Effect of chloride transients on corrosion of low-alloyed steel under oxygenated high-temperature water conditions

Martin Bojinov, Essi Jäppinen, Wolfgang Mayinger, Timo Saario, Maxim Selector, Konsta Sipilä

    Research output: Contribution to conferenceConference articleScientific

    Abstract

    Stress corrosion cracking (SCC) in low alloyed steels (LAS) has been extensively investigated during the last two decades. One finding from recent investigations with standard 1 CT specimen geometry is that even very small amounts (2.5 ppb) of chlorides increase tremendously the cracking susceptibility of LAS. However, no LAS cracking incidents in real plants have ever been attributed to a chloride transient. In the present work, the corrosion potential at the bottom of the crack tip of a 1 CT LAS specimen was calculated using a mixed-potential model for the corrosion reaction of low-alloyed steel at the metal/water interface allowing for active dissolution (mainly of iron), active-to-passive transition and dissolution in the passive state. The model was coupled to equations describing dilute solution transport for all the ionic and neutral species in the crevice associated with the crack tip. The chemical and electrochemical conditions at the bottom of the crevice, as well as an estimate of the enrichment factor of chloride in it, were obtained from the calculations. In the experimental part, the corrosion behavior of LAS in a crevice environment forming during a 50-ppb bulk water chloride transient was studied by in-situ electrochemical impedance spectroscopy (EIS) and mixed potential measurements, coupled to ex-situ characterization of the oxides by microscopic and surface analytical techniques as well as corrosion rate estimation from exposure coupons. The material studied was 20MnMoNi55 from the reactor coolant line of a German NPP. The general corrosion rate was found to increase several times when LAS was exposed to chloride from the start of the experiment, the effect vanishing after about 150 h. The EIS data revealed that the effect of chloride transients on an existing oxide film is moderate, concerns mostly the processes at the inner oxide layer/water interface and is to a major extent reversible. The SSRT experiments showed that LAS is susceptible to SCC in the crevice environment above a threshold potential of about -0.35 V (SHE). The model calculations revealed that the corrosion potential at the crack tip of a 1 CT specimen is about -0.24 V (SHE), almost irrespective of the crevice geometry. However, the chloride enrichment was found to depend strongly on the crevice geometry, increasing as the crevice width or depth increases. Thus, any limit concentration for chloride concentration based on 1 CT specimen laboratory SCC crack growth rate test results should be considered carefully and bearing in mind the possible differences in crevice width and depth between the 1 CT specimen and those of a realistic LAS flaw. Based on all the results obtained, it can be concluded that chloride transients up to 50 ppb in high temperature water do not result in any serious consequences for the corrosion of low alloyed steel with stainless steel cladding.
    Original languageEnglish
    Publication statusPublished - 1 Jan 2017
    Event20th International Corrosion Congress & Process Safety Congress 2017, Eurocorr 2017 - Prague, Czech Republic
    Duration: 3 Sep 20177 Sep 2017
    http://www.prague-corrosion-2017.com/

    Conference

    Conference20th International Corrosion Congress & Process Safety Congress 2017, Eurocorr 2017
    Abbreviated titleEurocorr 2017
    CountryCzech Republic
    CityPrague
    Period3/09/177/09/17
    Internet address

    Fingerprint

    Steel
    Chlorides
    corrosion
    chlorides
    steels
    cracks
    Corrosion
    Water
    water
    stress corrosion cracking
    crack tips
    Stress corrosion cracking
    Temperature
    Crack tips
    Corrosion rate
    Electrochemical impedance spectroscopy
    Oxides
    Geometry
    dissolving
    Dissolution

    Keywords

    • low-alloyed steel
    • pressure vessel
    • stress corrosion cracking
    • chloride impurity

    Cite this

    Bojinov, M., Jäppinen, E., Mayinger, W., Saario, T., Selector, M., & Sipilä, K. (2017). Effect of chloride transients on corrosion of low-alloyed steel under oxygenated high-temperature water conditions. Paper presented at 20th International Corrosion Congress & Process Safety Congress 2017, Eurocorr 2017, Prague, Czech Republic.
    Bojinov, Martin ; Jäppinen, Essi ; Mayinger, Wolfgang ; Saario, Timo ; Selector, Maxim ; Sipilä, Konsta. / Effect of chloride transients on corrosion of low-alloyed steel under oxygenated high-temperature water conditions. Paper presented at 20th International Corrosion Congress & Process Safety Congress 2017, Eurocorr 2017, Prague, Czech Republic.
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    title = "Effect of chloride transients on corrosion of low-alloyed steel under oxygenated high-temperature water conditions",
    abstract = "Stress corrosion cracking (SCC) in low alloyed steels (LAS) has been extensively investigated during the last two decades. One finding from recent investigations with standard 1 CT specimen geometry is that even very small amounts (2.5 ppb) of chlorides increase tremendously the cracking susceptibility of LAS. However, no LAS cracking incidents in real plants have ever been attributed to a chloride transient. In the present work, the corrosion potential at the bottom of the crack tip of a 1 CT LAS specimen was calculated using a mixed-potential model for the corrosion reaction of low-alloyed steel at the metal/water interface allowing for active dissolution (mainly of iron), active-to-passive transition and dissolution in the passive state. The model was coupled to equations describing dilute solution transport for all the ionic and neutral species in the crevice associated with the crack tip. The chemical and electrochemical conditions at the bottom of the crevice, as well as an estimate of the enrichment factor of chloride in it, were obtained from the calculations. In the experimental part, the corrosion behavior of LAS in a crevice environment forming during a 50-ppb bulk water chloride transient was studied by in-situ electrochemical impedance spectroscopy (EIS) and mixed potential measurements, coupled to ex-situ characterization of the oxides by microscopic and surface analytical techniques as well as corrosion rate estimation from exposure coupons. The material studied was 20MnMoNi55 from the reactor coolant line of a German NPP. The general corrosion rate was found to increase several times when LAS was exposed to chloride from the start of the experiment, the effect vanishing after about 150 h. The EIS data revealed that the effect of chloride transients on an existing oxide film is moderate, concerns mostly the processes at the inner oxide layer/water interface and is to a major extent reversible. The SSRT experiments showed that LAS is susceptible to SCC in the crevice environment above a threshold potential of about -0.35 V (SHE). The model calculations revealed that the corrosion potential at the crack tip of a 1 CT specimen is about -0.24 V (SHE), almost irrespective of the crevice geometry. However, the chloride enrichment was found to depend strongly on the crevice geometry, increasing as the crevice width or depth increases. Thus, any limit concentration for chloride concentration based on 1 CT specimen laboratory SCC crack growth rate test results should be considered carefully and bearing in mind the possible differences in crevice width and depth between the 1 CT specimen and those of a realistic LAS flaw. Based on all the results obtained, it can be concluded that chloride transients up to 50 ppb in high temperature water do not result in any serious consequences for the corrosion of low alloyed steel with stainless steel cladding.",
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    Bojinov, M, Jäppinen, E, Mayinger, W, Saario, T, Selector, M & Sipilä, K 2017, 'Effect of chloride transients on corrosion of low-alloyed steel under oxygenated high-temperature water conditions' Paper presented at 20th International Corrosion Congress & Process Safety Congress 2017, Eurocorr 2017, Prague, Czech Republic, 3/09/17 - 7/09/17, .

    Effect of chloride transients on corrosion of low-alloyed steel under oxygenated high-temperature water conditions. / Bojinov, Martin; Jäppinen, Essi; Mayinger, Wolfgang; Saario, Timo; Selector, Maxim; Sipilä, Konsta.

    2017. Paper presented at 20th International Corrosion Congress & Process Safety Congress 2017, Eurocorr 2017, Prague, Czech Republic.

    Research output: Contribution to conferenceConference articleScientific

    TY - CONF

    T1 - Effect of chloride transients on corrosion of low-alloyed steel under oxygenated high-temperature water conditions

    AU - Bojinov, Martin

    AU - Jäppinen, Essi

    AU - Mayinger, Wolfgang

    AU - Saario, Timo

    AU - Selector, Maxim

    AU - Sipilä, Konsta

    N1 - Only abstract reviewed Full article published on proceedings (USB flash drive) Project code: 114798

    PY - 2017/1/1

    Y1 - 2017/1/1

    N2 - Stress corrosion cracking (SCC) in low alloyed steels (LAS) has been extensively investigated during the last two decades. One finding from recent investigations with standard 1 CT specimen geometry is that even very small amounts (2.5 ppb) of chlorides increase tremendously the cracking susceptibility of LAS. However, no LAS cracking incidents in real plants have ever been attributed to a chloride transient. In the present work, the corrosion potential at the bottom of the crack tip of a 1 CT LAS specimen was calculated using a mixed-potential model for the corrosion reaction of low-alloyed steel at the metal/water interface allowing for active dissolution (mainly of iron), active-to-passive transition and dissolution in the passive state. The model was coupled to equations describing dilute solution transport for all the ionic and neutral species in the crevice associated with the crack tip. The chemical and electrochemical conditions at the bottom of the crevice, as well as an estimate of the enrichment factor of chloride in it, were obtained from the calculations. In the experimental part, the corrosion behavior of LAS in a crevice environment forming during a 50-ppb bulk water chloride transient was studied by in-situ electrochemical impedance spectroscopy (EIS) and mixed potential measurements, coupled to ex-situ characterization of the oxides by microscopic and surface analytical techniques as well as corrosion rate estimation from exposure coupons. The material studied was 20MnMoNi55 from the reactor coolant line of a German NPP. The general corrosion rate was found to increase several times when LAS was exposed to chloride from the start of the experiment, the effect vanishing after about 150 h. The EIS data revealed that the effect of chloride transients on an existing oxide film is moderate, concerns mostly the processes at the inner oxide layer/water interface and is to a major extent reversible. The SSRT experiments showed that LAS is susceptible to SCC in the crevice environment above a threshold potential of about -0.35 V (SHE). The model calculations revealed that the corrosion potential at the crack tip of a 1 CT specimen is about -0.24 V (SHE), almost irrespective of the crevice geometry. However, the chloride enrichment was found to depend strongly on the crevice geometry, increasing as the crevice width or depth increases. Thus, any limit concentration for chloride concentration based on 1 CT specimen laboratory SCC crack growth rate test results should be considered carefully and bearing in mind the possible differences in crevice width and depth between the 1 CT specimen and those of a realistic LAS flaw. Based on all the results obtained, it can be concluded that chloride transients up to 50 ppb in high temperature water do not result in any serious consequences for the corrosion of low alloyed steel with stainless steel cladding.

    AB - Stress corrosion cracking (SCC) in low alloyed steels (LAS) has been extensively investigated during the last two decades. One finding from recent investigations with standard 1 CT specimen geometry is that even very small amounts (2.5 ppb) of chlorides increase tremendously the cracking susceptibility of LAS. However, no LAS cracking incidents in real plants have ever been attributed to a chloride transient. In the present work, the corrosion potential at the bottom of the crack tip of a 1 CT LAS specimen was calculated using a mixed-potential model for the corrosion reaction of low-alloyed steel at the metal/water interface allowing for active dissolution (mainly of iron), active-to-passive transition and dissolution in the passive state. The model was coupled to equations describing dilute solution transport for all the ionic and neutral species in the crevice associated with the crack tip. The chemical and electrochemical conditions at the bottom of the crevice, as well as an estimate of the enrichment factor of chloride in it, were obtained from the calculations. In the experimental part, the corrosion behavior of LAS in a crevice environment forming during a 50-ppb bulk water chloride transient was studied by in-situ electrochemical impedance spectroscopy (EIS) and mixed potential measurements, coupled to ex-situ characterization of the oxides by microscopic and surface analytical techniques as well as corrosion rate estimation from exposure coupons. The material studied was 20MnMoNi55 from the reactor coolant line of a German NPP. The general corrosion rate was found to increase several times when LAS was exposed to chloride from the start of the experiment, the effect vanishing after about 150 h. The EIS data revealed that the effect of chloride transients on an existing oxide film is moderate, concerns mostly the processes at the inner oxide layer/water interface and is to a major extent reversible. The SSRT experiments showed that LAS is susceptible to SCC in the crevice environment above a threshold potential of about -0.35 V (SHE). The model calculations revealed that the corrosion potential at the crack tip of a 1 CT specimen is about -0.24 V (SHE), almost irrespective of the crevice geometry. However, the chloride enrichment was found to depend strongly on the crevice geometry, increasing as the crevice width or depth increases. Thus, any limit concentration for chloride concentration based on 1 CT specimen laboratory SCC crack growth rate test results should be considered carefully and bearing in mind the possible differences in crevice width and depth between the 1 CT specimen and those of a realistic LAS flaw. Based on all the results obtained, it can be concluded that chloride transients up to 50 ppb in high temperature water do not result in any serious consequences for the corrosion of low alloyed steel with stainless steel cladding.

    KW - low-alloyed steel

    KW - pressure vessel

    KW - stress corrosion cracking

    KW - chloride impurity

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    M3 - Conference article

    ER -

    Bojinov M, Jäppinen E, Mayinger W, Saario T, Selector M, Sipilä K. Effect of chloride transients on corrosion of low-alloyed steel under oxygenated high-temperature water conditions. 2017. Paper presented at 20th International Corrosion Congress & Process Safety Congress 2017, Eurocorr 2017, Prague, Czech Republic.