Upload
others
View
12
Download
0
Embed Size (px)
Citation preview
Summary of the Results of a German Research Project on Chloride Effects on the General Corrosion and Stress Corrosion Cracking Behavior of LAS under BWR Conditions
Matthias Herbst1, Armin Roth1, Martin Widera2 1 Areva GmbH, 2 RWE Power AG Ottawa, August 13th 2015
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.3
All rights are reserved, see liability notice.
Outline Background Investigations on the Effect of Chloride on General Corrosion
• Exposure Tests • Electrochemical Tests
Crack Initiation • SSRT-Tests (CERT) • Testing of Pre-Strained Specimens
Crack Growth • Crack Growth Rate Tests using CT-specimens • Crack Tip Microsampling
Summary and Discussion
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.4
All rights are reserved, see liability notice.
Background Can very small amounts (in the ppb-range) of dissolved chlorides cause a significant enhancement of corrosion cracking (environmentally assisted cracking) of low-alloy pressure vessel steels?
Laboratory results from Swiss Paul-Scherrer-Institut (PSI) raised concerns for the operational safety of BWR plants during chloride transients in water chemistry Actions levels for chloride in water chemistry guidelines
could be questioned GRS launched a „Weiterleitungsnachricht“
and requested utilities to reply AREVA was asked to support VGB to provide
a reliable answer for plant relevant conditions
Launching of R&D project by AREVA with significant co-funding from VGB Scientific clarification of corrosion phenomena and mechanistic aspects Assessment of component relevance for BWRs (operating at normal water chemistry)
Ref.: Seifert & Ritter (PSI)
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.5
All rights are reserved, see liability notice.
Objective Study chloride effects on general corrosion behavior and on EAC
The testing part of this work was divided into three parts, dedicated to the investigation of the effect of chloride on General Corrosion
• exposure tests at different continuously and temporarily increased chloride concentration • using unstressed coupon specimens
Crack Initiation (by EAC) • pre-strained C-ring specimens • actively strained SSRT-specimens
Crack growth (by EAC) • 1T-CT (CT25) specimens • including crack tip micro-sampling
Material: German low-alloy RPV steel (22 NiMoCr 3 7 ~ SA 508 Cl2) Element C Si Mn P S Cr Mo Ni Cu Weight-% 0.22 0.20 0.91 0.008 0.007 0.42 0.053 0.88 0.04
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.6
All rights are reserved, see liability notice.
Testing Conditions Simulated BWR normal water chemistry conditions (T = 288 °C, DO inlet = 400 ppb)
Pure water without chloride (EC inlet = 0.06 µS/cm)
Chloride concentrations are based on action levels of the VGB-Guideline “VGB-R 401J” (water chemistry guideline for primary circuits of BWR plants) • pure water without chloride • 5 ppb chloride • 20 ppb chloride • 50 ppb chloride
General Corrosion
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.8
All rights are reserved, see liability notice.
General Corrosion Test Matrix
Tests with continuously increased chloride concentrations (16 Tests)
►Tests with temporary chloride transients (12 Tests)
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.9
All rights are reserved, see liability notice.
General Corrosion Test Results
Decrease in Oxide Layer Thickness Change in Electrochemical Behavior
Increased Pitting with increasing Chloride conc. (Test Duration 1000 h)
without Cl-
50 ppb Cl-
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.10
All rights are reserved, see liability notice.
General Corrosion Summary of Results
Small additions of chloride contaminations caused significant changes of the electrochemical properties in this corrosion system (revealed on-line through EN and EIS during testing)
Chloride is absorbed at oxide surfaces (as revealed by ToF-SIMS) & causes thinning of the oxide layer (measurement of oxide layer thickness)
Chloride concentration is locally high and causes localized thinning of the oxide layer at those chloride islands (in particular for long-term increased chloride concentrations)
Temporary transients do not cause sustained changes of the corrosion behavior => No long-term memory effect (full recovery < 48 h)
Decreasing oxide layer thickness with increasing chloride concentrations and pronounced pitting can generally explain increased EAC susceptibility
Kaesche: Corrosion of Metals, 2003
Crack Initiation
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.12
All rights are reserved, see liability notice.
Crack Initiation Test Set-Up
Refreshing Loop ►Testing Autoclave ►SSRT-Specimen
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.13
All rights are reserved, see liability notice.
ε
σ
ε(oxide)~ 0.05% - 0.20 %(Oxide fracture elongation)
t
Cl-c
onc.
(p
pb)
50
Δσ (~ Δε)
20
5
trise/tfall ~ 10/1R = 0.9
0
dε/dt = 10-6 s-1
εmax~ 0.5 %
εmin~ 0.45 %
ε
ε min
~ 0.
45 %
ε max
~ 0.
5 %
Crack Initiation Testing Details
Pre-strained C-ring specimens were exposed to oxygenated HTW with up to 50 ppb Cl- and up to 1000 h Slow strain rate tensile tests (SSRT) were performed to apply the most severe mechanical load – Cl--effects were observed in σ- ε-curves Modified SSRT were performed to force repeated fracture of the protective oxide
SSRT-specimen before testing
SSRT-specimen after testing
Crack initiation (SSRT) tests at increased chloride conc. And after chloride transients
Procedure of modified SSRT testing – No crack initiation after test termination
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.14
All rights are reserved, see liability notice.
Crack Initiation Summary of Results
Constant strain (C-rings) No crack initiation at pre-strained C-ring specimens
up to 1000 hours with 5, 20 and 50 ppb chloride
Slow strain rate tensile tests (tensile specimens) Increasing chloride enhances crack initiation as revealed by the
stress-strain-behavior of strain-to-fracture tests at different chloride concentrations A clear increase of crack initiation sites occurs at 50 ppb
Modified SSRT “cyclic strain rate” (tensile specimens) Specimens cyclically strained above the oxide fracture elongation (εmax~ 0.5 %) Repassivation does not seem to be delayed
due to increased chloride concentrations (at un-notched flat surfaces) as revealed by EN-monitoring during modified SSRT (no transients)
Continuously increasing strain above the yield strength is necessary to cause crack initiation even at increased chloride concentrations (up to 50 ppb chloride)
Crack Growth
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.16
All rights are reserved, see liability notice.
Crack Growth Test Set-Up
Study the effect of different chloride contamination levels short term chloride transients superimposed cyclic loading or single unloading
on the CGR of LAS in oxygenated HTW
Testing procedure Two 1T-CT CGR specimens (T-S orientation)
in a daisy-chain to study • permanently increased chloride concentrations • short term chloride transients
(e.g. 24 hours, 50 ppb) • constant load or cyclic load
One specimen with a backbore that allows crack tip micro sampling
load
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.17
All rights are reserved, see liability notice.
Crack Growth Test Results (Testing at KI = 40 MPa√m)
KI = 40 MPa√m
Long testing time ►
without chloride & mechanical transients
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.18
All rights are reserved, see liability notice.
Crack Growth Summary of Results
Earlier results from PSI were confirmed, i.e. observation of a clear enhancement effect of chloride on crack growth of pre-cracked CT specimens under certain conditions.
The study performed by AREVA GmbH revealed a significant dependence from the specimen history throughout the cracks and led to the distinction between “arrested“ cracks and “dormant“ cracks Dormant cracks may be re-initiated by chemical Cl-transients alone, however crack growth
ceases some hours after the end of the transient Arrested cracks cannot be re-initiated by Cl-transients –
they need to be re-initiated by significant mechanical fatigue loading “Dormant” cracks: Onset of CGR due to Cl-
“Arrested” cracks: No effect of Cl--addition
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.19
All rights are reserved, see liability notice.
Crack Growth
Fractography and Microsampling
Testing Time / h
350 ppb
590 ppb
Fractography of CT specimens confirms well behaved crack growth behavior confirming high quality testing conditions & performance
A CT specimen with a backbore allowed crack tip micro-sampling
Chemical analysis of crack tip micro-samples revealed a significant increase (8x … ~ 20x) of the crack tip chloride concentration during Cl-transients in the bulk environment
Summary and Conclusion
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.21
All rights are reserved, see liability notice.
Summary and Conclusion (1/2) General Corrosion Clear effects of chloride were observed
• Electrochemical behavior (by EN and EIS) • Oxide thinning (and structure of the oxide layer)
Full recovery of chloride induced effects is observed after removal of chloride (after transients) – no memory effect is observed
Crack Initiation No enhancement in constant strain tests (C-ring specimens) Enhancement observed in rising load tests (SSRT),
but only at severe plastic deformation (above yield strain)
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.22
All rights are reserved, see liability notice.
Summary and Conclusion (2/2) Crack Growth Effects of chloride on crack growth were only observed under specific
conditions • Onset of crack growth depends on mechanical load and specimen history • The presence of chloride is a necessary but not a sufficient criterion
Cracks can be classified based on their behavior • „Dormant Cracks“ (i.e. cracks which are ready to re-start growing immediately)
- can be re-activated by increased chloride concentrations even under purely constant load at KI-levels ≥ 40 MPa√m
• “Arrested Cracks” (i.e. cracks with blunted crack tips and/ or filled with oxide) -are not re-activated by increased chloride concentrations (under constant load) up to KI = 40 MPa√m (and chloride concentrations up to 50 ppb)
The Chloride concentration in the crack and in particular at the crack tip is significantly higher as compared to the bulk (approx. 10 to 20 x higher, or even more)
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.23
All rights are reserved, see liability notice.
Conclusions for Practical Applications Crack Initiation in field applications at smooth surfaces At low stress and strain levels no increased crack initiation susceptibility at chloride
concentrations in the ppb range Conjoint occurrence of increased chloride concentration and severe mechanical
straining (plastification) of components should be avoided
Crack Growth (e.g. in RPV material) Short term chemical transients can cause only very limited crack advance under
specific conditions (“dormant crack”) and are not harmful regarding component integrity (no long-term memory effect of chloride transients)
For “dormant cracks” crack re-activation is possible due to increased chloride concentrations
Growth of “dormant cracks” is not stable under steady state full power operation, even postulated growing cracks will “arrest” during operation
Arrested cracks will not be re activated by an increase in the chloride concentration without additional mechanical transients
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.24
All rights are reserved, see liability notice.
Acknowledgement The financial contribution
as well as the many fruitful discussions with the experts of the
VGB Work Panel on Component Integrity
(AK KOM)
is gratefully acknowledged.
Summary of the Results of a German Research Project on Chloride Effects on the General Corrosion and Stress Corrosion Cracking Behavior of LAS under BWR Conditions
Matthias Herbst1, Armin Roth1, Martin Widera2 1 Areva GmbH, 2 RWE Power AG Ottawa, August 13th 2015
End of presentation
Effect of Chloride on EAC of LAS – 17th Environmental Degradation Conference , August 9 - 13, 2015 – Matthias Herbst - AREVA GmbH Proprietary - RESTRICTED AREVA - © AREVA - AL: N - ECCN: N - p.26
All rights are reserved, see liability notice.
Editor and Copyright 2015-08-13: AREVA GmbH – Paul-Gossen-Straße 100 – 91052 Erlangen, Germany. It is prohibited to reproduce the present publication in its entirety or partially in whatever form without prior written consent. Legal action may be taken against any infringer and/or any person breaching the aforementioned prohibitions.
Subject to change without notice, errors excepted. Illustrations may differ from the original. The statements and information in this brochure are for advertising purposes only and do not constitute an offer of contract. They shall neither be construed as a guarantee of quality or durability, nor as warranties of merchantability or fitness for a particular purpose. These statements, even if they are future-orientated, are based on information that was available to us at the date of publication. Only the terms of individual contracts shall be authoritative for type, scope and characteristics of our products and services.