Sulphate-induced corrosion of mild steel reinforcement in alkaline media remains poorly characterised relative to chloride attack, despite the prevalence of sulphate-contaminated groundwater and industrial effluents in contact with reinforced concrete. This study investigated the long-term (six-month) gravimetric corrosion behaviour of commercial-grade mild steel (3 mm flat bar, 0.07% C) immersed in sodium sulphate solutions at three concentrations (200, 1000 and 2000 mg/L), combined with chloride additions (0, 50 and 500 mg/L), at pH 9 and pH 12, representative of carbonated and fresh concrete pore solutions, respectively. The objectives were to quantify sulphate- and chloride-driven corrosion kinetics, characterise the pH-dependent corrosion morphology, and determine whether the resulting corrosion products are passivating. Mass loss was measured at weekly and monthly intervals over six months, and corrosion morphology was examined macroscopically and by stereomicroscopy. Mass loss increased linearly with time at both pH values (R² = 0.96–0.99), confirming non-passivating corrosion kinetics, with a maximum mass loss of approximately 287 mg recorded after six months. Corrosion was consistently more severe at pH 12 than at pH 9 for equivalent solute concentrations. A pH-dependent shift in morphology was observed: uniform etch corrosion occurred at pH 9, whereas at pH 12, pronounced pitting and tuberculation were observed. Increasing sulphate concentration increased corrosion rates at both pH values, and a chloride addition of 500 mg/L caused a disproportionately sharp increase compared with lower additions. These findings indicate that sulphate attack in highly alkaline conditions can be more damaging to mild steel than chloride-induced corrosion, with direct implications for the durability design of reinforced concrete in sulphate-bearing environments. The work also supports the UN Sustainable Development Goals 3, 6, 9, 11, 12, and 13 by informing the development of more resilient, resource-efficient infrastructure.

Keywords

Sulphate Corrosion; Alkaline Media; Gravimetric Corrosion Testing; Corrosion Kinetics; Carbon Steel, Concrete pore solution, Tuberculation