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 1000mg/L Cl- solution for 30 days bubbled with 10MPa CO2 at 40°C. In some cases tests were also carried out in supercritical CO2 containing 0.1MPa H2S. Microstructural characterization revealed that the carbon steel formed different scales siderite in pure 10MPa CO2 and mackinawite in CO2 containing 0.1MPa H2S. CRA coatings protected the steel substrate from CO2 corrosion when undamaged and no scale was seen.It was concluded that thermally sprayed CRA coatings can provide a cost-effective corrosion mitigation method for infrastructure likely to be in contact with wet supercritical CO2at 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.

Carbon capture and storage (CCS) is a promising technology that can keep the core value of fossil fuel power plants while significantly reduce CO2 emissions. Pipeline transportation is believed to be the most cost-effective and relatively safe solution in the context of CCS as it can transport large amounts of CO2 under well-controlled environments. However the transported sc-CO2 stream always contains certain amounts of corrosive impurities particularly SO2 and H2S. There is a stress corrosion cracking concern of sc-CO2 pipes because of the presence of high pressure CO2 stream and S-contained agents. Little work has been performed to address this issue. In this paper SCC investigation of pipeline steels in supercritical CO2 stream is conducted and the results are discussed. It is anticipated to support the development of CO2 pipeline standard and advance the deployment of CCS technology in a safe manner.

Various type of specimens were exposed at 450°C and 1100 psi in pure supercritical CO2(sCO2) for over 1 month. The exposure was performed in order to assess the effect of various variables on the oxidation of materials used in supercritical CO2 at high temperature.Variable such as temperature and pressure are fairly well covered in the literature. Other identified variables such as contaminants and coatings have been partially addressed. Additional variables of interest such as welding stress corrosion cracking galvanic issues or crevices have not been studied.Welding changes the local microstructure due to the high temperature in the vicinity of the weld. The chromium will diffuse to the grain boundaries and the chromium concentration in the matrix will drop significantly. Consequently the corrosion resistance near the weld will drop. Since welding will likely be used in the manufacturing process it is recommended to test coupons containing the heat affected zone near a weld.Galvanic corrosion occurs when two materials with different electrochemical potentials are in contact with a corrosive environment. There is usually very little change in corrosion rates when materials with similar composition are in contact. However there may be some issue in the case of nickel alloys/stainless steel couples. It has also been suggested in the literature that galvanic corrosion may not be an issue because sCO2is not considered an electrolyte. However it may be of interest to electrically couple two samples of different material (stainless steel and nickel alloy) and measure the weight change of each sample individually after exposure to assess galvanic corrosion.Stress corrosion cracking combines the effect of applied stresses and corrosive environment leading to accelerated crack growth of a susceptible material due to its microstructure.Crevice corrosion may occur within the occluded site of two sandwiched specimens of identical material.Mass loss micro-hardness and SEM/EDS inspection of the specimen cross sections were used to measure the extent of oxidation. The welded specimens wee of both martensitic stainless steel 410 and austenitic stainless steel 310. The galvanic corrosion specimens were stainless steel 410 coupled to nickel alloy 625. The crevice corrosion specimens were two martensitic alloy 410 specimens coupled together. Stress corrosion cracking was studied using C-ring specimens.