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This paper aims to construct the E-pH diagrams for AISI 316L stainless steel in chloride solutions containing SO₄²⁻ ions and therefore investigate the role of SO₄²⁻ ions on pitting corrosion of stainless steel.

A cyclic potentiodynamic polarisation method was performed to obtain polarisation curves at different pH. From these curves, corrosion, primary passivation, pitting and repassivation potentials were determined and plotted as a function of pH giving the E-pH diagram.

The addition of SO₄²⁻ ions to 10,650 ppm NaCl solution up to 3,000 ppm widened the passivation regime of the E-pH diagram mainly by shifting the pitting corrosion potential to the noble direction. This indicated the inhibiting role of SO₄²⁻ on the nucleation of new pits in the transpassive region. It also stabilised the pitting corrosion potential at the pH ranging from 5 to 11. However, at pH 7, it caused the pit area to increase, implying the catalytic role of SO₄²⁻ on the pit growth. Finally, it did not change the types of ions dissolved in solutions after pitting.

The diagrams can be used as a guideline in industries to determine the passivation regime of the AISI 316L stainless steel in chloride- and sulphate-containing solutions.

This paper reported the E-pH diagrams for the AISI 316L stainless steel in chloride solutions containing SO₄²⁻ ions. The roles of pH and SO₄²⁻ ions on pitting corrosion were innovatively discussed using a point defect model.