Corrosion of Carbon Steel in Supercritical CO₂/H₂S and Its Mitigation Using Coatings

Carbon steels are widely used in the oil and gas exploration and production service. These steels are prone to corrosion in CO₂ and cracking in H₂S. The propensity to cracking increases when high strength grades are employed. The use of corrosion resistant alloys incurs costs, and only when these costs are justified can such materials be used in industrial service. If these materials can be coated by an industrial method onto carbon steel then their industrial uptake can be increased. To understand the behaviour of CRA coatings on steel, several carbon steel coupons were sprayed using high velocity oxy-fuel (HVOF). Carbon steel specimens, with and without CRA coatings, were then tested in de-aerated 1000 mg/L Cl^- solution for 30 days, bubbled with 10 MPa CO₂ at 40 °C. In some cases, tests were also carried out in supercritical CO₂ containing 0.1 MPa H₂S. Microstructural characterization revealed that the carbon steel formed different scales, siderite in pure 10 MPa CO₂ and mackinawite in CO₂ containing 0.1 MPa H₂S. CRA coatings protected the steel substrate from CO₂ and H₂S corrosion when undamaged and no scale was seen.

It was concluded that thermally sprayed CRA coatings has the potential to provide a cost-effective corrosion mitigation method for infrastructure likely to be in contact with wet supercritical CO₂ and H₂S at 40 °C. The scales formed on the steel somewhat protected it from further corrosion in 10 MPa CO₂. However, it was evident that care must be taken to ensure that the thermally sprayed CRA layer does not have any through porosity or defect; else, such coatings may accelerate corrosion of the underlying steel substrate due to galvanic interactions.