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To mitigate carbon dioxide emissions CO2 is compressed and sequestrated into deep geological layers (Carbon Capture and Storage CCS). The corrosion of injection pipe steels is induced when the metal is in contact with CO2 and at the same time the geological saline formation water. Stainless steels X35CrMo17 and X5CrNiCuNb16-4 with approximately 17% Cr show potential as injection pipes to engineer the Northern German Basin geological onshore CCS-site. Static laboratory experiments (T = 60 ◦C, p = 100 bar, 700–8000 h exposure time, aquifer water, CO2-flow rate of 9 L/h) were conducted to evaluate corrosion kinetics. The anomalous surface corrosion phenomena were found to be independent of heat treatment prior to exposure. The corrosion process is described as a function of the atmosphere and diffusion process of ionic species to explain the precipitation mechanism and better estimate the reliability of these particular steels in a downhole CCS environment.
During capture and storage technology (CCS) as well as in geothermal energy production Steels need to withstand the corrosive environment such as: heat, pressure, salinity of the aquifer and CO2-partial pressure. 1.4542 shows unusual corrosion phenomena, but is still sufficiently resistant in corrosive environments. To better understand its behaviour differently heat treated coupons of 1.4542 and for comparison X20Cr13 and X46Cr13 were kept in the artificial brine of the Northern German Basin at T=60 °C. Ambient pressure as well as p=100 bar for 700 h - 8000 h in water saturated supercritical CO2 and CO2-saturated synthetic aquifer Environment was applied. Fatigue tests were performed via push-pull tests with a series of 30 specimens from 150 MPa to 500 MPa (sinusoidal dynamic test loads, R=-1; resonant frequency ~ 30 Hz). FeCO3 and FeOOH are corrosion products also after dynamic corrosion tests. Martensitic microstructure offers good corrosion resistance in geothermal environment. The S-N-curve showing no typical fatigue strength and very steep slopes of possible fatigue strength for finite life. Possible influencing artefacts, such as Al-inclusions could not be correlated to early rupture despite specimens containing inclusions at the fracture surface and cross section reached lower number of cycles. Applied potential proofed to enhance fatigue life tremendously.
Corrosion fatigue specimen with different surfaces (technical surfaces after machining and polished surfaces) of high alloyed martensitic stainless steel X46Cr13 (1.4043) and duplex stai nless steel X2CrNiMoN22 3 2 (1.4462) were compared at load amplitudes from 175 MPa to 325 MPa in the geothermal brine of the N orthern German Basin at 98 °C. Surface corrosion layers and pits reveal carbonate corrosion products on the surface such as FeCO 3 and FeOOH as the main precipitation phases with no dependence on the original surface roughness . At high stress amplitudes above 275 MPa technical surfaces (P50% at σa 300 MPa=5 × 10 5 ) resulted in more cycles to failure than polished (P50% at σa 300 MPa=1.5 × 10 5 ). The greater slope coefficient for technical surfaces k = 19.006 compared to polished surfaces k =8.78 demonstrate s earlier failure at given stress amplitude σa .
To resist the corrosive geothermal environment during carbon capture and storage CCS -such as: heat, pressure, salinity of the aquifer, CO2-partial pressure, properties of pipe steels-require certain specification. For evaluation samples of differently heat treated high alloyed stainless injection-pipe steels AISI 420 X46Cr13, AISI 420J X20Cr13 as well as X5CrNiCuNb16–4 AISI 630 were kept at T=60 °C and ambient pressure as well as p=100 bar for 700 h up to 8000 h in a CO2-saturated synthetic aquifer environment similar to a possible geological situation in the northern German Basin. Corrosion rates and scale growth are lowest after long term exposure for steels hardened and tempered at 600 °C to 670 °C and pits -indicating local corrosion- decrease in diameter but increase in number as a function of carbon content of the steel. Martensitic microstructure is preferred with respect to these particular conditions.
Samples of differently heat treated high alloyed stainless injection-pipe steels AISI 420 X46Cr13, AISI 420J X20Cr13 as well as X5CrNiCuNb16-4 AISI 630 were kept at T=60 °C and ambient pressure as well as p=100 bar for 700 h - 8000 h in a CO2- saturated synthetic aquifer environment similar to possible geological on-shore CCS-sites in the northern German Basin. Corrosion rates and scale growth are lowest after long term exposure for steels hardened and tempered at 600 to 670 °C and pits - indicating local corrosion- decrease in diameter but increase in number as a function of carbon content of the steel. Martensitic microstructure is preferred with respect to this particular CCS-site.
The process chain for Carbon Capture and Sequestration (CCS) includes tubing for injection of CO2 into saline aquifers. The compressed CO2 is likely to contain specific impurities; small concentrations of SO2 and NO2 in combination with oxygen and humidity are most harmful. In addition, CO2 saturated brine is supposed to rise in the well when the injection process is interrupted. The material selection has to ensure that neither CO2 nor brine or a combination of both will leak out of the inner tubing. In this comprehensive paper the investigated materials range from low-alloy steels and 13% Cr steels up to high-alloy materials. Electrochemical tests as well as long term exposure tests were performed in CO2, in brine and combination of both; pressure was up to 100 bar, temperature up to 60 °C. Whereas the CO2 stream itself can be handled using low alloy steels, combinations of CO2 and brine require more resistant materials to control the strong tendency to pitting corrosion. The corrosion behavior of heat-treated steels depends on factors such as microstructure and carbon content. For different sections of the injection tube, appropriate materials should be used to guarantee safety and consider cost effectiveness.
The appropriate strength of steels used for saline aquifer carbon capture and storage sites (CCS) is usually achieved by applying heat treatments. Thus, heat treatment influences the corrosion resistance for injection pipe steels with 13% chromium and different carbon content: 1.4034/X46Cr13 and 1.4021/X20Cr13 in CO2 saturated saline aquifer water at 60 °C, 1 and 100 bar. X46Cr13 shows better corrosion resistance with respect to corrosion rate, number of pits and maximum intrusion depth. Low corrosion rates are obtained for steels with martensitic microstructures exposed to supercritical CO2 at 100 bar, whereas normalized steels show better corrosion resistance at ambient pressure.