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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.
During carbon dioxide storage technology (carbon capture and storage, CCS) components are exposed to a corrosive environment and mechanical stress, which results in corrosion fatigue and inevitably followed by the a lifetime reduction of these components. In order to gain knowledge upon the corrosion fatigue strength of materials, Samples of high alloyed stainless injection-pipe steels AISI 420 X46Cr13, and X5CrNiCuNb16-4 AISI 630 were tested in a at T=60 °C and ambient pressure in a CO2-saturated synthetic aquifer environment similar to possible geological on-shore CCS-sites in the northern German Basin. Therefore a corrosion chamber applied to a resonant testing machine allowing for “in situ” test conditions was designed and successfully tested. In-situ tension compression experiments were established using a resonant testing machine at a frequency as low as 30 – 40 Hz. In addition technical CO2 was introduced into the closed system at a rate close to 9 L/h to keep stable environmental conditions. Simultaneously electrochemical testing was performed to get information on failure causes and the mechanism of failure during the injection of CO2 into deep geological layers. S-N plots, micrographic analysis, and surface analysis of the fracture surface were applied to obtain sustainable information on the corrosion fatigue behavior of injection pipe steels. Samples used have a surface roughness of Rz = 4, to simulate technically machined surfaces. X46Cr13 reached the maximum number of cycles (12.5 x 106) at a stress amplitude of 173 MPa. X5CrNiCuNb16-4 reached the maximum number of cycles (10 x 106) at a stress amplitude at 150 MPa. The scatter range of X5CrNiCuNb16-4 is very high (1:34); by comparison the scatter range of X46Cr13 is only 1:3.5.
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.
In geothermal power plants materials e.g. pumps are exposed to extremely corrosive thermal water. This results in corrosion fatigue and so inevitably the reduction of the lifetime of these components. Also in the field of the carbon dioxide storage technology (carbon capture and storage CCS) components are exposed to a corrosive environment and mechanical stress. In order to gain knowledge upon the corrosion fatigue strength of materials a corrosion chamber for "in situ" conditions was designed and successfully applied.Two different steels X46Cr13 and X5CrNiCuNb16-4 have been tested and their corrosion fatigue behavior was compared. To simulate the frequency of operating pumps (30 – 40 Hz) a resonant testing machine was used. In addition technical CO2 was introduced into the closed system at a rate close to 9 L/h to keep stable environmental conditions. The samples have a surface roughness of Rz = 4 to simulate technical machined surfaces. The calculated tensile strength of X46Cr13 with soft annealed microstructure (coagulated cementite in ferrite-perlite matrix) is about 680 MPa and the yield strength is about 345 MPa. The tensile strength of X5CrNiCuNb16-4 is about 1078 MPa and the yield strength about 928 MPa. Testing parameters are: corrosion media: saline aquifer water (Stuttgart Aquifer) temperature of the brine at 60 °C and load ratio of R=-1. For X46Cr13 a stress amplitude between 160 MPa to 270 MPa and for X5CrNiCuNb16-4 a stress amplitude between 150 to 500 was chosen. Cycles until crack initiation differ strongly and were found to start at 5 x 104 up to 12.5 x 106 cycles.X46Cr13 has reached a maximum number of cycles (12.5 x 106) at a stress amplitude of 173 MPa. X5CrNiCuNb16-4 has reached the maximum number of cycles (10 x 106) at a stress amplitude of 150 MPa. The range of scatter for X5CrNiCuNb16-4 is very high (1:34) in comparison the range of scatter for X46Cr13 (1:3.5).
Design of a high pressure system for in-situ tests on the corrosion fatigue of metallic materials
(2014)
In geothermal power plants pumps are exposed to corrosive and oscillating loads. This leads to corrosion fatigue and reduces the lifetime of certain components. Usually underwater power pumps are placed inside the borehole. In this case an installation depth of 600 to 800 meters below ground level is typical. In these depths an additional load due to high pressure and high temperature is introduced which is based on geothermal brine. Therefore the material which is used for the pump is steel.For fatigue testing at loads existing in deep geological layers an appropriate system for fatigue tests under in-situ conditions was set up. The requirements of the design are: temperature of the corrosion medium of 200 °C pressure of 200 bars and the complete immersion of the specimen in flowing corrosion medium containing mixtures of corrosive gasses during the entire test period.The test system has three main components the corrosion chamber the heating module and the reservoir. It is important that the corrosion chamber is only fixed onto the specimen which allows for highest flexibility of the test system operational in many different test machines. The heating module heats the corrosion medium up to 200 °C. The reservoir make sure that the volume of the corrosion medium satisfies requirements of the DIN 50905 Part 1 is conformed providing minimum required corrosive medium of 10 ml/cm² in relation to the sample surface.This innovative fatigue test system aims at performing in- situ fatigue corrosion tests simultaneously at high pressure and high temperature in liquid and gaseous corrosive environment. Results from the tests will allow determining the fatigue life of several materials influenced by pressure and temperature under corrosive conditions.