemical form of the contaminants governed combined behaviour: ammonium salts raised the combined threshold above that of Cl- alone, while mineral acids lowered it, better representing acidic, deposit-controlled boiler conditions. These findings establish a mechanistic baseline and relative threshold relationships to inform water chemistry guidelines.