Summary of the Results of a - envdeg2015.orgenvdeg2015.org/final-proceedings/ENVDEG/presentations/ENVDEG_PPT144.pdfSummary of the Results of a German Research Project on Chloride Effects

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



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)



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



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



General Corrosion Test Matrix

Tests with continuously increased chloride concentrations (16 Tests)

►Tests with temporary chloride transients (12 Tests)



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-



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



Crack Initiation Test Set-Up

Refreshing Loop ►Testing Autoclave ►SSRT-Specimen



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



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



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



Crack Growth Test Results (Testing at KI = 40 MPa√m)

KI = 40 MPa√m

Long testing time ►

without chloride & mechanical transients



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



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



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)



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)



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



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



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Summary of the Results of a - envdeg2015.orgenvdeg2015.org/final-proceedings/ENVDEG/presentations/ENVDEG_PPT144.pdfSummary of the Results of a German Research Project on Chloride Effects