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Oxidation of a single crystal nickel-base superalloy at 950 °C - a kinetic and microstructure study
(2008)
The nickel-base single crystal alloy investigated is a widely used material for first and second row blades in stationary gas turbines. Nickel-base superalloys are especially designed to resist high temperature oxidation by process gases. To determine this high temperature behaviour oxidation testing was carried out using samples cut perpendicular to (001)-direction. Microstructures were characterized by X-ray diffraction XRD, Light Microscopy LM, Scanning Electron Microscopy SEM, and X-ray energy dispersive spectroscopy (EDS), after a series of heat treatments (950 °C, 0 h - 1000 h). Mass gain and the respective thicknesses of the different oxide layers were measured to determine oxidation kinetics. In general, the isothermal oxidation behaviour at 950 °C, as defined by weight gain, follows a parabolic law with a parabolic rate constant around 1.4 · 10-2 mg2/(cm4 · h). A short incubation time is followed by the constitution of a multi-layered oxide scale. The oxide scale consists of a three layer structure. An outer scale contains a Ti-bearing thin film associated as TiO2 and NiTiO3 but mostly Cr attributed to Cr2O3, (Ni/Co)Cr2O4 beside NiTaO4. This outer scale is connected to a discontinuous layer of inner oxidation consisting mainly of Al2O3, which is followed by an area of γ´-depletion within the base material.
Isothermal oxidation behavior and microstructure of the single crystal superalloy PWA 1483 at 950 C
(2008)
The oxidation behaviour of single crystal PWA 1483 at 950 °C was investigated by means of XRD, SEM and EDS. The parabolic oxidation behaviour, as defined by mass gain and the respective oxide layer thicknesses, is characterized by a parabolic rate constant of about 4 × 10-6 mg²/(cm4 × s) and the formation of a multi-layered oxide scale. An outer scale contains a Ti-bearing thin film composed of TiO2 and NiTiO3 but mostly Cr in Cr2O3 and (Ni/Co)Cr2O4 besides NiTaO4. This outer scale is connected to a discontinuous layer of Al2O3 and an area of γ?'-depletion within the base material.
In corrosive environments such as CCS bore holes or geothermal power plants the materials loaded cyclically are also exposed constantly to the highly corrosive hot thermal water. The lifetime reduction of (X46Cr13, AISI 420C) is demonstrated in in-situ-laboratory experiments (T=60 °C, geothermal brine: Stuttgart Aquifer flow rate: 9 Nl/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 is most likely failure cause.
During the compression emission gasses in deep geological layers (Carbon Capture and Storage, CCS) CO2-corrosion will become a relevant safety issue. The reliability of the steels used at the geological onshore CCS-site at Ketzin, Germany, (heat treated steel 42CrMo4 (1.7225, AISI 4140) used for casing, and the martensitic stainless injection pipe steels X46Cr13 (1.4034, AISI 420 C), X20Cr13 (1.4021, AISI 420 J), X35CrMo17 (1.4122)) is demonstrated in 1 and 2 years laboratory experiments. Samples were kept in a synthetic aquifer environment similar to the geological CCS-site at Ketzin, Germany at T=60 °C. This corrosive environment is then saturated with technical CO2 at a flow rate of 3 l/h. Microstructures were characterized by X-ray diffraction, light microscopy, scanning electron microscopy, and energy dispersive X-ray, after a series of heat treatments (700 h to 2 years). Due to very slow mass loss at extended exposure times to CCS-environment one year is sufficient to predict stable surface corrosion rates from laboratory experiments. The non-linear isothermal surface corrosion behaviour of the steels reveals surface corrosion rates around 0.1 to 0.8 mm/year, when obtained by mass gain. The loss of the base material is higher when calculated from the corrosion layer magnitude due to the unpredictable local corrosion attacks. Severe pit corrosion (pit heights ca. 4.5 mm) are only located on the high chromium steels. Main phases of the continuous scales are siderite FeCO3 and goethite α-FeOOH. The formation of the non-protective layer is likely to form via a transient Fe(OH)2-phase.
The influence of heat treatment on pit corrosion needs to be considered to guarantee reliability and safety during the injection of compressed emission gasses – mainly containing CO2 – into deep geological layers (CCS-technology, Carbon Capture and Storage). In laboratory experiments different heat treated steels used as injection pipe with 13% Chromium and 0.46% Carbon (X46Cr13, 1.4034) as well as 0.2% Carbon (X20Cr13, 1.4021) were tested. Also X5CrNiCuNb16-4 (1.4542) was investigated as typical steel used for geothermal pumps. Keeping stable environmental conditions in laboratory experiments the samples were exposed to the distinct synthetic aquifer environment saturated with technical CO2 at a flow rate of 3 l/h for up to 6 months.
Independent of the exposure time the least amount of pits is found on hardened steels with martensitic microstructure where X5CrNiCuNb16-4 shows fewer pits than X46Cr13 and X20Cr13.
Regarding steels with similar Cr-content the higher Ccontent in 1.4034 results in fewer pits compared to 1.4021.
CO2-corrosion of injection pipe steels is a relevant safety issue when emission gasses are compressed in deep geological layers (CCS). The reliability of the steels used at the geological onshore CCS-site at Ketzin, Germany, is demonstrated in laboratory experiments under an equivalent corrosive environment (T=60 °C,p=1-60 bar, aquifer water, CO2-flow rate of 3 l/h, 700 h8000 h heat treatment). Corrosion kinetics and microstructures were characterized using samples of the heat treated steel 1.7225 (AISI 4140, 42CrMo4) used for casing, and samples of the martensitic stainless injection-pipe steel 1.4034 (AISI 420, X46Cr13).
During carbon capture and storage (CCS) CO2-corrosion of pipe steels is a relevant safety issue when emission gasses are compressed in deep geological layers. The reliability of the steels 42CrMo4, X20Cr13, X46Cr13 and X35CrMo17 is demonstrated in long term laboratory experiments up to 2 years of exposure time. Testings were carried out at ambient pressure under an equivalent corrosive environment as found at the geological onshore CCS-site at Ketzin, Germany (T=60 °C, aquifer water).
When emission gasses are compressed into deep geological layers (CCS) CO2-corrosion of injection pipe steels is a relevant safety issue. The reliability of the steels used at the geological onshore CCS-site at Ketzin, Germany, is demonstrated in 2 years laboratory experiments under an equivalent corrosive environment at ambient pressure (T=60 °C, aquifer water, CO2-flow rate of 3 l/h). Corrosion kinetics and microstructures were characterized using samples of the heat treated steel 42CrMo4 (casing), and samples of the martensitic stainless steel X46Cr13 (injection).
In engineering storage sites for Carbon Sequestration the CO2-induced corrosion of injection pipe steels is a relevant safety issue when emission gasses are compressed in deep geological layers. The reliability of the steels suitable for the geological onshore CCS-site (Carbon Capture and Storage) at Ketzin, Germany, is demonstrated in laboratory experiments in equivalent corrosive environment (T = 60 °C, p = 160 bar, aquifer water, CO2-flow rate of 3 L/h, 7008000 h exposure time). Corrosion kinetics and microstructures were characterized using specimens of the heat treated steel 42CrMo4 (1.7225) used for casing, and specimens of the martensitic stainless injection-pipe steel X46Cr13 (1.4034).
Properties of pipe steels for CCS (carbon capture and storage) technology require resistance against the corrosive environment of a potential CCS-site (heat, pressure, salinity of the aquifer, CO2-partial pressure). The influence of austenitzing in heat treatment routines of two different injection pipe Steels (1.4034, X46Cr13 and 1.4021, X20Cr13) was evaluated. Steel coupons were austenitized at different temperatures (900- 1050 °C) for different lengths of time (30-90 min) before quenching and annealing prior to long term corrosion experiments (60°C, 100 bar, artificial brine close to a CCS-site in the Northern German Basin, Germany). In general, fewer pits are found on X46Cr13. Comparing steels with 13% chromium each the higher carbon content of X46Cr13 (0.46% C) results in a lower number of pits compared to X20Cr13 (0.20% C). It is found that neither the carbon content of the steels nor austenitizing temperature has much influence, but local corrosion behaviour is most susceptible towards austenitzing time.
Properties of pipe steels for CCS technology require resistance against the corrosive environment of a potential CCS-site (heat, pressure, salinity of the aquifer, CO2-partial pressure). The influence of austenitizing in heat treatment routines of two different injection pipe steels (1.4034, X46Cr13 and 1.4021, X20Cr13) was evaluated. Steel coupons were austenitized at different temperatures (900 – 1050 °C) for different lengths of time (30–90 min) before quenching and annealing prior to long term corrosion experiments (60°C, 100 bar, artificial brine close to a CCS-site in the Northern German Basin, Germany). In general, fewer pits are found on X46Cr13. Comparing steels with 13% chromium each the higher carbon content of X46Cr13 (0.46% C) results in a lower number of pits compared to X20Cr13 (0.20% C). It is found that neither the carbon content of the steels nor austenitizing temperature has much influence, but local corrosion behaviour is most susceptible towards austenitizing time