HAL Id: tel-01940909
https://tel.archives-ouvertes.fr/tel-01940909
Submitted on 30 Nov 2018
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci-entific research documents, whether they are
pub-L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non,
correlation, acoustic emission and electrochemical noise
José Bolivar Vina
To cite this version:
N°d’ordre NNT : 2017LYSEI059
THESE de DOCTORAT DE L’UNIVERSITE DE LYON
opérée au sein de
L'institut national des sciences appliquées de Lyon
Ecole Doctorale
N° ED34
Ecole Doctorale matériaux de Lyon
Spécialité
: MatériauxSoutenue publiquement le 07/07/2017, par :
José Bolivar
Ingénieur en génie des matériaux
Experimental study of the behavior of
colonies of environmentally-assisted
short cracks by Digital Image
Correlation, Acoustic Emission and
Electrochemical Noise
Devant le jury composé de :
M. NOGUEIRA Ricardo Professeur – The Petroleum Institute Rapporteur
M. DESQUINES Jean Docteur HDR – IRSN Rapporteur
Résumé
Abstract
Table of contents
GLOSSARY... VI
Acronyms ... VI Notation for electrochemical variables ... VIII Notation for mechanical variables ... X
DEDICATION ... XI
ACKNOWLEDGMENT ... XII
I
GENERAL INTRODUCTION ... 1
I.1 Introduction ... 2
I.1.1 Context of the study ... 2
I.1.2 Objectives of this work ... 4
I.1.3 Overview of the manuscript ... 6
II
LITERATURE REVIEW ... 7
II.1 Introduction ... 9
II.2 Stress Corrosion Cracking mechanisms and models ... 9
II.2.1 Crack initiation ... 10
II.2.2 Crack propagation ... 11
II.2.3 Synopsis ... 16
II.3 Multiple cracking problem ... 17
II.3.1 Crack interactions and crack colony behavior ... 18
II.3.2 Multiple cracking and lifetime predictions ... 22
II.3.3 Synopsis ... 23
II.4 Intergranular stress corrosion cracking of Austenitic Stainless Steels (ASS) and Nickel base alloys in solutions containing polythionic acids ... 24
II.4.1 Polythionic Acids ... 24
II.4.2 PTA-SCC mechanism in ASS and Nickel base alloys ... 25
II.4.3 Synopsis ... 28
II.5 Experimental approach for a quantitative in-situ evaluation of multiple stress corrosion cracks propagation ... 30
II.5.1 Digital Image correlation ... 30
II.5.2 Acoustic Emission technique ... 34
Chapter Summary ... 56
III.
EXPERIMENTAL METHODOLOGY ... 59
III.1 Introduction ... 61
III.2 Material characterization and preparation ... 61
III.2.1 Material ... 61
III.2.2 Sample geometry ... 62
III.2.3 Characterization of the material ... 64
III.2.4 Study of the surface treatment for Digital Image Correlation (2D) measurements (surface patterning) ... 66
III.2.5 SCC Test solution ... 68
III.2.6 Preparation of the Long Tensile Samples (LTS) and the Counter-Electrodes for the in-situ Stress Corrosion Cracking tests at room temperature ... 68
III.2.7 Description of the Stress Corrosion Cracking tests on LTS samples ... 69
III.2.8 Description of the Stress Corrosion Cracking tests under four -point bending… ... 70
III.2.9 Preparation of the Small Tensile Specimens (STS) for the In-situ Stress Corrosion Cracking tests by using X-ray synchrotron tomography ... 71
III.2.10 Description of the in-situ Stress Corrosion Cracking tests with the STS samples (X-ray tomography) ... 72
III.3 In-situ monitoring techniques used during the Stress Corrosion Cracking tests ... 73
III.3.1 Digital Image Correlation measurements ... 73
III.3.2 Electrochemical Noise measurements ... 75
III.3.3 Acoustic Emission measurements ... 76
III.3.4 Electrochemical Impedance Spectroscopy (EIS) measurements ... 77
Chapter summary ... 77
IV
CHOICE AND VALIDATION OF THE EXPERIMENTAL CONDITIONS
FOR THE SCC TESTS AT ROOM TEMPERATURE AND PRESSURE . 78
IV.1 Introduction ... 79IV.2 Influence of the experimental conditions on the initiation and the development of a crack colony ... 79
IV.2.1 Characterization of the degree of sensitization according to the different applied Heat Treatments (HT) ... 79
IV.2.2 Effect of the loading mode on the crack colony morphology ... 84
IV.2.3 Suitability of SSRT and CL tests for AE, DIC, and EN measurements ... 88
IV.2.4 Effect of the pH of the solution on the crack colony morphology ... 89
IV.3 Choice and validation of the surface patterning compatible with DIC
measurements and SCC tests... 92
IV.3.1 Evaluation of surface treatment on Squared Samples ... 93
IV.3.2 Electrochemical characterization of the surface treatment ... 99
IV.3.3 Validation of the surface patterning on the Long Tensile Samples (LTS) . 100 Chapter Summary ... 103
V
STUDY OF CRACK PROPAGATION BY DIGITAL IMAGE
CORRELATION ANALYSIS ... 105
V.1 Introduction ... 107
V.2 Study of the propagation of a single stress corrosion crack ... 107
V.2.1 Presentation of the results ... 108
V.2.2 Discussion ... 117
V.2.3 Synopsis ... 120
V.3 Description of the propagation behavior of a colony of stress corrosion cracks . ... 121
V.3.1 Presentation of the results ... 121
V.3.2 Discussion ... 129
V.3.3 Synopsis ... 133
V.4 Validation by in-situ X-ray tomography and Digital Volume Correlation.. 133
V.4.1 Presentation of the results ... 135
V.4.2 Discussion ... 149
V.4.3 Synopsis ... 152
Chapter Summary ... 154
VI
ANALYSIS OF ELECTROCHEMICAL NOISE DURING STRESS
CORROSION CRACKING TESTS ... 156
VI.1 Introduction ... 157
VI.2 Theoretical background about the evaluation of the noise generated during Electrochemical Noise Measurements (ENM) ... 157
VI.2.1 Current partition according to Bertocci et al. model [172] ... 158
VI.2.2 Methodology for the evaluation of instrumental noise ... 161
VI.3 Evaluation of the instrumental noise generated by the REF600 Gamry potentiostat ... 164
VI.3.1 Results and discussion ... 164
VI.4 Application of ENM in SCC studies: influence of current partition and
instrumental noise ... 169
VI.4.1 Presentation of the results ... 171
VI.4.2 Discussion ... 173
VI.4.3 Synopsis ... 178
VI.5 Critical analysis of the rise in the asymmetry between electrodes during SCC process ... 179
VI.5.1 Results and discussions ... 180
Chapter Summary ... 184
VII
GENERAL SYNTHESIS, CONCLUSIONS AND FUTURE WORKS ... 186
VII.1 Introduction ... 187
VII.2 General Synthesis ... 187
VII.2.1 Influence of experimental parameters on the development of the crack colony. ... 187
VII.2.2 Development of the in-situ experimental methodology for the study of SCC crack colonies ... 189
VII.2.3 Crack detection and crack monitoring ... 190
VII.2.4 Evolution of intergranular stress corrosion cracks (Alloy 600 / tetrathionate).. ... 191
VII.2.5 Mechanisms involved for crack propagation in sensitized Alloy 600 in a solution containing tetrathionate ions ... 192
VII.2.6 Evaluation of Electrochemical Noise measurements in SCC experiments . 194 VII.3 Contribution of the experimental approach to modeling ... 196
VII.4 Transposition of the methodology towards intermediate temperature and pressure environments ... 200
A.
APPENDIX A ... 203
Issues during EN signal acquisition ... 203
B.
APPENDIX B ... 206
Signal conditioning before data analysis/interpretation in EN measurements ... 206
C.
APPENDIX C ... 208
Report MISTRAS RF 2017.01: Analysis of Acoustic Emission data collected during IG-SCC experiments ... 208
Assessment of Stress Intensity Factors from DIC measurements ... 232
E.
APPENDIX E ... 235
A phase field method for modelling stress corrosion crack propagation ... 235
F.
APPENDIX F ... 254
Description of the experimental apparatus for performing in -situ SCC experiments at intermediate temperatures and pressures ... 254
G.
APPENDIX G ... 258
Evaluation of the induced deformation by an out-of-plane movement: AF Micro 200 Lens ... 258
Glossary
Acronyms
ACF Auto-Correlation Function
AE Acoustic Emission
AIDE Adsorption Induced Dislocation Emission
ANR Agence National de la Recherche (in french)
AR As Received
ASS Austenitic Stainless Steel
BFM Body Force Method
BWR Boiling Water Reactor
CA Chemical Attack
CCD Charge-Coupled Device
CE Counter Electrode
CGR Crack Growth Rate
CL Constant Load test
CT Compact Tension sample
DIC Digital Image Correlation
DL-EPR Double-Loop Electrochemical Potentiokinetic Reactivation test
DVC Digital Volume Correlation
ECCOFIC Etude du Comportement de Colonies des Fissures Courtes de corrosion sous contrainte (in french)
ECN Electrochemical Current Noise
EDM Electrical Discharge Machining
EDX Energy Dispersive X-ray spectroscopy
EE Electro-Etching
EN Electrochemical Noise
EPN Electrochemical Potential Noise
FEM Finite Element Modeling
FFT Fast Fourier Transform
FIB Focused-Ion Beam
FIC Film Induced Cleavage
FS Flexion Sample
HDT Hit Definition Time
HE Hydrogen Embrittlement
HEDE Hydrogen -Enhanced Decohesion
HELP Hydrogen -Enhanced Localized Plasticity
HLT Hit Lockout Time
IDG Initiation Dominant Growth
IGA Inter-Granular Attack
IGSCC Intergranular Stress Corrosion Cracking
LEFM Linear Elastic Fracture Mechanic
LME Liquid Metal Embrittlement
LTNS Long Tensile Notched Specimen
LTS Long Tensile Specimen
MAS Mill Annealed and Sensitized
MEM Maximum Entropy Method
MER Maximum Energy Release rate method
MSE Minimum Strain Energy density method
MTS Maximum Tangential Stress method
NF No Failure
OCP Open Circuit Potential
OM Optical Microscopy
PCC Primary Cooling Circuit
PDT Peak Definition Time
PLEDGE Plant Life Extension Diagnosis General Electric
PTA-SCC Polythionic Acid Stress Corrosion Cracking
PWR Primary Water Reactor
PWSCC Primary Water Stress Corrosion Cracking
RBM Rigid Body Movement
RE Reference Electrode
RMS Root Mean Square value
ROI Region of Interest
SAS Solution Annealed and Sensitized
SCC Stress Corrosion Cracking
SCE Saturated Calomel Electrode
SD, σ Standard deviation
SEM Scanning Electron Microscope
SIF Stress Intensity Factor
SRLT Slow Rate Load Test
SS Stainless Steel
SS Square Sample
SSRT Slow Strain Rate Test
STS Small Tensile Specimen
TGSCC Transgranular Stress Corrosion Cracking
WE Working Electrode
ZRA Zero-Resistance-Ammeter
Notation for electrochemical variables
ia Instantaneous dissolution current
M Molecular weight
λ Mean number of charge transferred by unit of time
∆I Current fluctuations
𝜀̇𝑐𝑡 Instantaneous strain rate at the crack tip
F Faraday constant
fc Cutoff frequency
fmax Maximal frequency for analysis according to the Nyquist-Shannon theorem
fmin Minimal frequency for analysis according to the Nyquist-Shannon theorem
fs Sampling frequency
I0 Value of mean current
I Current
Icorr Corrosion current
k Boltzmann's constant
M Number of analyzed blocks
N Length of the analyzed block
ρ Material density
ΨI(f) Power Spectral density of the current noise
ΨV(f) Power Spectral density of the potential noise
q Electric charge
R Resistance
Rn Noise Resistance
Rp Polarization resistance
σI, σECN Standard deviation of the current signal
σV, σEPN Standard deviation of the potential signal
t Time
T Absolute Temperature
V0 Value of mean potential
v Volume
V Potential
Vpp Voltage peak -to-peak
z Number of electrons involved in the reaction
Notation for mechanical variables
𝑎̇, da/dt, Crack growth rate
h Separation of the longitudinal axis of the cracks
ci Half-length of the "i" crack
E Young modulus
εyy Strain according to the y-axis
f(x) Gray levels of the reference image
g(x) Gray levels of the deformed image
KI Stress Intensity Factor in mode I
KII Stress Intensity Factor in mode II
u, U Displacement field
Ux Displacement according to x-axis
Uy Displacement according to y-axis
Uyimposed Imposed y-displacement
Uymax Maximum displacement according to the y-axis
ωi Degree of freedoms
𝑥̅ Vector of coordinates
y y-axis
DEDICATION
This is for all my family. This would not have been possible without your support and all the good vibes that you gave me during all this time. I really thanks for always being th ere for me.
This is also for my life partner, my love, Amelia Fernandez. Thank you for all your care, the excellent moments shared and for all the love that we give to each other, each day.
Dedico esta obra a mi madre… Mujer increíble y que gracias a su tenacidad y a todo sus sacrificios soy la persona que soy. ¡Mis logros son tan míos como tuyos y sé que los festejas aún más que yo!
A mi padre que siempre me ha apoyado, formado e inculcado el valor del trabajo. ¡ Sé que esto representa algo muy importante para ti y sé que estarás muy orgulloso! Siempre me has dado lo mejor, de igual manera siempre he querido retribuirte dando siempre lo mejor de mí. A mi abuela Ana y a mi abuelo Antonio. Por su amor y cariño incondicional.
Al resto de mi familia, en particular a mis abuelos (María, Miguelina, Ángel), a mis tíos (Miguel, Nancy, Félix, Pedro, Maribel, Chepi, Evelyn, Kenya) y a todos mis primos y primas. A mis amigos, que me ensenaron el valor de la amistad con todos esos momentos compartidos. Y por último, pero no la menos importante, a mi compañera de vida…. A ti Amelia Fernández, por tu increíble personalidad, por el gran amor y el invaluable apoyo que me has dado durante todo este tiempo.
Con un inmenso cariño
ACKNOWLEDGMENT
No great achievement is carried out by itself, behind each achievement hides the support of a family, a workgroup and special people who contributed in one way or another.
It is not easy to write acknowledgments in your third language, but I hope that my gratitude will be clearly visible in the next lines.
I would like to thank my supervisor Marion Fregonese and my co-supervisor Julien Réthoré for their guidance throughout the duration of my Ph.D. I sincerely thank them for giving me their confidence to carry out my work, for encouraging my research and for allowing me to grow as a research scientist. I will always remember a frame plenty of respect, confidence, and motivation (a lot).
I would like to express my special appreciation and thanks to Pierre Combrade and Claude Duret for their scientific advice, knowledge, many insightful discussions, and suggestions made it during the numerous project meetings. Something that I will not forget, your humility. I am very thankful to François Huet who helped me with the Electrochemical Noise technique. He was always available for my questions and inquiries. His advice were priceless and very important for the development of my Ph.D.
I would like to thanks, Jean-Yves Buffière and Jerome Adrien, for their great support during the experiments carried out at SOLEIL synchrotron. Hard days but good moments.
I would also like to thank my committee members, Krzysztof Wolski, Ricardo Nogueira and Jean Desquines for serving as my committee members. I also want to thank you for letting my defense be an enjoyable moment, and for your brilliant comments and suggestions, thanks to all you.
I wish to express my gratitude to "Agence National de la Recherche" for sponsoring this project as well as to all the partners of the ECCOFIC project for their great collaboration. I am very thankful the staff of the MATEIS lab (Jalila, Yasmina, and Antonia), and especially the technicians (Robert, Ghani, Kader, Fred) in the workshop, were a great help and contributed to the advancement of my Ph.D.
Special thanks to my family. Words cannot express how grateful I am to my mother and father, to my grandparents, for all of the sacrifices that you’ve made on my behalf. I would also like to thank all of my friends and Amelia's family who supported me in writing and incented me to strive towards my goal.
At the end, I would like express appreciation to my life partner, my love, Amelia who always behaved kindly and warm when I was stressed and a little unpleasant. She was always supporting me in the moments when there was no one to answer my queries.
Many thanks to you all, José
Chapter I
I
General introduction
I.1 Introduction ... 2
I.1.1 Context of the study ... 2
I.1.2 Objectives of this work ... 4
I.1 Introduction
I.1.1 Context of the study
Climate change and global warming are some of the greatest problems facing humanity in this century. Our high energy dependency on fossil fuels is starting to show its consequences, in particular, the deleterious effects of greenhouse-gas emissions (issued from fossil fuels) both on climate and global society health. The gradual replacement of fossil fuels by sustainable and renewable sources of energy is an unavoidable process . In this context, emergents technologies based on wind-and-solar energies are in full development with the aim of reducing green-house-gas emissions. However, such sources are characterized by their natural intermittence which is non compatible with the requirement of modern industrial society (highly energy consuming). For the moment and despite all the controversies, nuclear power is one of the most promising sources able to provide huge energy amounts in a safely, economically, reliably and sustainable way [1]– [3].
France produces about 84 % of its electricity from the nuclear industry. In this manner, the per-capita greenhouse gas emission in France is among the lowest of the industrial nations worldwide. Nowadays, the French nuclear sector represents the third national industry, assembling 19 nuclear power plants and 58 nuclear reactors [4]. Most of these reactors were built with an initial design life of 40 years and are currently reaching the mean age of 30 years. In that context, an extension towards 60 or 80 years has been now envisaged. The main reason for such extension lies in political and economic reasons [4]–[6]. Obviously, this extension can only be possible if safe operating conditions are fully demonstrated by utilities. This requires further research, notably, the development of prediction tools that can assess the behavior face to the additional aging of such structures.
The list of the possible aging problems caused by the environmental degradation of structural materials in a nuclear reactor can be quite long. The extreme conditions of operation of the reactor (irradiation, high temperature and pressure) cause a very wide range of degradations such as: irradiation-induced hardening and softening, thermal aging, thermal fatigue and corrosion problems such as crevice corrosion, flow accelerated corrosion, fatigue corrosion and Stress Corrosion Cracking (SCC). Under the frame of this work, particular attention will be paid to the SCC phenomena.
core. The PCC works under extreme conditions because the water inside is at high temperature and pressure (i.e. ~ 290-345°C and 155 bars). Three or four boilers (steam generators) included in this system allows transferring the energy into steam in the secondary circuit. Finally, the produced steam is used to drive a turbine which produces electricity.
Figure I-1. Schematic representation of a Pressurized-Water Reactor (PWR)
The materials used in the PCC are Stainless Steels (SS) alloys such as austenitic stainless steels (304L SS, 316 L SS), and Nickel based alloy such as the Alloy 600 and the Alloy 690. A large number of non-catastrophic cases of SCC in the PCC occurred in the past. Most of them were reported for the Alloy 600 which presents high susceptibility to a type of intergranular SCC usually called PWSCC (Primary Water Stress Corrosion Cracking) in such environments (see Figure I.3). Some cases of transgranular SCC have also been observed in occluded parts of the circuit in the presence of impurities (mainly chlorides and sulfates).
In the case of PWSCC which usually occurs after very long incubation period, the main problem relies on the evaluation of the remaining lifetime of components. The current approach involves empirical model for crack initiation (see for example [8]) and crack propagation. In this context, numerous mechanistic studies and experimental data has been already obtained in order to describe the crack growth of one crack during in-service conditions. Such studies take into account Linear Elastic Fracture Mechanic concepts for the estimation of crack propagation rates of large cracks (see for example [9]). Nevertheless, the issue becomes more complicated and less controlled when multiple cracks initiate and propagate. The growth of cracks closely surrounded by other neighboring defects is expected to be very different, compared to the growth kinetics obtained in laboratory tests on notched specimens (single crack).
shielding effects, coalescence and irregular propagation). Therefore, the predicted lifetimes are often over-conservative, as the initiation stages and the interaction periods can constitute an important part of the cracking process. The "over-conservatism" of the predictions presents several safety advantages; notwithstanding, it leads to under-estimated lifetimes and therefore, to non-optimized both plant arrest planning and maintenance costs. Under this framework, there is an actual need for improving the understanding of the growth of multiple SCC cracks, particularly, for the enhancement of the actual predictive models, in the context of reactors lifetime extension.
Figure I-2. Examples of some cases of Alloy 600 SCC in the Primary Cooling Circuit of a nuclear reactor [10]. Left) Leakage due to PWSCC on the Reactor Vessel (Ohi, Japan 2001). Top) Cracks on t he Reactor Vessel Head of Ohi unit 3 (Japan 2001). Right) PWSCC on Nozzle for CRDM of the Palisades nuclear reactor (The
USA, 2008)
I.1.2 Objectives of this work
In this context, the present work is part of an ANR1 project relative to the study of the behavior of colonies of environmentally assisted short cracks, whose acronym due to its French name is "ECCOFIC" (Etude du Comportement des COlonies des FIssures Courtes de corrosion sous contrainte). The final objective of the project is to develop a more realistic methodology for the lifetime evaluation of components sensitive to Stress Corrosion Cracking in order to apply it for PWR life management. The aims of this project can be summarized as:
1. Enhance the understanding of the processes that are involved in the interactions stages of the life of a multiple stress corrosion cracks colony.
2. Simulate the behavior of a colony of cracks induced by stress corrosion cracking. Two approaches are developed in parallel within the ECCOFIC project: (i) an experimental one consisting in the characterization of a crack colony behavior and data collection and (ii) a numerical one for the development of a simulation methodology.
Within this framework, this PhD thesis is dedicated to develop the experimental part of ECCOFIC project, in order to contribute to the accomplishment of the two objectives described above. The objectives relative to this thesis can be listed as follows:
Choose one/or various experimental systems (material/environment) which cover a broad range of mode and mechanisms of Stress Corrosion Cracking (intergranular/transgranular SCC). The SCC systems should be able to give rise to a colony of cracks with different morphologies (e.g. crack densities and modes of propagation). The SCC process should be relatively easy to activate and compatible with an in-situ experimental characterization.
Develop an innovative in-situ experimental set-up to collect experimental data. The methodology should permit to obtain data in a fast an accurate way and to cover both the mechanical and electrochemical steps involved in the crack colony growth. The obtained data will be used for several purposes. Firstly, it will permit to characterize the different steps, stages, and processes involved in the propagation of a colony of stress corrosion cracks. The latter will allow identifying the parameters and tools that will be used for the construction of an SCC model (numerical approach). On the other hand, the input parameters intended to feed the model will be obtained from the collected data. Moreover, the experimental observations and measurements, resulting from the methodology developed here, will ensure the validation of the numerical approach.
I.1.3 Overview of the manuscript
This manuscript will be organized into seven chapters (including this introduction). Chapter II reviews the state-of-the-art related to stress corrosion cracking mechanisms and the multiple cracking phenomena. Moreover, this chapter includes a literature review of the SCC system studied during this work; this part permits to identify several parameters which control the development of the cracks. In addition, Chapter II also includes an extensive review of the experimental techniques used in this work, i.e. Digital Image Correlation (DIC), Electrochemical Noise (EN) and Acoustic Emission (AE). The experimental details and procedures are described in the third chapter.
Chapter IV presents the determination of the experimental conditions inducing the emergence of a crack colony. Several experimental parameters are studied (e.g. heat-treatments, sensitization, pH, mode of loading), and their influence on the morphology of the cracks is analyzed. Moreover, this chapter also describes the surface conditioning of the samples in order to allow DIC measurements. The results presented through this chapter allowed determining experimental conditions which offered crack colonies with different controlled features.
Chapter V describes the characterization of the growth of both a single stress corrosion crack and a crack colony. The description of the behavior is based on DIC measurements and correlation between AE and EN. The nature of the crack propagation mechanisms is discussed, as well as the effects of the crack interactions on the individual growth of the defects developing within the colony. A validation in 3D by microtomography is obtained. Chapter VI is entirely devoted to the application of EN measurements for SCC studies. Within this chapter, a special focus is made on the deleterious effects of both instrumental noise and electrode asymmetry on the measurements.
Chapter II
II
Literature review
II.1 Introduction ... 9 II.2 Stress Corrosion Cracking mechanisms and models ... 9
II.2.1 Crack initiation ... 10 II.2.2 Crack propagation ... 11 II.2.2.1 Slip dissolution model (or slip oxidation or film rupture model) ... 11 II.2.2.2 Film Induced Cleavage (FIC) model ... 13 II.2.2.3 Adsorption-based mechanisms ... 13 II.2.2.4 Models involving Hydrogen ... 15 II.2.3 Synopsis ... 16
II.3 Multiple cracking problem ... 17
II.3.1 Crack interactions and crack colony behavior ... 18 II.3.2 Multiple cracking and lifetime predictions ... 22 II.3.3 Synopsis ... 23
II.4 Intergranular stress corrosion cracking of Austenitic Stainless Steels (ASS) and Nickel base alloys in solutions containing polythionic acids ... 24
II.4.1 Polythionic Acids ... 24 II.4.2 PTA-SCC mechanism in ASS and Nickel base alloys ... 25 II.4.3 Synopsis ... 28
II.5 Experimental approach for a quantitative in-situ evaluation of multiple stress corrosion cracks propagation ... 30
II.5.3.4 Electrochemical Noise in Stress Corrosion Cracking studies ... 48 II.5.3.4.1 Electrochemical Noise sources in Stress Corrosion Cracking ... 48 II.5.3.4.2 Electrochemical Noise measurements during SCC tests ... 50 II.5.3.5 Synopsis ... 55
II.1 Introduction
his chapter presents a state-of-the-art of Stress Corrosion Cracking issues to which the present work is aimed at contributing. It will allow orienting the experimental choices that will be done.
It is divided into four main parts. First, the different mechanisms possibly involved in the initiation and propagation of single stress corrosion cracks are presented (Section II.2). The most mature models proposed to describe stress corrosion crack propagation are briefly discussed. A second section is devoted to the multiple cracking issue (Section II.3). Within this section, the literature works already done in this area, precisely those related to the influence of cracks interactions on their propagation, are detailed. Additionally, a third section (Section II.4) is devoted to the SCC system chosen for this study (Alloy 600 / tetrathionate solution). In this section, the mechanisms, as well as the various parameters affecting the crack morphology and propagation, are described.
Finally, as the main objective of this work is to develop an experimental methodology for the study of multiple cracks interactions in Stress Corrosion Cracking, the last part (Section II.5) is devoted to present a literature review of the most relevant experimental techniques. Within this section, their basis, their advantages and their applications for SCC studies are detailed. A conclusion allows defining the approach adopted for this work.
II.2 Stress Corrosion Cracking mechanisms
and models
Stress Corrosion Cracking (SCC) is the result of a synergic combination of mechanical, physical, chemical and electrochemical factors that induce the initiation and the propagation of brittle cracks. Depending on the alloy and the environment, cracks can propagate along grain boundaries and produce a fracture surface quite smooth. In this case, the process is called Intergranular Stress Corrosion Cracking (IG-SCC). Cracks can also progress throughout the grains, exhibiting few anodic dissolution patterns (or features). This second mode of propagation is called Transgranular Stress Corrosion Cracking (TG-SCC). Sometimes, combined forms of IG-SCC and TG-SCC can be involved in the development of a SCC defect.
The multidisciplinary characteristics and the complexity of the process are the main reasons why there is no universal mechanism able to describe the development of SCC in all materials and environments [11]. The SCC process is usually separated into two stages: (I)
initiation and (II) propagation of cracks. This distinction is often made because the conditions/mechanisms/controlling processes responsible for crack initiation are not necessarily the same as those needed to sustain crack growth [12].
II.2.1 Crack initiation
SCC risk mainly concerns passive alloys. The stability of the surface film plays an important role in the initiation and propagation of cracks [13]. The stability of this protective layer is characterized by how resistant the film is and by its self -healing capacity (rate of repair / repassivation). Such properties are determined by the composition of the alloy (e.g. Cr and Mo content) and environment factors such as pH, temperature, potential, ions concentration, strain, etc [14], [15]. The initiation of cracks is a complex process where suitable electrochemical (e.g. pH, potential, repassivation rate), mechanical (e.g. strain) and metallurgical conditions (e.g. composition, porosity, inclusions, etc.) need to be met for the destabilization of the surface film. Several preferential sites for stress corrosion cracks initiation have been already identified in the literature works [16]–[19]. Some of them can be listed as follows:
At surface discontinuities: SCC frequently initiates on preexisting or corrosion-induced surface defaults. These defaults may include grooves, laps, or burrs resulting from fabrication processes. The local environment around such discontinuities can favor the dissolution of metal and/or the formation of a weak film. In addition, they can act as stress concentration agents which induce the rupture of the protective layer.
By pitting: pits can form on inclusions that intersect the free surface or by a breakdown in the protective film. Pits act as occluded cells where environmental parameters such as pH and/or solution concentration (inside the cell) promote the further development of the pit. The transition between pitting and cracking depends on the same parameters that control SCC, i.e., the electrochemistry at the base of the pit, pit geometry, chemistry of the material, and stress or strain rate at the base of the pit.
II.2.2 Crack propagation
From an atomistic point of view, there exist essentially four fundamental processes that can produce a crack advance event: (1) the removal of atoms at the crack tip, (2) either shear movement of atoms at crack tips, emission of dislocations from crack tips or consumption of dislocations at crack tips; (3) the tensile separation of atoms at crack tips (decohesion) and (4) surface diffusion of atoms from crack tips [20]. Some of them can happen at the same time and could explain why there do not exist a universal SCC mechanism able to describe the crack growth process for all the material/environment systems.
Several mechanisms have been proposed to explain the crack propagation process, taking into account directly or indirectly, some of the above mentioned atomic processes. A common point between all of them is that all focus on the crack tip environment and on the local properties of the interfacial alloy-environment film. Depending on the nature of the model, they point out different controlling parameters of the crack tip environment such as localized or selective anodic dissolution, adsorption of specific species, surface mobility of atoms, rupture of the superficial film, vacancy injection in the base metal, local absorption of hydrogen leading to hardening and embrittlement or to local softening of the plastic zone, etc [18]. The next section presents only the most notorious and mature models hitherto proposed.
II.2.2.1 Slip dissolution model (or slip oxidation or film rupture model)
The basis of the current model was born in 1952 [21] and underwent several improvements and modifications, notably by Staehle, Parkins and Ford, until arriving to what it is today [22], [23]. This model [24] assumes that a slow but continuous plastic strain located at the crack tip periodically breaks the passive film, revealing a bare metal surface (see Figure II-1). Subsequently, the exposed fresh surface dissolves (atoms removing) until a new protective layer is formed [25].
In this model, the crack growth rate (ȧ, da/dt) is assumed to be described by the Faraday law and the density of the instantaneous dissolution current ia (Equation II-1).
𝑎̇ =𝑑𝑎 𝑑𝑡 =
𝑀
𝑧 ∙ 𝐹 ∙ 𝜌∙ 𝑖𝑎(𝑡, 𝜀̇𝑐𝑡, … ) Equation II-1
In this way, the crack growth takes into account the dissolution process, the film repair stage and the influence of the local strain rate at the crack tip (mechanical influence). The deterministic approach of the model is based on the knowledge of both the local chemistry at the crack tip and the crack tip strain rate. Both parameters can be really difficult to obtain properly by experimental approaches and are the main cause of the development of different versions of this model. The main difference between the several modifications lies in the dependence of the current density according to the local electrochemical environment inside the crack. The other discrepancy concerns the influence of the crack tip strain rate. Several equations have been already proposed to describe the current density taking into account different mechanical approaches [26]–[31]. Nevertheless, no consensus exists between the different analytic equations and their validity is still in discussion, mainly because they assume that crack tip strain is « homogeneous » while in fact it occurs along slip planes and it is fundamentally localized [22], [23], [32].
Figure II-1. Schematic representation of the crack advance process due to successive film breakdown/film repair transients, according to the slip-dissolution model: (Left) sequence of transients from Staehle and
(Right) resulting crack advance, from Ford.
aluminum alloys), and is generally not accepted as a mechanism of transgranular SCC [11]. It is in particular believed to be responsible for the IG-SCC of sensitized stainless steels and Ni-base alloys in aerated high temperature water, i.e. in BWR plants using the so-called “Normal Water Chemistry” and successfully modelled by the Plant Life Extension Diagnosis General Electric (PLEDGE) model developed by Ford and Andresen.
II.2.2.2 Film Induced Cleavage (FIC) model
The actual authors of this model are Sieradzki and Newman. The high growth rates (≈10-7
m/s) and the apparent arrest marks on transgranular fracture surfaces are not consistent with the slip dissolution model [33]. Those facts have encouraged some workers to argue that TG-SCC can be explained by an environmental phenomenon causing micro-cleavage [34]. This model assumes that an environmentally induced brittle film forms at the crack tip that fractures very fast in a brittle manner. The strain caused to base metal is too fast to be accommodated by the emission of dislocations and cracking continues to propagate across the ductile matrix for distance greater (10 – 10000 times) than the film thickness. The crack finally stops, suffers blunting and the film grows back to the conditions for cleavage (see Figure II-2). This model is able to explain the existence of crack arrest marking on the fracture surface, the cleavage like facets and discontinuous crack propagation. Ther efore, the model seems to be more adapted for TG-SCC than for IG-SCC.
The brittle surface films responsible for this form of SCC are generally nano -porous layers due to selective dissolution of an alloying element. This is believed to occur on copper alloys in ammonia, nitrite or chloride containing environments [35]. Film induced cleavage is also probably responsible for SCC of stainless steels in caustic environments where chromium can be selectively dissolved.
Figure II-2. Schematic representation of the crack advance process according to by the Film-Induced Cleavage model [36]
II.2.2.3 Adsorption-based mechanisms
interatomic bonds at the crack tips, hence promoting crack growth by decohesion along a cleavage plane or a grain boundary (see Figure II-3).
Figure II-3. Schematic representation of the adsorption-induced decohesion [36]
Another model based on adsorption of species was later proposed by Lynch; it was named Adsorption-Induced Dislocation Emission (AIDE). This model assumes that the adsorption of specific ions at the crack tip can affect the atomic neighborhood close to the surface within a crack (see Figure II-4). The adsorption of species reduces the force of the interatomic bond, hence, it also reduces the critical shear stress for dislocation mobility (rather than decohesion). Dislocations are easier to move locally and facilitate the formation of nano-voids in the plastic zone ahead of cracks, which can coalesce and produce a cleavage-like fractured surface. This model is in some way similar to the Hydrogen Embrittlement model (e.g. HELP see below) and can also explain many similarities between SCC and Liquid Metal Embrittlement (LME), where analogous fractured surfaces have been observed. In fact, several authors suggest that the rapid crack growth rates observed in such processes can only be explained by adsorption mechanisms because there is not enough time for the diffusion of species into the substrate or for other kinds of reaction than adsorption.
II.2.2.4 Models involving Hydrogen
Hydrogen-based models can be adopted to describe cracking in some SCC systems for several reasons [20]:
Hydrogen can be produced at the crack tip by electrochemical reactions and by chemical dissociation of water.
Some materials (High-strength steels, Ni-alloys, Ti-alloys, Al-alloys, and Mg-alloys), present degradation of the mechanical properties when tested under an hydrogen atmosphere (non-aqueous media). Moreover, these materials can suffer SCC in moist air, where dissolution reactions and other embrittlement phenomena can be neglected.
Besides, the similar characteristics of the fracture surfaces produced by SCC and HE in the materials mentioned above motivate the idea that the SCC mechanism could be the same as those proposed for HE. Hydrogen can be adsorbed at the surface of the material and due to its small size and its electronic configuration [38], the hydrogen atom can interact with the lattice of the material in several manners and promote cleavage, intergranular cracking and/or a highly localized plastic fracture. As noted, it can reduce the mechanical properties of the alloy and cause catastrophic cracking [39]. Yet this kind of environmental assisted cracking process cannot be explained by a single mechanism. Three main cases are usually encountered:
Hydride-induced embrittlement: this mechanism requires the possible and stable formation of metal hydrides within the material (as second phases). The materials that form hydrides are primarily the group V metals including Nb, V and Ta and also Zr, Ti, Mg and Pd and their alloys [40], [41]. The formation of these brittle phases is enhanced by the presence of hydrogen and the existence of a stress field at the crack tip. Hydrides form therefore at the crack tip, and due to their fragile nature, they fail by cleavage. The process is continuously repeated until final failure (see Figure II-5). This mechanism is supported by microscopic observations.
Figure II-5. Schematic representation of a crack growth process by hydride-induced embrittlement [42] Hydrogen-Enhanced Decohesion (HEDE): in this mechanism, it is considered that
interatomic cohesion forces. This model proposes that hydrogen can accumulate around regions with strong triaxiality as it is the case at the crack tip, decreasing the energy of bonding and facilitating crack propagation. This mechanism is debatable since no direct evidence demonstrated that dissolved hydrogen in metals decreases the interatomic forces. This model is applicable to non-hydride forming systems [43].
Hydrogen-enhanced localized plasticity (HELP): this model can be applied to systems where hydrides cannot be formed. This model establishes that the presence of hydrogen in solid solution decreases the barriers to dislocation motion, hence, increasing the amount of deformation but in a very local manner. In that case, hydrogen enhances plasticity, unlike the other models for which final failure is brittle [44].
II.2.3 Synopsis
This section gave a general idea about the different mechanisms proposed to account of the different stages of the SCC process: (I) crack initiation and (II) crack propagation. In the crack initiation stage, it has been noticed that the stability of the passive film (or surface layer) generally plays a major role. This stability is controlled in part by environ ment factors such as pH, electrochemical potential, local stress, strain, temperature, ions concentration and some other factors such as surface flaws and local Cr-depletion.
On the other hand, it was observed that the crack growth process could be reduced to three different kinds of interactions (happening the crack tip): (i) dissolution, (ii) adsorption of species and voids coalescence and (iii) hydrogen generation, diffusion, and segregation. The systems displaying IG-SCC seems to be more associated to a process involving dissolution. On the contrary, the systems presenting TG-SCC would be rather linked to a process of embrittlement promoted by the adsorption of species.
II.3 Multiple cracking problem
In some components, a single dominant crack may develop and lead to rupture. Nevertheless, there exist some situations where the final failure of a structural component is preceded by the initiation, propagation, and coalescence of multiple cracks [45]. Some examples of this kind of failure have already been reported in aging aircraft [46], pressure vessels (of nuclear reactors) [47], [48], and piping components (in high-pressure gas transmission) [49]–[51]. The domains where this process has occurred seem to be quite diverse. In fact, all the cases mentioned above share a common characteristic: the degradation process was associated to the nucleation of multiple surface cracks. Such feature are observed in fatigue [52], thermal fatigue [53], corrosion fatigue [54] and stress corrosion cracking [55].
Figure II-6. Example of multiple cracking problems in high-pressure gas transmission pipelines: (a) Failure in a buried gas transmission pipeline by IG-SCC [50] (b) Representation of the stress state of a pipeline and (c)
circumferential SCC crack colony in a pipeline [59]
Figure II-7. Schematic representation of the lifetime of a component subjected to multiple Stress Corrosion Crack initiation, coalescence, and growth [56]
II.3.1 Crack interactions and crack colony behavior
result from the reorganization of the stress field (see Figure II-8) ahead of the different cracks tips (stress concentration) and crack flanks (stress relaxation/shielding) [61]. Those complex interactions conduce to an irregular intermittent growth, periods of dormancy (or permanent arrest) and crack coalescences. Consequently, the effects of the interactions have direct implications on both the growth behavior of cracks and the remaining lifetime of the component.
Figure II-8. Schematic representation of the re-organization of the stress field around a surface crack Failure analysis involving multiple parallel cracks requires delimitating cracks that propagate as isolated ones from those that interact. Forsyth [62] was the first one to suggest that there exist an offset distance limit allowing to make such a distinction. He suggested that the limit must be proportional to the plastic zone size of the approaching crack tips and that the point of maximum interaction coincides with the overlapping of both plastic zones (see Figure II-9). Moreover, Wang et al. proposed a very simple rule for coalescence obtained from numerous experimental observations [58]. The coalescence criteria state that the separation of the parallel cracks (h) should be less than a fraction of the mean length of both cracks to coalesce (see Equation II-2).
ℎ < 0.14 × (2𝑐1+2𝑐2
2 ) Equation II-2
where c1 and c2 are the half-length of the approaching cracks.
Figure II-9. Changes of the crack tip plastic-zone size for approaching and passing offset cracks. The numbers denote simultaneous crack tip position [62]
Crack coalescence is characterized by a particular deviation of the approaching crack tips as depicted in Figure II-10 [63]. It has been theoretically demonstrated that such a deviation is produced by the emergence of mixed loading conditions at the crack tips (modes I and II). Several proposed theories describe the curve trajectory of the crack tips; among them, it can be cited the Maximum Energy Release Rate (MER), Maximum Tangential Stress (MTS) or the Minimum Strain Energy density (MSE). Wang et al. state that the introduction of a mode II stress have important consequences in a system where plasticity controls the crack growth [63]. In fact, the size of the plastic zone in mode II is larger than in mode I for comparable stress intensity factors (see Figure II-11). This can accelerate crack growth but can also involves plasticity-induced crack closure, thereby, reduce the crack propagation rate. Other authors [64], [65] argued that the enhanced plasticity can also induce the initiation of cracks at the crack tip and cause an apparent acceleration of the crack growth by the coalescence of these cracks.
Figure II-11. Crack tip plastic zone shapes estimated from elastic solutions and the von Mises yield criterion: (a) Mode I and (b) Mode II [67]
Figure II-12. Intensification of the SIF values of the approaching crack tips of two semi -elliptical cracks (subjected to a tensile stress): (Left) geometry of the interacting cracks and (Right) Normalized SIFs along the
crack front of the inner crack tip of the of the crack “1” (a1/c1=0,8) [61]
acceleration of the crack growth in the most part of the geometrical configurations [63]. Despite this, in the case of overlapping crack tips, the interaction level decreases due to the shielding effect1 [61]. Furthermore, Kamaya et al. proved that the intensity of the interactions varies across the position in the crack front of a 3D semi-elliptical crack [68]. The SIF values are therefore higher at the surface approaching crack tips than in the middle point of each crack in the depth direction [68]. At this point, it can be stated that the interactions between cracks are a complex process, and that the intensity of the interactions depends largely on the stage of growth. It means that it is constantly changing and needs to be evaluated permanently as the cracks approach to each other.
II.3.2 Multiple cracking and lifetime predictions
The first part of this literature review has described the different processes contributing to the growth of a stress corrosion crack. The slip dissolution model represents one of the most reliable deterministic mechanisms able to describe the crack advance under operating conditions (in Boiling Water Reactors) [72]. The model takes into account the contribution of electrochemistry and mechanical factors in order to estimate the crack growth rate. The slip dissolution model has shown good performances in predictive estimations [72]. Nevertheless, there is still missing a deterministic model able to take into account the nucleation and propagation of multiple cracks. Moreover, the Crack Growth Rates (CGRs) issued from the slip dissolution model can be highly impacted by the possible interactions that could exist between neighboring cracks. Therefore, using the estimated CGRs (from the slip dissolution model) in situations where multiple cracks are nucleated can lead to misleading conclusions. Even more if the period of initiation is neglected.
Realistic lifetime predictions of high-pressure pipelines have been already performed in the past [57]. Such structures are strongly susceptible to slightly alkaline multiple IG-SCC or near neutral multiple TG-SCC. The model used for such predictions was based on a Monte-Carlo approach [57], [66]. Such decision obeys to the stochastic nature of most of all the parameters necessary for modeling (initiation, CGRs, electrochemistry, crack morphology). The input values were obtained through numerous statistical characterizations of the distributions (Weibull distributions) of the initiation of cracks, cracks lengths, and CGRs. In this model, crack initiation is controlled by a power law (determined experimentally) and by a Weibull distribution of random crack sizes. The Weibull distributions are determined empirically for the particular material, environment and stress conditions. On the other hand, the crack velocities are derived from Weibull distributions which are a function of the crack length, the stress, and the environment. For each selected time period, the coordinates of each randomly located crack are determined, then a test is conducted to calculate the probability of dormancy and subsequently the appropriate value of crack
1The shielding effect represents the diminution in the CGR once the crack tip enter s to the stress
velocity is applied to the selected growing cracks. After that, the cracks are tested for the coalescence criterion (Equation II-2). The program continues to integrate multiple initiation and growth until a crack reaches a pre-set critical size criterion. The process is then repeated many times to determine the distribution of failure times.
Yet, the model presents several drawbacks that can impact the confidence limits of the results. Firstly, the model does not take into account the effect of local stress in the nucleation rate of cracks. Secondly, the model does not take into account the possible change of cracks growing towards the depth-direction. Additionally, the interactions are strongly simplified: they are basically neglected until the coalescence criterion is reached. Moreover, the dormancy of cracks is entirely established by statistical parameters, without taking into account local stresses (stress shielding). Some of these points have already been taken into account by Kamaya et al. and included in a simulation procedure, particularly, the effect of local stresses on the initiation kinetics [65]. Nevertheless, a coupling with a deterministic approach able to describe the evolution of a crack colony by the synergic effect of the electrochemistry and mechanical factors is still missing.
II.3.3 Synopsis
This section introduced the influence of the interactions between several cracks on their individual behavior. The latter has been demonstrated by both experimental observation and analytical approaches. The experimental observations dealt principally with the study of crack interactions as a function of crack parameters such as length, offset distance, crack growth rate and coalescence events. On the other hand, the study of crack interactions has also been performed numerically by the evaluation of the SIF values. Such evaluation has only been performed in the cases of coplanar and parallel cracks (2D or 3D). To summarize, it was observed that the interactions modify the CGRs of the interacting crack tips and tha t the intensity of the interaction is a function of the relative position of cracks, relative length, and relative shapes (crack front shape and dimensions). In other words, the interaction level changes as the cracks develop. This implies that the interactions between the cracks have to be permanently evaluated.
Although a lot of progress have been made in the mechanistic SCC studies devoted to the propagation of a single crack, very few works have taken into account the effects of the interactions between several cracks for a predictive analysis. A deterministic/empiric model able to describe the initiation of multiple cracks and their propagation that takes into account the specificities of the crack growth processes involved in SCC, i.e., the synergy between the electrochemistry and the mechanical factors is still missing.
permit the possibility to modify the crack colony morphology (e.g. cracks density) in o rder to obtain a varied input data able to describe different situations. As detailed in Section II.2, the modification of electrochemical (e.g. pH, potential), environmental (severity, temperature, pressure) and mechanical factors (e.g. stress, strain rate) can have consequences: (i) on the initiation kinetics of cracks (i.e. crack density) and (ii) on the contribution of electrochemical and mechanical processes implied in crack growth. A system which meets the conditions mentioned above is the alloy 600 in polythionate solutions. In fact, this alloy can be sensitized at different degrees; additionally, the severity of the corrosive media can also be easily modified. The above allows initiating multiple cracks in the form of colonies which is the object of this work. The next section is therefore devoted to describe the SCC of the alloy 600 in polythionate solutions; the section includes also sensitized austenitic stainless steels because, in polythionic solutions, both materials behave in very similar way and their SCC mechanism is considered to be the same.
II.4 Intergranular stress corrosion cracking of
Austenitic Stainless Steels (ASS) and
Nickel base alloys in solutions containing
polythionic acids
II.4.1 Polythionic Acids
Polythionic acids have the general formula of H2SxO6 with x being 3, 4 or 5 for the most
usual. Both of the protons have very strong acidic properties so that in aqueous solution they exist under the form of oxy-sulfur anions (e.g. SxO62-). Moreover, polythionates are
metastable anions and their chemistry is very particular and complex. They can be oxidized, reduced and can disproportionate or decompose under varying electrochemical and/or chemical conditions. Figure II-13 depicts a simplified potential-pH diagram for the S-H2O
system. In this diagram, only a group of the possible reactions were considered for practical reasons. It can be observed that the regions of stability of the thiosulfate and tetrathionate (S2O32-, S4O62-) are very narrow. In fact, it is very likely that such metastable species coexist
with other sulfur compounds such as S, H2S, HS-, and HSO3 - in aqueous solutions [73].
Besides, it can be noticed that thiosulfate is more stable at neutral-basic pH while tetrathionate is more stable at acidic pH values.
oxygen during shutdowns to form polythionic acids (see Equation II-3) and other oxyanions, such as thiosulfate (S2O32−) [74].
8FeS + 11O2 +2H2O 4Fe2O3 + 2H2S4O6 Equation II-3
On the other hand, the thiosulfate ion can also be accidentally found in the main cooling system of nuclear power plants, where solutions of borate with thiosulfates are stored to be used as sprays in containment building following a serious nuclear accident (thiosulfate anions are used to react with the iodine fission products) [75], [76]. Many cases of localized corrosion such as pitting, crevice and stress corrosion cracking have already been registered in such environments. Therefore, Polythionic Acid Stress Corrosion Cracking (PTA-SCC) has been intensively studied in the last 30 years. Nevertheless, the SCC mechanisms remain in discussion [73], [77].
Figure II-13. Simplified potential-pH diagram for the system S-H2O. The total concentration of sulphur in
water is 0.204 grams S per kg H2O [78] II.4.2 PTA-SCC mechanism in ASS and Nickel base alloys
It has been reported that the polythionate solutions can cause stress corrosion cracking of structural materials such as austenitic stainless steels and nickel base alloys [79], [80]. It is highlighted that in such systems, a sensitized microstructure is essential to promote intergranular cracking [81].
films and facilitates the initiation of cracks [85]. The Cr-depleted zones become more anodic than both the grains and the chromium carbides, hence, intergranular corrosion and/or intergranular stress corrosion cracking are promoted.
Figure II-14. Schematic representation of the intergranular stress corrosion cracking of austenitic stainless steels alloys in polythionic acids [86]
In this system, the crack initiation kinetics is strongly dependent on both the sensitizing degree and the associated grain boundary chromium depletion. Figure II-15 shows that IG-SCC occurs when grain boundary Cr concentration is lower than a critical value and becomes more severe as grain boundary Cr concentration decreases [79], [80]. Crack initiation takes place by film rupture over the Cr-depleted zones produced by the sensitization treatment [87]. The film rupture occurs due to Inter-Granular Attack (IGA) and/or due to the emergence of slip steps on the surface, over the Cr-depleted region. Crack initiation kinetics has been reported to increase with decreasing the pH [88]. In stainless steel alloys the stability of the passive film is given by the potential and pH domains at which the material is exposed [15], [89], [90]. Acidic media weaken the passive film and compromise the conditions of re-passivation. Therefore, not only the crack density (crack initiation kinetic) can be controlled by changing the pH, but the contribution of mechanical and electrochemical factors on the crack propagation can also be managed. Moreover, it has been reported that this system exhibits short incubation time for crack initiation at acidic pHs [91].
The crack propagation process in this system is not very clear and remains in discussion [73]. Nevertheless, a general consensus exists concerning some points. It is generally accepted that the crack propagation is improved by an important anodic dissolution of the active-path, which is formed between the Cr-depleted zones and the matrix. Stress has a purely mechanical effect, increasing crack opening, enhancing the transport of species and increasing anodic dissolution [92]. This dissolution seems to be enhanced by the deposition of elemental sulfur at the crack tip which prevents repassivation and increases anodic dissolution [93]. The elemental sulfur is produced by the reduction of the metastable anions (tetrathionate, thiosulfate) according to Equation II-4 and Equation II-5.
S4O62- + 12H+ + 10e 4S + 6H2O Equation II-4
S2O32- + 6H+ + 4e 2S + 3H2O Equation II-5
Several works have demonstrated that the chemisorption of sulfur significantly enhances the dissolution rate of iron and nickel [93], [94]. Nevertheless, it remains difficult to explain the high CGRs registered during cracking only by anodic dissolution. In that context, some authors argued that a mechanical step must also be involved (cleavage) while some others state that hydrogen-embrittlement mechanisms must be implicated [77], [80]. Newman et al. studied the SCC process of a sensitized 304 Stainless Steel (SS) in thiosulfate solutions at relatively high pH [80]. They stated that the highest current density obtained from scratching test can only produce Crack Growth Rates (CGRs) 50 to 100 times smaller than those experienced during the SCC tests. Based on thermodynamic arguments, they disregarded the hydrogen embrittlement mechanism for this system. They explained the results on the basis of intermittent brittle fracture of the grains boundaries, assisted by local formation of strain generated martensite and sharpening effect of anodic dissolution. Concerning alloy 600, where the martensite does not form, Newman et al. suggested that the process must involve localized fracture of the grain boundaries.
Finally, it would be interesting to note that depending on pH, sensitized SS can suffer either significant generalized intergranular corrosion or IG-SCC. The above could correspond to change in the mechanisms according to the solution pH. On the left of the Figure II-16, cracking could be due to a simple opening of IG penetration. On the right, cracking could be more related to film rupture. Similar observations can be done on the effect of potential (see Figure II-17).
Figure II-16. Cross-sectional micrographs of sensitized 304 SS U-bended specimens tested in a solution of 1 wt. % K2S4O6 at different pH values (adjusted by H2SO4 addition) [96]
Figure II-17. Stress corrosion cracking test performed in a sensitized 304 SS at various potential in a polythionic acid solution. Figures indicate potential and time to failure. NF stands for no failure. 10 X, 100 X
[97]
II.4.3 Synopsis
process were listed. Firstly, it was noted that the material susceptibility is highly related to the sensitization degree. The incubation time was observed to diminish when increasing the sensitization state. Secondly, it was also noticed that the properties of the passive film control both initiation and propagation of cracks. It was mentioned that such properties a re controlled by the pH of the solution and the corrosion potential. Considering all above, it can be argued that the morphology of the cracks can be controlled by the pH, the potential and the degree of sensitization.
Moreover, it was noticed that the processes involved in the crack propagation advance are not yet well-understood. Clearly, the mechanism must imply an important anodic dissolution of the active-path. Nonetheless, the measurements of the current involved during the cracking step are not able to explain the rapid crack growth rate experienced. This fact suggests that other processes (cleavage, HE) also support the cracking mechanisms. A mechanistic study is not the aim of this work. Nevertheless, the experimental methodology developed through this work can potentially elucidate some of the missing parts.
In that context, it can be stated that the polythionate solutions and the alloy 600 are good candidates for the experimental part of this investigation. The main reasons are that this system quickly produces IG-SCC, enabling the feasibility of monitoring techniques and producing the initiation of multiple cracks on smooth specimens. It is expected that by modifying experimental parameters such as pH and the degree of sensitization of the material, different crack colony morphologies will be obtained.
II.5 Experimental approach for a quantitative
in-situ evaluation of multiple stress
corrosion cracks propagation
A key point in the study of multiple cracks growth is the development of a methodology for quantifying the rate of damage and the different phases involved in the evolution of the crack colony. Experimental approaches for studying single long environmental assisted cracks are well established. In this case, standards are available for evaluating crack growth rates under Linear Elastic Fracture Mechanics (LEFM) on notched Compact-Tension (CT) specimens (e.g. ASTM E 1681-03 [98]). However, as far as small numerous cracks are concerned, there are no standards that guide the measurement process, neither in the case of a single defect nor for multiple growing cracks.
In this context, the next section aims at introducing some experimental techniques able to provide statistical data and local measurements concerning the development of a crack colony. The information obtained through the implementation of the techniques here described should allow characterizing the crack colony behavior and the interactions between the cracks. Among the most relevant techniques, Digital Image Correlation (DIC) seems to be quite promising in SCC studies. This technique gives access to local measurements of deformation and to the early detection of growing defects on the surface. However, the technique provides only surface mechanical information about the process (2D); its extension to 3D is possible but need tomographic imaging techniques (e.g. high -resolution X-ray tomography). In order to cover both the mechanical and the electrochemical steps involved in the crack colony growth (in a three-dimensional manner), complementary techniques such as Acoustic Emission (AE) and Electrochemical Noise (EN) can be coupled to DIC. The latter should provide a broad range of mechanistic information (mechanical and electrochemical) in three dimensions.
DIC, EN and AE techniques are therefore presented in this section. Their basis, their advantages and their applications for SCC studies are detailed
II.5.1 Digital Image correlation
