Mechanism of transformation of ion-selectivity of pyrrhotite by hypervalent anion (SO42−) and its effect on corrosion behavior

The corrosion behavior of P110 steel in H₂S environments was investigated, revealing that the ion-selectivity of the corrosion products significantly influences corrosion behavior. At 60°C and 90°C, the corrosion products formed on P110 steel in the NaCl solutions and the NaCl+Na₂SO₄ solutions were cation-selective mackinawite, which provided good protection for the metal substrate, resulting in low corrosion rates and uniform corrosion. However, at 120°C, 160°C, and 205°C, the corrosion products formed in the NaCl solutions were anion-selective pyrrhotite, which failed to protect the metal substrate effectively, leading to high corrosion rates and pitting corrosion. In contrast, in the NaCl+Na₂SO₄ solutions, the cation-selective pyrrhotite exhibited excellent protective performance, significantly reducing the corrosion rate and preventing pitting corrosion. By calculating the surface energy of Cl⁻ and SO₄²⁻ adsorption on the (001) plane of pyrrhotite, it was found that SO₄²⁻ adsorption changed the exposed atoms on the (001) plane from Fe to S, transforming pyrrhotite from anion-selective to cation-selective. Additionally, the morphology of pyrrhotite crystals evolved from elongated hexagon to regular hexagon. According to the Gibbs-Wulff theorem, by simulating the Wulff structure of pyrrhotite crystals, the relationship between crystal morphology and surface energy was established. The law of crystal morphology evolution of pyrrhotite was revealed.