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Geothermal Corrosion: High-Temperature Pitting of Stainless Steels and Ni-Alloys

The temperature dependence of the pitting corrosion resistance of a number of stainless steels, Fe-Cr-Ni and Fe-Cr-Ni-Mo materials in a series of high-temperature high-pressure chloride solutions, with or without addition of various (inhibitive) anions: SO4=, HCO3-, PO43-, OH-, etc.

Product Number: 51317--9269-SG
ISBN: 9269 2017 CP
Author: Walter Bogaerts
Publication Date: 2017
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It is well recognized that geothermal environments are highly corrosive to metals and alloys. The high corrosiveness of the environment typically arises from the combination of elevated temperatures (e.g. up to 150 200 … 300°C) with the presence of a high concentration of chloride ions and sometimes sulfur species. Chloride-induced pitting corrosion is an imminent materials failure mechanism under these conditions. Accordingly a major factor in the economic exploitation of geothermal resources will be the cost-effective selection of materials that have sufficient resistance to pitting and related corrosion phenomena such as stress corrosion cracking to maintain component integrity.In this paper we describe the temperature dependence of the pitting corrosion resistance of a number of traditional Fe-Cr-Ni and Fe-Cr-Ni-Mo materials (e.g. Type 304 & Type 316 stainless steels Alloy 800 825 600 625) in a series of high-temperature high-pressure chloride solutions with or without addition of various (inhibitive) anions: SO4= HCO3- PO43- OH- etc. Tests have mainly been carried out by means of potentiodynamic electrochemical methods in a recirculating autoclave system. Test temperatures varied between 50 and 300°C with pressures up to 120 bar (12 MPa). After the tests samples have been analyzed by means of various microscopic and surface-analytical methods (optical microscopy SEM EDX AES-SAM etc.).The different results show some remarkable tendencies: In the lower temperature range i.e. between 50 and ca. 150°C the results of the electrochemical tests are fully in line with known literature data and practical experience and show a sharp decrease of the pitting potentials (Ep) with increasing temperatures. However for most test solutions this trend gradually diminishes as temperature rises above » 150 to 175°C and no further decrease of pitting potentials is observed above 200°C in none of the test solutions.At these higher temperatures Ep-values may even shift again to higher values and the typical pitting morphology may change into a more generalized type of corrosion. This means that pitting potentials (and also the corresponding repassivation potentials) show a minimum value at about 175-200°C. This is often accompanied by the formation of thick oxide layers and more shallow types of (localized) attack including under-film of ‘filiform’-like types of corrosion etcetera. These fairly remarkable results are in line with some older data for stainless steel Type 304 in high-temperature high-pressure aqueous environments.A possible explanation might be related with transpassive dissolution phenomena of the Fe-Cr-Ni-(Mo) alloys and the effect on their protective surface films. Also the chloride reactions related to the pitting initiation process as well as the ‘occluded cell’ pit growth mechanisms might change at these higher temperatures. This paper will discuss this in some more detail. 

Key words: pitting, geothermal, high-temperature, stainless steel, nickel alloys

 

It is well recognized that geothermal environments are highly corrosive to metals and alloys. The high corrosiveness of the environment typically arises from the combination of elevated temperatures (e.g. up to 150 200 … 300°C) with the presence of a high concentration of chloride ions and sometimes sulfur species. Chloride-induced pitting corrosion is an imminent materials failure mechanism under these conditions. Accordingly a major factor in the economic exploitation of geothermal resources will be the cost-effective selection of materials that have sufficient resistance to pitting and related corrosion phenomena such as stress corrosion cracking to maintain component integrity.In this paper we describe the temperature dependence of the pitting corrosion resistance of a number of traditional Fe-Cr-Ni and Fe-Cr-Ni-Mo materials (e.g. Type 304 & Type 316 stainless steels Alloy 800 825 600 625) in a series of high-temperature high-pressure chloride solutions with or without addition of various (inhibitive) anions: SO4= HCO3- PO43- OH- etc. Tests have mainly been carried out by means of potentiodynamic electrochemical methods in a recirculating autoclave system. Test temperatures varied between 50 and 300°C with pressures up to 120 bar (12 MPa). After the tests samples have been analyzed by means of various microscopic and surface-analytical methods (optical microscopy SEM EDX AES-SAM etc.).The different results show some remarkable tendencies: In the lower temperature range i.e. between 50 and ca. 150°C the results of the electrochemical tests are fully in line with known literature data and practical experience and show a sharp decrease of the pitting potentials (Ep) with increasing temperatures. However for most test solutions this trend gradually diminishes as temperature rises above » 150 to 175°C and no further decrease of pitting potentials is observed above 200°C in none of the test solutions.At these higher temperatures Ep-values may even shift again to higher values and the typical pitting morphology may change into a more generalized type of corrosion. This means that pitting potentials (and also the corresponding repassivation potentials) show a minimum value at about 175-200°C. This is often accompanied by the formation of thick oxide layers and more shallow types of (localized) attack including under-film of ‘filiform’-like types of corrosion etcetera. These fairly remarkable results are in line with some older data for stainless steel Type 304 in high-temperature high-pressure aqueous environments.A possible explanation might be related with transpassive dissolution phenomena of the Fe-Cr-Ni-(Mo) alloys and the effect on their protective surface films. Also the chloride reactions related to the pitting initiation process as well as the ‘occluded cell’ pit growth mechanisms might change at these higher temperatures. This paper will discuss this in some more detail. 

Key words: pitting, geothermal, high-temperature, stainless steel, nickel alloys

 

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