Investigation of Iron Sulfide Surface Layer Growth in Aqueous H2S/CO2 Environments

The nature of the iron sulfide films formed on steel surfaces during H₂S/CO₂ corrosion is a key parameter for pH stabilisation in sour gas pipelines, and will also affect the efficiency of corrosion inhibitors. On this background, experimental studies of iron sulfide corrosion film growth in aqueous H₂S/CO₂ environments have been carried out, using gravimetric (mass loss) and electrochemical methods, FE-SEM and X-ray diffraction analyses.

X65 carbon steel samples were exposed to sour conditions with different alkalinities/pH values for various periods of time up to 10 days. Evidence of iron sulfide layer formation governed by the parabolic film growth rate law was found for certain test conditions, indicating that the corrosion and film growth rates in these cases were limited by the electronic conductivity of the film or mass transport, probably Fe²⁺ diffusion, across the corrosion film.