Effect of Cl⁻ Concentration on the Corrosion Behavior of 16Mn Steel in Saturated H₂S/CO₂ Solution

Abstract

The corrosion behavior of 16Mn steel was studied in saturated H₂S or H₂S/CO₂ solutions containing different Cl⁻ concentrations at 80 °C. The microstructure and chemical composition of the corrosion products were investigated through scanning electron microscopy, energy-dispersive X-ray spectroscopy, EPMA, and X-ray diffraction. Results showed that the corrosion rate decreased with increasing Cl₋ concentration in saturated H₂S or H₂S/CO₂ solution at pH 4. Conversely, the corrosion rate increased with increasing Cl⁻ concentration in saturated H₂S solution at pH 6. The relative H⁺ concentration decreased because of the increase of Cl₋ concentration at pH 4, and Cl⁻ acted as a catalyst in the corrosive medium at pH 6 because the net H⁺ concentration decreased obviously compared with the condition at pH4. Cl⁻ promoted the formation of Fe-deficient iron sulfide at pH 4, and the opposite effect was observed in the nearly neutral solution. The corrosion rate increased firstly with increasing Cl₋ concentration and then decreased in the saturated H₂S/CO₂ solution at pH 6. The corrosion products were mainly composed of two kinds of iron sulfide. Sulfide FeS_1−x was a kind of tetragonal crystal, whereas the other was the hexagonal/monoclinic iron sulfide Fe_1−xS. The corrosion film that was mainly composed of FeS_1−x did not confer a protective effect on the base metal. The atomic ratio of Fe/S was more than 1 for FeS_1−x. The appearance of sulfide FeS_1−x resembled a square block or small, needle-like, flocculent particles. The atomic ratio of Fe/S was less than 1 for Fe_1−xS, and the corrosion film mainly composed of Fe_1−xS conferred some protective property on the base metal. The sulfide FeS_1−x exhibited a long claviform morphology with a hexagonal or quadrilateral cross-sectional shape.

Keywords

16Mn steel / acid corrosion / weight loss / microstructure / corrosion products