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Aims to investigate the effect of chlorine on corrosion behaviours of stainless steels.

Very complicated thermodynamic calculations are needed to establish the E‐pH diagrams of commercial alloys, because they comprise of many elements. To avoid these complex calculations and facilitate corrosion prevention of AISI 316L stainless steel, the potentiodynamic method was used to construct the E‐pH diagram. The polarization curves were carefully experimented at the scan rate of 0.1 mV/s. The experimental conditions were aqueous solutions saturated with air (oxygen concentration 7.8‐8.5 ppm) containing chloride 0, 50, 500 and 5,000 ppm, pH 2, 4, 6, 8, 10 and 12, and at 25°C. The transpassive or pitting potential, the protection potential, the primary passive potential and the corrosion potential were determined from the polarization curves and plotted with respect to the pH of the solution. The ions in solution were investigated by qualitative chemical analysis and stated in the E‐pH diagrams.

The constructed E‐pH diagrams showed clearly the effect of chloride concentration in the tested conditions on the transpassive or pitting potential, the protection potential of AISI 316L stainless steels. The ion states after pitting corrosion were different at low and high pH. This may be useful information for further investigation of pitting corrosion mechanisms.

The E‐pH diagram was originally based on thermodynamic equilibrium. The potentiodynamic method was kinetically controlled and not in equilibrium. However, the experiments were kept at near stationary state as much as possible. The investigated E‐pH diagrams were limited for the solutions saturated with air containing chloride 0, 50, 500 and 5,000 ppm and at 25°C. The effects of temperature and other ions such as Fe³⁺, Mg²⁺, Ca²⁺, etc. on the transpassive or pitting potential, the protection potential, the primary passive potential and the corrosion potential should be further investigated, because natural water may contain those ions and is at high temperatures which could affect on the corrosion of AISI 316L stainless steels.

The investigated E‐pH diagrams may be applicable to avoid corrosion of AISI 316L stainless steels in similar conditions. The useful application may be for fields where natural water is not able to be treated, as is carried out in industry.

There have been several investigations on the effect of chloride on the corrosion behaviours of AISI 316L stainless steels. However, those investigations were carried out in different conditions. Very few experimental E‐pH diagrams of AISI 304L have been found, but not for AISI 316L stainless steels. The investigated diagrams showed also the ion states in pitting corrosion region which were influenced by pH. This may indicate the different pitting corrosion mechanism at different pH.

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.