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51316-7378-Corrosion Fatigue of X2CrNiMoN22-5-3 exposed to the Geothermal Environment of the North German Basin

Product Number: 51316-7378-SG
ISBN: 7378 2016 CP
Author: Marcus Wolf
Publication Date: 2016
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In geothermal power plants the materials used in pumps are loaded cyclically and exposed constantly to the highly corrosive hot thermal water. This leads to corrosion fatigue and thus inevitably to the reduction of the lifetime of these components. In the field of geothermal energy production materials have to provide a high resistance to corrosion and mechanical stress.The corrosion fatigue of duplex alloy X2CrNiMoN22-5-3 (manufactured via pulltrusion) was investigated on a horizontal axial fatigue testing machine equipped with a specially designed corrosion chamber. The hourglass-shaped specimens underwent purely alternating compressive-tensile cyclic load to failure. The experiments were carried out in in-situ geothermal conditions introducing the highly saline Northern German aquifer at 96°C. An additional electric insulation of the subsystem specimen/corrosion chamber was employed to evaluate the sufficiency of the setup's electrical grounding. Additionally these results were verified using a distinct vertical fatigue testing machine confirming the results of the main set of experiments.The S-N curves were determined for both electrically insulated and uninsulated experimental series using the method of linear regression. The S-N curve obtained via electrically insulated setup exhibited a negative shift of the corrosion fatigue life expectancy. Furthermore an increase of the degradation rate was observed. The continuous measurements of the free corrosion potential during the tests were accomplished by an Ag/AgCl electrode. This electrode was verified via in-situ conditions prior to the fatigue testing.The electrical insulation leads to more positive (more noble) initial values and an overall more stabilized free corrosion behavior. The obvious drop of the free corrosion potential was characteristic for all experiments indicating an upcoming final mechanical failure of the specimen.Corrosion pits in combination with cracks were observed via scanning electron microscope. Further examinations of microstructural cross sections thorough the pit’s cavities revealed grains that were attacked horizontally to the load direction. The investigation of the microstructural cross section of the crack propagation region revealed considerable corrosion degradation of the austenitic phase within the alloy for the specimen.
In geothermal power plants the materials used in pumps are loaded cyclically and exposed constantly to the highly corrosive hot thermal water. This leads to corrosion fatigue and thus inevitably to the reduction of the lifetime of these components. In the field of geothermal energy production materials have to provide a high resistance to corrosion and mechanical stress.The corrosion fatigue of duplex alloy X2CrNiMoN22-5-3 (manufactured via pulltrusion) was investigated on a horizontal axial fatigue testing machine equipped with a specially designed corrosion chamber. The hourglass-shaped specimens underwent purely alternating compressive-tensile cyclic load to failure. The experiments were carried out in in-situ geothermal conditions introducing the highly saline Northern German aquifer at 96°C. An additional electric insulation of the subsystem specimen/corrosion chamber was employed to evaluate the sufficiency of the setup's electrical grounding. Additionally these results were verified using a distinct vertical fatigue testing machine confirming the results of the main set of experiments.The S-N curves were determined for both electrically insulated and uninsulated experimental series using the method of linear regression. The S-N curve obtained via electrically insulated setup exhibited a negative shift of the corrosion fatigue life expectancy. Furthermore an increase of the degradation rate was observed. The continuous measurements of the free corrosion potential during the tests were accomplished by an Ag/AgCl electrode. This electrode was verified via in-situ conditions prior to the fatigue testing.The electrical insulation leads to more positive (more noble) initial values and an overall more stabilized free corrosion behavior. The obvious drop of the free corrosion potential was characteristic for all experiments indicating an upcoming final mechanical failure of the specimen.Corrosion pits in combination with cracks were observed via scanning electron microscope. Further examinations of microstructural cross sections thorough the pit’s cavities revealed grains that were attacked horizontally to the load direction. The investigation of the microstructural cross section of the crack propagation region revealed considerable corrosion degradation of the austenitic phase within the alloy for the specimen.
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