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Microelectrochemical Approach Towards the Analysis of Electrochemical Noise Signals Related to Intergranular Stress Corrosion Cracking of Austenitic Stainless Steel

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Microelectrochemical Approach Towards the Analysis of Electrochemical Noise Signals Related to Intergranular Stress Corrosion Cracking of Austenitic Stainless Steel 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 ................................................
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