Dissolution of iron during the initial corrosion of carbon steel in aqueous H2S solutions

The dissolution of iron from carbon steel in aqueous H 2 S solutions has been studied as a function of time and temperature at 0.1 MPa H 2 S pressure using the rotating disc technique. The iron sulfide film formed on the disc surface has been identified by X-ray diffraction analysis. The solubility and dissolution rate of synthesized mackinawite, FeS (1−x) , have also been determined to help establish the mechanism of release of iron from carbon steel corroding in aqueous H 2 S solutions.The rate data have been analysed by an equation for a joint chemical and transport controlled process. At 22 °C and 0.1 MPa H 2 S, the maximum rate of release of Fe 2+ , obtained by extrapolating the data to infinite rotation speed, is 7.4 ± 0.5 μmol/m 2 s. This is similar to the dissolution rate determined for synthesized mackinawite powders, but is 20 times faster than the rate of dissolution for troilite and more than 1000 times faster than the dissolution rates for pyrrhotite and pyrite. Thus, troilite and iron sulfide phases that form subsequently are not major contributors to Fe 2+ release during the corrosion of carbon steel.At 22 °C the diffusion coefficient for the FeSH + ion is (1.4 ± 0.2) × 10 −9 m 2 /s and the activation energy of the diffusion process is 25 ± 7 kJ/mol. The chemical reaction occurring at the surface has an activation energy of 77 ± 14 kJ/mol.It is concluded that the release of iron from corroding carbon steel is governed by the dissolution rate of mackinawite, the initial corrosion product. This dissolution is controlled by the chemical reaction between mackinawite and H + and by the transport of the complexed FeSH + from the interface to the bulk solution.