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Eingeladener Vortrag
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Mechanical properties of superheater materials after ageing and corrosion in CO2 rich flue gases
(2010)
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 resistance of structural steels against corrosion in CO2-H2O and CO2-H2O-O2
atmospheres at temperatures above 550°C was observed with high magnification methods.
The test atmosphere is the basic composition of an oxyfuel coal combustion atmosphere. The
analytical information base was applied to develop reaction schemes for the oxidation of
steels. It was demonstrated that the formation of phases and the kinetics of oxidation reactions
in this atmospheres is affected by the formation of intermediate iron hydroxide and a high
carbon activity. In the temperature regime above 550°C the thermal equilibrium between H2O,
CO2 and Fe reacted and formed CO. CO triggers the formation of FeO and carbides and
increases the reaction kinetics. Porosity at the interface is established by gaseous phases
such as Fe(OH)2 or H2O. H2 formation is not limited to the steel - oxide interface but can occur
in the whole oxide scale. Examples of the experimental results were compared with a
thermodynamic approach. The objective of the work was to derive qualitative rules of the basic
corrosion reactions.
The most efficient construction materials for boiler water walls and superheaters in
steam power plants are ferritic and martensitic steels. In practical operation tubes are
simultaneously exposed to combustion gas and air/steam on their opposite surfaces. The
corrosion behaviour of ferritic-martensitic steels under such dual atmospheres is nondistinctive
and has therefore been investigated in a special designed test equipment between 500 and
620°C. The power plant conditions were simulated wi th a flowing and pressurised (80 bar)
combustion gas on the inner side of the tube which mainly consists of H2O and/or CO2. On the
outer side, the tube material was exposed to air. It was discovered that under similar
temperatures the oxides formed on the air side under dual atmosphere conditions were
significantly different to the oxide scales formed when the alloy was exposed to air only. It is
assumed that the anomalous corrosion behaviour during the dual atmosphere exposure is due
to the hydrogen transport through the bulk alloy from the combustion gas side to the air side.
Influence of pressure and chromium content on corrosion reactions at 600 C in a CO2-H2O atmosphere
(2008)
In future power plant technologies, oxyfuel, steels are subjected to steam rich and carbon dioxide rich combustion gases. The effect of simulated combustion gases H2O/CO2/O2 (30/69/1 mol%) and H2O/CO2 (30/70 mol%) on the corrosion behavior of low alloyed steels, 9-12% chromium steels and an austenitic steel were studied. It was discovered that the formation of protective chromium rich oxides is hampered due to the carburization of the base material and the formation of chromium rich carbides. The kinetics of corrosion and carburization are quantified. The effect of temperature and the effect of gas pressure are analyzed statistically.
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.
The materials in oxyfuel power plant will be subjected to CO2– and SO2–rich gases on the fireside. The oxidation behaviour of two 9–12 % Cr steels T92 and VM12 was studied under dry oxyfuel environment in the temperature range of 580–650 °C for up to 1,000 h. The oxide structure and morphology were analyzed using various experimental techniques. A complex temperature dependence of oxidation rate is observed for both T92 and VM12 whereby the oxidation rate decreased with increasing temperature. This is attributed to increased Cr-enrichment in the inner scale with increasing temperature. T92 and VM12 alloys are also susceptible to carburization in an oxyfuel environment.