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

Abstract

The presence of chloride (Cl^-) and sulphate (SO₄^2-) in boiler water poses a significant threat to carbon steel integrity, yet their specific corrosion mechanisms and interaction at industrially relevant high temperatures remain poorly characterized. This study quantifies the accelerated corrosion thresholds and elucidates the distinct 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. In situ corrosion rates were determined via linear polarization resistance (LPR), complemented by thermochemical equilibrium modelling (FactSage™). Results demonstrate that the corrosion threshold for SO₄^2- is 1.5 to 3 times higher than for Cl^-, validating industry heuristics. Mechanistic analysis reveals that Cl^- primarily acts by acidifying the solution and increasing iron solubility, whereas SO₄^2- accelerates corrosion through hydrothermal reduction to hydrogen sulfide. Furthermore, the chemical form of which the contaminants were introduced critically influences their combined effect: ammonium salts exhibit a buffering interaction that raises the combined threshold, while mineral acids produce a synergistic effect that lowers it. These findings provide quantitative thresholds and mechanistic clarity that establish a foundational basis for improving water chemistry guidelines and models.