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Degradation of AISI 630 exposed to CO2-saturated saline aquifer at ambient pressure and 100 bar
(2018)
In general high alloyed steels are suitable as pipe steels for carbon capture and storage technology (CCS), because they provide sufficient resistance against the corrosive environment of CO2-saturated saline aquifer which serves as potential CCS-site in Germany. High alloyed martensitic steel AISI 630 has been proven to be sufficient resistant in corrosive environments, e.g. regarding heat, pressure, salinity of the aquifer, CO2-partial pressure), but reveals a distinct corrosion pattern in CCS environment. Therefore coupons of AISI 630 heat treated using usual protocols were kept at T=60 °C and ambient pressure as well as p=100 bar up to 8000 h in an a) water saturated supercritical CO2 and b) CO2-saturated synthetic aquifer environment similar to on-shore CCS-sites in the Northern German Basin. AISI 630 precipitates a discontinuous ellipsoidal corrosion layer after being exposed for more than 4000 hours. Best corrosion resistance in the CO2-saturated synthetic aquifer environment phase is achieved via normalizing prior to exposure. In water saturated supercritical CO2 tempering at medium temperatures after hardening gives lowest corrosion rates. Corrosion fatigue via push-pull tests with a series of 30 specimens was evaluated at stress amplitudes between 150 MPa and 500 MPa (sinusoidal dynamic test loads, R=-1; resonant frequency ~ 30 Hz). The endurance limit of AISI 630 is reduced by more than 50% when exposed to CCS environment (maximum number of cycles (10 x 106) at a stress amplitude of 150 MPa).
With CO2 being one reason for climate change carbon capture and storage (CCS) is discussed to mitigate climate change. When emission gases are compressed into deep geological layers CO2-corrosion can easily cause failure of injection pipes. Different steels 42CrMo4, X46Cr13 and X20Cr13 were tested as well as X35CrMo17 and X5CrNiCuNb16-4 in a laboratory Environment similar to the conditions of the CCS engineering site at the Northern German Bassin. Samples were exposed to synthetic aquifer water saturated with technical CO2 at a flow rate of 3 NL/h.
Corrosion rates obtained via mass loss vary in a wide range (0,005 to 2.5 mm/year). The precipitations within the corrosion scale revealed a complicated multiphase layer containing siderite FeCO3, goethite α-FeOOH, lepidocrocite γ-FeOOH, mackinawite FeS and akaganeite Fe8O8(OH)8Cl1,34 and spinelphases of various compositions.
The commercial usage of latent thermal energy storages primarily depends on the development of a suitable phase change material (PCM). For industrial high temperature applications above 400 °C multicomponent chloride eutectics are promising and therefore discussed seriously. The profound thermodynamic investigation of such eutectics requires a much greater amount of specimen material than conventional calorimeter can handle. Therefore, a special adiabatic calorimeter was developed and designed. With a specimen mass of > 100 g the typical thermodynamic measurements with a commercial calorimeter can be extended by cycle stability measurements, which are often decisive for practical application of PCM. Furthermore, by implementing corrosion specimens inside the calorimeter high temperature corrosion experiments according to ISO 21608 can be performed inside the calorimeter. Adiabatic measuring conditions can be provided by using two separate heating systems. Therefore, the outer “protective system” follows the temperature curve of the inner “measuring system” minimizing the temperature difference between the heating systems and simultaneously preventing heat losses from the measuring systems.
The lifetime reduction of cyclically loaded AISI 420C (X46Cr13, 1.4034) constantly exposed to highly corrosive CO2-saturated hot thermal water is demonstrated in in situ-laboratory experiments (60 °C, brine: Stuttgart Aquifer, flowing CO2: 30 L/h). SN plots, micrographic-, phase-, fractographic- and surface analysis were applied to obtain sustainable information on the corrosion and corrosion fatigue behavior. Maximum number of cycles (here 12.5 × 106 cycles to failure) is reached at σa = 173 MPa. Hydroxide and siderite layers were found on pits and crack surfaces. No typical fatigue limit exists and pit corrosion prior to crack initiation may be identified as failure cause.
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 .
In CCS environment (carbon capture and storage) pipes are loaded statically and/or cyclically and at the same time exposed constantly to the highly corrosive hot thermal water. Experimental procedures such as ambient pressure immersions tests, in-situ corrosion fatigue experiments using a flexibly designed corrosion chamber at ambient pressure and a specially designed corrosion chamber at high pressure. Experimental set-ups for push/pull and rotation bending load are introduced. The corrosion behavior and lifetime reduction of high alloyed steels (X46Cr13, 1.4043), (X5CrNiCuNb16-4, 1.4542) and (X2CrNiMoN22-5-3, 1.4462) is demonstrated (T=60 °C, geothermal brine: Stuttgart Aquifer flow rate: 9 l/h, CO2 ).
High-temperature-high pressure stress-strain testing of materials in CO2-containing saline solutions
(2015)
In-situ stress-strain testing under corrosive environment, such as corrosive gasses (e.g. CO2) and highly saline water, is a challenge in testing corrosion fatigue of materials, e.g. for geothermal application or CCS (carbon capture and storage). The first corrosion chamber system was designed for performance at ambient pressure up to 100 °C. The second allows for corrosion fatigue testing at high pressure up to 200 bar and 400 °C. The highly flexible corrosion chambers allow for fast changing and easy alignment of test samples, visual monitoring, CAD-camera monitoring electrochemical measurements, O2-partial pressure or gas partial pressure measurement. Novelty is the fixing of the corrosion chamber directly onto the specimen, that guarantees best fitting and enables the test system to be modified easily suiting a variety of fatigue test machines. All parts of the test system are conforming to the technical rules.
In the field of water pipelines, geothermal energy production as well as carbon capture and storage technology (CCS) materials have to provide a high resistance to corrosion and mechanical stress. The combination of cyclic load and corrosive aqueous environment leads to corrosion fatigue of pipes and components (e.g. pumps) and thus inevitably to the reduction of the lifetime of these components. To estimate the reliability of components from adjusted in-situ-laboratory experiments a corrosion chamber was designed and tested with CO2 saturated corrosive aqueous media flowing at a steady rate. Unique feature of this special chamber is its installation directly onto the sample and thus providing flexible usability in almost every testing machine. This allows simultaneous mechanical loading of the sample, operation at temperatures up to 100 °C and exposure to fluid flow of corrosive liquids and gases. The lifetime reduction of AISI 420C (X46Cr13, 1.4034) is demonstrated at T=60 °C, geothermal brine: Stuttgart Aquifer flow rate: 9 l/h, CO2. S-N plots, micrographic-, phase-, fractographic- and surface analysis were applied to obtain sustainable information on the corrosion fatigue behavior. Maximum number of cycles (here 12.5 x 106 cycles to failure) is reached at σa =173 MPa. No typical fatigue strength exists and passive corrosion fatigue may be identified as failure cause.
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.