Microelectrochemical approach towards the analysis of electrochemical noise signals related to intergranular stress corrosion cracking of austenitic stainless steel

Mikroelektrochemischer Ansatz zur Analyse elektrochemischer Rauschsignale der interkristallinen Spannungsrisskorrosion von austenitischem rostfreiem Stahl

Der Technischen Fakultät der Universität Erlangen-Nürnberg zur Erlangung des Grades

D O K T O R – I N G E N I E U R

vorgelegt von Mathias Stefan Breimesser M.Sc.

Erlangen – 2012

Als Dissertation genehmigt von der Technischen Fakultät der Universität Erlangen-Nürnberg

Tag der Einreichung: 12.06.2012 Tag der Promotion: 24.09.2012 Dekanin: Prof. Dr.-Ing. M. Merklein Berichterstatter: Prof. Dr. sc. techn. S. Virtanen, WW IV Jun.-Prof. Dr.-Ing. A. Heyn, Uni Magdeburg

2 Zusammenfassung

Die elektrochemische Rauschmesstechnik ist eine vielversprechende Methode zur Untersuchung und Früherkennung von Korrosionsprozessen. Zum besseren Verständnis der Rauschsignale, die während der Initiierung und des Wachstums von interkristalliner Spannungsrisskorrosion in austenitischem rostfreiem Stahl auftreten, wurde eine Kombination von zwei experimentellen Techniken angewandt: Einerseits wurden makroskopische elektrochemische Rauschmessungen durchgeführt. Andererseits wurden mittels der elektrochemischen Mikrokapillartechnik die Initiierung und das Wachstum einzelner Mikrorisse untersucht, mit dem Ziel, ein besseres Verständnis für die makroskopischen Rauschsignale zu erreichen und diese zu modellieren. Um vergleichbare Resultate zu erhalten, muss ein experimentelles System gewählt werden, dass sowohl in mikro- wie in makroskopischen Experimenten anwendbar ist. Eine 0.01 M Lösung von Kaliumtetrathionat mit einem pH-Wert von 2.2 erfüllt diese Anforderung. In Kombination mit der Mikrokapillartechnik ist es möglich, Probenoberflächen systematisch abzusuchen und aktive Oberflächenpositionen anhand des Korrosionspotentials zu identifizieren, und das Stromsignal eines einzelnen wachsenden Mikrorisses aufzuzeichnen. Mikrokapillarmessungen auf geschliffenen Proben unter konstanter Verformung zeigen, dass sich Probenoberflächen grösstenteils passiv verhalten. Wenige spezifische Punkte sind aktiv und zeigen Rissinitiierung. Aktive Punkte treten überwiegend an den Grenzflächen zwischen Mangansulfid-Einschlüssen und dem Metall auf. Rissinitiierung hängt jedoch in hohem Masse von der Oberflächenbehandlung ab. Ist die Oberfläche oxidiert, so tritt Rissinitiierung verzögert auf. Werden geschliffenen Oberflächen zusätzlich vibrationspoliert, so zeigen alle Korngrenzen aktives Verhalten und interkristalline Korrosion, auch auf unbelasteten Proben. Stromsignale einzelner Mikrorisse bestehen aus Überlagerungen von Transienten mit abruptem Anstieg und langsamerem Abfall. Die gemessenen Ladungsmengen korrelieren dabei gut mit den Mikrorisslängen und ermöglichen ein Abschätzen der Dicke der aufgelösten Metallschicht entlang einer Korngrenze. Die gefundenen Stromsignale können als Serie von Passivfilmrissen, gefolgt von anodischer Metallauflösung und Repassivierung gedeutet werden. In makroskopischen Versuchen kann aufgrund der Aggressivität des Elektrolyts die Initiierung von Spannungsrisskorrosion unter konstanter Last unterhalb der Streckgrenze des Materials innert Stunden oder weniger Tage beobachtet werden. Elektrochemische Rauschmessungen können dabei klar den Übergang von passivem zu aktivem Verhalten detektieren. Risswachstum ist gekennzeichnet durch charakteristische Strom- und Spannungstransienten. Drei Haupttypen

3 von Transienten können identifiziert werden, welche sich metastabiler Lochkorrosionsbildung, Initiierung und Wachstum von Mikrorissen, sowie beschleunigtem Risswachstum zuordnen lassen. Die Stromrauschsignale, welche Initiierung und Wachstum von Mikrorissen anzeigen, gleichen dabei in hohem Masse den potentiostatischen Stromsignalen, die mit der Mikrokapillare auf einzelnen Mikrorissen gemessen werden können. Ein makroskopisches Rauschsignal lässt sich somit nicht alleine durch Aufsummierung von mikroskopischen Risswachstumssignalen simulieren, einzelne Komponenten des Rauschsignals können jedoch anhand der Mikrokapillarexperimente erklärt werden. Es ist nicht abschliessend möglich, aufgrund der vorliegenden Ergebnisse einen bestimmten Mechanismus der Spannungsrisskorrosion auszuschliessen oder definitiv zu bestätigen. Ein Mechanismus basierend auf Passivfilmbruch, anodischer Metallauflösung und Repassivierung kann die beobachteten Ergebnisse jedoch schlüssig erklären.

4 Abstract

The electrochemical noise technique is a promising method for the study and early detection of corrosion processes. To gain a better understanding of the electrochemical noise signals related to the initiation and growth of intergranular stress corrosion cracks in austenitic stainless steel, a combination of two experimental techniques was applied: First, macroscopic electrochemical noise measurements were performed. Second, the electrochemical microcapillary technique was applied to study the initiation and growth of single micro cracks, with the aim to gain a better understanding of macroscopic noise signals and to model them. To obtain comparable results, an experimental system has to be chosen, which is applicable for both micro- and macroscopic experiments. A 0.01 M solution of potassium tetrathionate with a pH value of 2.2 fulfils this requirement. In combination with the microcapillary technique it allows a systematic scanning of a sample surface, identification of active surface spots by measurement of the open circuit potential, and the subsequent measurement of current signals caused by single growing micro cracks. Microcapillary measurements on ground samples under constant deformation conditions show that most part of a sample surface exhibits passive behaviour. Crack initiation occurs at few specific active spots. These active spots are mostly found on the boundary between manganese sulphide inclusions and metal. However, crack initiation depends to a high degree on surface preparation. On oxidised surfaces, crack initiation can occur delayed. Vibration polishing of ground surfaces on the other hand renders all grain boundaries susceptible to intergranular corrosion, independent of applied stress. The current signal of a single micro crack can be explained as a superposition of transients with fast rise and slower decay. The detected charge values correlate well with micro crack lengths and allow estimating the width of the dissolved metal layer along a grain boundary. These current signals can be interpreted as a series of passive film rupture events, followed by anodic metal dissolution and repassivation. In macroscopic noise measurements, the aggressive electrolyte allows the monitoring of the initiation of stress corrosion cracks under constant loading conditions in a matter of hours or few days. Electrochemical noise measurements can clearly detect the transition from passive to active behaviour. Crack propagation is indicated by characteristic current and potential transients: Three main transient types can be identified and assigned to metastable pitting, micro crack initiation and growth, and accelerated growth steps. Current noise signals indicating micro crack initiation

5 and growth highly resemble the potentiostatic current signals measured with the microcapillary on single micro cracks. A macroscopic noise signal can therefore not be modelled by simply adding up microcapillary signals, but microcapillary measurements can explain certain components of a macroscopic signal. It is not possible to finally confirm or dismiss a specific mechanism for stress corrosion cracking based on the presented results. However, a mechanism based on passive film rupture, anodic metal dissolution and repassivation coherently explains the found results.

6 Contents

Zusammenfassung...... 3 Abstract ...... 5 Contents...... 7 List of abbreviations and symbols...... 9 1 Introduction...... 11 1.1 Background and motivation ...... 11 1.2 Objectives and experimental approach ...... 12 1.3 Relevance of the work...... 14 1.4 Outline of the thesis ...... 15 2 Theoretical background...... 16 2.1 Stress corrosion cracking ...... 16 2.1.1 Definition and phenomenology...... 16 2.1.2 Mechanism of IG SCC of sensitised stainless steel ...... 18 2.1.3 Loading methods...... 25 2.2 The electrochemical microcapillary technique ...... 26 2.2.1 Overview of microelectrochemical measuring techniques ...... 26 2.2.2 Features of the electrochemical microcapillary ...... 28 2.2.3 Applications of the electrochemical microcapillary technique...... 29 2.3 The EN technique...... 31 2.3.1 Definition ...... 31 2.3.2 Measuring methods ...... 32 2.3.4 Application of the EN technique to monitor SCC ...... 36 2.4 Conclusion...... 39 3 Experimental procedures and development of an experimental system ...... 40 3.1 Material ...... 40 3.2 Macroscopic EN measurements...... 42 3.2.1 Sample preparation...... 42 3.2.2 Measuring setup...... 42 3.3 Microcell measurements ...... 44 3.3.1 Sample preparation...... 44 3.3.2 Measuring setup (analogous to [23])...... 45 3.4 Initial experiments with neutral sodium thiosulphate electrolyte ...... 47 3.4.1 Macro scale EN measurements ...... 47 3.4.2 Microcell tests...... 48 3.5 Evaluation of new electrolytes...... 49 3.5.1 Selection criteria...... 49 3.5.2 Acidic thiosulphate solutions...... 50 3.5.3 Chloride solutions...... 51 3.5.4 Potassium tetrathionate solutions...... 54 3.6 Final measuring procedures ...... 58 3.6.1 Macroscopic CERT tests (Figure 27a)...... 58 3.6.2 Macroscopic CL tests (Figure 27b)...... 59 3.6.3 Microcapillary measurements ...... 59 3.6.4 Post test inspection...... 60 3.7 Summary ...... 61 4 Investigation of initiation and propagation of IG cracks on the micro scale ...... 62 4.1 Signal characteristics...... 62 4.1.1 Passive behaviour...... 62

7 4.1.2 Active surface spots ...... 63 4.1.3 Discontinuous crack growth behaviour and repassivation...... 65 4.2 Location of crack initiation sites...... 66 4.3 Influence of applied stress and DOS on the crack growth behaviour...... 70 4.3.1 Influence on crack sizes ...... 70 4.3.2 Influence on current signal...... 71 4.3.3 Crack growth direction...... 71 4.4 Quantitative comparison of current signals and micro crack dimensions...... 72 4.5 Systematic array measurement...... 76 4.5.1 Distribution of active sites ...... 76 4.5.2 Variation of OCP values ...... 77 4.6 Influence of the surface state...... 79 4.6.1 Behaviour of polished surfaces ...... 79 4.6.2 Influence of oxide film formation over time...... 81 4.7 Discussion ...... 83 4.7.1 General validity of the results ...... 83 4.7.2 Variation of OCP values on passive surface spots...... 83 4.7.3 Micro crack initiation...... 84 4.7.4 Micro crack propagation...... 85 4.8 Summary ...... 88 5 Macro scale EN measurements ...... 90 5.1 Results of CERT tests ...... 91 5.2 Results of CL tests ...... 93 5.2.1 General behaviour of solution annealed samples...... 93 5.2.2 General behaviour of sensitised samples ...... 94 5.3 Interpretation of current transient shapes...... 102 5.3.1 Classification of transient types ...... 102 5.3.2 Transition to macroscopic cracking ...... 109 5.3.3 Integration of characteristic current signals ...... 109 5.4 Further analysis of EN signals ...... 111 5.4.1 Comparison of statistical parameters ...... 111 5.4.2 Analysis in the frequency domain...... 112 5.5 Discussion ...... 113 5.5.1 General results...... 113 5.5.2 Crack initiation under CERT and CL conditions...... 114 5.5.3 Crack initiation and propagation: mechanism and influence of stress.... 114 5.5.4 Identification of IG SCC and use of EN for monitoring applications..... 116 6 Comparison of macro and micro scale measurements...... 118 6.1 Direct upscaling: Macrocapillary measurements...... 118 6.1.1 Passive surface behaviour ...... 118 6.1.2 Current signals of active surfaces ...... 119 6.1.3 Comparison with microcapillary measurements...... 121 6.2 Discussion of microcapillary and macro scale EN measurements...... 123 6.2.1 Comparability of macro and micro scale results...... 123 6.2.2 Comparison of current transients during crack growth...... 124 6.2.3 Mechanistic considerations...... 127 Summary and conclusions...... 130 Outlook...... 132 Acknowledgement...... 134 References...... 135 Appendix...... 144

8 List of abbreviations and symbols a) Abbreviations AC Alternating current AISI American Iron and Steel Institute CE Counter electrode CERT Constant extension rate tensile CL Constant load(ing) DOS Degree of sensitisation EBSD Electron backscatter diffraction ECN Electrochemical current noise EMPA Eidgenössische Materialprüfanstalt EN Electrochemical noise EPN Electrochemical potential noise FFT Fast Fourier transformation IG Intergranular OCP Open circuit potential PSD Power spectral density PSD I Current power spectral density PSI Paul Scherrer Institut RE Reference electrode SCC Stress corrosion cracking SCE Saturated calomel electrode SEM Scanning electron microscope/microscopy TG Transgranular WE Working electrode

9 b) Symbols a Surface crack length

Acrack Crack wall area b Micro crack depth c Width of dissolved metal along a micro crack F Faraday constant: 96485 J·K-1·mol-1 M Molar mass I Current i Current density

ia Activation current density

ir Reactivation current density

Rn Noise resistance U Potential

Zn Noise impedance ρ Density σ Strain

σy Yield stress

10 1 Introduction

1.1 Background and motivation Stress corrosion cracking (SCC) is a phenomenon, which is observed on susceptible metals and alloys, when subjected to stress in the presence of an aggressive medium. The combination of stress and corrosion causes quasi-brittle fractures in normally ductile alloys, and cracking well below yield stress σy. One of the first documented examples of SCC was the so called “season cracking”, observed by British troops in India in the 19th century: brass cartridge cases that were stored for extended times in horse stables during the monsoon months suffered cracking, especially where the cartridges were crimped to the bullets. The combination of the ammonia rich, humid atmosphere and significant residual stress in the cartridge cases could be identified as the cause of this phenomenon in 1921 [1]. Today, SCC is still a common phenomenon and a dangerous material degradation process in many fields of engineering. The use of better materials and the avoidance of potentially harmful operating conditions reduced the number of SCC failures, but they could not be totally avoided so far. Stress corrosion field failures are a major problem in the petroleum, chemical, and nuclear sectors [2-10], where austenitic steels and nickel-based alloys find widespread application as construction materials, often under aggressive environmental conditions. While resistant against uniform corrosion, these materials can suffer SCC under certain specific conditions. In the nuclear industry, intergranular (IG) SCC has been reported to occur under boiling water and pressurised water reactor conditions [6, 9, 10], affecting the integrity of various reactor components, e.g. pressure vessel penetrations, coolant piping or heat exchanger tubing. SCC in these components often results in significant economic losses, and in few cases can also affect system integrity. The early detection of SCC could significantly reduce potential corrosion damage and would increase plant operation safety. Therefore, there is a high demand for a reliable, continuous and non-destructive monitoring system for the early detection of localised corrosion phenomena in industrial applications. A measuring technique that might fulfil these requirements is the electrochemical noise (EN) technique. EN is defined as fluctuations of potential (electrochemical potential noise, EPN) or current (electrochemical current noise, ECN), typically in a low frequency region (< 10 Hz), which are produced from a corroding electrode on its own, without need of external current or potential sources. The EN technique is a promising tool for standardised testing of the corrosion resistance of metals and protective coatings, or for monitoring and diagnostic applications [11]. The fundamentals of corrosion monitoring are found

11 in [12] and [13]. Extended field studies were done at Hanford Site [14]: EN measurements were performed inside storage tanks for high-level nuclear waste. The aim of these measurements was to detect SCC and control the rate of uniform corrosion in the highly aggressive nitrate/hydroxide system. Various difficulties arose during these tests [15, 16], illustrating the possible challenges that might occur in a monitoring application: Probe design had a significant influence on the outcome of a noise measurement. Only certain sample designs that were tested, exhibited SCC, while others only showed uniform corrosion. Furthermore, certain samples showed SCC, but crack propagation could not be detected during the EN measurement. This was attributed to the cathodic reactions taking place mostly on the working electrode, due to an unfavourable design. It is concluded that a series of problems can occur during the implementation of the EN method in a real setup, but in the experience of the authors, none of these problems are insurmountable, if enough time and money is invested in development. Further examples of the application of EN probes in industrial environments are presented in [17-21]. The use of EN as a diagnosis tool to identify critical chemical conditions is described in [22]. The basic understanding of EN signals is still limited, and the identification of specific corrosion processes by analysing these signals is challenging. While a differentiation between uniform corrosion, localised corrosion and passive states has been possible in certain cases, particular localised corrosion processes, such as pitting and SCC, are still difficult or even impossible to discern.

1.2 Objectives and experimental approach The main objectives of this thesis are to improve the understanding of EN signals produced during IG SCC of stainless steel, and to gain new insight in the underlying mechanisms of crack initiation and propagation. Characteristic EN patterns should be found, which allow the reproducible and unambiguous identification of the early stages of IG SCC. A novel twofold approach to study the initiation and propagation of IG SCC should be applied. In a “top-down” approach, EN measuring technique should be used to study IG SCC on macroscopic samples. A “bottom-up” approach aimed at the study of single EN sources with the electrochemical microcell technique. Combination and analysis of the two approaches is expected to lead to new insights into the phenomenon of IG SCC itself, especially initiation and early propagation of cracks, as well as a better understanding of the resulting EN signals.

12 Top-down approach On the macroscopic scale, classic EN measurements were performed on dynamically and statically stressed tensile specimens. In this “top-down” approach, the EN signals of a multitude of active sites on the working electrode were measured simultaneously. Both current and potential EN signals were recorded and the corrosion behaviour of the sample could be monitored. Macroscopic EN measurements are well established for some corroding systems, but the production of meaningful data is far from being trivial. The macro scale experiments in this thesis were based on preliminary work performed at the Paul Scherrer Institute (PSI). In the course of this thesis, the existing test setup for EN measurements at room temperature was utilised and optimised. EN data was analysed in time and frequency domain, to derive characteristic signal parameters indicating SCC (initiation).

Bottom-up approach On the micro scale, single cracking events were investigated using the electrochemical microcapillary technique. This part of the thesis was done at the Swiss Federal Institute of Materials Science and Technology (EMPA) in Dübendorf. The idea behind this “bottom-up” approach is to decrease the surface area exposed to the electrolyte and to measure the current and potential signals of single active corrosion sites on a surface. Due to the small area exposed to the electrolyte (1 - 1000 µm2), it is often possible to directly correlate surface damage to a measured current signal. Signals of single active sites might then be extrapolated to model a macroscopic signal. The electrochemical microcapillary technique has been used before to study the influence of stress on pitting corrosion [23-25]. However, it has not been used so far to monitor the initiation and early propagation of single micro cracks. This was a new and untested application of the technique, which could open up new applications for the electrochemical microcapillary technique.

Experimental system The basis for this study is a well-understood system. American Iron and Steel Institute (AISI) 304 austenitic stainless steel was chosen, due to its widespread use in many industrial applications, and its known susceptibility to SCC in chloride or sulphur containing environment. Additional thermal treatment of the solution annealed base material further increased its tendency to suffer from IG SCC.

13 Although known for a long time, a variety of effects on IG SCC of austenitic stainless steels are still subject of research: The influence of applied [26] and local stress values (including the effect of crack coalescence) [27], surface preparation [28], inhibiting and accelerating effects of different metal cations [26, 29, 30] and anions [26, 30-32], cold work [33], or thermal sensitisation [26, 34], to name a few. To produce meaningful and comparable results with both approaches, experiments on both scales had to be performed under reasonably comparable experimental conditions. This limits the choice of parameters. The microcell setup is not suitable for measurements under pressure or at temperatures above the boiling point of the electrolyte. All experiments were therefore performed under atmospheric pressure and at room temperature. Furthermore, experiments were mainly performed under constant loading (CL) conditions. Initially, an aqueous solution of sodium thiosulphate was considered as a suitable electrolyte, which was successfully applied in preliminary experiments [35]. After a series of tests on both setups, this electrolyte was dismissed, and other electrolytes had to be considered. An acidic solution of potassium tetrathionate was finally found to be suitable for both approaches.

1.3 Relevance of the work The combination of electrochemical methods on the macro and micro scale to characterise IG SCC, presented in this study, is new. The monitoring of single micro crack initiation and propagation by electrochemical measurements has not been reported yet. The results should help to clarify if characteristic signals exist, which clearly indicate and identify IG SCC. The ability to detect smallest current and potential fluctuations also allows an investigation of the very early signs of single cracking events. Information gained by these experiments might clarify some open questions concerning the underlying mechanism of IG SCC (initiation) for the investigated system. This thesis is part of the KORA I and II1 projects of the Laboratory for Nuclear Materials at the PSI. The results gained in this fundamental study will complement experiments on IG SCC in high-purity water at elevated pressure and temperature. Aim of these studies is the application of the EN technique as a monitoring tool under boiling water reactor conditions.

1 The KORA research project is considered with environmentally-assisted cracking in light water reactor structural materials. It is split up into three sub-projects. In KORA I, corrosion fatigue of austenitic stainless steels is studied. KORA II is focussed on the optimisation of the EN technique for monitoring applications. Sub-project III is focussed on SCC in weld seams between Nickel-base alloys and low-alloy steels.

14 1.4 Outline of the thesis Following this introduction, a background chapter will provide some basic information on the topics covered in this thesis. An introduction into SCC is given, and an overview of the applied measuring techniques is presented. The main body of the thesis is split into three parts. First, the applied experimental conditions and methods are presented and the process of establishing a suitable experimental system for further experiments on both macro and micro scale is described. The following chapters present the results of systematic electrochemical microcell measurements and macroscopic EN measurements. EN signals are analysed and correlated with corrosion processes. In a final chapter, the results of micro and macro scale measurements are compared, and general conclusions and suggestions for possible future work are made.

15 2 Theoretical background

2.1 Stress corrosion cracking 2.1.1 Definition and phenomenology The cracking of metals below their elastic limits in an aggressive environment is known as environment assisted cracking. According to [36], this term includes the phenomena of SCC, corrosion fatigue and hydrogen embrittlement.

SCC can be defined as the formation (often at stress levels below σy) and sub-critical growth (at stress intensity factors well below the fracture toughness in air) of cracks in a material under the simultaneous and synergistic interaction of approximately constant mechanical tensile stress and a corrosive environment. It is considered to be the most important, as well as the most dangerous type of corrosion, and causes major economical losses. It is a synergistic process of mechanical and chemical factors, which causes damages that would not occur under load alone in absence of an aggressive electrolyte and susceptible material. The stress can be externally applied shear or tensile stress, or residual stress in the material, induced by mechanical (cold work) or thermal treatment. SCC often affects metals and alloys that are protected against uniform corrosion by a passive layer. In fact, SCC occurs in certain well known combinations of materials and chemical environments. As illustrated by the presented example of SCC on brass cartridges exposed to an ammonia rich atmosphere, SCC occurs under very specific conditions, when a susceptible material is subjected to (internal or external) stress and is exposed to specific environmental conditions (chemical species, temperature, humidity). To sustain crack growth, corrosion must proceed at the crack tip, while the crack walls remain passive. Important material environment combinations causing SCC are shown in Table 1. Depending on material and environments, IG or transgranular (TG) crack propagation is observed. Corrosion fatigue describes a reduction of fatigue initiation life and an accelerated fatigue crack growth in a material under cyclic or fluctuating mechanical stress in a corrosive environment. Contrary to SCC, corrosion fatigue occurs over a broad range of environmental, material and loading conditions. This phenomenon is not considered further in this study.

16 Table 1: SCC susceptible material-environment combinations (taken from [37]).2 Material Environment Concentration3 Temperature3 Mode4 Carbon steel Hydroxides High High IG Nitrates Moderate Moderate IG Carbonate/bicarbonate Low Moderate IG Liquid ammonia Low TG CO/CO2/H2O Low TG Aerated water Very high TG Low-alloy steel Water Moderate TG (e.g. Cr-Mo, Cr-Mo-V) High strength steels Water Low Mixed Chloride Low Mixed Sulphide Low Mixed Austenitic stainless steels Chloride High High very TG (incl. sensitised) Hydroxide High High Mixed Sensitised austenitic Aerated water Low Very high IG stainless steels Thiosulphate or polythionate Low Low IG Duplex stainless steels Chloride High Very high TG Martensitic stainless Chloride + H2S high High Moderate TG steels Chloride ( usually + H2S high) Moderate Low TG High-strength aluminium Water vapour Low Low TG alloys Chlorides Low IG Titanium alloys Chlorides High Low TG Methanol Low TG N2O4 high Low TG Copper alloys (excluding Ammoniacal solutions and Low Low IG Cu-Ni) other nitrogenous compounds Nitrate Moderate Low TG Cupric sulphate Moderate Low TG Nickel-based alloys Water Very high IG Caustic High Very high IG Pb2+ Very high IG Chloride + H2S High Very high IG

Hydrogen embrittlement describes quasi-brittle fracture of higher strength materials, which can occur with relatively low hydrogen concentrations [38]. Atomic hydrogen in metals has a small solubility and large mobility, a high molar partial volume, and a large electronic interaction, strongly influencing the surrounding metal atoms. Hydrogen tends to segregate at trap centres such as impurities, grain boundaries or regions of high hydrostatic stress, and it reacts in a variety of (sometimes contradictory) ways with metals and alloys. The most important effect of dissolved

2 The table presents the systems for which SCC problems are well established and of practical importance. The absence of a metal-environment combination does not mean that SCC has not been observed. 3 There are rarely well-defined temperature or concentration limits for SCC, and the ratings given here are indicative only. As an approximate guide the terms used equate to the following ranges of values. Low: up to 10-2 M, ambient temperature; moderate: up to 1 M, below 100 °C; high: ~1 M, ~100 °C; very high: near saturation, above 100 °C. Significantly increased local concentrations may be obtained under the influence of local boiling or evaporation, or by accumulation in pits and crevices, and cracking is often obtained for nominal concentrations that are much lower than is indicated here. 4 The fracture mode is classified as IG, where cracks go along the grain boundaries, TG where cracks go across the grains, or mixed where there is a combination of the two modes, or where the mode can vary depending on the conditions. There are often circumstances that can cause the fracture mode to change (e.g. chloride SCC of sensitised austenitic stainless steel may give IG cracking).

17 hydrogen on the mechanical properties of steels is the production of brittle fractures where normally ductile behaviour would be expected [38]. Different theories explain this effect, and not all predict an embrittlement: The hydrogen-enhanced localised plasticity mechanism explains crack propagation by highly localised slip due to local softening by hydrogen at the crack tip. The hydrogen-enhanced decohesion model explains hydrogen embrittlement by a weakening of interatomic bonds in the metal, allowing brittle fracture. Other possible effects of hydrogen are the formation of brittle hydrides (e.g. in titanium alloys), the reaction of hydrogen with certain alloy components (e.g. reaction with iron carbide in steels and formation of methane), or the recombination of hydrogen in voids and other micro structural defects, resulting in very high overpressures, which can assist cracking. Hydrogen embrittlement can accompany SCC at the crack tip, and the discerning of both phenomena is not always possible. In these cases, hydrogen embrittlement can be considered as one possible mechanism of SCC.

2.1.2 Mechanism of IG SCC of sensitised stainless steel Several mechanistic models have been developed. No single model can explain all observed phenomena, and it seems likely that several different mechanisms account for SCC in different systems. This section introduces mechanistic aspects that are currently discussed for IG SCC of thermally sensitised austenitic stainless steel. As indicated in Table 1, these materials are susceptible towards IG SCC at room temperature, when exposed to certain sulphur species. The susceptibility towards IG attack depends on the properties of a grain boundary, i.e. grain boundary chemistry, misorientation, and precipitates. The effect of grain boundary geometry on IG crack growth is e.g. presented in [39], for different austenitic materials. It is shown that twin boundaries are crack-resistant, and the intersection of a twin with a random boundary may act as a barrier to crack advance. No statistically valid conclusion could be made, if any other grain boundary geometry had a beneficial effect on the resistance towards cracking, and it is emphasised that chemical characteristics must be considered as well. Grain boundary chemistry of austenitic stainless steels highly depends on thermal treatment: At temperatures between 500 – 700 °C the formation of chromium-rich carbides (Cr23C6) along the grain boundaries is observed [34, 40-42]. This “sensitisation” leads to a chromium depletion of a narrow seam along the grain boundaries, and renders the grain boundaries vulnerable towards corrosion attack if the chromium content is below a critical value (~12% Cr). Extended experiments and theoretical considerations on the chromium depletion theory are described in

18 [42], and it is concluded that this theory is capable of interpreting all effects related to IG corrosion. Other theories for IG corrosion are rebutted. A model to predict the extent of grain boundary depletion is described in [41]. Diffusion processes of chromium at the grain boundary towards growing carbides and the evolution over time of chromium concentration are analysed. A distinction is made between chromium concentrations at the interface between bulk metal and carbides, and the concentration at points between adjacent grain boundary carbides. These concentrations can be different, and it is concluded that both have to be lower than a critical value for sensitisation to occur.

Crack initiation SCC is usually split up into an initiation and a propagation stage. This differentiation is not well defined [43]. For a technical/engineering definition, “crack initiation” can be based on detection limits, or on a minimum crack length related to the spacing of some metallurgical features, e.g. grain size. Andresen proposed a more scientific definition [44]: He defines crack initiation as the “formation of a mechanical distinct geometry that will tend to grow in preference to its surroundings”. An interpretation of SCC initiation as a sequence of different phases, as proposed by Staehle [45], is shown in Figure 1.

Figure 1: Possible definition of SCC initiation phases according to Staehle [45].

A number of typical initiation sites are documented, which can lead to the formation of such a distinct geometry: The origin of a single crack is usually some kind of weakness or irregularity on a sample surface. Slip steps, pits, machining defects or microstructural heterogeneities, e.g. (sulphide) precipitates, which are intersected by the surface, are possible initiation sites. If a material is susceptible to IG corrosion, SCC can be initiated at attacked grain boundaries. Crack

19 initiation is therefore strongly dependant on the surface state. Surface roughness and structure, as well as residual stress or cold work, which is introduced during fabrication or grinding can cause crack initiation as well. A mechanically induced rupture of a passive film can also serve as an initiation point. Crack initiation at pits can occur, when a high enough stress concentration is reached at the bottom of the pit. Trethewey mentioned in [46] that stress concentration at the pit base is more concerned with the shape of the pit than its depth: If dissolution at the pit walls continues at the same speed as at the pit base, the pit will grow and maintain a hemispherical shape, which will not increase the stress at the base much. For crack initiation to occur, the corrosion at the pit bottom has to accelerate, while pit walls repassivate. In general, SCC initiation mechanisms are more difficult to investigate than crack propagation: Crack initiation often exhibits long initiation times and strong variation between samples. A high number of time consuming experiments are necessary to produce statistically valid results. Crack propagation on the other hand can be studied on pre-cracked samples, hence bypassing the initiation time, and the variation of crack growth rates typically shows less scatter than crack initiation.

Crack propagation Two main mechanisms are considered for crack growth: cracks either proceed by preferential anodic dissolution at the crack tip, or by a sequence of brittle fracture events. Both mechanisms are still discussed for the case of IG SCC of austenitic stainless steels. Different variations of the film rupture or slip-dissolution model [47] exist, which explain crack growth by anodic metal dissolution. Cracking by brittle fracture is mostly explained by hydrogen embrittlement [48]. Both mechanisms will be discussed further below, regarding IG SCC of sensitised stainless steels. Other mechanisms, such as the surface mobility model [49] or the film induced cleavage model [50] have been developed, but they find use for systems other than sensitised stainless steel: The film induced cleavage model is typically applied to noble alloy systems, such as Au-Ag or Au-Cu, where SCC proceeds through brittle fracture of de-alloyed surfaces. The surface mobility model explains SCC as a surface diffusion process where atoms diffuse away from the crack tip. It was applied to a wide range of systems. It is stated in [51], that the surface mobility model is inadequate for the description of systems with pre-existing crack paths, such as sensitised austenitic stainless steels. SCC in such cases is often better explained by an anodic dissolution mechanism.

20 Crack propagation by film rupture The film rupture model explains crack growth as a stepwise process: the local rupture of a passive surface film is followed by anodic dissolution of the freshly exposed metal at the crack tip, and finally repassivation of the active crack tip occurs. Metal dissolution proceeds along the sensitised grain boundaries, while the crack walls remain passive, due to their higher chromium concentration. The film rupture model is often used to explain results of slow strain rate tensile (SSRT) tests. Crack propagation rates can then be modelled as a function of strain rate and the fracture strain of the passive oxide layer [52]. The film rupture mechanism requires dynamic plasticity at the crack tip. For crack propagation under CL conditions, this can be due to low- temperature creep. Another explanation is that dynamic strain is caused by crack growth itself [53]: A plastic strain distribution exists ahead of a crack tips, with stress inversely proportional to the distance from the crack tip. Each crack growth step shifts this stress distribution, causing a strain transient in front of the crack. A special case of the film rupture mechanism is the slip-dissolution model, incorporating the effect of slip planes: The periodic breaking of the passive film is explained by local plastic strains at the crack tip, which lead to the formation of slip steps (Figure 2). The shear stress in such a case depends on the orientation of the slip plane and the applied stress.

Figure 2: The slip-dissolution mechanism explains crack propagation by film rupture due to material dissolution along slip planes [36].

As stated by Gutman in [54], the term “slip-dissolution model” is often incorrectly used as a synonym for “film rupture mechanism”. The basic film rupture mechanism does not require the

21 presence of a slip plane, and brittle passive films can already rupture at loads below σy of the base material. Gutman further critically discusses the film rupture model for SCC and points out inconsistencies in certain models for crack growth speed and the rates of initiation and repassivation events. Nevertheless, the film rupture model is widely accepted and is used in many studies on austenitic stainless steels: Roychowdhury et al. studied IG SCC of AISI 304 steel in thiosulphate solutions [55] and explained crack growth by a film rupture mechanism. They stated that a balance between film rupture events and film formation could be established by changing the thiosulphate concentration and the application of potentials. IG and TG SCC of stainless steel under boiling water reactor conditions was attributed to a film rupture mechanism by Arganis-Juarez et al. in [56] (Figure 3). A film rupture mechanism is also proposed to explain the EN signals of IG and TG SCC of stainless steel in thiocyanate solution in [57] (Figure 4): The sudden exposure of bare metal surface leads to immediate metal dissolution, which is indicated by a sudden rise in current, accompanied by a potential drop of the corroding metal. With proceeding repassivation the current signal drops and the potential rises back to the initial value. This results in characteristic current and potential transients. Further details on the EN signals related to SCC are described in Section 2.3.4.

Figure 3: Stress-time curve and EN time series for solution annealed AISI 304 stainless steel at 288 °C, 8 MPa, air saturated water (approximately 4000 ppb oxygen and +200 mV/standard hydrogen electrode, outlet conductivity 0.59 μS/cm. Transients were explained by slip-dissolution, due to their shape and polarity [56].

22

Figure 4: EPN, ECN and displacement rate of a tensile specimen during a slow-rate-load-tensile test in aqueous thiocyanate solution, measured by Leban et al. [57]. Transients are explained by a slip-dissolution mechanism.

Crack propagation by hydrogen embrittlement As explained in 2.1.1, hydrogen embrittlement can be a phenomenon on its own, or it can act as a mechanism of SCC. It explains crack propagation as a series of brittle fractures, induced by absorbed hydrogen in front of the crack tip. The cathodic corrosion half reaction provides the hydrogen by reduction of protons. The locally produced hydrogen then diffuses into the metal. It accumulates in the plastic zone in front of the crack tip, in the region of highest hydrostatic stress. The absorbed hydrogen induces slip steps, finally leading to quasi-brittle fracture events. This mechanism explains highly discontinuous crack propagation. Gomez-Duran and Macdonald postulate a hydrogen embrittlement mechanism for SCC of AISI 304 stainless steel in aqueous thiosulphate solution [58, 59]. They state that the discrete crack propagation events they found during square wave loading experiments are of the size of grain boundaries (35 – 108 µm), and are too large to be explained by slip-dissolution events. Therefore, the detected crack growth steps are better explained by hydrogen embrittlement than by a slip-dissolution mechanism.

23 Effect of aggressive sulphur compounds Most experimental work presented in this thesis was done using acidic or neutral solutions of thiosulphate or tetrathionate. Cragnolino und Macdonald review the effects of various sulphur species on austenitic stainless steel in [60]. A whole class of sulphur species exist, which can induce IG SCC in sensitised steels, e.g. hydrogen sulphide, sulphur dioxide, thiosulphate, polythionates or thiocyanate. These substances can form complex systems, as many of them are not stable in aqueous solution and can react with each other. A variety of disproportionation reactions are described for tetrathionate and thiosulphate:

+ 2− ←⎯⎯ − [60, 61] H + S2O3 ⎯⎯→ S + HSO3 (1)

+ 2− ←⎯⎯ 2− [61] 6H + 5S2O3 ⎯⎯→ 2S + 2S4O6 + 3H 2O (2)

+ 2− ←⎯⎯ 2− [61] 2H + 3S2O3 ⎯⎯→ 4S + 2SO4 + H 2O (3)

2− − ←⎯⎯ 2− [62] S4O6 + 2e ⎯⎯→ 2S2O3 (4)

←⎯⎯ [62] H 2 S2O3 ⎯⎯→ H 2 S + SO3 (5)

←⎯⎯ [62] H 2 S + SO3 ⎯⎯→ H 2O + SO2 + S (6)

2− ←⎯⎯ 2− + [62] H 2O + 2O2 + S2O3 ⎯⎯→ 2SO4 + 2H (7)

It is still under debate, how these sulphur species accelerate SCC. It is proposed, that the metastable sulphur oxy-anions disproportionate in the acidic pH at the crack tip, and atomic sulphur is released (reactions 1 – 3, 6). Sulphur then adsorbs on the metal, where it can have multiple effects [63]: it can catalyse the anodic dissolution of the metal, and it blocks the surface sites required for the adsorption of oxygen and thereby inhibits repassivation. Passive films formed in the presence of adsorbed sulphur additionally exhibit more defects than those formed under sulphur-free conditions. High concentrations of sulphur species may also directly cause the breakdown of a passive film, when the formation of metal sulphides is thermodynamically more stable than the oxide. Considering hydrogen embrittlement, it is known that sulphide species, which are adsorbed to the metal surface, can accelerate the absorption of hydrogen [2, 64]. Furthermore, the inhibition of repassivation can further facilitate hydrogen uptake. The observed acceleration of IG SCC by sulphur compounds therefore does not dismiss or favour one of the two mechanisms.

24 2.1.3 Loading methods The type of applied stress and the specimen shape highly influence the outcome of any SCC test. An overview of different specimen designs and testing procedures is given in [65]. The applied stress can either be static or dynamic, where “dynamic” usually means the application of slow strain rates.

Static loading Historically, most SCC testing was done under static loading. Static loads can be applied easily by bending a suitable specimen, either applying a constant displacement or CL to a sample. Constant displacement tests can suffer from relaxation and exhibit a decrease in stress with propagating cracks. In CL tests, on the other hand, stress increases with growing cracks, due to a reduction of the sample cross section. Typical specimens are bent beams, U-bend specimens or C-rings [65]. Depending on the cell design, the stress values may vary significantly over the surface of a bent sample, and for all bent specimens the stress varies through the sample profile. Statically loaded specimens can be tested in the elastic or plastic region: The advantage of elastic straining is that the magnitude of the applied stress can be calculated from the measured strain and modulus of elasticity. Plastic strain tests are generally faster due to the more aggressive loading conditions, but the stress conditions are not known precisely. For experiments dealing with the monitoring of SCC initiation and propagation, systems under static loading can be difficult to use, as time to crack initiation may be extremely long, and vary between samples. Kovac et al. describe the monitoring of IG SCC of austenitic stainless steels with a combination of EN, acoustic emission, digital imaging and specimen elongation under CL in [66]. In these experiments, it was necessary to increase the load several times during a measurement, which caused transients in the EN signals (marked regions at 15 h and 25 h in the blue and red curves in Figure 5). Crack initiation occurred directly after such a loading step. The following crack growth could then be monitored by all applied techniques, although increased acoustic emission activity was detected only after a crack reached a certain length (Figure 5, purple curve).

25

Figure 5: Monitoring of IG SCC under CL, using elongation of specimen, EN and acoustic emission. EN transients occurred, when the load was increased stepwise at t = 15 and 25 h. Crack growth set in after the second increase, clearly detectable with all applied measuring methods [66].

Dynamic loading Dynamic loading is generally a more severe testing method than static loading, and it can produce SCC in a reasonable time frame in systems where static loading cannot. This allows the use of less aggressive environments. Constant extension rate tensile (CERT) tests are commonly performed in the study of SCC on the laboratory scale. In these tests, slow strain rates are applied. A too high strain rate may lead to ductile fracture without any contribution of SCC. Typical rates range from 10-7 to 10-5 s-1. A drawback of CERT tests is that the application of slow strain rates needs more sophisticated testing equipment. These setups allow the application of uniaxial tensile stress, which leads to a uniform stress over the cross section of a sample.

2.2 The electrochemical microcapillary technique 2.2.1 Overview of microelectrochemical measuring techniques Corrosion processes are mainly studied on a macroscopic scale. Typical laboratory tests are performed on samples in the region of mm2 – cm2. But corrosion can be a highly localised phenomenon. The electrochemical behaviour of two surface spots at a distance of a few µm can be totally different, depending on the microstructure of the metal. Furthermore, the miniaturisation of electronic and mechanical devices creates microstructures, which cannot be

26 studied with macroscopic methods. To perform electrochemical measurements on this scale, the application of microelectrochemical methods becomes necessary. One group of micro scale methods are scanning techniques [67]: Macroscopic surfaces are immersed in an electrolyte, and micro- or ultramicroelectrodes are used to scan the surface and measure inhomogeneities in potential or current density. A range of methods, based on this concept, have been developed, e.g. scanning vibrating electrode technique [68], or the local electrochemical impedance spectroscopy [69]. The lateral resolution of these techniques depends on the conductivity of the electrolyte. The main drawback of these methods is that they do not allow the selective polarisation of a small surface region. A combination of a scanning technique and a small area technique (discussed below) is the free droplet method [70]: An electrolyte droplet, hanging freely on the tip of a capillary is brought into contact with the sample surface. This technique can be applied on non-wetting surfaces, e.g. most electropolished metal surfaces. Capillary diameters typically range from a few µm to some 100 µm. The dimension of the wetted area is in the same order of magnitude. The electrolyte droplet can be moved over the surface during a measurement, therefore surface scanning is possible. The technique is suited for very sensitive surfaces, as no mechanical stress is applied by the capillary. The scanning droplet cell was used to study the different passivation potentials of various polycrystalline materials, depending on grain orientation (Zn: [71]. Zr, Ta: [72]. Au: [73]). A drawback of this technique is the constant evaporation of electrolyte at the droplet and resulting concentration build up. The second group of microelectrochemical methods are the small area techniques: In these methods the surface area exposed to the electrolyte is reduced to the region of µm2 or even nm2 in the case of nanoelectrochemical techniques. This allows the polarisation of a limited surface area and the investigation of local corrosion behaviour. The localisation of corrosion processes on a small surface area allows a direct correlation of measured corrosion currents and the corrosion attack observed after an experiment. Different small area techniques are applied: A common method is to reduce the size of the working electrodes, e.g. by using embedded metal wires as electrodes [74-76], or by masking techniques [77-80]. Early techniques used micro hardness imprints to damage isolating surface films and thereby create micro spots of exposed metals [77]. More sophisticated coating techniques have been developed, and advanced electrochemical cell designs allow the combination of electrochemical and optical monitoring of micro spots, as demonstrated by Chiba et al. [81]. The electrochemical microcapillary technique, developed at ETH Zürich and further improved at EMPA Dübendorf [82-84], uses silicone coated glass microcapillaries as electrochemical cells.

27 The capillaries containing the electrolyte are positioned on a working electrode, exposing only an area of the size of the capillary opening to the electrolyte. Reference and counter electrode are connected to the wide end of the capillary. This cell design allows a reduction of the exposed surface to a few µm2. Installation of the microcell in a modified socket of a light microscope allows precise positioning on the surface. It is possible to precisely perform measurements on single surface features (e.g. grain boundaries, inclusions, precipitates) and study the effects of these surface features on the local anodic behaviour of the bulk metal. When a three electrode setup is used, all common electrochemical measurements can be performed, e.g. potentiodynamic and potentiostatic current measurements, open circuit potential (OCP) measurements or cyclic voltammetry. The capillary tips are coated with a soft silicone rubber to create a flexible seal, which prevents leaking even on rough surfaces. (see Figure 6) The silicone is hydrophobic in nature, which reduces the tendency of crevice corrosion to occur. The silicone coatings also protect sample surfaces from damage by the glass capillary5. Many of the specific features of the microcapillary technique rely on the properties of the silicone coating at the tip, which makes it a key point for reproducible and meaningful measurements.

Figure 6: A 100 µm microcapillary with silicone sealant. a) Surface quality of the silicone sealant; b) Deformability of the silicone sealant [84].

2.2.2 Features of the electrochemical microcapillary Microcapillary measurements show significantly less background noise than macroscopic measurements, due to the reduced number of EN sources present on a studied surface spot. The

5 Soft metal surfaces, e.g. aluminium alloys can be damaged, when uncoated glass capillaries are pressed on the surface. This can even happen with coated capillaries, if the coating is thin and if the capillary is pressed on the surface too hard, or if a very sensitive surface is studied.

28 reduction of the surface area, together with the use of a specially developed high resolution low noise potentiostat, allows the detection of single events in the fA-range. Low pass filtering of the signals further reduces noise. A Faraday cage and screened cables protects the signal from interferences. The microcapillary is a stationary measurement, as opposed to scanning techniques like the scanning droplet method. However, by performing consecutive measurement in an array pattern on a surface, a discontinuous step-by-step scanning of a surface is possible [82, 85]. The incorporation of microsensors into the microcapillary cell allows monitoring of additional experimental parameters. The possibility of simultaneous potentiodynamic current measurements and pH measurements was demonstrated in [84]. A main feature of the microcell technique compared to other small area techniques is its applicability on a wide variety of surfaces. Small area techniques usually either require complicated sample preparation (surface masking) or they are limited to very specific samples (e.g. embedded wires). The microcapillary technique uses a different approach and reduces the size of the electrochemical cell. Sample preparation is the same as for a macroscopic measurement or even simpler. Measurements can be performed on a variety of polished or rough surfaces, as long as the specimen fits in the setup. The silicone seal of the capillary serves as the outer boundary of the investigated working electrode. It is not necessary to apply any covering lacquer on the sample to prevent crevice corrosion. 2.2.3 Applications of the electrochemical microcapillary technique Local OCP and pitting potentials The microcapillary has been used to study the initiation of pitting corrosion at MnS-inclusions in AISI 304 stainless steel in [86]. Potentiodynamic measurements were performed on single inclusions. It was shown that pitting behaviour depended on the geometry of each inclusion: Shallow inclusions showed dissolution, accompanied by metastable current transients. On narrow, deep MnS-inclusions, the metastable events were followed by stable corrosion. SEM images of attacked inclusions revealed that corrosion proceeded mainly along the edges of the inclusions. This selective dissolution led to the formation of micro crevices. The microelectrochemical investigation of pit initiation on Al2024 is described in [87]. The pitting potential of various intermetallic particles was identified, and micro cracks in the bulk material were found to be initiating points for pitting. Further studies on aluminium alloys and high purity aluminium are described in [88-95], illustrating the potential of the microcapillary technique to study localised corrosion phenomena and identify the influence of different intermetallic particles on corrosion behaviour.

29 Influence of stress The microcapillary technique has also been applied to study the influence of surface stress on the initiation of pitting [23-25]. Similar to the work described in [86], pitting of 304 stainless steel in 1 M NaCl was studied in [23] as a potential initiation step for SCC. Microcapillary measurements were performed on stressed and unstressed stainless steel samples to evaluate the influence of applied stress on the pitting behaviour. A three point bending cell was used to apply constant deformation to the sample. It was found that certain shallow MnS-inclusions would lead to metastable pitting on unstressed surfaces, but could lead to stable pitting on stressed surfaces. With increasing stress, the dissolution rate of MnS-increased and the dissolution potential was shifted to more negative values. The influence of residual stress on pitting sensitivity was studied by Oltra and Vignal in [24]. Microelectrochemical measurements were combined with stress mapping on a resulphurised AISI 316L stainless steel. Localised electrochemical measurements allowed a correlation of pit initiation and stress gradients. Krawiec et al. investigated the influence of stress on the local electrochemical behaviour of an AlCu4Mg1 aluminium alloy [25]. After the application of plastic strain, surfaces showed high densities of micro cracks, especially in precipitates, along matrix/precipitate interfaces and along the grain boundaries. It was found that pitting potentials of stressed surfaces were systematically lowered compared to unstressed surfaces. Stressed surfaces appeared to be highly heterogeneous: The most active surface spots corresponded well with the location of extended micro cracks and surface damages.

Microelectronics Local electrochemical surface conditions on micro engineered parts are important quality factors: The quality of insulating layers or the purity of conducting lines can be studied with the electrochemical microcapillary technique. Such an application in microelectronics is shown in [84]: Microcapillary measurements on Au lines showed lower overpotentials on the edges of a line, compared to the centre, indicating a higher concentration of impurities.

In summary, the electrochemical microcapillary has been successfully used to study differences in local electrochemical behaviour on several systems. The technique was especially useful for measurements of OCP and pitting potentials on different intermetallic particles and surface defects, on both stressed and unstressed samples. It was also possible to study the initiation

30 mechanism of stable pitting corrosion. So far, the technique has not been used to track the electrochemical signals of single growing stress corrosion cracks.

2.3 The EN technique The first application of the EN technique for monitoring of corrosion was published by Iversion in 1968 [96]. In an earlier study, he observed the formation of “hollow whisker” on metals exposed to acidic solutions of potassium hexacyanoferrate (II) and (III), which indicated that bursts of ions were formed on point anodes on the metal surfaces. As these bursts appeared sporadically, he concluded that it must be possible to measure the potential or charge fluctuations created during formation of these whiskers. Using a high impedance voltmeter, connected to a chart recorder, he measured the potential fluctuations between a platinum foil and various metal sheets, immersed in different electrolyte solutions. Since this early work, EN measurements have been widely used to study the corrosion behaviour of a variety of metal/electrolyte combinations. Better measuring equipment with smaller instrument noise has allowed measuring EN of passive systems and even under extreme experimental conditions, e.g. high purity water at temperatures and pressures common for nuclear reactors. New data analysis methods have been developed, and methods for automated online monitoring are being developed. In this section, different measuring and data analysis methods of EN measurements are introduced, and examples of the successful application of the EN technique for corrosion monitoring in different environments are summarised.

2.3.1 Definition EN describes electrochemical current and potential fluctuations, which are produced on an electrode undergoing an electrochemical reaction. EN occurs naturally, and does not need any external stimulation. The nature of these fluctuations depends on the ongoing anodic and cathodic processes on the electrode surface. EN measurements record these fluctuations, which the corroding system constantly produces by itself. EN signals can then be analysed to gain information on the ongoing corrosion processes. EN sources in corroding systems are small, stochastic events that cause sudden charge bursts and change the electrode potential [97]. Typical examples are local oxide film breakage, subsequent metal dissolution and repassivation during pitting corrosion, or the slow formation and sudden detachment of a hydrogen bubble from a metal surface. A general theoretical description of EN has not yet been established. However, there exists an analysis for the special case of shot noise [97-99]. Shot noise analysis can be applied when a

31 corrosion signal consists of discrete events, such as the current transients of metastable pitting. Each transient provides a certain charge to the power of the signal. This charge value and the number of transients over time define the total power of the signal and can be calculated from EPN and ECN signals [99]. For shot noise to be applicable, it must be assumed that single corrosion event are uncorrelated, and the system under investigation is stationary. The shot noise theory therefore does not fully explain typical corroding systems, in which these assumptions usually are not valid. Depending on the system, EN signals are usually in the range of µV to mV, and nA to µA. An example of a single transient, which was detected during an experiment in the course of this thesis, is shown in Figure 7. The current signal shows a rapid rise of 55 nA, lasting approximately 3 min before the signal drops to the initial level. The potential shows a rapid drop of 150 mV, until the current stops flowing. The slow potential rise back to its initial value indicates the recharging of the double layer capacitance. This transient illustrates that the current is the primary parameter, and the potential just responds to the local corrosion currents [99].

Figure 7: Typical EPN (green) and ECN (red) transient, detected on sensitised stainless steel in a 0.01 M potassium tetrathionate solution at pH 2.2, at room temperature. The fast current peak indicates a growth step of a micro crack or formation of a metastable pit.

2.3.2 Measuring methods ECN and EPN can be measured separately or simultaneously. If only EPN is measured, it can either be measured between two equivalent electrodes or between one working electrode (WE) and a low-noise reference electrode (RE). Similarly, ECN is either measured between a working electrode and counter electrode (CE) under potentiostatic control, or it is measured between two equivalent electrodes. In this case, both electrodes may corrode, and the source of a single event

32 can be deduced from the transient direction/polarity. When EPN and ECN are measured simultaneously on a freely corroding system, a three electrode setup is applied. ECN is measured between working and counter electrode, which are at the same potential. Their potential is measured against the reference electrode. Possible configurations are shown in Figure 8 [100].

U

I

galvanostat U potentiostat RE

WE 1 WE 2 WE 3 RE R ¨ WE 2 WE 1 RE amplifier

I WE 2 WE 1 computer amplifier amplifier

computer computer (a) (b) (c) Figure 8: EN measurement principles: a) EPN measurement under galvanostatic conditions; b) ECN measurement under potentiostatic conditions; c) ECN measurement (I) without external polarisation and possibility to measure EPN (U) simultaneously [100].

The measurement of both ECN and EPN signals allows the calculation of noise resistance Rn or noise impedance Zn (see Section 2.3.3). Eden stated in [101], that many problems in simultaneous ECN and EPN measurement can be solved if a setup with three identical electrodes is used. The advantages of this setup are that no Haber-Luggin capillary has to be used, which eliminates possible sources of contamination and the impedances of the electrodes are (nearly) identical.

With identical impedances it is actually possible to calculate Zn from the data. For certain applications, it is not possible to use two identical electrodes for ECN measurements. This is the case when corrosion is limited to one electrode, e.g. during typical SCC experiments, where only one electrode is stressed. Under these conditions, it is not possible to obtain the properties of one single electrode, because a system with asymmetric electrodes contains too many unknowns [102, 103]. Another important point in cell design is the surface area of the electrodes, because the ECN power is proportional to the area, while the EPN power is inversely proportional to the area. The electrode area also determines the number of corrosion sites that can be monitored. The influence

33 of the electrode area, as well as other parameters, such as measuring frequency and experiment duration, are discussed in [104]. A critical factor in EN measurements are external interferences. A typical interference is the 50 Hz power line hum. Low-pass filtering of the measured data is therefore always necessary. The maximum frequency that can be analysed, called Nyquist frequency, is determined as one half of the scanning frequency. Any frequencies above this value cannot be detected and might cause aliasing effects, if not filtered out. Other interferences may be caused by vibrations or electromagnetic field fluctuation in the proximity of the measuring setup. To avoid the pick up of external interferences, EN measurements should be performed in a Faraday cage, and cables between the data processing unit and the electrodes should be properly shielded and kept as short as possible. Further interferences may arise from the formation of ground loops: Ground loops are created when elements of an electrical circuit (or electrochemical cell) are grounded through multiple connections. Potential differences between the different ground connections can then induce current flow through the circuit. To avoid ground loops, the whole experimental setup should be connected to ground at a single point.

2.3.3 Data analysis The main aim of the analysis of EN data is the identification of rate and type of corrosion processes. While the measurement of EN signals is basically straightforward, the interpretation and analysis of EN data is a rather complex task. No standards exist, and a multitude of different methods for EN analysis are available: Visual examination and peak counting, statistical methods of the time domain data as well as transformation to the frequency domain have all been used and proved their usefulness for certain systems. It is not yet decided which parameters are suited best to analyse EN data, and often a method that works well in one study gives only poor results in a different case. An overview of the different possible methods is given in [99]. Some of these methods shall be introduced:

Visual analysis: Direct interpretation of transients The most straightforward method and at the same time a very powerful approach to analyse EN data is the direct examination of signals in the time domain. EN signals of localised corrosion phenomena often exhibit characteristic transients. The analysis of transient shapes, amplitudes and distribution over the time record can help discerning between different corrosion processes. Transient shapes are intensely investigated to understand the underlying mechanisms of different corrosion reactions [105, 106]. The visual analysis also directly reveals trends or long term

34 fluctuations, which especially occur in EPN measurements, and it often reveals signal interferences. The downside of this analysis technique is that it is usually difficult to automate.

Statistical methods A further possibility to analyse EN data are basic statistical methods. In these calculations, EN measurements are treated as sequences of unrelated points. Detailed description of statistical parameters can be found in [99]. Among the most often used parameters are: Mean: The mean is easily understood as the average signal value. For the current I, the mean is calculated as follows: t 1 max mean = I = ∫ I(t)dt (8) tmax 0 Standard deviation, variance and root mean square: These parameters are related and basically give information on the extent of signal fluctuations. For statistical signal analysis in this thesis (Section 5.4.1) the standard deviation was used. The standard deviation of a current signal is calculated as follows:

tmax 1 2 std. dev. = ∫ (I(t) − I ) dt (9) tmax o

By dividing the standard deviation of the EPN by the standard deviation of the current, Rn can be calculated. Rn can be used to estimate the corrosion rate of a system, similar to the polarisation resistance. Skew and kurtosis: These are nondimensional higher-order statistical parameters. The skew is a measure for the shape of a distribution of values. The kurtosis is a measure of the shape of a distribution relative to the normal distribution. A weakness of the statistical methods is the indifference towards baseline trends and changes of the signal characteristics over time. Leban et al. stated in [107], that the non-stationarity of most corrosion processes renders analysis with the mentioned statistical tools unreliable. While passive state, localised corrosion and uniform corrosion could show significant differences in the estimated parameters, differentiating between different kinds of localised corrosion seemed not possible. It is further proposed that mathematical techniques to improve a signal stationarity can lead to incorrect interpretations.

35 Analysis in the frequency domain EN data is commonly analysed in the frequency domain. The most prominent frequency-based technique is the estimation of power spectral densities (PSD). PSD plots show the distribution of the signal power as a function of frequency. The units of the PSD are V2/Hz or A2/Hz. The two methods commonly used for the estimation of power spectra are the fast Fourier transformation (FFT) and the maximum entropy method. For the calculation of the FFT, the following equations are applied [108]:

∞ −2iπf t X ( f ) = ∫ x(t)e dt (10) −∞ The corresponding PSD is:

2 2 Ψ ( f ) = X ( f ) (11) x T T The equations for the transformation in the maximum entropy method can be found in [99]. The frequency range that can be analysed with these methods is limited by two factors. The upper limit is defined by the Nyquist frequency. The low frequency limit of PSD spectra is determined by the duration of the measurement. However, in real corroding systems, the non-stationarity of many corrosion processes limits the usefulness of the frequency domain analysis of very long measurements. Typical EN power spectra consist of a plateau at low frequency values, followed by a power drop in proportion to 1/fn. The roll-off slope was expected to be characteristic of the type of corrosion occurring, but so far, analysis of the roll-off slope has often been found to be an unreliable parameter [97]. When ECN and EPN are simultaneously recorded, the potential PSD can be divided by the current PSD for each frequency, to derive a value with the unit of Ω2. The square root of this value is known as the noise impedance Zn. Just like Rn, it can be used to estimate the corrosion rate. Zn is related to conventional electrochemical impedance, and it can be used to estimate the AC impedance spectrum of a system. This application is limited by the low signal power above 1 Hz, which is typically encountered in EN measurements, but noise measurements can provide very low frequencies, which might offer an advantage over conventional AC impedance measurement.

2.3.4 Application of the EN technique to monitor SCC The use of EN measurements to monitor SCC processes on the laboratory scale has been demonstrated in several cases [56-59, 66, 107, 109-113], many of them using stainless steels as a

36 model system. SSRT tests were predominantly performed to ensure a reproducible initiation of SCC. Fewer studies were done on experimental systems using CL. Various electrolytes and electrode materials were used by several groups of scientists to measure EN signals of passive phases, pitting corrosion and SCC. Only a few examples are given in the following paragraphs, and some examples have already been presented in earlier sections: EN measurements under simulated boiling water reactor conditions are presented in [56] (see also

Figure 3). Transients, appearing above the σy, were attributed to slip-dissolution events. A transition from IG to TG SCC to ductile fracture was monitored. Further studies in boiling and pressurised water reactor conditions are presented in [112] and [113]. IG SCC of sensitised 304 stainless steel, exposed to thiocyanate solutions was studied in [57] (see also Figure 4). Different transient shapes were detected during SSRT tests: Fast, sharp transients were interpreted as fracture events of metal ligaments. Slow current fluctuations were interpreted as crack propagation along the grain boundaries, according to a slip-dissolution mechanism. Cyclic loading/unloading experiments on pre-cracked specimens of the same material are described in [58, 59], and a hydrogen embrittlement mechanism was proposed for IG SCC of austenitic steel in thiosulphate solutions. Different corrosion processes are directly compared in [107]. Statistical and spectral analysis techniques showed significant differences between passive state, localised and uniform corrosion, but it was not possible to discern the different localised corrosion processes. The reason for this shortcoming was attributed to the non-stationarity of the system. SSRT tests on sensitised 304 stainless steel in aqueous thiosulphate solutions are described in [35] and [109]. Single current transients were attributed to the rapid crack propagation along single grain boundaries. Crack growth is assumed to arrest at grain junctions, where the growth direction changes. Crack tip dissolution widths and current densities were estimated from transient analysis, calculating the amount of metal that was dissolved during crack growth. Furthermore, a jumping probability is introduced in [109], and a model for crack propagation rate based on this jumping probability is proposed. The differentiation of localised and uniform corrosion processes is also presented in [110], for Alloy 690 that was subjected to SSRT tests at 90 °C in a thiosulphate electrolyte: EN analysis showed current peaks with high intensity and low frequency during SCC, while passive or uniformly corroding samples showed signals of lower intensity, but higher frequency. It is concluded that EN is a powerful method to detect initiation and propagation of SCC, but it is difficult to identify a specific mechanism based on the signal shape. Interrupted SSRT tests are described in [111]: Using 304H stainless steel and a thiosulphate electrolyte, repeated SSRT cycles were applied and interrupted shortly after reaching σy. Fast transients were interpreted as

37 single rupture events, and a rise in baseline current indicated continuous metal dissolution of grain boundaries. It was found that crack propagation stopped during phases of static strain, unless stable macro crack growth had already started. A comparison of these studies shows that EN signals often exhibit characteristic singular transients, indicating stepwise cracking events. These transients typically occurred when certain loading conditions were applied, e.g. right at a stepwise change of the applied load, or during dynamic straining phases above a certain load level. Current and potential transients usually lay in the region of nA-µA and mV, respectively. Frequency and amplitude as well as baseline signal noise varied considerably between the studied systems. This variation is easily understandable, as different sample materials, surface preparations and environmental as well as loading conditions were studied. Hence, single transients are generally not directly comparable between different studies.

Combination of EN with other monitoring techniques EN signals are often difficult to analyse, and it is normally not possible to unambiguously correlate a specific EN signal pattern or transient to a certain corrosion process or a particular stage of the underlying mechanism. An important step towards the understanding of EN signals and the application of reliable monitoring systems is the combination of EN measurements with other monitoring techniques: The combination of EN measurements with in situ atomic force microscopy on corroding samples of coated stainless steels samples is described in [114]. The growth of metastable pits could be observed with in situ atomic force microscope scans and compared to characteristic EN signals. Depending on the coating metastable pit generation occurred with different rates. However, the two measuring techniques were not simultaneously performed on the same sample. In recent years, the combination of EN with acoustic emission was applied for the monitoring of IG SCC. Several studies describe this approach [66, 115-117]. SCC initiation and its early stage propagation could be detected by EN measurements, whereas the acoustic emission technique detected rapid crack propagation events, which caused relatively large plastic deformation of a specimen. In [66], EN and acoustic emission measurements are further combined with simultaneous elongation measurements (see also Figure 5) and digital image correlation, creating a comprehensive detection and monitoring system for SCC.

38 2.4 Conclusion The phenomenon of IG SCC of thermally sensitised stainless steel is basically well understood. Cracks are known to propagate along the sensitised grain boundaries, and crack initiation occurs on well known weak points on a surface. However, it is still very difficult to predict, where and when crack initiation will occur. Even for a well studied system as AISI 304 stainless steel in aqueous thiosulfate solutions at room temperature, numerous studies, by means of EN measurements or other methods, could not yet definitely determine the mechanism responsible for crack propagation. Film rupture mechanisms are widely accepted, especially for experiments where dynamic loads are applied, but the theory is incomplete, and the cause of the film rupture and repassivation events is still unclear. Some researchers also suggest hydrogen embrittlement as the dominating mechanism. The present study may not definitely answer all of these open questions, but the combined use of EN measurements and microcell experiments might clarify some points for the studied system. The existing data from microcapillary studies on AISI 304 stainless steels so far mostly covered the initiation of pitting corrosion. Experiments were predominantly performed in chloride containing electrolytes. The study of single micro cracking events is a new application of the technique and, if successful, might trigger further studies on similar systems. However, it has to be demonstrated first, that the initiation of a crack under the microcapillary is possible and reproducible. EN measurements on stressed samples are well established on the laboratory scale, and various methods for data analysis are available. In general, EN measurements allow differentiating between passive behaviour, uniform corrosion and localised corrosion. The identification of specific types of localised corrosion has been reported in several studies, but results seem to be very specific for the applied experimental conditions. No universally valid criteria for signal interpretation exist. The monitoring of IG SCC subjected to SSRT tests in thiosulfate solutions is documented in several studies. This system is well suited for EN measurements, because IG SCC can be initiated in a controlled time span and little additional corrosion processes occur. Less data is available for EN measurements under CL conditions, as crack initiation can exhibit very long incubation times, making laboratory experiments tedious and unpredictable. In the planned twofold approach, the need to perform experiments on both macro and micro scale under comparable conditions, require either CL or constant deformation. This raises the question, if crack initiation can be observed on sufficient samples, and how the EN signals of crack propagation under CL conditions will differ from the transients observed under dynamic loading.

39 3 Experimental procedures and development of an

experimental system

3.1 Material All samples were machined from a rod of austenitic stainless steel (Deutsche Industrie Norm DIN 1.4301, UNS S30400). The material chosen for this study was selected to be suitable for the study of IG SCC initiation: The steel had a high carbon content (0.06 wt.%) to ensure good thermal sensitisation properties. The selected steel showed a sulphur content of 0.029 wt.% and a high number of MnS-inclusions. The chemical composition is shown in Table 2. The material was solution annealed at 1050 °C and water quenched. The material exhibited a mean grain size of

30 - 100 µm and a hardness of 150 HV15. The yield stress σy and the ultimate tensile strength of the material were 291 MPa and 686 MPa respectively (at 25 °C). For further heat treatment the material was split up in different batches. One batch was kept as solution annealed reference material. The other batches were thermally sensitised at 620 °C for 1 to 24 h. The degree of sensitisation (DOS) for each material was measured by the double loop electrochemical potentiokinetic reactivation test, according to JIS G 05801986:

Samples of each batch were immersed in an aqueous solution of 0.5 M H2SO4 and 0.01 M KSCN at a constant temperature of 30 °C. An Ag/AgCl electrode and a platinum coupon were used as reference and counter electrodes for the following measurement. A potential sweep from - 500 mV/Ag/AgCl to +330 mV/Ag/AgCl with a rate of 100 mV/min was applied (activation), followed by a second sweep in the opposite direction (reactivation). DOS values were calculated from the resulting polarisation curves according to equation 12: i DOS % = r ⋅100 (12) ia where ir and ia indicate the maximum current densities during reactivation (sweep in cathodic direction) and activation (sweep in anodic direction), respectively. The resulting polarisation curves and DOS values are shown in Figures 9 and 10. The heat treatments and DOS values of all sample batches are listed in Table 3.

Table 2: Chemical composition of the investigated material (in wt.%). C Si Mn P S Cr Mo Ni Co Cu Nb Ti Fe AISI 0.062 0.15 0.53 0.023 0.029 18.3 0.273 8.59 0.169 0.26 0.019 0.001 Balance 304

40 Table 3: DOS values and heat treatments of the material batches.

Material designation Heat treatment DOS

C Solution annealed: 1050 °C / 30 min / water quench 0.25 %

F Sol. ann. + sensitised: 620 °C / 1 h / water quench 1.2 %

IRK, IRO Sol. ann. + sensitised: 620 °C / 5 h / water quench 11 %

G, GI Sol. ann. + sensitised: 620 °C / 9 h / water quench 15.2 %

D, HRK, HI Sol. ann. + sensitised: 620 °C / 24 h / water quench 22.4 %

Figure 9: Double loop electrochemical potentiokinetic reactivation tests of all material treatments (0.5 M H2SO4, 0.01 M KSCN, T = 30 °C sweep rate: 100 mV/min.) The sweep direction is indicated by arrows.

Figure 10: DOS values against sensitisation time for all material treatments.

41 3.2 Macroscopic EN measurements All macroscopic tensile tests with EN measurements were performed at PSI, using an electromechanical tensile machine with an electrolyte recirculation loop.

3.2.1 Sample preparation Round bar specimens (gauge length = 36 mm, diameter = 6 mm) with a V-shaped or U-shaped notch in the centre were machined from the different heat treated material batches (Figure 11). A connecting wire was spot welded on each specimen for connection to the zero resistance ammeter and voltmeter. Samples were degreased with ethanol in an ultrasonic bath and then dried using hot air. The clean samples were covered with shrinking tape and/or covering lacquer approximately 1 mm above and below the notch, to leave only the centre area of approximately 65 mm2 around the notch exposed to the electrolyte. Care had to be taken to avoid crevices at the protective covering. The protected specimens were then fixed in an electromechanical tensile machine (CORMET 10kN SSRT Unit). A cell with the counter electrode and the reference electrode was assembled around the sample. The setup was shielded by a Faraday cage against electromagnetic interference.

Figure 11: Schematic of the used samples with U- or V-notch. Linear dimensions are given in millimetres. Samples were protected with shrinking tube and covering lacquer (grey area); only a small area around the notch was exposed to the electrolyte.

3.2.2 Measuring setup ECN and EPN were recorded simultaneously using a zero resistance ammeter and high resistance voltmeter, specifically designed for EN measurements (EcmNoise, IPS Germany) and verified according to a guideline for EN measuring devices [108]. EN data was recorded with sampling frequencies of either 2 or 10 Hz. Accordingly, low-pass filtering at 0.8 or 4 Hz was applied to avoid aliasing effects. Besides the raw data, a high-pass filtered (0.01 Hz) signal was recorded as well. High-pass filtering removed any slow current and potential trends or “direct current”

42 components from the signal. The remaining “alternate current” (AC) signals only showed fast signal changes, such as the transients typically detected during localised corrosion processes. ECN was measured between the notched sample and a C-ring made of the same steel (solution annealed) as the samples. The polarity of the ECN measurement was chosen in a way, that positive current indicate anodic processes on the working electrode. In the initial setup a Pt-wire was used as reference electrode. For later experiments, EPN was record with the help of a second steel coupon or a saturated calomel electrode (SCE), which was connected to the experimental cell by a Haber-Luggin capillary and an additional glass frit to inhibit electrolyte contamination. A schematic of the test setup (with a saturated calomel electrode in place) is shown in Figure 12. The cell configuration with a Pt-wire as reference electrode is shown in Figure 13.

Figure 12: Schematic of the experimental setup for simultaneous ECN and EPN measurement. The whole cell was protected by a Faraday cage. The electrolyte recirculation loop is not shown.

Figure 13: Electrode setup of the EN measuring cell, with tensile specimen as working electrode (centre), solution annealed stainless steel C-ring as counter electrode (connection to the right, surrounding the working electrode) and a Pt-wire as reference electrode (connected to the left, in between the other two electrodes). The electrolyte outlet can be seen in the upper right corner.

43

Connecting cables between the cell and the measuring instruments were shielded and kept as short as possible. To avoid ground loops, the Faraday cage, all cables and the voltmeter and ammeter were connected to the same mains ground. Some specific interference signals that were encountered during measurements are presented in Appendix I. For a measurement, all electrodes were connected to the ammeter and voltmeter. The electrolyte recirculation loop was connected to the cell and the Faraday cage was closed. The electrolyte recirculation loop consisted of a 5 L storage container, the electrolyte pump, a cooling loop and the cell. The cooling loop cooled the electrolyte to 21 °C before entering the sample cell. The cell volume was 0.2 L. The electrolyte flow rate was adjusted to 0.9 L/h, establishing quasi-stagnant electrolyte flow around the sample. Nominal stress values for macroscopic experiments were calculated from the force applied by the electromechanical tensile machine and the area cross-section of the round notched samples at the notch root. These values are stated in the experiments presented later. Local stress values varied greatly from these nominal values, due to the geometry of the notch. Finite element calculation of stress distribution for nominal loads typically applied during CL experiments in this study (230 – 275 MPa) showed stress values above 600 MPa at the notch root and values below 200 MPa on the notch walls.

3.3 Microcell measurements All experiments on the micro scale were performed at EMPA Dübendorf. The development of the measuring and capillary making procedures introduced in this section is described in the literature [82-84].

3.3.1 Sample preparation 0.9 mm thick coupons of 2.5×1 mm size were cut from the heat treated bulk material. For the majority of the experiments, the samples were ground with 600 – 4000 grit SiC emery paper and tap water, until a mirror finish was reached. Surfaces prepared this way showed few scratches, which did not interfere with the measurements. Samples were then rinsed with ethanol and dried with hot air. Measurements were performed directly after grinding. The surface of a sample after preparation is shown in Figure 14. The influence of a change of the surface treatment on sample behaviour was studied in a limited number of experiments. These measurements are discussed in section 4.6.1.

44

Figure 14: Specimen after surface preparation with 600 - 4000 grit SiC emery paper. Surfaces prepared this way showed scratches from the grinding procedure. MnS-inclusions were clearly visible as dark grey particles (shown in the centre of the picture).

3.3.2 Measuring setup (analogous to [23]) Microcapillaries were manufactured from borosilicate glass capillaries (GB150F-8P, Science Products GmbH) with a ratio of outer to inner diameter of 1.5 mm/0.84 mm. The glass capillaries were pulled to a tip with less than 1 µm diameter, using a pulling machine (WPI PUL-1). Capillary shapes could be influenced by regulating the temperature applied to the middle of the glass tube. Pulled capillaries were ground to the desired end diameter by hand, using a 4000 grit SiC emery paper and water as lubricant. When the desired diameter was reached, the capillary was flushed thoroughly to remove any residual glass particles. The ground tips were coated with a transparent silicone rubber (DOW CORNING ® 732 multi-purpose sealant) to form a rim around the capillary edge. To do that, capillaries were repeatedly dipped into fresh droplets of silicone sealant and flushed with ethanol. This way, the rubber seals could be built up over time. For most experiments capillary diameters between 50 and 100 µm were used, exposing surface areas of 2000 - 8000 μm2 to the electrolyte. These sizes were in the region of single grains, so most measurements hit at least one grain boundary. Smaller capillaries made it increasingly difficult to find grain boundaries or active surface spots. Larger capillaries often hit more than one active spot and made signal interpretation difficult. Good capillaries showed very little dripping and allowed for several measurements on the same sample without accidentally contaminating the surface with electrolyte. The microcell setup consists of an acrylic glass cell, which can be fixed in the objective revolver of a light microscope, and has an opening for a microcapillary at the bottom and for the counter electrode and the reference electrode at the top. To prepare the setup for a measurement, a capillary was inserted into the microcell, and the counter and reference electrodes were connected. A saturated calomel electrode, connected through a Haber-Luggin capillary was used as reference and a Pt wire was used as counter electrode (see Figure 15). The cell was filled with

45 the electrolyte and was then mounted in the modified socket of the light microscope objective carousel. A sample was fixed on the microscope stage. By rotating the objective revolver, the microcell could be switched to an ordinary microscope objective, and the surface could be searched for suitable measuring locations. Once a desired position was found, the capillary could be switched back in for the measurement. A high resolution potentiostat/galvanostat (Jaissle IMP 83 PC T-BC) with a current detection limit of 1 fA was used. A scanning frequency of 9 Hz was applied. A Faraday cage shielded the experimental setup from external electromagnetic interference. Measurements were performed under atmospheric pressure and at room temperature (23 °C). Figure 12 shows the measuring setup used in this study.

Figure 15: Experimental setup of the electrochemical microcapillary and bending cell [23].

The application of stress was limited by the experimental setup, as the whole loading unit had to fit on the light microscope table. A three point bending cell was used to bend the samples and create the specified surface stress (also shown in Figure 12). The same setup has already been used for microcapillary measurements to study the effect of stress on pit initiation on stainless steels [23]. Deformation was applied by turning a screw underneath the sample coupon. The nominal stress was calculated from the deformation, assuming only elastic deformation. For some experiments, nominal stress values of up to 720 MPa are stated, although σy of the material was much lower (291 MPa). Obviously, significant plastic deformation was induced in these experiments. Before each measurement, the capillary was flushed with fresh electrolyte. Using the light microscope, a suitable position for a measurement was selected, and the capillary was positioned. The Faraday cage was then closed and the power cables to the potentiostat and the equipment inside the cage (lighting, microscope) were unplugged for minimising external noise.

46 3.4 Initial experiments with neutral sodium thiosulphate electrolyte 3.4.1 Macro scale EN measurements The initiation of IG SCC under dynamic loading conditions in thiosulphate solutions has been proven in a series of studies [58, 59, 109, 111] (see also Section 2.3.4). Thiosulfate concentrations as low as 6·10-7 M [118] have been reported to induce IG SCC. Early CERT measurements showed stable current and potential signals during unloaded phases. Increased activity occurred when high stress levels were reached. An exemplary CERT measurement on a highly sensitised specimen in 0.1 M Na2S2O3 is shown in Figure 16. Figure 17a and b highlight the first rapid current and potential transients. A slow current rise and potential drop, which set in at approximately 250 MPa, indicates increasing activity of the strained surface. Subsequent cracking events, indicated by fast current transients and simultaneous potential drops, are likely due to mechanical initiation of film rupture events. The sample was unloaded when 300 MPa were passed and after a current transient reached 1 µA.

Figure 16: EPN (green) and ECN (red) measurement during a CERT test on a fully sensitised sample (DOS: 22%) in

0.1 M Na2S2O3. Applied strain is indicated in blue. Current and potential transients and a rise in base line current and potential drop occurred shortly before 300 MPa were reached.

47

Figure 17: a) Detail view of the transient signals shown in Figure 16. Current transients (red) show a fast rise and exponential decay. The potential (green) shows a less pronounced response b) The high-pass filtered (0.01 Hz) AC signal clearly shows the two events as potential drops and current oscillations.

3.4.2 Microcell tests For the first microcell experiments, the same electrolyte was used as for the preliminary macroscopic experiments. Potentiodynamic current measurements were performed on random surface spots with 40 µm capillaries in aqueous 0.01 – 0.5 M sodium thiosulphate solution. Exemplary current curves, measured on highly sensitised material (DOS 22%) at various degrees of deformation in 0.01 M Na2S2O3, are shown in Figure 18. Current curves indicated passive behaviour. OCP values varied between -350 and -250 mV/SCE, and anodic current densities around 10-4 mA/mm2 were measured. Surface spots exhibiting MnS-inclusions showed typical dissolution peaks around 400 mV/SCE. Comparison of current signals on bulk metal and on grain boundaries showed no change in behaviour. While neutral Na2S2O3 solutions of concentrations well below 0.01 M have been successfully used in macroscopic experiments, concentrations up to

48 0.5 M did not show any sign of crack initiation with the microcapillary technique, even at high applied deformations and anodic polarisation of the sample. As mentioned before, the small surface areas exposed during microcapillary measurements exhibit a more noble behaviour than a macroscopic surface, because fewer surface weaknesses are present. More important, the more stable behaviour of samples in microcapillary experiments is attributed to the applied types of loading. Crack initiation and propagation in these electrolytes only occurs under dynamic loading conditions by mechanical film rupture, which could not be done during microcapillary measurements.

Figure 18: Potentiodynamic current measurements in 10 mM thiosulphate solution on differently stressed samples of the highly sensitised steel (DOS: 22%) wit a potential sweep rate of 1 mV/s. All samples show a passive behaviour. Dissolution of MnS-inclusions (if present on the micro spot) sets in at 400 mV/SCE. The applied load does not show any influence.

3.5 Evaluation of new electrolytes 3.5.1 Selection criteria Downscaling of the exposed sample surface and switching from a macroscopic electrochemical cell design to the microcell had two major effects: • The application of load to the sample was restricted to a three point bending cell. Samples could only be strained to a fixed value of deformation at the beginning of an experiment, and no adjustment of stress was possible during a measurement.

49 • With decreasing surface area the number of structurally “weak” points present on the investigated surface is reduced, and the chance to hit an “active” surface spot decreases. A sufficiently aggressive system has to be chosen for micro scale measurements, which may therefore be overly aggressive on the macro scale. For experiments with the microcapillary technique, a chemical environment had to be found that fulfilled the following requirements: • A suitable electrolyte has to rapidly induce IG SCC under the predefined working conditions (room temperature, atmospheric pressure and constant deformation applied to the sample) to make a detection of active spots with the microcapillary possible. And while IG SCC is a highly localised type of corrosion, where even a small number of initiation points can be very dangerous in practice, a high number of initiation sites considerably facilitates their study. • To unambiguously correlate certain signals with IG SCC, other localised corrosion processes occurring simultaneously have to be minimised. In particular, crevice and pitting corrosion have to be avoided. • Ideally, the electrolyte should be non-toxic and should not exhibit an extreme pH value. The stability of the electrolyte and a possible decomposition during long term experiments must be considered.

3.5.2 Acidic thiosulphate solutions To increase the aggressiveness of the initially used electrolyte based on sodium thiosulphate, sulphuric acid was added to lower the pH value. Macroscopic as well as microscopic measurements at pH 3 showed similar results as measurements in neutral electrolytes. A further lowering of the pH was limited by the stability of the thiosulphate ion under acidic conditions: At room temperature and a pH around 7, aqueous solution of thiosulphate were reported to be stable over days to weeks [119]. Air oxidation occurred only under elevated temperature and high oxygen pressure. Under acidic conditions, thiosulphate easily decomposes and forms elemental sulphur (see equations 1-3 in Section 2.1.2). A rate law for the disproportionation reaction 1 of thiosulphate to sulphite and sulphur is deduced in [120]. The found reaction rate is first order + - with respect to H , and second order with respect to S2O3 :

+ 2− 2 −2 −1 R0 = k0 ⋅[H ]⋅[S2O3 ] k0 = 0.66 mol s (13)

50 Thiosulphate decomposition in the presented exposure tests was indicated by an opaque precipitate of colloidal sulphur in the electrolyte and the formation of hydrogen sulphide, readily detectable by its characteristic smell. The applicability of such unstable, continuously decomposing electrolytes was deemed unsuitable for further study.

3.5.3 Chloride solutions Pitting corrosion induced by chloride ions has already been studied extensively with the microcell [23, 83]. Chloride solutions were therefore considered for measurements in the present system.

Macroscopic exposition tests on stressed samples in concentrated acidic MgCl2 and NaCl solutions showed severe corrosion damage on the exposed surfaces (Figure 19a and b). Grinding of the damaged surface after an experiment and additional bending revealed IG SCC in both environments (Figure 19c and d). These aggressive electrolytes had the potential to initiate IG SCC under the applied conditions, but the simultaneous occurrence of pitting made them unsuitable for further experiments. The result of an exposure test in a less concentrated electrolyte is shown in Figure 20. A highly sensitised sample was exposed to 1 M NaCl at pH 7 and stressed to nominal 360 MPa. No signs of cracking were observed after 72 h. The deformation was increased to nominal 540 MPa. After additional 72 h, the surface around the area of highest deformation showed IG cracking. The surface also exhibited additional corrosion attack in some regions.

51

Figure 19: a and b) Light microscope images of two sample surfaces (DOS 22%) after 24 h of exposition to

5 M MgCl2 (a) and 5 M NaCl (b), both at pH 1.5. c) and d) Light microscope images of the same sample surfaces shown in a) and b), after polishing. Stress corrosion cracks are revealed on both samples.

Figure 20: a) SEM image of IG cracking on a macroscopic exposure specimen at nominal 600 MPa, after 6 d exposure to 1 M NaCl. Cracks appeared around the area of highest deformation on the sample. b) Additional corrosion occurred on some parts of the surface.

Microcell measurements were performed using 0.001 – 1 M sodium chloride solutions at pH values 2 – 7. Potentiodynamic current measurements on unstressed samples often showed pitting

52 or crevice corrosion in the region of 200 – 800 mV/SCE, both on sensitised as well as solution annealed samples (Figure 21).

Figure 21: Potentiodynamic current measurement on unstressed material, using the electrochemical microcapillary and a 1 M NaCl solution at pH 2. A potential sweep rate of 1 mV/s was applied. Both solution annealed (a) and sensitised (b, DOS: 22%) material suffered stable pitting corrosion in the range of 100 – 800 mV/SCE.

While macroscopic immersion tests in chloride solutions showed the occurrence of IG SCC, it was not possible to isolate this phenomenon from pitting corrosion and to study it exclusively with the microcell technique. Macroscopic EN measurements that were performed in chloride solutions, showed micro crack formation on sensitised samples, but also extended crevice corrosion. The initiation of IG SCC did not occur faster than with thiosulphate. These electrolytes therefore did not provide any advantages for macroscopic measurements, and crack propagation was not observed on the micro scale, the use of chloride for further experiments was dismissed.

53 3.5.4 Potassium tetrathionate solutions The tetrathionate ion is closely related to thiosulfate in its potential to initiate IG SCC in sensitised steels, but it exhibits a higher stability at low pH. Therefore, stable electrolytes with low pH values can be prepared. The initiation of IG SCC on stainless steel under CL conditions in acidic tetrathionate solutions is described in [121]: a 0.1 M solution of potassium tetrathionate with pH 2 initiated IG SCC on passively deformed stainless steel samples. These results could be reproduced in the current study. Stressed samples exposed to this electrolyte showed clear IG SCC in the region of highest load after hours of exposure. Additionally, exposure to the electrolyte caused a dark blue/black deposition of sulphides on the sample surfaces, mostly around the freshly formed IG cracks. With the macroscopic EN measurements in mind, tests with more diluted electrolytes were performed. Applied nominal stress in the elastic (180 MPa) region, just beyond the transition from elastic to plastic deformation (360 MPa), and in a highly plastic deformation region (> 360 MPa) were chosen. Table 4 summarises the immersion experiments performed with various tetrathionate solutions and stress values.

Table 4: Results of macroscopic immersion tests of different tetrathionate electrolytes.

c(K2S4O6) c(H2SO4) Nominal stress Surface inspection 0.1 M 0.01 M 180 MPa IG SCC, IG corrosion, dark precipitates 0.01 M 0.01 M 180 – 900 MPa IG SCC, IG corrosion, dark precipitates 0.01 M 0.005 M 180 – 360 MPa IG SCC, precipitates 0.01 M 0.001 M 360 MPa No corrosion on surface 0.01 M 0 360 MPa No corrosion on surface 0.001 M 0.01 M 360 MPa Black precipitate layer, extended IG corrosion, more prominent in region of highest load. 0.001 M 0.005 M 180 – 360 MPa IG SCC, dark precipitates Control test without tetrathionate: 0 0.01 M 720 MPa Grain boundaries attacked at centre of highest loading 0 0.05 M 720 MPa Grain boundaries attacked at centre of highest loading Control test without load: 0.1 M 0.01 M 0 IG corrosion

High concentrations of tetrathionate and acid caused IG corrosion and IG SCC at stress values below σy. Sample surfaces showed dark blue precipitate films, which can be attributed to the formation of sulphides on the surface [122]. For constant tetrathionate concentrations, raising the pH generally reduced IG corrosion. At pH 2.2 (0.005 M H2SO4), IG cracks could still be observed at 180 MPa nominal stress (Figure 22b). At higher pH values, crack initiation below σy was not reproducible any more.

54 Tetrathionate concentrations below 0.01 M gave contradictory results: A 0.001 M solution at pH 1.8 caused severe IG corrosion on the whole specimen (Figure 22c). This increased corrosion was accompanied by formation of a very dense precipitate layer, compared to higher tetrathionate concentrations. At pH 2.2, IG SCC occurred with moderate precipitate deposition on the surface. It is unclear why the 0.001 M solution at pH 1.8 showed an increase in aggressiveness compared to 0.01 M solutions. But the tendency to attack all grain boundaries of a sample and the dense precipitate layer made it difficult to identify micro cracks.

A tetrathionate concentration of 0.01 M and pH values of 1.8 and 2.2 caused IG SCC below σy of the material, with little IG corrosion on the unstressed surface and with moderate sulphide precipitation on the surface. These electrolytes were considered for further experiments.

Figure 22: Light microscope images of samples exposed to various tetrathionate electrolytes over 24 h. a) 0.1 M

K2S4O6, pH 1.8, 180 MPa: IG cracks formed over the whole width of the sample. The surface shows dark blue sulphide depositions. b) 0.01 M K2S4O6, pH 2.2, 180 MPa: single cracks appeared near the centre of highest deformation. The surface shows colour changes around cracks and MnS-inclusions. c) 0.001 M K2S4O6, pH 1.8, 360 MPa (after removing of sulphide deposits): extended IG corrosion. Cracks cannot be identified.

55 Macro scale EN measurements A CERT test (labelled RTL 30) was performed on a highly sensitised specimen exposed to

10 mM K2S4O6 at pH 1.8. The EN signals are shown in Figure 23: Already during the initial stabilising phase at 10 MPa, the current exceeded 1 µA. The ECN signal jumped to 10 µA at 160 MPa, causing a signal overload. After reaching 200 MPa, complete fracture of the sample set in. The potential shows a corresponding fast drop at 160 MPa stress, followed by a continuous decrease until the sample broke. Inspection of the fracture surface after the experiment revealed almost complete IG fracture of the sample cross section (Figure 24). In some regions of the sample, small pits were found, following the grain boundaries (Figure 24b).

Figure 23: EPN (green) and ECN (red) measurement during a CERT test on highly sensitised material (DOS: 22%) in 0.01 M K2S4O6 at pH 1.8 (RTL 30). Current signal overload occurs at 15 h. The sample breaks apart after 18 h.

Figure 24: SEM image of the completely fractured round notched sample after the CERT test shown in Figure 23. a) The fracture surface shows almost complete IG fracture. b) Some regions showed additional pitting and IG corrosion.

In a series of follow-up experiments it was tried to optimise experimental procedures and sample preparation. All further experiments were conducted in 0.01 M K2S4O6 at pH 2.2. Results are presented in detail in Section 5. In the process of optimising experimental conditions, current

56 signal fluctuations during the stabilising phase could usually be limited to a region of 0 – 5 nA. In some cases, better values were achieved (stable current signals of 850 ±50 pA over several hours).

Potentiodynamic current measurements with tetrathionate electrolytes on the micro scale initially gave results very similar to those obtained in thiosulphate solutions. After several measurements on highly deformed samples, a different behaviour emerged. Distinct surface spots showed an OCP shifted to more negative values and immediate active dissolution in the anodic region, while the largest part of a surface showed a passive behaviour, identical to unstressed samples (Figure 25). Inspection of surface spots that exhibited active signals revealed micro cracks along a few grain boundaries. Cracks propagated predominantly in the direction perpendicular to the applied load (Figure 26). Based on these results, 10 mM K2S4O6 at pH 2.2 was chosen for the subsequent systematic studies.

Figure 25: Potentiodynamic current measurement (potential sweep rate = 1 mV/s) on a passive surface spot (black), and on an active spot (red), which resulted in IG SCC under the microcapillary. Active spots generally showed a lower OCP and immediate active behaviour in the anodic region. The current rise in the passive curve at 500 mV/SCE can be attributed to dissolution of a MnS-inclusion.

57

Figure 26: SEM image of the first micro crack that was initiated under the microcapillary, measured on highly sensitised material (DOS 22%) in 0.01 M K2S4O6 at pH 1.8 and a nominal stress of 180 MPa. The direction of applied load is predominantly horizontally, resulting in a crack growth in vertical direction on the image. The surface area exposed to the electrolyte underneath the capillary is indicated by the dotted circle.

3.6 Final measuring procedures 3.6.1 Macroscopic CERT tests The experimental procedures for CERT experiments is shown in Figure 27a. The electrolyte recirculation loop was filled with the appropriate test solution from the beginning. An initial load of 1000 N (~50MPa) was applied to the sample. The system was allowed to stabilise under these constant conditions for 48 – 72 h 6. The conditioning phase was also used to identify interferences in the signal and adjust the system. After the system had stabilised, and ECN and EPN measurements showed stable signals, a pull rod extension rate of 3.5·10-9m (nominal strain rate: 2·10-7 s-1) was initiated. At the end of an experiment, either due to complete failure of a sample, or when a certain maximum stress was reached, the specimen was unloaded. The measurement was continued for some hours, to monitor the change of the noise signal after unloading: A sudden decrease in noise activity after unloading confirmed that the detected noise signals were caused by the applied stress. The specimen was finally removed from the setup and prepared for surface inspection.

6 In exceptional cases, the conditioning phases could be shortened or had to be extended.

58 3.6.2 Macroscopic CL tests The experimental procedures for CL experiments is shown in Figure 27b. CL experiments were performed in aqueous solutions of potassium tetrathionate at pH 2.2, adjusted with sulphuric acid. Any dynamic loading of the specimen exposed to the tetrathionate was avoided. For these experiments the electrolyte recirculation loop was initially filled with distilled water and the specimen was stressed to the desired nominal load. The system was allowed to stabilise for 48 – 72 h. When potential and current signal were stable, the missing components of the electrolyte were added to the loop. Measurement durations then ranged between a few hours and up to 10 days, before the experiments were stopped.

Figure 27: Illustration of the experimental procedure for macroscopic EN measurements. a) CERT test: The sample is exposed to the complete electrolyte from the beginning. During a stabilising phase a constant pre-load is applied. At t = 0 a constant extension rate is initiated. b) CL test: The sample is stressed to the desired level from the beginning. During a stabilisation phase, the sample is exposed to distilled water. At t = 0, tetrathionate and sulphuric acid are added.

3.6.3 Microcapillary measurements For potentiodynamic scans, samples were prepolarised at -500 mV/SCE for 1 min. The scan rate was 1mV/s and scans ranged from -500 to 1000 mV/SCE. For potentiostatic measurements, initial OCP measurement was performed for 1 min before the current measurement at a defined constant potential was started. Potentiostatic measurements lasted between a few seconds and several hours. For selected experiments, extended OCP measurements were performed, monitoring the fluctuations of the OCP over up to 2 h. To find the few potential crack initiation sites, which were spread over a mostly passive sample surface, the surfaces had to be systematically scanned. To quickly discern active from passive areas, the distinct difference in OCP values, which was found in the initial potentiodynamic current measurements, could be used: Locations exhibiting OCP values in the range of -200 mV/SCE often showed crack initiation, while surface spots with OCP values near

59 0 mV/SCE usually remained passive. The use of the microcapillary for systematic scanning has already been explained in detail [82, 85]. To perform measurements selectively on active sites, the capillary was placed spot by spot in an array of rectangular shape (see Figure 28). Opposite to electrochemical droplet cells, which can change position while still in contact with the surface, the microcapillary has to be lifted off the surface after each positioning and put in place again at the next one. After positioning, the OCP drift was observed on the potentiostat for a few seconds to minutes. If the potential rose to potentials around 0 mV/SCE no measurement was performed and the capillary was moved to the next stop. If the OCP on a spot stayed around -200 mV/SCE for a few seconds, a measurement was started. This procedure allowed a fast identification of active spots over a selected area. Measurements could be selectively performed on active surface spots.

Figure 28: Illustration of the point by point scanning technique using the microcapillary technique. The capillary has to be lifted from the surface and repositioned after each measurement.

3.6.4 Post test inspection After a completed experiment, the exposed surfaces of the tensile specimens were investigated using SEM and stereo light microscopy. SEM measurements were performed using a LEO 440 or a LEO 1455 model. Additional stressing of the samples under air after an experiment above σy could be applied to improve the visibility of cracks. Selected samples were mechanically overloaded and fractured in air. The resulting fracture planes often revealed the crack walls, which allowed their direct inspection and an estimation of the crack wall areas. For microcapillary experiments, the first assessment of corrosion damage was done by light microscope immediately after lifting the capillary from the surface. The highly localised measurements allowed the direct identification of any corrosion damages on the exposed surface. Intergranular cracks were often directly visible, using an objective with 20x magnification. The localisation of investigated surface spots on cleaned specimens in the SEM was challenging, as sample surfaces showed little features that would facilitate orientation. Microcapillary

60 specimens were usually removed from the bending cell for SEM inspection. This caused cracks to partially close again. To increase their visibility in the SEM, microcapillary samples were often plastically deformed after a measurement, to open the micro cracks. The extensions of selected micro cracks were measured by preparing cross sections of the cracks and grinding them layer by layer. Depth measurements allowed an estimation of the crack wall area.

3.7 Summary The first aim of this thesis was the development of a system that allowed the monitoring of IG SCC of austenitic stainless steel on the micro and macro scale, using the electrochemical microcell technique and EN measurements during macroscopic tensile tests. The combination of these two measuring techniques required the definition of experimental conditions for both scales, which were reasonably similar to be able to compare the result. The initially proposed system of neutral thiosulphate solutions was well suited for CERT tests on the macro scale, but this system was not aggressive enough to initiate IG SCC on passively stressed samples in a reasonable time frame. Microcapillary experiments under potential control showed no difference in behaviour between the sensitised grain boundaries and the bulk material, and no IG cracking was observed. A more aggressive chemical environment had to be found for the micro scale experiments, but not overly aggressive on the macro scale. Lowering the pH of the thiosulphate was limited by the decomposition of the electrolyte. Chloride solutions showed crack initiation under constant deformation in a matter of days, but pitting corrosion predominantly occurred during potentiodynamic current measurements. Current signals related to micro crack propagation could therefore not be isolated from pitting corrosion processes Potassium tetrathionate solutions at pH values below 3 were finally found to be suitable electrolytes, as they fulfilled the initially stated requirements very well: They were easy and safe to handle, showed IG SCC initiation under constant deformation, and it showed only little other corrosion processes besides IG SCC. Besides these requirements, this experimental system showed crack initiation only at few, well defined surface spots. Active spots could be easily identified by a shift in OCP compared to passive surface spots, and they showed almost immediate crack initiation and growth when in contact with the electrolyte. This allowed making measurements selectively on active spots.

61 4 Investigation of IG crack initiation and

propagation on the micro scale

The growth of single micro cracks was studied by potentiostatic current measurements. A data set was established by scanning sample surfaces and performing measurements selectively on active spots. All experiments were performed in 0.01 M K2S4O6 at pH 2.2 as described in Section 3.5.4. Samples were polarised at potentials in between the OCP of typical active and passive surface spots (Figure 25). Measurements were performed on material with DOS of 22% and 15% and lasted between a few seconds up to several hours. The resulting current – time curves are analysed and compared to SEM images of micro cracks that had formed on the surface during the measurement. Finally, potential and limitations of the microcapillary technique for the study of IG SCC are discussed.

4.1 Signal characteristics This section illustrates the typical signal characteristics that were observed. A statistical signal analysis will be presented in sections 4.3 - 4.5. Immediately after a freshly ground surface spot came in contact with the electrolyte, OCP values around -400 mV/SCE were measured. This potential was stable for a few seconds. A rapid potential rise occurred over the following minutes. The same reaction of AISI 304 stainless steel exposed to tetrathionate solutions was reported for macroscopic samples by Kowaka and Kudo in [122]. After this initial potential drop, surface spots either exhibited active or passive behaviour, indicated by the OCP:

4.1.1 Passive behaviour OCP values measured on the majority of surface spots for both solution annealed and sensitised material showed a continuous potential rise towards a constant value in between 0 and 200 mV/SCE (Figure 29a). Exact values varied between samples, and between surface spots on the same sample. No corrosion attack was observed on these “passive” surface spots after a measurement. The same behaviour was found on bulk material, grain boundaries and inclusions, for all tested materials. Potentiostatic current measurements on passive surface spots showed negative current signals, due to the applied potential below the OCP of the passive surface. Signals often showed a slight trend towards less negative values over several hours. Cathodic

62 current densities after 10 min, measured with a 60 µm capillary on passive surfaces, ranged between -0.4 - 0 mA/cm2 (Figure 29b).

Figure 29: a) Typical OCP curve of a surface spot exhibiting passive behaviour, measured on a highly sensitised sample (DOS: 22%) with a 60 µm capillary over 2 h in 0.01 M K2S4O6 (pH 2.2). b) Typical potentiostatic current measurement on a passive surface spot at -150 mV/SCE, measured on a highly sensitised sample (DOS: 22%) with a

60 µm capillary over 2 h in 0.01 M K2S4O6 (pH 2.2).

4.1.2 Active surface spots Surface spots which showed an OCP anywhere between -400 and -100 mV/SCE for longer than a few seconds are designated “active” (Figure 30a). These surface spots were found on sensitised material exclusively. Active surface spots were mostly found on stressed surfaces. Very few spots showed initial active behaviour without stress. Potentiostatic current measurements on these spots showed fluctuating current signals immediately after polarisation. Single current transients showed fast rise and often irregularly shaped decay (Figure 30b and c). Initial currents could reach amplitudes above 250 nA directly after polarisation for very short time spans. Current peak amplitude then declined and showed values up to 100 nA.

63 Active surface spots on stressed samples showed intergranular cracks after an experiment. Crack growth was not stopped at the capillary rim, but continued underneath the silicone seal of the capillary, and occasionally even beyond it. Experiments at low stress (below 150 MPa) showed very short cracks, which were difficult to find.

Figure 30: a) OCP measurement on an initially active surface spot on highly sensitised material (DOS: 22%), measured with a 60 µm capillary over 2 h in 0.01 M K2S4O6 (pH 2.2). b) Potentiostatic current measurement on an active surface spot on highly sensitised material (DOS 22%) at -150 mV/SCE, measured with a 60 µm capillary over 2 h. c) Potentiostatic measurement on an active spot on partially sensitised material (DOS 15%), measured under the same conditions.

64 4.1.3 Discontinuous crack growth behaviour and repassivation The classification of active and passive surface spots is inaccurate, as repassivation commonly occurred during measurements. Current signals showed a slow decay towards a constant or slightly drifting passive value in the cathodic current region, equivalent to the current signals measured on surface spots that were passive from the beginning. Samples could passivate after significant crack growth, or just seconds after a measurement on an initially active surface spot was started (Figure 31). SEM images of such surfaces after the measurements showed only limited surface damage (Figure 32).

Figure 31: Potentiostatic current measurement (-150 mV/SCE, DOS: 22%, 0.01 M K2S4O6 (pH 2.2) on a surface spot that showed initial activity, indicated by an OCP value of -290 mV/SCE after 1 min. The measurement shows mostly passive behaviour, only interrupted by two dissolution events.

Figure 32: SEM image of the surface spot after potentiostatic measurement over 1 h, shown in Figure 31. The MnS- inclusion to the right shows dissolution along the inclusion-metal interface. A 10 µm long micro crack has formed.

Furthermore, reinitiation after a passive phase was observed occasionally, directly proving that micro crack growth is a discontinuous process, even at constant deformation conditions. This behaviour is shown in the current signals in Figure 30b and c. Reinitiation usually occurred by a

65 fast current rise. It is not clear, if an initially passive surface spot would show the transition to active behaviour over time. Such behaviour was not observed in any measurements on passive surface spots, however their number and duration was limited. Therefore it might still be possible for crack initiation to occur delayed on certain passive surface spots.

4.2 Location of crack initiation sites An important feature of the presented experimental system is the potential to identify crack initiation sites on a mostly passive metal surface. The nature of these active spots was further investigated. The presence of a sensitised grain boundary was obviously a necessity for IG SCC initiation, but apparently it was not sufficient on its own: The majority of the grain boundaries on a surface did not show active behaviour. The question arises, what conditions have to be fulfilled to create an active surface spot. Post-test inspection of micro cracks revealed, that in most cases, crack initiation occurred at MnS-inclusions. In some of these cases the MnS-inclusions were still intact after the measurement (see Figure 33). This is in accordance with the results from potentiodynamic scans: MnS-inclusions showed dissolution at potentials above 400 mV/SCE, while IG SCC could be initiated below 0 mV/SCE. In other cases the inclusions were attacked (as seen in Figure 32), which could lead to the partial or complete break out of an inclusion. Cracks could also initiate at defects, which were created or revealed during surface preparation (Figure 34). These were rare cases, as the applied surface preparation procedure usually did not create such large defects. For certain micro cracks, no clear initiation point could be identified. The causes of crack initiation in these cases could not be determined. It is likely that very small MnS-inclusions, or some kind of weakness in the oxide layer of the surface, are responsible for these cases of crack initiation. Typical weak spots for the case of pitting corrosion are e.g. local variations of oxide film structure and composition, thermally induced film rupture events or electrostriction [123], and the same surface features probably also account for crack initiation. A final identification of the cause of crack initiation in the described cases went beyond the scope of this work. It also remains unclear, if certain grain boundaries were more susceptible to crack initiation, due to an orientation that leads to very high shear stress.

In an attempt to quantify the occurrence of active surface spots, the results of an array measurement with 99 investigated surface spots were evaluated (see also Section 4.5), comparing the number of active spots with the number of MnS-inclusions. The results are listed in Table 5. Of 99 measurements, 4 surface spots showed crack initiation in the absence of a visible MnS-

66 inclusion or other particular surface feature, while crack initiation at an inclusion was observed in 20 cases. This underlines the importance of MnS-inclusions for the initiation of IG cracks in the present material.

Figure 33: SEM image of a short crack after 15 s of polarisation at -150 mV/SCE. The crack initiated right at the intersection of a grain boundary and a MnS-inclusion, indicated by a small pit. The inclusion is still intact.

Figure 34: SEM image of a crack that initiated at a large defect. These cases were rare, as the samples rarely exhibited such defects.

67 Table 5: Evaluation of 99 consecutive measurements on the same sample (DOS: 22 %, load: 360 MPa), concerning the influence of MnS-inclusions on crack initiation.

MnS-inclusion present No visible inclusions total

Passive behaviour: 9 57 66 No crack, only MnS- Crack initiation at Cracking without MnS- total: inclusion attacked MnS-inclusion inclusion present Active dissolution: 9 20 4 33

All measurements 99

For most experiments, a detailed investigation of a surface spot before a microcapillary measurement was not possible: The scarce distribution of active points made it necessary to scan large surface areas with the microcapillary. These areas were too large to be inspected by SEM in detail prior to the measurements. However, it was possible in two cases, to obtain SEM images of surface areas that would exhibit active behaviour in the following measurements: A sample was stressed to a nominal load of 720 MPa, and SEM images were recorded of random “suspect” surface spots on the stressed surface. The selected surface spots exhibited MnS-inclusions, which facilitated the later identification and increased the chance for active behaviour. Microcapillary measurements were then performed on the exact same positions. Two surface spots were hit precisely and showed active behaviour: Figure 35a and b show SEM images of these two MnS- inclusions on a stressed surface. The inclusions show no cracks or defects, which might serve as initiation sites. Potentiostatic current measurement on the two surface spots (Figure 36) indicated active metal dissolution. After the measurements, both inclusions showed cracking, but no other sign of corrosion (Figure 35c and d). This implies that the first step in the initiation of IG SCC is a chemical attack of the surface.

68

Figure 35: Two examples of MnS-inclusions before and after a potentiostatic current measurement. a) and b) show MnS-inclusions on an already stressed surface before the measurements. No sign of mechanical damage can be seen prior to the measurement. c) and d) show the same inclusions after potentiostatic measurement at -150 mV/SCE for 30 min (c) and 15 min (d). Besides being cracked, both inclusions are still intact and do not show any significant corrosion attack.

Figure 36: a and b) Corresponding potentiostatic current measurements to the micro cracks shown in Figure 35c and d. Measurements were performed with a 50 µm capillary on highly sensitised material (DOS: 22%) at -150 mV/SCE in 0.01 M K2S4O6 (pH 2.2). Currents of 40 – 80 nA indicate active metal dissolution.

69 4.3 Influence of applied stress and DOS on the crack growth behaviour To quantify the influence of applied stress and of the sensitisation of the material on the crack growth behaviour, crack lengths and integrated current signals were compared for partially and fully sensitised material, and for two stress values. All experiments were performed with a 60 µm capillary, and samples were polarised to -150 mV/SCE for 2 h. The stress values selected were 720 and 360 MPa. Both values were in the region of plastic deformation and usually produced large and clearly visible cracks.

4.3.1 Influence on crack size Mean crack surface lengths for the four possible combinations of the two applied stress values and the two material treatments are shown in Figure 37. Mean crack lengths were higher for highly sensitised materials and for higher DOS of the material. The largest cracks clearly appeared at high stress. Differences between the other material – stress combinations are smaller, and error bars are large for these calculations, due to the large spreading of crack lengths and the limited number of experiments. Mean crack growth velocities are 0.04 µm/s for highly sensitised material at high stress, and in the range of 0.02 µm/s for the other combinations of material and stress.

Figure 37: Mean surface crack lengths after 2 h potentiostatic current measurements for differently sensitised materials and applied stress values. Both sensitisation and increased stress leads to longer cracks.

The observed influence of stress on crack growth confirms the earlier macroscopic immersion tests of sample coupons. Stress is clearly a driving force for crack propagation.

70 4.3.2 Influence on current signal Integration of current signals gave the dissolution charge values for each investigated crack. A baseline correction was applied to account for the negative sum currents measured during passive phases: a positive current value was added to each signal, so that passive phases showed a value of 0 nA. All active phases then exhibited positive current values, and integration of the corrected signal resulted in the total anodic charge produced by active metal dissolution. The same trend as for the crack lengths is expected for the mean charge value of the differently stressed materials. The values are shown in Figure 38. Again, the highly stressed material shows clearly the highest charge values. The differences between the other values are even smaller than for the crack lengths. This can be explained by additional current that originated from dissolving MnS-inclusions. The number and shape of MnS-inclusions varied greatly between samples, and crack initiation on the tested partially sensitised material mainly occurred on large MnS- inclusions, which were often attacked during tests. The resulting dissolution currents were independent of the applied stress and therefore obscured its influence on crack growth.

Figure 38: Mean charge values after 2 h potentiostatic current measurements for differently sensitised materials and applied stress values. A combination of high stress and high sensitisation caused the highest charge values. The other combinations of stress and sensitisation show similar values.

4.3.3 Crack growth direction Crack growth was limited to grain boundaries, and therefore the orientation of the grains determined the exact growth direction of a crack. In all SEM images of micro cracks presented, the applied main load was in horizontal direction. Accordingly, crack growth proceeded predominantly vertically, i.e. orthogonal to the direction of the highest stress. Due to the

71 geometry of the used three point bending cell, additional stress components in vertical direction were also present: The screw used to deform the specimens touched the sample in one point in the centre. Stress is highest directly above the screw and decreases radially around this centre point. As stress was applied to the sample only in one point, and measurements were spread over an area around this centre of highest deformation, the sample surface was subjected to biaxial stress, and both intensity and direction of stress varied between surface positions. This explains the deviation of crack growth from the expected direction in some cases, such as the small cracks in Figure 35 d, which proceeded to the right.

4.4 Quantitative comparison of current signals and micro crack dimensions So far, the comparison of current signals with micro cracks was mainly made qualitatively. To quantify the correlation between crack dimensions and current signals, charge transfer values from current data were compared to the estimated crack wall areas, using Faraday’s law. To estimate crack wall dimensions, some assumptions had to be made: Cracks were found to initiate at distinct weak points, and crack growth had to proceed from these points, following the grain boundaries. Dissolution of the crack walls was restricted to the sensitised regions along grain boundaries. The width of these regions, and with it the width of the dissolution zone, were predetermined by the extent of the chromium-depleted zone. This width was assumed to be constant. Extended dissolution and crack propagation were therefore limited to two dimensions in the metal: crack propagation in depth and in length along the grain boundaries, mainly perpendicular to the applied load. With a constant dissolution rate along the grain boundaries from the initiation point in every direction, the result would be a semi-circular disc, with a radius of half the visible surface crack length. Growth speed might be higher at the surface due to higher stress (stress is declining with crack depth in the case of constantly deformed samples). Therefore cracks were expected to have approximately a semi-elliptical shape. The result of these assumptions is a model shape for a micro crack of a semi-elliptical disc with a constant width. The volume of a crack can then be derived from the width of the metal dissolution zone (marked c in Figure 39) and the area of a crack wall (marked A in Figure 39). A similar calculation has already been described by Wells et al. [109]. There the calculation was based only on the measurement of surface crack lengths, and a semicircular crack shape was assumed.

72

Figure 39: Illustration of the crack wall area calculation: Cracks were assumed to have a semi-elliptical shape with crack walls along the grain boundaries. The crack wall area A can be estimated from the visible crack length a and the maximum crack depth b. The width of the dissolved metal layer c can be calculated from A and the volume of dissolved metal, derived from the current signal using Faraday’s law.

To confirm the assumption of semi-elliptical crack shape, and to estimate the mean crack depth, the cross sections of 11 selected cracks were investigated using light microscopy and SEM: Sample profiles were progressively ground with SiC paper to reveal the depth profile along the surface crack. After each grinding step the depth of the crack was measured. A combination of two SEM images, showing the surface crack and the depth profile, is shown in Figure 40. Cracks showed highest depths around the centre of the surface crack line, and depth decreased continuously towards the crack tips, confirming a semi-elliptical shape. A mean ratio of maximum crack depth to surface length of 0.47 ± 0.07 was found. As expected the main depth was smaller than half of the visible crack length on the surface. This value of 0.47 was used to estimate the depths of all other investigated cracks and to calculate their wall areas. The measured values as well as the linear fit are shown in Figure 41.

73

Figure 40: Combination of two SEM images to illustrate the estimation of crack shapes and ratios between the length on the surface a and the depth in the crack centre b.

Figure 41: Maximum depths (measured optically on ground sample profiles) vs. surface lengths for selected cracks, measured optically on the sample profiles. A ratio of 0.47±0.07 between surface length and maximum crack depth was found.

Current values from potentiostatic measurements were used to calculate electrode charge transfer. Again, a baseline correction was applied to each current measurement to account for the cathodic currents measured during passive phases. The resulting wall area and charge transfer values are plotted in Figure 42. A best fit line for the data has a slope of 2.2 mC/mm2, which is the amount of charge flown per area of the crack wall. For the calculation of the trend line, several data points (open data points in Figure 42) were omitted: For these particular experiments, surface investigation after the measurements revealed extended corrosion of MnS-inclusions, leaking of the capillary or other types of corrosion. The current signal from these corrosion processes

74 perturbed the current signal related to crack propagation. Accordingly, these data points mostly exhibit a higher charge than would be expected from the crack size. The final ratio of 2.2 mC/mm2 was compared to passive charge transfer values measured on freshly ground steel samples. The charge of passivation per area ranged between 0.03 and 0.09 mC/mm2, i.e. values are more than 20 times smaller than the ratio found for crack walls: Significant additional metal dissolution must occur during active crack growth in the chromium- depleted regions along the grain boundaries.

Figure 42: Charge vs. estimated crack wall areas of 50 micro cracks (both degrees of sensitisation). A trend line was calculated for the filled symbols. Open symbols indicate measurements that were omitted from the calculation of the trend line.

The ratio between charge and crack wall area can directly be used to calculate the width of the dissolved metal layer along the grain boundary. Using Faraday’s Law the corresponding volume of dissolved metal was calculated from the charge as follows:

M ⋅Q V = (14) ρ ⋅ z ⋅ F

(V: volume; M: molar mass; Q: charge; ρ: density; z: charge number; F: Faraday constant)

The molar mass M and density ρ of iron and a charge number z of 2 were assumed for the calculation. The width of the dissolution front could then be calculated for each crack, simply by dividing the obtained volume V by the estimated crack wall areas. A mean width of 80 nm was found, which is in the expected range for critical chromium depletion (< 12 wt.%) along sensitised grain boundaries, [124, 125]. Similar values for dissolution widths along grain

75 boundaries were found in macroscopic studies of IG SCC of stainless steel in thiosulphate solutions [35, 109, 111]. The presented calculations incorporated measurements on both material treatments (DOS 15% and 22%). As can be seen in Figure 42, similar values were found for both partially and fully sensitised material. This is surprising, as the degree of sensitisation directly influences the concentration profile of chromium at the grain boundaries, and therefore should define the dissolution width. It is possible, that the number of measurements on the partially sensitised material was too small to statistically detect a variation. Another possibility is that the width of the area with a critical chromium concentration (e.g. < 12 wt.%) is similar for both degrees of sensitisation: As shown in [41], the chromium concentration profile of a grain boundary shows a very steep depletion for short sensitisation times. With increasing time, the width of the depleted zone increases, while the depth of the zone decreases. The profiles for two different sensitisation times therefore intersect at one point. If this point is close to the critical chromium concentration, both degrees of sensitisation may lead to similar metal dissolution.

4.5 Systematic array measurement As introduced earlier, an array measurement was performed to study the distribution of active surface spots on an area of approximately 1.4 mm2: A series of short measurements (t = 10 min) were performed in an array of 10×10 spots on a single sample (DOS 22%, nominal stress 360 MPa). Measurements were carried out on each spot, independent of the initial OCP value. On the first column OCP measurements were performed over 10 min. On the subsequent 9 columns the OCP was measured for 90 s, followed by a potentiostatic current measurement at -150 mV/SCE over 10 min. All 99 measurements (one measurement was skipped) were performed on three consecutive days without any regrinding of the surface between the measurements. 4.5.1 Distribution of active sites An image of the array after the measurements is shown in Figure 43a. The array area with a dimension of 1.1×1.3 mm was marked using a micro hardness imprinter. These imprints served as orientation points in the following analysis. An overview over the number of active and passive spots on the array was already shown in Table 5. The scanned surface area exhibited a high number of large MnS-inclusions (> 10 µm), and many of the inclusions, on which a measurement was performed, were attacked. Crack initiation mostly occurred at these inclusions. Only four measurements showed cracking in the absence of MnS-inclusions.

76 Figure 43b shows the array surface after vibration polishing. Most traces of the microcapillary measurements disappeared. Only few large cracks are still visible. MnS-inclusions can be seen clearly as longish, dark grey particles that are horizontally oriented. The position of each microcapillary measurement is indicated by a circle. Surface spots exhibiting active metal dissolution during a measurement are indicated by red circles. They seem evenly distributed over the surface, and no pattern emerges.

Figure 43: a) Surface after 99 consecutive microcapillary measurements with a 60 µm capillary on highly sensitised material (DOS: 22%), at -150 mV/SCE in 0.01 M K2S4O6 (pH 2.2). The scanned array area was marked at the corners and along the sidelines with micro hardness imprints. b) The same surface area, after vibration polishing for 30 min in silica emulsion. Positions of measurements are indicated by circles. Red circles indicate spots which showed activity. Black circles indicate passive spots.

4.5.2 Variation of OCP values The presented array measurement produced a set of 99 OCP values, measured on the same surface, under the same experimental conditions and with the same capillary. The only factor that changed was the time span between surface preparation and each measurement. Such a dataset allows a comparison of OCP values of a large number of both active and passive surface spots. This gives insight in the variation of OCP values over a sample surface, which is typically observed with the microcapillary technique. The first 60 s of OCP measurements for all data points of the array are plotted in Figure 44a. Curves that led to localised corrosion are plotted in red. Black curves indicate passive surface spots, on which no significant corrosion damage occurred.

77

Figure 44: a) OCP measurements over 1 minute on 99 positions on the same sample. b) Distribution of OCP values after 1 min. OCP values of passive surface spots are concentrated around 0 mV/SCE. OCP values of active surface spots are distributed over region of -450 - -100 mV/SCE. c) Distribution of OCP values relative to the time passed since the first measurement. No trend appears, indicating that the time that passed after surface preparation did not significantly influence the behaviour of the surface.

OCP values are spread over a region of 500 mV, but a separation into two potential regions can be seen for active and passive spots: Figure 44b shows the distribution of OCP values after 1 minute. Passive OCP values are concentrated in the potential region between -80 and 90 mV/SCE, indicated by the grey band in Figure 44a, with few exceptions. A mean OCP of 2 mV/SCE and a standard deviation of 27 mV were found. Active spots showed a higher variation. OCP values are spread between -420 and -150 mV/SCE, with a mean value of -300 mV/SCE. A concentration of values is observed in the region of -420 to -300 mV/SCE, indicated by the red band in Figure 44a. The high variation between active spots is coherent with the described crack propagation mechanism. Potential values are expected to fluctuate during crack initiation and early growth, due to the discontinuous nature of the process. The variation of OCP values on passive surfaces indicates inhomogeneities of the oxide layer that formed on the surface during grinding. With the applied surface preparation, using SiC grinding paper and water, the OCP of the passive surface varied by 170 mV between different surface spots on the same sample. Figure 44c shows the OCP values of all measurements after 1 min, relative to the time that passed since the first measurement of the array. Similar distribution of OCP values for passive and active surface spots occurred. No decline in the number of active surface spots appeared over time. Apparently, time spans of 50 h did not significantly influence the behaviour of the surface.

78 4.6 Influence of the surface state Surface preparation has a large effect on the topography of the surface, residual stresses and structure of the oxide layer, and therefore on corrosion resistance of a surface [126-128]. Morach demonstrated in [129], how the formation of metastable pitting corrosion on austenitic stainless steel varied with the used grinding paper. Corrosion resistance of ground surfaces continuously increased from 320 to 4000 grit SiC paper, which was explained by a decrease of both surface roughness and plastic deformation of the top layer. A similar effect of surface roughness to the formation of pitting corrosion was reported by Burstein and Pistorius [130]. Furthermore, it was shown in [129] that supposedly identical manual surface preparation, performed by different operators, can lead to systematically different results, due to different amounts of pressure applied to a sample during grinding. Hence, an optimisation of grinding parameters, such as the type of abrasive paper, grinding pressure and speed or the lubricant can significantly increase corrosion resistance of a surface [131]. Possible effects of surface preparation on SCC initiation are exemplarily described in [28]: The initiation frequencies of cracks on four different mechanical surface finishes are compared. Cracks predominantly initiated at pits, which often occurred at local defect sites, such as deep grooves created during grinding. However, the surface roughness values did not necessarily correlate with initiation rate. Depending on the grinding method, lower roughness was accompanied by higher residual tensile stress, which could lead to more pitting. In this study, care was taken from the beginning to prepare all samples for microcapillary measurements by the same procedure, explained in detail in Section 3.3.1.: Coupons were ground up to 4000 grit SiC in water and then degreased in ethanol and dried with hot air. The effect of a deviation from this procedure is described below. Additionally, the effect of the time between surface preparation and start of a measurement was studied.

4.6.1 Behaviour of polished surfaces It was evaluated how fine polishing changed the behaviour of surfaces. These tests should clarify if it was possible to perform microcapillary measurements on surfaces with very little residual deformation. This would allow EBSD measurements on specimens after electrochemical measurements, and it would be possible to analyse grain orientations adjacent to the micro crack and determine their effect on crack growth direction. The additional surface preparation consisted of consecutive polishing steps with 3 µm and 1 µm colloidal diamond suspension and a final step of vibration polishing in silica suspension. Vibration polished surfaces were scratch-free in the light microscope. Surfaces prepared in this

79 manner were found to be highly vulnerable to IG corrosion: Microcapillary measurements showed selective dissolution of grain boundaries, both on stressed and unstressed samples (Figure 45). Under these conditions, crack initiation can occur anywhere on the grain boundaries, or at several locations at once, and the additional current signal from the widespread IG corrosion obscures the current signs of crack initiation and propagation.

Figure 45: SEM image of an unstressed sensitised sample (DOS: 22%) that was vibration polished and then subjected to a potentiostatic current measurement for 1 min (60 µm capillary, -150 mV/SCE, 0.01 M K2S4O6, pH 2.2). The surface shows extended IG corrosion.

This behaviour can be explained by chemical and mechanical etching of the grain boundaries during vibration polishing with the alkaline polishing agent. This dependence of the sample behaviour on the surface preparation (grinding vs. polishing) illustrates how sensitive the studied system reacts to changes of parameters, and how an apparently improved surface quality can increase corrosion Hence, micro electrochemical measurements on vibration polished surfaces did not produce any meaningful results. This limited further surface analysis, and it was not possible to electrochemically monitor the growth of micro cracks and then perform EBSD measurements on the same surface. For EBSD measurements, samples have to be polished after the electrochemical measurements. This in turn changes the top surface layers, and a direct comparison of both measurements is not possible for very short cracks and their initiation points (see also Appendix II).

80 4.6.2 Influence of oxide film formation over time Series of microcapillary measurements were normally started immediately after grinding. The number of experiments performed on one sample varied with the time it took to find a suitable position for a measurement, and with the duration of the measurements. As shown in Figure 44c, time differences of up to 50 h did not seem to have an effect on the OCP values or the occurrence of active surface positions. Further experiments were performed to determine, if longer time periods between surface preparation and the start of a measurement influenced the sample behaviour. Samples were ground and then immersed in distilled water for several days. They were then stressed, and measurements were performed. Figure 46a shows typical OCP signals measured on a sample that had been immersed in aerated distilled water for 17 d. The sample was stressed directly before the measurements. All measurements showed markedly higher initial OCP values compared to freshly ground surfaces. After a period of a few seconds up to several minutes, the potential dropped, and the curves subsequently showed either active or passive behaviour, just as on freshly ground surfaces. A second series of experiments were performed, where stress was applied before the exposure period: Figure 46b shows the corresponding OCP signals for measurements on a sample that was stressed directly after grinding, and was then immersed in aerated distilled water for 20 d. The same behaviour as before was found. SEM investigation of micro cracks for both series of experiments showed no particularities. Initiation occurred at MnS- inclusions (Figure 46c and d). Apparently the oxide films, which had formed over several days, did not prevent crack formation, but initially slowed it down. As mentioned in Section 2.1.2, sulphur-containing electrolytes can cause the breakdown of a passive film, by replacing metal oxides by sulphides. The observed delay in crack initiation might therefore be due to this reaction. Variations in initiation time can then be attributed to variations in oxide film thickness or structure. Furthermore, it was observed that the outcome of an experiment was not influenced by the timing of the application of stress: Crack initiation still occurred on samples, which were deformed before an oxidation period of several days. This indicates that the first step of crack initiation in the studied system is a chemical process, which does not require a mechanically induced surface defect right before the measurement.

81

Figure 46: a) Exemplary OCP measurements in 0.01 M K2S4O6 (pH 2.2) on a highly sensitised sample (DOS: 22%) which was additionally oxidised in aerated water for 17 d after grinding and stressed directly before the measurements. Initial high potential were found, which did not occur on freshly ground surfaces. After the potential dropped, the distinct behaviour of active and passive surface spots was found. b) Exemplary OCP measurements in

0.01 M K2S4O6 (pH 2.2) on a highly sensitised sample (DOS: 22%) which was stressed and then oxidised in aerated water for 20 d. The same behaviour was found as before. c) SEM image of the micro crack formed during the active OCP measurement shown in a). d) SEM image of the micro crack formed during the active OCP measurement shown in b).

82 4.7 Discussion 4.7.1 General validity of the results The presented experiments were successful, as they allowed the initiation of single micro cracks and the monitoring of current signals during micro crack initiation and early propagation. Crack depth measurements confirmed the assumption of basically semi-circular micro crack shapes, as it was assumed in earlier studies [109]. The inspection of micro cracks in the SEM revealed exclusively IG SCC on most surface spots. In some cases, additional corrosion along large inclusions occurred, but these cases could be easily identified. Together with the correlation of charge transfer values with micro crack dimensions, this leaves little doubt that the measured current signals were caused by anodic dissolution of metal along the grain boundaries, and the course of the current signal reflects the propagation of a micro crack. Crack propagation was not restricted to the surface area exposed to the electrolyte: The electrolyte continuously filled the growing crack enclave, and crack growth continued underneath the silicone seal of the capillary. An influence of the increasing solution resistance with crack growth was not found: Cracks that grew well beyond the walls of the capillary still showed the same ratio of charge to estimated crack wall area.

4.7.2 Variation of OCP values on passive surface spots A factor of uncertainty for the microcapillary technique is the quality of the microcapillary tip, and the comparability of measurements with different capillaries: Any inconsistency in experimental results is easily attributed to influences of flawed capillaries. This uncertainty was mostly avoided in the presented study: Almost all measurements were performed with the same 60 µm capillary. Comparability of measurements was improved further during the array measurement presented in Section 4.5. These measurements were not only performed with the same capillary, but also on the exact same sample surface, and any differences between samples introduced during surface preparation were avoided. The found variation of OCP values on passive surfaces, ranging from -100 to +100 mV/SCE, can therefore most likely be attributed to inhomogeneities of the surface oxide layer. Small variations in the exchange current density could then result in the observed potential variation. This would be intrinsic to the studied surface and could be influenced by the type of surface preparation. Another possible reason for the observed OCP fluctuations between measurements is a variation in oxygen concentration at the capillary tip between measurements. The oxygen concentration was not measured and no data on its variation is therefore available.

83 Currently, a work in progress with J. DeRose at EMPA studies this variation of OCP and pitting potentials during microcapillary measurements on stainless steel and aluminium. It could already be established, that the use of ethanol instead of water during grinding of the surface reduces the spreading of OCP values on stainless steel. This can be explained by a more homogeneous oxide layer forming during surface preparation. Further studies are needed to quantitatively assess these effects.

4.7.3 Micro crack initiation

Crack initiation occurred on sensitised samples above and below σy. The current signals related to the very first initiation events could not be monitored, as initiation often set in immediately when the capillary was positioned. The location of crack initiation on the studied material generally confirmed the known possible weak spots, as described in literature [36, 46]: Initiation was observed at inclusions, surface defects and, in case of vibration polished surfaces, on any grain boundary due to widespread IG corrosion. For this special case, the initial statement in Section 4.2 has to be corrected: Apparently the presence of a grain boundary can be sufficient for IG SCC initiation, depending on the surface preparation. Crack initiation obviously strongly depends on the surface state. Any observation concerning the surface weaknesses responsible for crack initiation might only be valid for one specific type of surface preparation. This has to be taken into account for the comparison of microscopic and macroscopic results, which are discussed in Section 6. For samples that were ground with 4000 grit SiC paper, MnS-inclusions were by far the dominating initiation sites on the surface. The results shown in Section 4.2 imply that MnS- inclusions were still intact after plastic deformation of the surface. Fracture of inclusions occurred during measurements and was very likely not involved in the initiation process. This observation, together with the tendency of MnS-inclusions to be attacked even on unloaded specimens, heavily implies that crack initiation was caused by localised corrosion at the metal-inclusion interface. Besides cracks growing from MnS-inclusions, a small number of cracks initiated on surface spots, which did not exhibit any visible weakness. The underlying cause for crack initiation on these sites could not be found. An initiation due to solely mechanical film rupture seems unlikely. The experiments presented in Section 4.6.2 have shown that crack initiation could still occur on surfaces that were exposed to aerated water for several weeks after grinding and deformation. During this time, any ruptured surface oxide film would have had ample time to reform. A more

84 probable explanation for crack initiation would be the formation of metastable pitting at a grain boundary triple point or a weakness in the surface oxide layer.

4.7.4 Micro crack propagation The main characteristics of current signals during micro crack propagation are current transients with a fast rise and slower decay, and the occurrence of passive phases, followed by reactivation, without any external change of the applied stress. A detail view of a potentiostatic current measurement, illustrating these features, is shown in Figure 47. A mechanism must account for these findings. As described in Section 2.1.2, two main theories for crack propagation need to be considered: a mechanism based on anodic dissolution, and a mechanism based on brittle fracture.

Figure 47: Detail view of a superposition of current peaks. This crack propagation event exemplarily shows the typical fast current rise, fluctuating current signal consisting of several superimposed events, and repassivation.

Anodic dissolution In the case of anodic dissolution, each current peak during a measurement represents a dissolution event at the crack front. The superposition of many single transients indicates several simultaneous dissolution events along the crack front, which continuously appear and repassivate. This is in good agreement with the studies from Newman et al. [132]. They concluded that along a crack front, even within one grain, all possible stages of the film rupture mechanism are present simultaneously. While single events are occurring along a crack front, stress distribution along the crack front is constantly changing, and the crack opens further, which might trigger further film rupture events. When all single dissolution events repassivate and no new active dissolution events are formed,

85 crack growth arrests, and the surface shows fully passive behaviour. This might happen, when a complete grain boundary is dissolved and the triple points of the next grains are reached, where metal dissolution becomes less favourable. As soon as the passive film on the crack front is ruptured, crack growth reinitiates and proceeds. Integration of the transient group shown in Figure 47 gives a charge of 8 µC, equivalent to a crack with a surface length of approximately 90 µm. This is in the typical size range of grain boundaries. The single transients that form the whole superposition therefore account only for a fraction of metal dissolution of a single grain boundary. A crack growth mechanism based on anodic dissolution depends on the formation of a corrosion cell at the crack front. In the studied system the formation of a corrosion cell can be explained by the microstructure of the studied material: The crack front consists of the grain boundary metal, which is depleted in chromium and is therefore susceptible to dissolution. The crack walls quickly repassivate, due to their higher chromium concentration. Hence, crack propagation is limited to a narrow metal layer, and an electrochemical opening of the crack by metal dissolution is not possible. The occurrence of IG corrosion on highly polished samples (see Section 4.6.1), independent of applied stress, further supports an anodic dissolution mechanism. Obviously the electrolyte is capable of attacking the sensitised grain boundaries even in the absence of external stress. Application of stress might merely promote this process by opening the formed cracks and accelerating the diffusion of electrolyte and reaction products, and by mechanical rupture of the passive film. While the used electrolyte is already quite aggressive, the crack geometry further promotes the formation of a chemical environment which favours crack propagation. The narrow crack crevice limits diffusion processes. Dissolved oxygen in the electrolyte is quickly consumed, which hinders repassivation. Additionally, dissolved metal ions are hydrated and release protons, lowering the pH in the crack and creating a more aggressive local chemistry. The main question remaining is the nature of the film rupture events after a complete passivation of the crack front. It is not clear, which effect is responsible for this reactivation. Film rupture occurred under constant deformation conditions, a purely mechanical cause therefore seems less likely. The formation of local aggressive chemical environments, i.e. a “chemical” weakening of the passive film is a probable trigger for film breakdown and re-initiation of a passive crack front. For such a mechanism, no need arises to invoke the effect of slip planes on crack growth reactivation.

86 Brittle fracture A mechanism based on hydrogen embrittlement might explain the observed current signals as well. Atomic hydrogen is formed inside a crack by reduction of protons. This is promoted by highly acidic conditions inside the cracks due to metal dissolution and subsequent hydrolysis of metal ions. Reactivation after a passive phase can be readily explained by a brittle fracture event that takes place after a sufficient amount of hydrogen is absorbed in front of the crack tip. Possible mechanisms for embrittlement along the grain boundaries are hydrogen formation and enrichment along the grain boundaries, or the reaction of hydrogen with segregated alloy components and subsequent decohesion of the grain boundary. After a brittle fracture event, the crack opens and crack walls are attacked by the electrolyte. The chromium-depleted metal is dissolved, leading to the observed ratio of charge and crack dimensions. Repassivation of the crack walls occurs when the chromium content of the base metal exceeds a minimal value. The accelerating effect of the tetrathionate electrolyte can be explained by the catalysis of hydrogen absorption by adsorbed sulphur, as well as a retarded repassivation (see also Section 2.1.2). The acidity of the electrolyte might further promote hydrogen evolution. According to Gomez-Duran and Macdonald [58], hydrogen embrittlement would likely cause fracture dimensions that correspond to the spacing of certain microstructural features, e.g. the grain size. The current signals found in microcapillary measurements are composed of much smaller single events, which are superimposed and form the peak groups as shown in Figure 47. This would imply brittle fracture events of only fractions of grain boundaries.

A final dismissal of any mechanism is not possible. Both mechanisms may explain the observed IG cracking along boundaries, and the correlation of crack dimensions and detected charge. A mechanism based on film rupture and anodic dissolution does not explain the nature of the reactivation events under constant deformation conditions, but this can be attributed to low temperature creep or chemical reactions with the oxide film. The occurrence of IG corrosion on certain surfaces proves the susceptibility of the sensitised grain boundaries towards anodic dissolution, and the application of stress might accelerate the dissolution process along a crack path. The hydrogen embrittlement mechanism does not explain IG corrosion on unstressed polished surfaces, and it would suggest larger current transients, which correspond to fracture dimensions in the range of whole grain boundaries. The film rupture mechanism and crack propagation by anodic dissolution therefore suggests itself more than a hydrogen embrittlement mechanism. Further studies are necessary to better understand the chemical reactions of tetrathionate and its decomposition product at the crack tip.

87 4.7.5 Limitations of the presented experiments The established measuring technique required a rather time consuming scanning process of the surface to find active surface spots. This made it difficult to establish a large data set for statistical evaluation. Furthermore, current signals measured on active cracks showed large variations due to the local microstructure, e.g. orientation of grain boundaries and presence of inclusions. Therefore only few parameters were investigated, and statistical evaluation showed large error bars. On the other hand, the fact that these variations between initiation points can be studied with this method is remarkable, and no other method so far allows the study of single micro crack growth. It was avoided so far to make a statement on the density of active sites on the differently sensitised materials. It would be an interesting question, how the applied stress or the DOS of the material influence the number of initiation sites. But the number of active sites on a selected area depended to a high degree on the abundance and size of MnS-inclusions, two parameters that varied greatly between samples, independent of stress and DOS. This is not a problem of the technique, but a property of the studied phenomenon. Multiple array measurements, as described in Section 4.5, with arbitrary positioning of the capillary, could establish a database large enough to make a valid conclusion on this point. Another limitation was the limited control of the applied stress. The simple three point bending cell used for all microcell measurements only allowed for constant deformation at the beginning of a measurement. A more advanced experimental setup, with the ability to apply active loading, might allow CERT tests with less aggressive electrolytes on a microscopic scale.

4.8 Summary It has been demonstrated that the early stages of crack growth of single micro cracks can be monitored over hours by electrochemical means. The microcapillary technique proved to be well suited for the study of single micro cracks in stainless steel. The flexible positioning and spot by spot scanning for active behaviour made it possible to measure selectively on active crack initiation sites. The specific current signals of single growing cracks can be explained satisfactorily by a crack growth mechanism of film rupture and anodic metal dissolution along the sensitised grain boundaries. However, the results of the presented experiments did not allow proving or dismissing a specific mechanism for IG crack propagation in the studied system. Some information was obtained on the location and cause of crack initiation, but experiments with varying surface preparation illustrated, that this information is specific for the investigated

88 sample and its surface state only. A comparison of initiation sites between microcapillary experiments and the corresponding macroscopic experiments was therefore limited. The initiation of cracks in absence of any visible surface weakness, such as a MnS-inclusion, could not be explained satisfactorily. A detailed surface analysis to assess such potential weak spot would have to be done before the actual electrochemical measurement.

89 5 Macro scale EN measurements

This section presents the results of EN measurements, performed on macroscopic samples under CL and slow strain rate, exposed to the tetrathionate electrolyte. Selected measurements are presented to illustrate the variety of EN signals recorded. Typical signal features that were repeatedly encountered during these measurements are analysed and assigned to different corrosion processes. A series of challenges arose during these tests, partially due to the change from the initially used thiosulphate electrolyte to the more aggressive tetrathionate solution (Section 3.5.4). Despite the large number of experiments performed, only a small number were regarded as successful. These experiments, running between 24 and 250 h, provided a large data set of different EN signals, which are introduced in this section. For identification, all measurements were labelled “RTL” (room temperature loop), followed by a two digit number. An overview of all experiments presented in this section is given in Table 6.

Table 6: Overview of selected macroscopic EN measurements. All measurements were performed in 0.01 M K2S4O6 at pH 2.2, except RTL 30 (pH 1.8). Designation DOS Load type Time Remarks on EN signal Surface Shown in under CL inspection RTL 30 22% CERT High activity during the severe IG SCC, Figure 23 (pH 1.8) whole measurement, pitting, I-transient > 10 µA, IG corrosion U < -300 mV/SCE RTL 47 22% CERT I-rise and U-drop during severe IG SCC, Figure 48 loading ramp extended pitting RTL 42 15% CERT fast transients during micro cracks Figure 49 loading ramp RTL 53 0.3% CL 250 h passive signal - Figure 50 RTL 44 15% CL 215 h stepwise signal changes micro cracks Figure 51 after 170 h. no large transients. RTL 43 22% CL 24 h stepwise signal changes, severe IG SCC, Figure 52 I-transient > 1 µA pitting RTL 49 22% CL 72 h high density of micro severe IG SCC, Figure 54 transients extended pitting two I-transients > 1 µA RTL 51 22% CL 72 h Stepwise signal change, severe IG SCC, Figure 56 I-transient > 10 µA pitting

90 5.1 Results of CERT tests Initial experiments were performed under constant extension rate conditions. The first macroscopic EN measurement in tetrathionate solution was already introduced in Section 3.5.4

(Page 54): A highly sensitised specimen was exposed to 10 mM K2S4O6 at pH 1.8. Large fracture events occurred during this CERT test after 15 h of dynamic loading, reaching 160 MPa. Failure of the sample occurred after 18 h. Besides extended IG SCC, the sample exhibited IG corrosion on parts of the exposed surface (Figure 24). These additional corrosion processes contributed to the overall EN signal and caused the high currents immediately after the sample was exposed to the electrolyte. For all following experiments a pH value of 2.2 was chosen to reduce the aggressiveness of the electrolyte. CERT measurement performed under these milder conditions exhibited passive phases with few transients before the constant extension rate phase, as can be seen in Figure 48 (RTL 47). Accelerating crack growth at high loads was indicated by several fast transients and finally a current rise above 1 µA and current drop by 400 mV. Slow current rise and potential drop, as well as single current transients of several hundred nA, indicated approaching macroscopic cracking (Figure 48c). Unloading stopped crack growth, and accordingly the current and potential signals returned to their initial values. Post-test inspection of these samples in the SEM still revealed extended pitting corrosion on the whole exposed surface and IG SCC at the notch root of the sample.

91

Figure 48: EPN (green) and ECN (red) measurement during a CERT test (RTL 47) on highly sensitised material

(DOS: 22%) in 0.01 M K2S4O6 (pH 2.2). Applied strain is indicated blue. a) Overview over 65 h. b) High-pass filtered (0.01 Hz) AC signal of the measurement. c) Detail view of transients that occurred before current rises above 1 µA and potential drops by 400 mV. Transients indicated with numbers were integrated for further evaluation (Section 5.3 and Appendix III).

CERT experiments on partially sensitised material (15% DOS) showed markedly less activity than on the highly sensitised material, as shown in Figure 49 (RTL 42). No continuous crack growth set in. Current transients with fast rise and exponential decay appeared during the constant extension rate phase, at stress levels above σy, and no further transients appeared after unloading of the sample. The raw EPN signals in Figure 49 only shows a very weak potential response to fast current transients, which cannot be discerned easily from the base line potential fluctuations. The high-pass filtered signal (Figure 49b) clearly shows corresponding potential transients. SEM investigation revealed micro cracks and very few pits.

92

Figure 49: a) EPN (green) and ECN (red) measurements of a CERT test (RTL 42) on partially sensitised material

(DOS: 15%) in 0.01 M K2S4O6 (pH 2.2). Applied strain is indicated blue. b) The high-pass filtered (0.01 Hz) AC signal clearly shows both potential and current transients.

5.2 Results of CL tests CL tests were performed according to the schematic shown in Figure 27 (page 59). Samples were stressed to the desired level from the beginning of an experiment. During the stabilising phase, the experiments were exposed to distilled water only. In ideal cases, EN signals during these stabilising phases reached constant values after a few hours. At the end of the stabilising period, a concentrated solution of tetrathionate and sulphuric acid was added to the electrolyte loop to establish the final concentration of 0.01 M K2S4O6 and pH 2.2. The manipulations on the setup during addition caused vibrations of the system and induced potential and current fluctuations for a limited period of time. 5.2.1 General behaviour of solution annealed samples The solution annealed material showed very little activity when exposed to tetrathionate. It did not suffer from IG SCC in the investigated time frame, and metastable pitting corrosion only occurred scarcely. A typical measurement on solution annealed material is shown in Figure 50

(RTL 53). The sample was stressed to 275 MPa, close to σy (290 MPa). Current fluctuations over 250 h are below 10 nA, and the signal exhibited stable phases below 1 nA over hours.

93

Figure 50: EPN (green) and ECN (red) measurement of a CL test (RTL 53) at 275 MPa on solution annealed material (DOS: 0.3%) in 0.01 M K2S4O6 (pH 2.2). The signal exhibits very few fluctuations over a time span of 250 h (11 d).

5.2.2 General behaviour of sensitised samples EN measurements on sensitised samples generally showed increased activity compared to solution annealed material, with highly sensitised material being more active than the partially sensitised one. Little activity was detected during the stabilising phase in distilled water. Transients often appeared directly after the addition of tetrathionate to the electrolyte recirculation loop. ECN measurements showed distinct current transients in positive directions and EPN measurement showed corresponding potential drops as seen in CERT tests. Under CL conditions, samples exhibited highly non-stationary signal behaviour: In many cases, activity increased with time, indicating consecutive corrosion attack and increasing crack growth. Transient shapes, amplitudes and occurrence frequencies varied not only between the different materials, but between each measurement, even under identical experimental conditions. CL experiments on partially sensitised material (DOS 15%) Two CL experiments on partially sensitised material are shown in Figure 51a and b. The first experiment, designated RTL 44, was performed at 230 MPa. EN signals showed fluctuations over 150 h of exposure to tetrathionate, after which rising current and a potential drop were observed. During a mostly passive period, the reference electrode leaked and ran dry, leading to data loss of the EPN signal. The following experiment, designated RTL 48, was performed at 275 MPa, but showed less activity than the previously presented experiment. This contradictive result indicates that variations in surface micro structure have a large influence on the corrosion resistance of a specific sample. Few single transients occurred over 200 h of passive behaviour. SEM investigation revealed micro cracks and few pits on both samples, spread over the whole exposed surfaces.

94

Figure 51: EN EPN (green) and ECN (red) measurement during CL tests on partially sensitised material (DOS:

15%) in 0.01 M K2S4O6 (pH 2.2). a) RTL 44, performed at 230 MPa. b) RTL 48, performed at 275 MPa.

Selected CL experiments on fully sensitised material (DOS 22%) CL tests on fully sensitised material at nominal loads of 230 MPa or higher showed macroscopic cracking in the studied time range. RTL 43, shown in Figure 52, was performed at a constant stress of 230 MPa. The disturbance of the signals at 0 h was caused by manipulation on the setup during the addition of the electrolyte. As in the case of RTL 47 (Figure 48), macroscopic cracking was preceded by a baseline current increase and fast transients of several hundred nA, and corresponding potential drop. The sample was unloaded and removed after 24 h. SEM images of the samples taken after the measurement confirmed IG SCC in the notch exposed to the electrolyte (Figure 53). Only little pitting corrosion appeared on the sample surface. The sample was mechanically overloaded to complete fracture, to reveal the crack wall areas. Figure 53b shows a fracture plane. The region of IG cracking can clearly be discerned from ductile fracture.

95

Figure 52: EPN (green) and ECN (red) measurement during a CL test (RTL 43) on highly sensitised material (DOS:

22%) at 230 MPa in 0.01 M K2S4O6 (pH 2.2). a) Overview over 3.5 h, until large transients occurred. b) High-pass filtered (0.01 Hz) AC signal of the measurement. c) Detail view of two transient groups that occurred before a large current rise and potential drop. Transients indicated with numbers were integrated for further evaluation (Section 5.3 and Appendix III). d) High-pass filtered detail view.

96

Figure 53: SEM images of the sample from RTL 43. No cracks were visible directly after unloading. a) Two IG cracks appeared in the notch after additional mechanical loading in air after the experiment. b) Mechanical overloading to complete fracture of the sample revealed the crack wall area.

In a subsequent experiment, labelled RTL 49, the load was increased to 275 MPa (Figure 54). A series of current transients below 40 nA occurred directly after the addition of tetrathionate and the current baseline showed a slow increase. These small signals were not accompanied by an increasing baseline current, and they were followed by a passive phase. As already observed for experiments on partially sensitised material, a higher load did not necessarily lead to faster macroscopic cracking. Initial fast transients < 40 nA are shown in detail in Figure 54c. Such small and fast transients often did not appear in the unfiltered potential signal. The high-pass filtered signals in Figure 54b and d clearly show both current and potential transients. These fast signals are likely caused by metastable pitting, which set in immediately after the surface came in contact with the tetrathionate solution. SEM images of RTL 49 (Figure 55) are in agreement with this interpretation. Besides extended IG SCC, the surface showed circular pits over large areas of the exposed notch (Figure 55b).

97

Figure 54: EPN (green) and ECN (red) measurement during a CL test (RTL 49) on highly sensitised material (DOS:

22%) at 275 MPa in 0.01 M K2S4O6 (pH 2.2). a) Overview of the whole measurement. b) High-pass filtered (0.01 Hz) AC signal of the measurement. c) Detail view of the initial phase, exhibiting a high number of small current transients. Transients marked by numbers were integrated for further analysis (Section 5.3 and Appendix III). d) High-pass filtered signal of fast transients.

98

Figure 55: SEM image of RTL 49 after the measurement. a) The mechanically overloaded sample reveals extended IG cracking along the notch. b) The surface exhibited a high number of pits. Some of them were scattered over the whole exposed area and exhibited mostly circular shapes, while others were concentrated along grinding grooves, forming surface cracks.

RTL 51, presented in Figure 56, was conducted under identical conditions as RTL 49, shown before, at a nominal stress of 275 MPa. It exhibited a distinctly different behaviour, which was confirmed by the EN signal and the inspection of the sample in the SEM after the experiment. After an initial disturbance during addition of tetrathionate, this measurement exhibited very clear stepwise transitions from passive behaviour to different levels of activity, finally resulting in a current jump above 11 µA and potential drop below -300 mV. After this large transient, constant current rise and potential drop indicated the transition to continuous, accelerating crack propagation. High-pass filtering of the signal clearly shows the large current and potential transients, as well as the transition from passive to active behaviour at 25 h (Figure 56b and d). Detail view of the transition from passive behaviour to macroscopic crack growth is shown in Figure 57. It illustrates the order of magnitude of the stepwise change in current. The initial change in current by 50 nA is barely visible, and the following current jumps of 150 and 200 nA are still small compared to the final current jump by 10 µA at 55 h.

99

Figure 56: EPN (green) and ECN (red) measurement during a CL test (RTL 51) on highly sensitised material (DOS:

22%) at 275 MPa in 0.01 M K2S4O6 (pH 2.2).a) Overview of the measurement. b) High-pass filtered (0.01 Hz) AC signal of the measurement. c) Detail view of the transition from passive to active behaviour. d) Detail view of the high-pass filtered signal.

100

Figure 57: Detail view from Figure 56. The transition from passive behaviour to micro crack propagation and to macroscopic cracking illustrates the different scales of signal jumps. After macroscopic cracking sets in, the current rises continuously until the sample is unloaded.

SEM investigation of the sample revealed IG cracking almost completely around the circumference of the sample (Figure 58a). Mechanical overloading in air after the measurement revealed that IG SCC extended over ca. 70% of the sample cross section (Figure 58b).

101

Figure 58: a) SEM images of RTL 51 after the measurement. IG cracking occurred along the notch. Additional pits can be found in some regions of the notch. b) The sample was mechanically overloaded to complete fracture after the measurement. The fracture plane reveals extended IG SCC over approximately 70% of the sample cross section.

5.3 Interpretation of current transient shapes The main tool for the interpretation of EN data was the analysis of single transients. Current transients were characterised by amplitude, duration and shape. Selected signals were integrated separately to calculate the respective charge transfer values and estimate the extent of the corrosion damage related to these discrete events.

5.3.1 Classification of transient types Each macroscopic EN measurement exhibited a unique signal pattern. To facilitate the interpretation of the various signals, which occurred over the course of these measurements, classes of similar current transients are introduced. Figure 59 gives an overview of the three main types of signals that were extracted from ECN signals. These classes are by no means absolute or complete. However they encompass reasonably well the majority of transients that occurred.

102

Figure 59: Typical current transient shapes encountered in CERT and CL tests. Type I: Peaks of short duration (1 – 5 s) and low amplitude (< 50 nA). These signals might indicate metastable pitting or micro crack initiation and growth steps. Type II: Series of superimposed transients, typically exhibiting fast rise and slow decay, and lasting 10 – 30 min. No single transients can be isolated. Amplitudes are in the range of 30 – 50 nA. Type III: Single transients of longer duration (5 s – 5 min) and high amplitude (50 – 1000 nA). These signals were often detected before larger current transients (> 1µA).

Type I incorporates fast and small current peaks that were encountered during many measurements on sensitised material (both 15% and 22% DOS). Only few of these transients appeared on solution annealed material. They often appeared directly after the addition of tetrathionate to the electrolyte system during CL tests. Transients exhibited durations of a few seconds. Amplitudes were mostly below 20 nA, occasionally reaching higher values. Transient shapes showed variations and fell generally in two categories: Either a slowly increasing current rise was followed by an abrupt decay, or the inverse shape occurred, with a sudden current rise and a slower (exponential) decay (Figure 60). These transient shapes might be explained by two different underlying mechanisms: Type Ia (Figure 60a) shows the characteristic shape expected for a film rupture event, with sudden exposure of bare metal causing fast current rise, followed by exponential decay due to continuous repassivation [133]. Such transients might be related to micro crack initiation and growth steps, IG corrosion events or the formation of closely packed pits along grinding groves. Type Ib (Figure 60b) on the other hand exhibits a transient shape that is characteristic for metastable pitting. As described in [134], the current rises fast with pit growth after initiation. Pits grow by undercutting the passive film, until the pit breaks open and the local composition of electrolyte inside the pit is disrupted. This immediately stops the corrosion process, leading to a sudden current drop.

103

Figure 60: Type I current peaks, exhibiting similar amplitude and duration, but inverse shapes: a) Fast rise and exponential decay. b) Slowly accelerating rise and sudden decay.

Large numbers of small pits were found on samples after experiments which exhibited high densities of these transients. A particularly high number occurred during experiment RTL 49 (Figure 54). This corresponds with a high number of circular pits scattered over the whole surface, found after the experiment (Figure 55b). Fewer pits occurred on the partially sensitised material to some degree. No pitting corrosion was found on crack wall areas of IG cracks that had formed (Figure 61a). Only the machined outer surface was attacked. Occasionally very high densities of shallow pits occurred, ultimately resulting in uniform corrosion of limited surface areas (Figure 61b). Pits in the notched area were often lined up in grinding groves or surface scratches; closely packed, they formed kinds of micro cracks through the top layer of the surface (Figure 55b). These attacked groves could therefore serve as initiation sites for IG cracking. As opposed to typical pits, these surface damages often did not exhibit circular shapes. They more closely resembled very small cracks. It is likely that stress accelerated the formation of these specific pits via film rupture.

104

Figure 61: a) High density of metastable pits along grinding groves, next to an IG crack. The crack wall area shows no significant pitting. b) Extended formation of shallow micro pits caused uniform corrosion on some surface areas.

Type II transients prominently appeared in experiment RTL51, shown in Figure 56, where a clear transition from a passive signal to an active signal occurred. No single transient can be discerned, as many events are superimposed, but a grouping of events can be observed, exhibiting fast current rise and slow decay. Between these transient groupings, the decreasing current signal approaches the passive level, suggesting a discontinuous behaviour of the underlying processes. Maximum amplitudes of the transient groups are in the range of 30 – 50 nA, and their duration ranges between 10 – 30 min. The current signal in experiment RTL 51 shows a transition at 41 h (Figure 56c and detail views in Figure 62a and b). The frequency of events increases and the grouping becomes less clear. The signal does not reach values close to the passive level any more, which leads to an overall rise in baseline current (Figure 62b). Similar signals appeared in several measurements, often at an elevated base current (RTL 44, Figure 62c). Such signals are likely caused by simultaneous metal dissolution processes at multiple active sites. The fast rise and slow decay of the observed transient groups might be explained by a film rupture mechanism exhibiting extended periods of anodic dissolution before repassivation occurs. This corresponds to the propagation of IG micro cracks along single grain boundaries: Film rupture initiates the dissolution, which then proceeds along one or several grain boundaries until it stopped, possibly at a grain boundary triple point. This interpretation is supported by the current signals detected with the electrochemical microcapillary technique under potentiostatic conditions: They showed very similar transient patterns, which could be attributed to the propagation of single micro cracks.

105

Figure 62: Type II transient series over 10’000 s (3 h). a) Excerpt from the current measurement of RTL 51, shown in Figure 57 (27 – 30 h). Single transients could usually not be discerned, but transient groupings appear. The signal baseline is close to 0 nA. b) Excerpt from the same experiment (41 - 44 h): An increase in transient density occurred, leading to a rise in baseline current. c) Excerpt from a measurement on partially sensitised material (RTL 44): Superposition of an increased baseline current with type II transients.

Type III transients (Figure 63) reached amplitudes of 50 – 1000 nA. Transients lasted from a few seconds to several minutes. Typically, a fast current rise is followed by differently shaped decays. Current decays exhibited different shapes, e.g. a sudden drop to the initial base line, an exponential decay, or a “chair”-like shape, where current slowly approaches an elevated value, and final repassivation is delayed (Figure 63a-c). Some type III transients did not show a complete decay back to the initial current level. Instead, baseline current stabilised at an increased

106 level, creating a step shape in the signal (Figure 63d). Step sizes that occurred during CL experiments before large current transients ranged between 5 and 300 nA. The distribution of type III signals during measurements was non-uniform. Single transients could occur occasionally during a measurement, but more often these transients were grouped together: During some CERT and CL experiments on fully sensitised material, such transient groups appeared right before large current jumps (as seen in Figure 48 and Figure 52), where they apparently mark the transition of micro crack propagation to macroscopic cracking. A possible explanation for these transients under CL conditions is the coalescence of micro cracks, leading to the accelerated dissolution of metal along a whole grain boundary, or even several grains. Series of type III transients could then be explained as a cascade of coalescence events, which increase the real stress on the sample, due to the continuously decreasing surface cross section during crack growth. These series of transients could then finally culminate in macroscopic cracking. Wells et al. [109] uses a simple geometrical model to explain the shapes of current transients, assuming a semicircular crack front progressing along a grain boundary. A transient shape similar to the one shown in Figure 63c was derived, explaining the “chair-like” shape as caused by the limitation of a single grain boundary. Type III transients also prominently occurred during CERT tests on partially sensitised material (Figure 49). Under these conditions, type III transients were likely triggered by mechanical rupture of the protective oxide layer on the surface.

107

Figure 63: Illustration of typical type III transients. a) Fast rise and fast decay. b) Fast rise and exponential decay. c) Fast rise and delayed current drop. d) Series of type III current transients occurring before a current jump above 1 µA. Dotted lines indicate stepwise baseline current increase following some of the transients.

As initially stated, the classification of transient types into three categories is incomplete and to some extent ambiguous. Transients of type Ia and type III could exhibit the same typical shape of fast rise and exponential decay: The only discerning criterion may then be the amplitude. Current transients with amplitudes in the range of 20 – 100 nA could therefore not clearly be attributed to one or the other type. The transition from type Ia to type III is continuous, which implies that micro crack growth events range from fractions of single grain sizes to whole grain boundaries or even several grains.

108 5.3.2 Transition to macroscopic cracking Macroscopic cracking occurred on highly sensitised samples and was indicated by fast current and potential fluctuations with amplitudes of several µA and several hundred mV. Repassivation could still occur after these events during CL experiments, but SEM inspection showed IG cracks with lengths of several hundred µm. The detail view of such a transient in Figure 64 shows that the current rise, albeit steep, is not sudden, but rather smooth and constantly accelerating. The current rises over a period of ca. 240 s. This indicates that this large transient is not caused by a single brittle fracture event of the sample, which is followed by repassivation of the exposed crack walls. The transient shape is better explained by a continuously growing crack. With the expanding crack front, the number of film rupture events rapidly increases, and metal dissolution along grain boundaries is accelerated. Finally, repassivation becomes faster than crack growth/metal dissolution, and the current drops.

Figure 64: Detail view of the large transient that occurred during experiment RTL 51 (Figure 56).

5.3.3 Integration of characteristic current signals To verify the interpretation of current transients introduced above, typical current transients of all types were integrated over time to calculate the amount of charge. The equivalent volume of dissolved metal was calculated using Faraday’s Law, as described in Section 4.4 (Page 72). Macroscopic ECN curves generally showed too much activity to assign single surface defects to certain current peaks. A proper correlation of crack wall area and charge value was only possible for experiment RTL 51: the sample was overloaded in air after the measurement and the area of

109 the sample cross section that showed IG cracking was estimated using the SEM. To account for the intergranular surface roughness, an estimated correction factor of 1.41 was applied. A crack wall area of 58 µm2 was found. Integration of the ECN curve gave total charge of 0.15 C, which is equivalent to 5.5 · 10-3 mm3, using Faraday’s law. From these values, a mean crack width of 94 nm was calculated. This estimation of the cracked surface was conservative, as only the visible crack wall area could be taken in account. Therefore the result agrees reasonably well with the value of 80 nm that was calculated on the base of microcapillary data. For further calculations, this value is used. Charge values, equivalent metal volumes, theoretical crack sizes and crack growth rates for selected current transients and transient groupings are listed in Appendix III. For the calculation of crack lengths and their propagation rates, crack shapes were approximated to be semi-elliptical, with a ratio of maximum depth to surface length of 0.47 7, as described in 4.4. Integration of type I transients gave values equivalent to metal dissolution of 0.026 – 6.8 µm3. Based on the interpretation presented earlier, such volumes would account for hemispherical pits with diameters of 0.5 – 3 µm, as proposed for type Ib, or micro crack growth steps of 1 – 15 µm for type Ia. Both results agree with the expected sizes of these events. Integration of type II signal sequences such as the one shown in Figure 62a gives dissolution charge values of 120 – 150 µC. This is equivalent to crack lengths of 380 – 430 µm and would account for quasi-continuous crack propagation along a few grain boundaries. Crack propagation rates range from 0.053 to 0.067 µm/s. Signal sequences during the subsequent phase of increased activity (Figure 62b) show slightly higher values of 0.067 – 0.071 µm/s. For single micro cracks, monitored with the microcapillary, mean propagation rates of 0.02 – 0.04 µm/s were calculated, and crack lengths varied between a few µm up to 450 µm. The values are in a similar order of magnitude, which supports the proposed interpretation of these transients. The higher rates measured on the macroscopic scale indicate, that either several micro cracks were propagating in parallel, or several active spots along the crack fronts of micro cracks were active at the same time. Integration of type III transients gives theoretical crack lengths of 21 – 106 µm, which corresponds to the typical grain sizes of the material (30 – 100 µm). These events exhibited growth rates of 0.4 – 6.2 µm/s, which is considerably faster than type II transients. This implies the formation of cracks along a few grain boundaries in a matter of seconds or minutes.

7 An alternative would be the assumption of semi-circular crack shapes, as it was done by Wells et. al.[109]. By assuming a semi-elliptical shape with 0.47, calculated surface crack lengths increase by 3.2%, which is insignificant, compared to the large variation between the transients.

110 5.4 Further analysis of EN signals 5.4.1 Comparison of statistical parameters Mean and standard deviation were calculated for selected measuring periods of 1 h (from different tests), exhibiting the characteristic transient types introduced in 5.3. Table 7 shows mean and standard deviations for passive phases and phases exhibiting transients of types I-III. Mean potential values allow an estimation of the activity of a specimen, with lower values indicating higher activity. This worked well for the comparison of different sequences of the same measurement, e.g. the phases of different activity during RTL 51. The comparison of mean potential values between different experiments was less reliable, as OCP values varied between samples. Mean current values gave a more reliable comparison of surface activities. While mean values do not reflect the signal shape, the standard deviations of current and potential signals give an indication of signal fluctuation: The lowest values for both current and potential were found for passive states. Increasing values were found from type I to type II and up to type III transients, which exhibited the highest values. A remarkable difference can be seen between type III signals measured during CERT tests on partially sensitised material (RTL 42 and 35), and the CERT and CL measurements on fully sensitised material (RTL 43 and 47). Signals measured on the fully sensitised material exhibited the highest mean currents and lowest mean potentials, as well as the highest standard deviations. The sequences on partially sensitised material, although exhibiting high transients, showed low mean current values and markedly lower standard deviations, confirming significantly less corrosion on this material. The results presented in Table 7 generally depended on the selected time interval. Single transients were unevenly distributed, and depending on the number of transients in a specific time interval, the statistical analysis will give differing results. The non-stationarity of the typical EN signals therefore limits the validity of statistical parameters.

111 Table 7: Mean and standard deviation for selected EPN and ECN signals exhibiting characteristic EN patterns. Signal Sequence Sensitisation loading Potential Current type Mean Std.dev. Mean Std.dev.

(mV/SCE) (mV) (nA) (nA) RTL 51, 14 – 15 h Fully sens. CL Passive 205 0.0955 -2.18 0.278 RTL 53, 175 – 176 h Sol. ann. CL Passive 190 0.239 -0.620 0.0228 RTL 49, 1 h – 2 h Fully sens. CL Type I 259 1.55 -0.141 1.24 RTL 42, 40 h – 41 h Part. Sens. CERT Type I 246 0.526 4.66 0.0905 RTL 51, 28 h – 29 h Fully sens. CL Type II 172 4.76 15.5 8.20 RTL 51, 29 h – 30 h Fully sens. CL Type II 165 4.52 17.5 6.99 RTL 51, 46 h – 47 h Fully sens. CL Type II 137 1.77 31.3 4.38 RTL 51, 47 h – 48 h Fully sens. CL Type II 137 2.07 29.5 4.71 RTL 44, 207 h – 208 h Part. Sens. CL Type II 68.8 0.930 159 4.73 RTL 42, 47 h – 48 h Part. Sens. CERT Type III 197 10.5 10.8 10.3 RTL 35, 24 h – 25 h Part. Sens. CERT Type III 201 0.206 9.45 9.41 RTL 43, 2 h – 3 h Fully sens. CL Type III 89.6 46.9 36.2 42.2 RTL 47, 63 h – 64 h Fully sens. CERT Type III 35.9 9.78 75.5 28.6

5.4.2 Analysis in the frequency domain The macroscopic EN signals were transformed into the frequency domain, using FFT. PSD I curves were calculated for the measuring periods selected in Section 5.4.1, according to equation 11 (Section 2.3.3):

2 2 Ψ ( f ) = X ( f ) (11) x T T Typical curves are shown in Figure 65.

Figure 65: PSD I of 1 h periods of EN signals, exhibiting different characteristic transients. a) Comparison of PSD of passive state (measured on solution annealed material) and type I-III transients (measured on highly sensitised material). b) Comparison of different periods during the same experiment, exhibiting passivity and different phases of type II transients.

112 The curves in Figure 65a show clearly different PSD levels and roll-off slopes for the different signals. The highest PSD was calculated for type III signals, exhibiting a characteristic fluctuation in the roll-off slope, which is due to the single events dominating the current signal [99]. Clearly the lowest PSD is calculated for a passive phase on solution annealed material. Figure 65b compares PSD spectra calculated from 1 h periods during phases of experiment RTL 51 (Figure 56). The passive phase (14 – 15 h) exhibits the lowest PSD. The other passive periods showed similar values. It could not be discerned between the initial phase of type II transients, which showed clearly grouped transients (28 – 29 h and 29 – 30 h), and the later phase of increased frequency of type II events (46 – 47 h and 47 – 48 h). Type II signals at higher current (e.g. shown in Figure 62c) give similar PSD curves. The transformation of ECN signals to the frequency domain generally allowed distinguishing phases of different activity, but it did not provide any advantage over the analysis of the signal in the time domain for the present case. Like the statistical analysis, the results depend on the selected time interval, due to the non-stationarity of the observed EN signals.

5.5 Discussion 5.5.1 General results The presented macroscopic EN measurements confirm that the chosen electrolyte can initiate IG SCC under CL conditions in a matter of hours or days. This allowed the monitoring of the transition from a passive surface to micro crack formation and growth and finally to macroscopic cracking without changing the applied load. Additional corrosion besides IG SCC occurred during these measurements in the form of metastable pitting corrosion. Due to the rather high corrosion activities of sensitised samples, it was not possible to assign a certain signal transient to a specific surface damage. Thus, there is some degree of uncertainty in the interpretation of EN signals. Signals showed a highly non-stationary behaviour. Therefore the most part of signal analysis was done by visual inspection of EN signals in the time domain and by integration of current transients to calculate the equivalent dissolved metal volumes. Signal analysis in the frequency domain or by statistical parameters highly depended on the selected time frame. Furthermore, the signal shape and the time until initiation of IG SCC under CL conditions showed high variation. This limited the possibility to derive an initiation mechanism or to predict the occurrence of IG SCC for the investigated system (see also Section 5.5.4). Additional macroscopic experiments are necessary to derive a statistical evaluation of crack initiation times.

113 5.5.2 Crack initiation under CERT and CL conditions Metastable pitting corrosion was likely part of the initiation mechanism of SCC on the studied samples. SEM images of cracked samples showed closely packed pits along grinding grooves. The coalescence of these pits could lead to micro cracks in the upper surface layers, which could then serve as initiation points for IG SCC. For experiments under CL, crack initiation was therefore very likely triggered by ongoing corrosion processes on the surface, i.e. by “(electro)chemical” initiation. This type of initiation was mainly governed by the electrolyte and the surface micro structure of each sample. Variations in surface roughness and oxide thickness between samples could then explain the observed variation of EN signals and crack initiation time between experiments. While metastable pits are most likely initiation points for crack growth under CL, crack initiation under CERT tests could also result from mechanical rupture of the surface film at particular weak points, i.e. by “mechanical” initiation. This occurred reproducibly at nominal stress levels near

σy, which made CERT experiments more predictable and therefore easier to perform. It was also possible to initiate cracks on partially sensitised samples in CERT tests, while CL tests on this material showed only little activity during the studied time spans. Concerning a possible correlation of the shown results with IG SCC in real applications, both “(electro)chemical” and “mechanical” initiation are not directly comparable to the conditions present in a real system. Under real conditions, initiation is typically a very slow process and may be caused by a combination of both mechanisms. But irrespective of the initiation event, the EN signals measured during corrosion of a real system are expected to be similar to the signals detected on the presented experimental system, as long as cracks in the real system propagate by the same mechanism of anodic dissolution along corrosion-sensitive pathways. It cannot be expected that systems which tend to suffer from other forms of SCC (e.g. TG SCC of 304 stainless steel in environments containing chloride [135]), exhibit comparable signals.

5.5.3 Crack initiation and propagation: mechanism and influence of stress The EN signals encountered in this study could be satisfactorily explained as caused by anodic metal dissolution. Small transients with slow increase and sudden decrease were attributed to the formation and repassivation of metastable pits, as described in the literature [134]. Transients with fast rise and slower decay were interpreted as events of film rupture and subsequent repassivation [133]. While pitting corrosion was usually avoided in other studies by choosing less aggressive electrolytes, the characteristic transient shape of film rupture events was reported in a multitude of studies on IG SCC in AISI 304 stainless steel (see also Section 2.3.4): Wells et al.

114 [109] performed EN measurements on AISI 304 stainless steel under CERT conditions in thiosulphate solution: their ECN signals featured very low baseline currents, ranging below 2 nA. This allowed the clear detection of distinct current signals similar to type II and III transients. Current transients were successfully correlated with micro cracks. As mentioned in Section 5.3.1, transient shapes for dissolution of single grain boundaries were derived from a simple geometrical model, with results corresponding to type III transients with “chair-like” shape, that were found in this study. Current transients detected on the same material in thiocyanate solution under CERT conditions again showed the common transient shape of fast rise and slow decay [57]: Transients exhibited similar amplitudes as the type III transients in the present study. The amplitudes were reported to correlate with the sample displacement rate, which was measured in parallel. A second signal type was reported to occur, characterised as smooth current fluctuations, which often occurred directly after a large transient. These smooth fluctuations were attributed to anodic dissolution along grain boundaries. A rather high DC current above 7 µA is reported for these EN measurements, which is attributed to pitting corrosion. Such high base current signals make it highly unlikely to detect signals similar to type II transients, which might be attributed to the propagation of a single micro crack. The explanation of IG SCC proceeding only via film rupture and anodic dissolution is at odds with the studies of Gomez-Duran and Macdonald, which were already introduced in section 2.1.2: In particular, EN signals similar to the type III transients, were attributed to hydrogen embrittlement: These transients were found to be equivalent of IG cracks with lengths of 35 – 108 µm, similar to the values found in the present study. The authors claimed that such fracture dimensions are too large to be consistent with a slip dissolution mechanism and are better explained by brittle fracture events along single grain boundaries, noting that the dimension of brittle fracture events should correspond to the spacing of some metallurgical asperity, such as precipitates or grain boundary triple points [58]. The EN measurements in the present study showed that during CL tests, these large transients of type III were preceded by the much smaller type I and II transients and rising baseline currents. For these small transients, the argument of fracture dimensions is not valid, and an explanation based on anodic dissolutions is more likely. Type III transients are then explained as anodic dissolution events that are accelerated by high local stress. IG SCC in the studied system can therefore be described as a sequence of different phases, with varying duration and intensity between experiments: Initiation and micro crack growth (type I and II transients), micro crack coalescence (type III transients, stepwise signal changes), large

115 fracture events (current transients beyond 1 µA), and transition to macroscopic cracking and sample failure. A mechanism based on hydrogen embrittlement cannot be ultimately dismissed for certain aspects of crack propagation, but it is not necessary for the explanation of the encountered EN data. It was expected that an increase in stress leads to faster macroscopic crack growth during CL experiments, as it was found for microcapillary experiments. Such an influence of stress did not clearly emerge from the performed experiments: Experiments at nominal loads below 230 MPa did not show macroscopic cracking in the studied time frame. But increasing the stress from 230 MPa to 275 MPa did not necessarily lead to faster cracking in the experiments performed in this study. And while most cracks on round notched samples initiated near the notch root, where the highest local stress values occurred, cracks were also found on the notch walls, in regions of significantly lower stress. Other influences, such as the specific surface morphology, apparently had a large influence on crack initiation and propagation as well. A larger data set is required to confirm a statistically significant effect of stress on the occurrence of macroscopic cracks.

5.5.4 Identification of IG SCC and use of EN for monitoring applications The transition from passive behaviour to active metal dissolution could be detected very well in both EPN and ECN signals. EPN signals required the application of high-pass filtering to resolve fast transients with durations of only few seconds. No further signal treatment was necessary to detect these fast events in the ECN signal. For small transients, an unambiguous differentiation between metastable pitting and SCC initiation was not possible in the studied system. For environments where no metastable pitting occurs, signal interpretation is expected to be much clearer, allowing a reliable early detection of film rupture events and subsequent micro crack growth. Considering CL and CERT experiments on fully sensitised material, type III current transients indicated the onset of macroscopic cracking and might serve as a warning sign. In RTL 47 (see Figure 48), these transients appeared over a period of 12 min before a large current transient exceeded 1 µA. Likewise, a warning time of 30 min passed in RTL 43 (Figure 52) between the occurrence of type I transient series and a current jump above 1 µA. In RTL 49 and 51, no such transients announced the upcoming macroscopic cracking events. Instead, stepwise signal changes, which preceded macroscopic cracking, could serve as warning signs in these cases. Type III transients on partially sensitised material occurred only during CERT tests at high stress, probably due to mechanical film rupture events. No continuous crack propagation set in after these events. Type III transients in these cases therefore did not indicate imminent macroscopic

116 cracking. Their significance as warning signs for proceeding crack growth therefore depended on the experimental conditions.

117 6 Comparison of macro and micro scale measurements

6.1 Direct upscaling: Macrocapillary measurements For the direct upscaling from micro to macro scale SCC testing, measurements with unpulled capillaries were performed: The glass capillaries were not pulled to tips, but left at their initial diameter, cut to the appropriate length, and coated with the silicone rubber in the same way as for microcapillaries. The result was a macrocapillary with a diameter of 900 µm, exposing surface spots approximately 250 times larger than the normally used 60 µm micro capillaries. The application of these macrocapillaries allows a direct comparison of small and large scale measurements using an identical experimental setup. The same measuring procedure as for micro capillaries could be applied. Measurements with the two capillary types could therefore be performed under identical conditions.

6.1.1 Passive surface behaviour Unstressed samples generally showed passive behaviour (Figure 66). Passive surfaces could also be found on few stressed samples, but most stressed surface spots showed active behaviour when scanned with the large capillary diameter of 900 µm. The potentiostatic current measurements on passive spots showed current densities that were similar to those found in microcapillary measurements (-0.2 – 0 mA/cm2).

118

Figure 66: a) Potentiostatic current measurement with a 0.9 mm macrocapillary on a passive surface spot of an unstressed sample (DOS 22%) at -150 mV/SCE in 0.01 M K2S4O6 (pH 2.2). After an initial high current response to polarisation, the current drops and shows a slow trend to more positive values. b) Detail view of the current signal, showing fluctuations below the nA-range.

6.1.2 Current signals of active surfaces Potentiostatic current measurements on active surface spots are shown in Figure 67a – d. Active spots showed current densities in the range of -0.1 – +0.05 mA/cm2. The current densities on active spots are lower than on active micro cracks, due to the much smaller ratio of active to passive surface. Active surface spots exhibited additional anodic current of several hundred nA superimposed on the cathodic sum current of passive surface sites. Single active cracks in microcapillary experiments typically provided currents below 100 nA. Multiple cracks were therefore active during these measurements. Due to the overlap of several signals, no passive phases appeared during these experiments. Only few single transients appeared, and in most measurements, no individual growth events could be discerned. After the measurements, the investigated spots exhibited a slight dark sulphide precipitation. SEM images confirm that multiple cracks were active simultaneously on the surface spot (see Figure 68a and b).

119

Figure 67: Typical current measurements with the macrocapillary on active surface spots, performed on fully sensitised material (a and b, DOS: 22%) and partially sensitised material (c and d, DOS: 15%) at -150 mV/SCE in

0.01 M K2S4O6 (pH 2.2).

120

Figure 68: SEM Images of the investigated surface spots after the measurements shown in Figure 67. Multiple cracks are visible across the area exposed to the electrolyte. Dotted circles indicate the inner rim of the capillary.

6.1.3 Comparison with microcapillary measurements The data obtained from the array measurement described in Section 4.5 were used to model the current signal of a macroscopic measurement: It was taken advantage of the fact that during the array measurement, scans were made on both passive and active surface spots. This allowed the summation of all current signals to model the behaviour of the whole area. The total area exposed during the array measurements was 99 times the surface area of the 60 µm capillary, giving a value of 0.28 mm2. The area of the presented macrocapillary measurement was approximately two times larger (0.65 mm2). The resulting current signal is similar to the measurements performed with the macrocapillaries (Figure 69). This shows that the direct upscaling from micro scale results to the macro scale is possible. Simultaneous crack growth at multiple active sites leads to a raise in baseline current, and no single transients can be discerned. Analysis in the frequency domain confirmed the similarities of the signals. PSD I curves were calculated by FFT for signals measured with the macrocapillary as well as the simulated current signal derived from the microcapillary array measurement. Calculation over the first 10 minutes of these current measurements gives overlapping PSD curves (Figure 70).

121

Figure 69: a) Potentiostatic current measurement on a highly sensitised sample (DOS: 22%), with a 0.9 mm macrocapillary, at -150 mV/SCE in 0.01 M K2S4O6 (pH 2.2). b) Sum current of 99 potentiostatic current measurements with a 60 µm microcapillary under the same conditions as above, resulting in a total area of 0.28 mm2.

Figure 70: PSD spectra of the current curves shown in Figure 67 (brown, dotted) and the overlay of 99 micro spots shown in Figure 67 (black). All curves show similar values.

122 6.2 Discussion of microcapillary and macro scale EN measurements 6.2.1 Comparability of macro and micro scale results The macro and micro scale experiments introduced in Sections 4 and 5 had to be performed under different conditions, due to the specific requirements and limitations of the two setups. The effects of these differences on the comparability of results are listed below. It is clear that a direct comparability of the results is limited. Nevertheless, it was possible to identify transient types that appeared in both experimental approaches. This indicates that the underlying mechanisms of micro crack growth are identical for both studied approaches, and are not influenced by the differences in surface preparation, type of stress or measuring method.

Surface state Initial grinding of coupon samples was necessary for microcapillary samples, because they exhibited very rough surfaces after machining. While microcapillary measurements could be performed on such rough surfaces, the identification of micro cracks on them would not have been possible. The surfaces of round tensile specimens directly after machining were smoother, and no additional surface treatment was necessary. A reproducible and uniform grinding of the surface areas in the sample notch before a measurement was not possible. While the causes for crack initiation varied between the different sample designs, the different surface states were necessary for the comparison of early micro crack propagation on both scales. A high number of active sites and immediate initiation were necessary on the micro scale. Macroscopic measurements on such surfaces did not show any discernable signal types, as shown in Section 6.1. On macroscopic samples fewer active sites were favourable for the identification of different transients and their comparison to microcapillary measurements.

Sample Polarisation Microcapillary measurements were performed under potentiostatic conditions, measuring only the current while the potential was kept constant. A few OCP measurements on active surfaces have been performed, shown in 4.1. Active surface spots exhibited potentials in the range of -200 – -300 mV/SCE during the growth of a single micro crack. Similar potentials on the macro scale were measured only after the onset of macroscopic cracking. Hence, a comparison of potential signals between macro and micro scale experiments is not useful, as the ratio of active and passive surface area was much higher for microcapillary measurements. Signal analysis was therefore limited to current transients.

123 Application of stress Due to limitations of the experimental setup, only constant deformation could be applied during microcapillary experiments, whereas CL or constant extension rates were possible in macroscopic experiments. Crack formation under CL and constant deformation conditions could undoubtedly be assigned to SCC, while under dynamic loading conditions single fracture events might be interpreted as mechanical cracking, merely accelerated by additional corrosion. Local plastic deformation occurred in macroscopic CL experiments at the notch grounds, but crack initiation also occurred on notch walls, where local stress values below σy were calculated. Crack initiation below σy was also observed in microcapillary experiments. But as described in Section 3.6.3, plastic deformation was applied for most experiments. This compensated the decrease in local stress due to relaxation during crack propagation and caused larger cracks.

6.2.2 Comparison of current transients during crack growth As already introduced in Section 5.3, type II transients during macroscopic experiments closely resembled the typical signal shape of micro crack propagation, measured with the microcell technique (Figure 71a and b). Certain microcapillary measurements also exhibited single current transients similar to type Ia transients, exhibiting fast rise and slower decay (Figure 71c and d). These signals could be assigned to the formation of micro cracks. Single transients account for crack propagation by a fracture of a grain boundary, which can be explained by microscopic film rupture and anodic dissolution events. Type Ib transients, which would suggest the formation of metastable pits, rarely appear during microcapillary measurements. Accordingly, these samples did not exhibit pitting corrosion after microcapillary measurements.

124

Figure 71: Comparison of potentiostatic current measurements with the microcapillary and macroscopic ECN measurements. a) Macroscopic measurement, exhibiting type II transients. b) Microcapillary measurement, exhibiting similar current transient groups. c) Single type Ia transient, exhibiting fast rise and slower decay, detected during a macroscopic ECN measurement. d) Detail view of single current transients detected during a microcapillary measurement.

Comparison of type II signals from macroscopic and microcapillary tests in the frequency domain further confirms their similarity. Figure 72 shows that the current power spectral densities for different type II signals (a) are overlapping with curves calculated from microcapillary measurements (b). Macrocapillary measurements on the other hand, which were performed under

125 identical conditions as the microcapillary experiments, exhibit clearly higher activity. Macroscopic noise measurements, performed under different conditions therefore allow a better comparison of characteristic signal shapes than the macrocapillary measurements.

Figure 72: PSD I curves for different type II transients measured during macroscopic EN measurements (a), microcapillary measurements (b), macrocapillary measurements and the combination of several microcapillary measurements (c).

Type III transients during macroscopic EN measurements were identified as important warning signs for the onset of macroscopic cracking under CL conditions. The lack of similar transients in microcapillary measurements is in accordance with the interpretation of these transients as accelerated crack growth events, which were induced by the coalescence of micro cracks or active loading. Neither one of these possible causes was present during measurements with the microcapillary, as only single cracks were investigated, and constant deformation was applied. Hence, microcapillary measurements provided the possibility to identify certain elements of a macroscopic noise measurement, although different experimental conditions had to be applied. A

126 complete macroscopic signal can not be modelled, because certain signal features, which were attributed to crack coalescence events, did not occur in microcapillary measurements. Direct modelling of a macroscopic signal based on microcapillary measurements was possible for the measurements performed with the macrocapillary. These measurements illustrated how several active micro cracks lead to an elevated current signal, which does not exhibit any discernible single transients.

6.2.3 Mechanistic considerations Based on experiments with the microcapillary and the macroscopic setups, the initiation of IG SCC varied between different surface preparations. Cracks were either initiated by a chemical attack of the surface by the aggressive electrolyte, or by mechanical rupture of a surface film, due to dynamically changing stress during CERT experiments. Initiation points for chemical initiation on polished surfaces were grain boundaries. On ground coupons, the dominating initiation points were MnS-inclusions, further crack initiation was observed on surface defects and on surface areas that did not exhibit any visible weakness. In these cases, grain boundary triple points or local weaknesses of the oxide layer are possible reasons for crack initiation. Metastable pitting was likely responsible for crack initiation on round notched samples. Most initiation points have in common, that an initial local corrosion event or mechanical film rupture occurred, which in turn exposed a vulnerable point on a grain boundary. Independent of the surface preparation, a single crack propagation mechanism explains the current signals measured with the microcapillary as well as the observed noise signals for macroscopic CL and CERT experiments: Crack propagation was limited to a narrow metal zone along the grain boundaries, which was defined by the concentration profile of chromium. Micro cracks propagated in a stepwise process of film rupture, anodic dissolution and repassivation, forming semi-elliptical micro cracks. Along a single crack front, multiple propagation and repassivation events could occur in parallel, until complete repassivation occurred. Occasionally, corrosion could re-initiate on a micro crack, even under constant deformation condition. The reason for these re-initiation events might be attributed to diffusion processes inside the micro crack or low-temperature creep at the crack tip. More research is needed to clarify this behaviour. In the presence of other growing micro cracks, re-initiation can be readily explained by the constantly changing surface stress conditions. With proceeding crack growth, sudden signal steps on the macro scale indicated accelerating crack growth: Coalescence of micro cracks caused accelerated metal dissolution, indicated by large current transients. The resulting large crack fronts often exhibited additional active spots,

127 which led to a rise in base line current and further potential drop. Crack growth continued in this stepwise fashion, causing increasingly large signal transients. Finally, the decrease of sample cross section reached the point, where continuous crack growth set in, leading to continuous current rise and potential drop, until complete sample failure occurred.

Influence of sensitisation A variation of the degree of sensitisation in the studied range of 15 – 22% did not appear to have a large effect on the microscopic cracking behaviour: Crack initiation for both material treatments occurred mainly on MnS-inclusions. It could not be verified, if the sensitisation had an effect on the occurrence of crack initiation on surface spots in absence of these inclusions. Crack propagation showed a tendency towards shorter cracks for lower sensitisation. Macroscopic EN measurements on the other hand showed clearly less activity on the partially sensitised material and no transition to macroscopic cracking in the studied time frame. Further studies and the measurement of local chromium profiles are necessary to clarify the influence of sensitisation on IG SCC initiation and propagation on the different scales.

Early detection of IG SCC SCC during CL experiments exhibited a high variability, and the prediction of the time until macroscopic fracture and sample failure was generally not possible. The measurement of EN signals allowed the detection of warning signs for accelerating crack growth, and for certain environments, it might be possible to identify very early signs of cracking, and estimate the dimension of growing micro cracks: Such a favourable situation would be the sudden transition from a passive signal to type II transients. When the base line signal during passive behaviour is stable, a sudden signal change can be detected, and the characteristic shape of type II transients might be perceptible. The integration of type II transient groups would then allow the calculation of crack lengths, based on a presumed dissolution width. Under these conditions, the stepwise change from passive to active behaviour would allow a very early detection of IG SCC. If these transients cannot be detected clearly, type III transients and large stepwise signal changes can serve as warning signs. When these signals appear, IG SCC might already have caused extended damage, and the transition to macroscopic cracking might follow soon. On the other hand, a single type III transient might just be a unique event, caused by a fluctuation of stress, or the rupture of a surface oxide film due to the impact of a particle.

128 EN measurements are a suitable tool for the detection of earliest signs of corrosion, but the meaning of a single transient always depends on the specific system, its mechanical and chemical properties, and the fluctuations of experimental parameters.

129 Summary and conclusions

In the present work, electrochemical microcapillary and macroscopic EN measurements were combined to study the characteristic EN signals during initiation and early propagation of IG SCC of stainless steel. Initial work focussed on the development of an experimental system that allowed the study of IG SCC on two scales: Macroscopic study of IG SCC was already well established, but the use of the microcapillary technique for the monitoring of single stress corrosion cracks was a novelty. The chosen material, thermally sensitised AISI 304 stainless steel with high sulphur content, proved to be a good choice for study, as it provided a high density of micro crack initiation sites. Just as important were the use of a suitable electrolyte and the surface treatment of the samples. The critical point was the initiation of IG SCC. It had to occur in a short time frame, with minimal occurrence of other types of localised corrosion, which would interfere with the measurements. As only constant deformation could be applied during microcapillary measurements, an aggressive electrolyte had to be chosen. This in turn increased the risk of “unwanted” corrosion processes to occur. Acidic solutions of potassium tetrathionate fulfilled these criteria and proved to be well suited for the study of single micro cracks with the microcapillary technique. Systematic current measurements were performed on active surface spots where cracking appeared, to reveal typical signal patterns: Distinct active and passive signal periods showed that crack growth proceeds discontinuously, even on the scale of single grain boundaries. The characteristic current signal of a growing micro crack is composed of superimposed transients with fast rise and slow decay. This signal shape can be explained easiest by parallel events of film rupture, anodic metal dissolution along the sensitised grain boundaries, and repassivation. The influence of the surface state emerged later during the study. Grinding up to 4000 grit SiC paper without further polishing led to surfaces that exhibited distinct active locations, on which crack initiation occurred, while the rest of the surface remained passive. Immediate response of active spots to the electrolyte allowed the identification of such positions in a matter of seconds. Further polishing of surfaces was found to render all grain boundaries vulnerable to corrosion. The result was extended intergranular corrosion, independent of the applied stress. Hence, surface preparation heavily influenced crack initiation, and measurements were limited to certain specific types of surface preparation. In these cases, it was possible to identify typical crack initiation sites. Distinct active surface spots were found, mostly on the interface of bulk metal and MnS- inclusions but also on positions without any visible weakness. The direct transfer of these

130 observations to other surfaces is not necessarily valid. The specific requirements of surface treatment also limited further study of the surface, e.g. by EBSD measurements.

Upscaling of the surface area showed different results, depending on the sample and experimental conditions: macrocapillary measurements, performed under conditions identical to the microcapillary experiments, could be directly modelled from current data measured with the microcapillary: The simple addition of a sufficient number of microcapillary current signals, both on active and on passive surface spots, approximated the typical signals of macrocapillary measurements. A different situation was observed when round notched tensile samples without any prior surface treatment were used in EN experiments. These samples showed markedly less activity than ground coupon samples, which were used for the experiments with the micro- and macrocapillary. And compared to capillary measurements, the signal characteristics of macroscopic EN measurements varied greatly between experiments. The development of IG SCC on a macroscopic scale could not be predicted precisely, especially under CL conditions. Macroscopic ECN signals exhibited distinct transient types, which were assigned to the formation of metastable pits, the initiation and propagation of micro cracks, and to accelerated cracking events of whole grain boundaries. Transients indicating micro crack initiation and propagation were also observed regularly on the micro scale. Accelerated growth steps were attributed to coalescence of micro cracks, or, in case of CERT tests, mechanically induced film rupture. They could therefore not occur under the experimental conditions applied for microcapillary experiments. Large current transients which were assigned to these events are commonly documented in the literature as signs of IG SCC. For monitoring purposes, these large single transients can be easily detected and clearly discerned from baseline signal fluctuations. However, they are not the very first sign of IG SCC. Under favourable conditions, for systems with low background noise, type II transients indicating the very early stage of IG SCC initiation might be detected. Concerning the mechanism of IG SCC under the studied conditions, all observations indicate a film rupture mechanism. However, the present data does not allow a final dismissal of other mechanisms, particularly hydrogen embrittlement, as the characteristic current signals are not fully conclusive. To finally resolve this question, electrochemical methods alone, even when obtained on both micro and macro scale, might not be sufficient.

131 Outlook

As the aim of this project was the combination of two experimental techniques, much effort was put into the development of an experimental system suitable for both approaches. While the link between the two techniques could be made, many aspects of the two approaches could not be investigated in depth, leaving a number of open questions and possible topics for further studies. The main limiting factor of the presented macroscopic work is the small amount of experiments performed with the finally introduced tetrathionate electrolyte under suitable loading conditions. These experiments were time consuming and prone to interferences, and usually a conflict arose between stopping an experiment after the first transients for identification of corrosion damage, or waiting until macroscopic cracking set in, to monitor the development of the EN signals over time. But the few successful experiments under CL conditions already proved to produce EN data that is likely not obtainable in CERT measurements; therefore the introduced system should be studied further. Additional experiments are necessary to refine the presented interpretation of current transients and clarify the role of pitting in crack initiation. Extended experiments, lasting for weeks or months, could provide data on the initiation of macroscopic cracking for less sensitised material, which did not exhibit large current jumps in the studied time frames up to 200 h. Considering the microcapillary technique, it is tempting to find additional systems that allow the monitoring of SCC, to confirm the presented results, or to expand the technique to different cracking mechanisms. A comparison of the characteristic current signals of different SCC mechanisms on the micro scale might lead to new insight and better identification of these mechanisms on the macroscopic scale. Especially the study of hydrogen embrittlement on the micro scale by introduction of hydrogen into a sample could provide useful information. Another open question is the influence of surface preparation. This aspect was only brought up briefly in this work, as difficulties were encountered during measurements on highly polished surfaces. The influence of surface oxide layers on the electrochemical behaviour of a surface, and the differences in surface structure between micro spots need further study to improve the understanding and predictability of microcapillary measurements. For future development of the microcell technique, an improvement of the cell design to include the simultaneous visual monitoring of a surface spot seems to be an ambitious but promising goal. The parallel recording of electrochemical and visual information during micro crack propagation could significantly improve the understanding of localised corrosion processes. This

132 would however require a redesign of the microcell and likely the whole experimental setup. Furthermore, the implementation of a dynamic loading device could open new possibilities for the study of SCC on the micro scale.

133 Acknowledgement

First and foremost, my thanks go to Prof. Dr. Sannakaisa Virtanen for giving me the possibility to write this thesis. Her supervision and motivation during the whole thesis was invaluable. I thank the examination committee for their time and effort. I would like to thank my supervisors at PSI and EMPA, Hans-Peter Seifert and Thomas Suter, for their guidance. Special thanks go to Stefan Ritter for his dedication to this project and his continuous support. The financial support of this thesis by the Swiss Federal Nuclear Safety Inspectorate (ENSI) is gratefully acknowledged.

This thesis would not have been possible without the help and technical support from my co- workers: My colleagues at PSI, especially Beat Baumgartner, Leonard Nue, Roger Schwenold, Urs Tschanz, and Patrick Simon. I thank Hans Leber for the EBSD measurements and an introduction to metallography, and Klaus Reichlin for the finite element calculations of stress concentrations on round notched samples. And my colleagues at Empa, especially Alessandra Beni, Patrik Schmutz, Martin Sauder and Ronald Lay. I thank James Derose for the many fruitful discussions and his help with proof reading during writing.

Finally, I thank my family and friends for their continuous support over more than four years of woe and distress: My mother and father, for all their love and encouragement. My brother and fellow outdoorsman, for having my back. Katrin, Ursula, Anna, Raph and Katarina, for accompanying me on my adventures.

134 References

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142 Publication list

M. Breimesser, S. Ritter, H.-P. Seifert, T. Suter, S. Virtanen, Electrochemical noise of SCC in austenitic stainless steels: A combined macro- and microelectrochemical approach, in: EUROCORR 2010, Moscow, Russia, 2010.

M. Breimesser, S. Ritter, H.-P. Seifert, S. Virtanen, T. Suter, Application of the electrochemical microcapillary technique to study intergranular stress corrosion cracking of austenitic stainless steel on the micrometre scale, Corrosion Science 55 (2012) 126-132.

M. Breimesser, S. Ritter, H.-P. Seifert, S. Virtanen, T. Suter, Application of electrochemical noise to monitor stress corrosion cracking of stainless steel in tetrathionate solution under constant load, Corrosion Science, accepted for publishing.

143 Appendix

Appendix I: Signal interferences in macroscopic EN measurements

A series of different problems and interferences were encountered during macroscopic EN measurements, usually appearing after a detail of the setup or the measuring procedure was changed. Some of the interferences that occurred repeatedly could be identified and eliminated:

Bubble formation in the electrochemical cell Depending on the weather, humidity and temperature in the laboratory, air bubbles appeared in the electrochemical cell in some experiments. Generally, the incorporation of air in the electrolyte during preparation should be avoided. Degassing of the distilled water by heating it to 80 °C for 2 h was done in some cases. During a measurement, the cell was checked regularly for bubbles. If they appeared, they were removed to avoid blockage of the Haber-Luggin capillary (see Figure I- 1).

Figure I-1: Air bubbles can block the Haber-Luggin capillary partially or completely and therefore interfere with potential measurement.

Blockage of glass frit between cell and reference electrode If a saturated calomel electrode was used as a reference, it was observed that from one experiment to the next the fluctuation of the potential signal could increase. It was found that the potassium tetrathionate tended to block the frit that was used for the connection of the reference electrode with the setup. If not corrected, this could lead to wrong potential measurements. Cleaning of the frit surface or replacing the frit re-established conductivity.

144

Figure I-2: A blocked frit between the electrochemical cell and the reference electrode heavily disturbs the potential measurement. After removing the frit just before t = -44 h, the potential signal stabilises at the correct value.

Interference induced by thermocouple The thermocouple that was used to measure the electrolyte temperature induced interferences in both potential and current signals. Temperature measurements were therefore performed only at the beginning and at the end of an experiment. During a noise measurement the thermocouple was kept unplugged.

Figure I-3: Characteristic interference caused by the thermocouple during a noise measurement. These peaks disappeared after disconnection of the thermocouple.

145 Vibration of the cell due to electrolyte flow While the electrolyte pump itself often did not cause any interference, the outflow of electrolyte to the storage tank could significantly distort the signal. When a thin outlet tube was used, the electrolyte tended to collect in the tube, due to surface tension, and then suddenly flow out. This caused periodic vibrations of the whole cell and distorted both current and potential signal. The transients were unevenly spaced, and their amplitude varied. An example is shown in Figure I-4, but the appearance of this interference could vary. This interference was identified by removing electrolyte from the cell during a running experiment, to stop flow out. The interference vanished, and it reappeared when the cell was completely filled again and electrolyte flow out of the cell continued. The use of a larger outlet tube and adjusting the length and angle of the tube enabled a more continuous electrolyte flow and reduced this interference. However, depending on the positioning of the outlet tube, this interference often reappeared.

Figure I-4: Characteristic signal interference caused by periodic electrolyte outflow. Each current and potential transient was directly correlated with a quantity of electrolyte flowing out of the outlet tube.

Interference peaks from the loading unit The loading unit occasionally caused characteristic interference peaks. These signals showed constant amplitude in negative direction and lasted only one data point, even at a data acquisition frequency of 100 Hz. Changing of the detection limit also changed the amplitude of these transients. Peaks were not evenly distributed, but occurred in high frequency for limited time periods and then disappeared again for hours or days. They appeared in the potential and current measurement, but not necessary in parallel with each other. These peaks were caused by control signals from the Cormet system to the loading unit and interfered directly with the ECMNoise device through the grounding of the faraday cage. The

146 interference could be minimized by changing the grounding of the setup: The Faraday cage, which was in contact with the loading unit, was disconnected from the ECMNoise ground. Instead, it was grounded on the mains ground. Additionally, the insertion of a 400 Ω resistor in the ground of the Faraday cage shielded the measuring setup against these interference signals. Due to their constant amplitude and duration, these peaks had a very characteristic shape and could be easily identified and removed from the data.

Figure I-5: a) A cluster of characteristic fast interference peaks from the Cormet loading unit. Peaks appeared in both potential and current signal for a limited time. b) Signal after manually removing the interference peaks.

147 Appendix II: Attempt to correlate crack growth direction with grain orientation by EBSD measurements

EBSD measurements were performed on the systematically scanned surface introduced in Section 4.5, to establish a map of grain orientation and active surface spots. It should be evaluated, if crack initiation and early propagation could be attributed to a certain grain orientation, and if specific grain boundaries showed a higher susceptibility towards IG SCC. EBSD measurements of good quality can only be obtained, if the top layer (10 – 50 nm) of the specimen is strain-free and clean from contamination. The array specimen did not meet these requirements after the microcapillary measurements. The surface was polished with 3 µm and 1 µm diamond suspension 6 min each. The sample was then vibration polished with silica emulsion for 3 h. After this polishing procedure, an EBSD measurement on the array was performed (Figure II-1):

Figure II-1: Microstructure of the scanned surface area, shown in inverse pole figure with overlay of scanned micro spots. Active spots and micro cracks are indicated in black. Passive spots and micro hardness imprints are indicated grey.

148 The EBSD measurement still exhibits areas with low quality, but already several µm of material were abraded from the surface, and as a consequence, most micro cracks were removed, and the positions of grain boundaries on the polished surface are shifted. Only on a few positions, grain boundaries showed orientations similar to the cracks (Figure II-2), but the assignment of cracks to these grain boundaries is difficult. These results show that the combination of electrochemical microcapillary measurements with EBSD measurements to study IG SCC in the present system was severely limited. The two methods had opposed requirements concerning surface: They could not be performed on the same surface, and any changing of the surface between the two measurements severely alters the results. It might be possible to follow the course of larger cracks, which are allowed to grow deeper into the material and are therefore still visible after surface polishing. The correlation of initiation and early crack growth with grain orientation, which was the main point of these experiments, was not possible.

Figure II-2: Detail view of three surface positions. Capillary positions and formed cracks are indicated as black lines. Dotted lines indicate possible grain boundaries, along which the initial cracks might have formed.

149 Appendix III: Integrated charge values and resulting theoretical crack lengths and crack growth rates for selected current transients

Table III-1: Type I transients. Crack wall Volume of Surface crack length Theoretical Experiment Charge area (µm2) for dissolved (µm), calculated for crack growth (peak number) (µC) const. width metal (µm3) semi-elliptical cracks rate (µm/s) of 80 nm

RTL 49-c 0.058 2.1 27 8.5 1.4

RTL 49-d 0.013 0.48 6.0 4.0 2.7

RTL 49-e 0.026 0.96 12 5.7 1.0

RTL 49-f 0.008 0.29 3.7 3.2 2.1

RTL 49-g 0.12 4.4 55 12 1.4

RTL 49-h 0.053 1.9 24 8.1 1.5

RTL 49-i 0.01 0.37 4.6 3.5 2.4

RTL 49-j 0.002 0.074 0.9 1.6 3.2

RTL 46 0.062 2.3 28 8.8 0.88

RTL 46 0.018 0.66 8.3 4.7 0.59

RTL 46 0.040 1.5 1.8 7.1 1.0

RTL 46 0.011 0.40 5.1 3.7 0.52

RTL 46 0.0015 0.055 0.7 1.4 2.7

RTL 46 0.0007 0.026 0.3 0.9 3.1

RTL 46 0.19 6.8 85 15 0.29

150 Table III-2: Intervals of 2 h, exhibiting superposition of type II transients. Crack wall Volume of Surface crack length Theoretical Experiment, Charge area (µm2) for dissolved (µm), calculated for crack growth time interval (µC) const. width metal (µm3) semi-elliptical cracks rate (µm/s) of 80 nm RTL51, 150 5500 69’000 430 0.066 26 h – 28 h; RTL51, 120 4400 55’000 380 0.053 28 h – 30 h; RTL51, 150 5400 68’000 430 0.059 30 h – 32 h; RTL51, 190 6800 85’000 480 0.067 40 h – 42 h; RTL51, 200 7400 93’000 500 0.070 42 h – 44 h; RTL51, 210 7700 96’000 510 0.071 44 h – 46 h; RTL44, 1200 42’000 530’000 1200 0.17 198 h- 200 h RTL44, 950 35’000 440’000 1100 0.15 200 h- 202 h RTL44, 100 38’000 480’000 1100 0.16 202 h- 204 h

151 Table III-3: Type III transients. Crack wall Volume of Surface crack length Theoretical Experiment Charge area (µm2) for dissolved (µm), calculated for crack growth (peak number) (µC) const. width metal (µm3) semi-elliptical cracks rate (µm/s) of 80 nm

RTL 47-c 0.42 15 190 23 1.8

RTL 47-d 0.68 25 310 29 4.1

RTL 47-e 0.50 18 230 25 6.2

RTL 47-f 0.42 15 190 23 4.6

RTL 47-g 0.37 13 170 21 4.3

RTL 47-h 0.36 13 170 21 3.0

RTL 46 9.1 330 4200 106 0.39

9.1 0.89 RTL 46 330 4200 106

RTL 43-c 1.3 48 600 40 5.8

RTL 43-d 5.4 200 2500 81 1.4

RTL 43-e 2.1 78 970 51 2.3

RTL 43-f 0.44 16 200 23 3.3

RTL 43-g 4.8 170 2200 77 3.5

RTL 43-h 0.81 30 370 32 2.1

RTL 43-i 1.3 46 570 39 2.2

152