This study investigates the corrosion behavior of three stainless steel grades at two H<sub>2</sub>SO<sub>4</sub> concentrations, namely 1 and 10 wt.%, with varying NaCl concentrations in the range from 500 to 10000 mg/L. Dissociation of sulfuric acid yields the hydrogen (H<sup>+</sup>) and sulfate (SO<sub>4</sub><sup>2-</sup>) ions, the former of which lowers the pH value of solution while the latter increases the concentration of sulfate ions that act as a corrosion inhibitor. The equilibrium chemistry of the solutions was defined at the test temperatures of 22, 50, 90, and 130°C, and correlated with the observations on the electrochemical and microstructural examination of the materials. The results showed clear differences in the main corrosion form between the two H<sub>2</sub>SO<sub>4</sub> concentrations. In 1 wt.% H<sub>2</sub>SO<sub>4</sub>, pitting was the major form of corrosion attack in the presence of chlorides, whereas uniform corrosion dominated in 10 wt.% H<sub>2</sub>SO<sub>4</sub>. The pitting corrosion tendency for the three stainless steel grades under various test conditions was consistent, but there were differences in their resistance to uniform corrosion. The chloride-to-sulfate activity ratio, a(Cl<sup>-</sup>)/a(SO<sub>4</sub><sup>2-</sup>), was found to be the key parameter in defining the occurrence of pitting corrosion for all three alloys. In H2SO4-NaCl systems, no pitting occurred at the activity ratio a(Cl<sup>-</sup>)/ a(SO<sub>4</sub><sup>2-</sup>) below 10, with higher values inducing pitting attack particularly in 1 wt.% H<sub>2</sub>SO<sub>4</sub>. The described novel results are presented and discussed in this paper.

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