The Role of Sulfide Inclusions in Hydrogen Entry During the Exposure of Steels to Acids

Measurements of hydrogen permeation rates through mild steels containing elongated (FeMn)S inclusions, cathodically charged at 20 μA mm⁻² in 1N H₂SO₄ at 2 and 50 C, show that H₂S produced at the metal surface by dissolution of the inclusions promotes hydrogen entry into the steels. Permeation rates increase with S content of the steel in the range 0.002 to 0.24% but only where the H₂S supply can be maintained by inclusion dissolution. At 2 C, the corrosive attack is confined essentially to the inclusions. Hence, the H₂S supply is reduced as initially exposed inclusions are dissolved. Permeation rates are lower through specimens oriented with the inclusions parallel to the surface as these are removed much more rapidly than inclusions oriented perpendicular to the surface. At 50 C, there is less anisotropy because matrix dissolution is faster and fresh inclusions not originally exposed by metallographic preparation are revealed and contribute to the H₂S supply.