Hydrogen embrittlement in ferrous and non-ferrous alloys is, and has been for over a century, a prominent issue within many sectors of industry. Despite this, the mechanisms by which hydrogen embrittlement occurs and the suitable means for its prevention are yet to be fully established. As hydrogen fuel becomes a prominent feature in modern concepts of a sustainable global energy infrastructure and nuclear power enters its renaissance, with commercially viable fusion plants on the horizon, hydrogen embrittlement is becoming an ever more pertinent issue. This has led to a considerable demand for novel alloys resistant to hydrogen embrittlement, notably within the bearings industry, where the commonly conflicting properties of high strength and hydrogen embrittlement resistance are required. This work investigates the mechanisms through which hydrogen embrittlement and irradiation damage occur in steels and nickel-based alloys respectively, with novel alloys designed for improved resistance. Through the engineering of secondary phases, optimised for helium and/or hydrogen trapping capacity, the novel alloys present the benefits of such trapping species with respect to embrittlement resistance. Such species have been studied in depth with respect to their interactions with hydrogen, establishing a novel mechanism of hydrogen embrittlement - the hydrogen enhanced dissolution and shearability of precipitates, leading to enhanced localised plasticity.

Advanced high strength steels (AHSS) have high susceptibility to hydrogen embrittlement, and are often exposed to hydrogen environments in processing. In order to study the embrittlement and recovery of steel, tensile tests were conducted on two different types of AHSS over time after hydrogen charging. Concentration measurements and hydrogen microprinting were carried out at the same time steps to visualize the hydrogen behavior during recovery. The diffusible hydrogen concentration was found to decay exponentially, and equations were found for the two types of steel. Hydrogen concentration decay rates were calculated to be -0.355 /hr in TBF steel, and -0.225 /hr in DP. Hydrogen concentration thresholds for embrittlement were found to be 1.04 mL/100 g for TBF steel, and 0.87 mL/100g for DP steel. TBF steel is predicted to recover from embrittlement within 4.1 hours, compared to 7.2 hours in DP steel. A two-factor method of evaluating recovery from embrittlement, requiring hydrogen concentration threshold and decay rate, is explained for use in predicting recovery after exposure to hydrogen. Anisotropic hydrogen diffusion rates were also observed on the surface of both steels for a short time after charging, as hydrogen left the surface through <001> and <101> grains faster than grains with <111> orientations. This could be explained by differences in surface energies between the different orientations.

Cette étude porte sur la sensibilité à la fragilisation par l'hydrogène (FPH) d'un alliage base nickel, l’Inconel 718 issu d’un procédé de fabrication additive (FA), sous sollicitation cyclique. Cematériau est utilisé pour la réalisation de certains composants des ensembles propulsifs d’Ariane qui sont fabriqués par Ariane Group. Dans ce domaine, certaines pièces sont sollicitées sous « ambiancehydrogène ». Ainsi, le risque de fragilisation de ces pièces est une problématique de premier ordre.Pour cela, nous avons caractérisé l’état métallurgique d’un Inconel 718 FA à différentes échelles structurales afin d’observer un possible impact du procédé de mise en œuvre sur la microstructure, puis d’envisager sa conséquence sur le comportement mécanique et la sensibilité à la FPH de l’alliage. Notons que nous avons conduit une étude plus réduite sur un alliage forgé afind’avoir des éléments de comparaison. Afin de comprendre les mécanismes de fragilisation par hydrogène de l’Inconel 718 FA, il est nécessaire de connaitre l’état et la mobilité de l’hydrogène dans le matériau. Des analyses deperméation électrochimique (PE) et gazeuse (PG), ainsi que de spectroscopie de désorption thermique (TDS) apportent des éléments de compréhension des mécanismes de piégeage et de diffusion de l’hydrogène. Ils permettent de discuter l’implication des joints de grains dans les mécanismes dediffusion ainsi que l'effet des hétérogénéités microstructurales sur les mécanismes de piégeage dans l’Inconel 718 FA.D'autre part, l’influence de l’hydrogène sur le comportement mécanique a été questionnée en traction et en fatigue oligocyclique en terme de comportement élasto-viscoplastique, de processus d’initiation, de propagation de fissure en fatigue et de ténacité. Dans ce cadre nous présentons, dans unpremier temps, l’étude du comportement en traction et sa sensibilité à l’hydrogène pour l’Inconel 718 FA. Nous questionnons lors de cette partie l’effet de la vitesse de sollicitation sur les mécanismes defragilisation ainsi que sur les différentes interactions hydrogène/matériau. Des essais de traction interrompue ont été réalisés afin de questionner l’effet de l’hydrogène sur le comportement viscoplastique et notamment questionner les interactions hydrogène/plasticité.Dans un deuxième temps, le comportement en fatigue de l’Inconel 718 FA en présence d’hydrogène gazeux est interrogé à l’aide d’essais de fatigue pour un rapport de charge, R de 0,1 sous une pression d’hydrogène de 300 bar. Il est clairement mis en évidence une réduction du nombre decycles à rupture en présence d’hydrogène ainsi qu’un changement de mode de rupture. L’impact de l’hydrogène a été évalué sur les étapes d’initiation et de propagation de fissures ainsi que sur la ténacité du matériau en fonction de la métallurgie de l’alliage.L'ensemble des résultats obtenus permettent une discussion de la sensibilité du matériau à la FPH, tenant compte en particulier des interactions hydrogène/plasticité. / This study focuses on the hydrogen embrittlement sensitivity (HE) under cyclic loading of a nickel based alloy, Inconel 718, manufactured by the additive manufacturing process (AM). This material is used in some components of Ariane cryogenic engines that are manufactured by ArianeGroup. Some of these components are solicited under "hydrogen atmosphere", and the risk of embrittlement is a major problem.The metallurgical states of the Inconel 718 AM alloy have been characterized at different structural scales in order to observe a possible impact of the manufacturing process on the microstructure and discuss the possible consequences on the mechanical behaviour of the alloy underhydrogen pressure. Moreover, a forged alloy has also been studied for comparison.To discuss the mechanisms of hydrogen embrittlement in the material, it is first necessary to study the hydrogen behaviour in the material. Electrochemical and gaseous permeation analyses as well as TDS were performed to provide insights into the mechanisms of hydrogen diffusion. Fromthese results, the involvement of grain boundaries in the diffusion mechanisms and the effect of microstructural heterogeneities on the trapping mechanisms in this material have been discussed.On the other hand, the influence of hydrogen on the mechanical behaviour has been investigated under monotonic and cyclic loading in terms of elasto-viscoplastic behaviour, crack initiation process, fatigue crack propagation and toughness. In a first part, the tensile behaviour of the Inconel 718 AMunder hydrogen pressure has been considered. The effects of the loading rate on the mechanisms of embrittlement have been addressed. Moreover, interrupted tensile tests have been carried out to identify the effect of hydrogen on the viscoplastic behaviour and allowing to discuss hydrogen /plasticity interactions.The fatigue behaviour of Inconel 718 AM in the presence of hydrogen gas was investigated by cyclic tests for a 0.1 load ratio (R) under a 300 bar H2 at room temperature. It has been shown that hydrogen leads to a clear decrease of the number of cycles to rupture and to a change in failure mode.The impact of hydrogen has been evaluated on the fatigue crack initiation and propagation stages as well as on the toughness of the material.Finally, all the obtained results allowed a discussion of the hydrogen embrittlement sensitivity of the material, taking into account the hydrogen / plasticity interactions.

Environmentally Assisted cracking (EAC) is a very critical materials science problem that concerns many technological areas such as petrochemical engineering, aerospace operations and nuclear power generation, in which cracking or sudden failure of materials may happen at stress far below the tensile strength. This type of corrosion is initiated at the microscopic level and is complicated due to the combination of chemistry (reaction caused by corrosive agents) and mechanics (varying load). As EAC is generally related to the segregation of impurity elements to defects (mainly grain boundaries), the symptoms of risk may not be apparent from the exterior of the metal components: hence EAC remains latent and gives no sign of warning until the failure occurs. Due to its intricate nature, conducting experiments on this phenomenon involves difficulties and requires much effort. In this work, we employed advanced molecular simulation techniques to study EAC in order to give insight into its atomistic behavior. First, Density-Functional Theory (DFT) method was used to investigate the fundamental processes and mechanism of EAC-related issues at the nanoscale level, with two case studies concerning the stress corrosion in iron and hydrogen embrittlement in palladium. When segregating to the grain boundary (GB) of iron, different impurity elements such as sulfur, phosphorus and nitrogen raise corrosion failures in a variety of ways. Hydrogen atoms, due to their mobility and small atomic size, are able to form high occupation at crystal defects, but show different interactions to vacancy and GB. Then, we used the classical Molecular Dynamics (MD) method to gain an understanding of the dynamic response of materials to mechanical load and the effects of temperature, strain and extreme conditions (high pressure shock compression) on structural properties. The MD simulations show that hydrogen maintains the highest localization at grain boundaries in the vicinity of ambient temperatures, and grain boundaries are the preferred nucleation sites for dislocations and voids. This computational work, using DFT and MD techniques, is expected to contribute to the better understanding on chemistry and mechanisms of complex environment-assisted cracking phenomenon at a fundamental level in order to beneficially complement conventional laboratory approaches.

Το τεχνολογικό προβλήμα της υποβάθμισης της δομικής ακεραιότητας “γηρασκόντων” αεροσκαφών αποτελεί σήμερα αντικείμενο έρευνας αιχμής τόσο για τις αεροπορικές βιομηχανίες και τους διεθνείς και εθνικούς οργανισμούς ασφάλειας πτήσεων, όσο και για την επιστημονική κοινότητα. Οι μέχρι σήμερα προσπάθειες εστιάζονται κυρίως στην μελέτη της επίδρασης των καταστάσεων πολλαπλής βλάβης και ευρείας έκτασης βλάβης κόπωσης στην δομική ακεραιότητα των γηρασμένων αεροσκαφών. Σε πολλές όμως περιπτώσεις εκτός από καταστάσεις πολλαπλής βλάβης και ευρείας έκτασης βλάβη κόπωσης, παρατηρούνται επίσης εκτεταμένα προβλήματα διάβρωσης παρά την καλή προστασία που γενικά προσφέρουν οι χρησιμοποιούμενες σήμερα μέθοδοι αντιδιαβρωτικής προστασίας. Πρόσφατες έρευνες έδειξαν ότι η προκαλούμενη διάβρωση δεν προκαλεί μόνο μείωση της διατομής των υλικών αλλά και ψαθυροποίηση λόγω υδρογόνου η οποία υποβαθμίζει την δυσθραυστότητα του υλικού και την ικανότητα του να αποταμιεύσει μηχανική ενέργεια πρίν την θραύση. Στην παρούσα διατριβή: - Παρουσιάζονται επιπρόσθετα αποδεικτικά στοιχεία για την ψαθυροποίηση λόγω υδρογόνου που προκαλείται από την διάβρωση στο κράμα αλουμινίου 2024, με βάση πειράματα εφελκυσμού σε διαβρωμένα και αδιάβρωτα δοκίμια του κράματος. Τα αποτελέσματα των δοκιμών εφελκυσμού υποστηρίζονται από εκτενή μεταλλογραφική και στερεοσκοπική μελέτη, ανάλυση των επιφανειών θραύσης καθώς επίσης και από μετρήσεις του εκλυόμενου υδρογόνου. Επιπρόσθετα μελετάται η επίδραση του πάχους των ελασμάτων στην μηχανική συμπεριφορά σε εφελκυσμό των διαβρωμένων δοκιμίων. - Επιβεβαιώνεται η αντιδιαβρωτική προστασία που παρέχει στο κράμα 2024 η επικάλυψη καθαρού αλουμινίου (Alcladding), αλλά επιπλέον, διαπιστώνεται ότι η επικάλυψη προσφέρει επίσης προστασία από την ψαθυροποίηση λόγω υδρογόνου που συνοδεύει την διάβρωση. - Η κύρια συμβολή της εργασίας έγκειται στην παροχή δεδομένων ότι η τοπική επικάλυψη (local Alcladding) της επιφάνειας των δοκιμίων με καθαρό αλουμίνιο σε περιορισμένα ποσοστά της επιφάνειας αυτών, αρκεί για μια αποτελεσματική προστασία τόσο έναντι της βλάβης διάβρωσης όσο και έναντι της προκαλούμενης ψαθυροποίησης λόγω διάχυσης και παγίδευσης υδρογόνου στο εσωτερικό του κράματος. - Τέλος, εισάγεται μεθοδολογία για την εκτίμηση της επίδρασης της διάβρωσης και της εξ’ αυτής προκαλούμενης ψαθυροποίησης του υλικού σε φαινόμενα κόπωσης του κράματος 2024 μέσω της τροποποίησης του “Χάρτη Βλάβης Κόπωσης”, ώστε ο τελευταίος να μπορεί να αξιοποιηθεί για την περίπτωση διαβρωμένων δοκιμίων. / A possible integrity loss represents a not tolerable scenario for aging aircraft structures. To face the mentioned technological and scientific problem essential efforts have been undertaken by the scientific community as well as the aircraft industries and the international and national flight safety organizations. Nowadays research focuses to the study of the effects of widespread fatigue damage (WFD) and multiple site damage (MSD) scenarios on the structural integrity of the aging aircrafts. However, in numerous cases, additionally to WFD and MSD, extensive corrosion problems have been observed. Recent investigations have shown that the corrosion attack does not cause only a reduction of the cross-section of the structural member as well as a possible onset of fatigue cracks, but also a corrosion induced hydrogen embrittlement which reduces the fracture toughness of the material and its ability to store mechanical energy before fracture. In the present thesis: - Evidence is presented for a corrosion-induced hydrogen embrittlement of the alloy 2024 also in the absence of mechanical loads. A parametric study including series of tensile tests carried out on both corroded and uncorroded 2024 aluminum alloy specimens has been performed. The tensile tests results are supported by an extensive metallographic and stereoscopic study, analysis of the fracture surfaces, as well as hydrogen measurements. The effect of the sheet thickness on the tensile behaviour of corroded aluminum alloy 2024 specimens has been investigated, as well. - The corrosion protection offered by the aluminum coating (Alcladding) on the substrate alloy 2024 is confirmed. Additionally, evidence is provided on the protection offered by the aluminum coating against hydrogen embrittlement that accompanies corrosion. - The main contribution of the thesis is the provision of data for the case of local coating on the specimen surface, indicate that aluminum coating in limited percentages on the specimen surface for the alloy 2024 is sufficient for an efficient protection against corrosion damage, as well as against the induced embrittlement stem from the diffusion and trapping of hydrogen in the material interior. - Finally, a methodology is introduced for the estimation of the effect of corrosion and the corrosion-induced hydrogen embrittlement on the fatigue behaviour of the aluminum alloy 2024, through the modification of the Fatigue Damage Map (FDM), in order to utilize it for the case of corroded members.

Zirconium alloys are susceptible to engineering problems associated with the uptake of hydrogen throughout their design lifetime in nuclear reactors. Understanding of hydrogen embrittlement associated with the precipitation of brittle hydride phases and a sub-critical crack growth mechanism known as Delayed Hydride Cracking (DHC) is required to provide the engineering justifications for safe reactor operation. The nature of bulk zirconium hydrides at low concentrations (< 100 wt. ppm) is subject to several contradictory descriptions in the literature associated with the stability and metastability of γ-phase zirconium hydride. Due to the differing volume expansions (12-17%) and crystallography between γ and δ hydride phases, it is suggested that the matrix yield strength may have an effect on the phase stability. The present work indicated that although yield strength can shift the phase stability, other factors such as microstructure and phase distribution can be as or more important. This suggests that small material differences are the reason for the literature discrepancies. DHC is characterised by the repeated precipitation, growth, fracture of brittle hydride phases and subsequent crack arrest in the ductile metal. DHC growth is associated primarily the ability of hydrogen to diffuse under a stress induced chemical potential towards a stress raiser. Knowledge of the factors controlling DHC are paramount in being able to appropriately describe DHC for engineering purposes. Most studies characterise DHC upon cooling to the test temperature. DHC upon heating has not been extensively studied and the mechanism by which it occurs is somewhat controversial in the literature. This work shows that previous thermo-mechanical processing of hydrided zirconium can have a significant effect on the dissolution behaviour of the bulk hydride upon heating. DHC tests with γ-quenched, furnace cooled-δ and reoriented bulk hydrides upon heating and DHC upon cooling suggest that the amount of hydrogen in solution is the primary factor controlling the occurrence of DHC and consistent with the postulation that the stress induced chemical potential is the driving force for DHC. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-01-24 06:14:14.152

The influence of steel metallurgy on rock bolt SCC was studied using a series of commercial carbon and low-alloyed steels. The chemical composition, their mechanical properties and the microstructures of these steels varied considerably in order to gather information for the discussion of the metallurgical influences under Hydrogen Embrittlement (HE) and Stress Corrosion Cracking (SCC) conditions. In order to understand the metallurgical influences on Rock Bolt SCC, an evaluation was carried out to fifteen commercial steels. The experiments reproduced the Stress Corrosion Cracking condition at which commercial rock bolts had failed in Australians mines. Due to the selected materials, stress and electrolyte condition it is expected that Hydrogen Embrittlement (HE) will affect the steel failure. The approach was to use the Linearly Increasing Stress Test (LIST) and exposing the sample to a dilute pH 2.1-sulphate solution, in accordance with prior studies. Stress Corrosion Cracking was evaluated by analysing the decrease in tensile strength, loss of ductility and fractography observed using Scanning Electron Microscopy (SEM). The initial series of test to the fifteen steels were performed at the free corrosion potential (f.c.p.) vs. Ag/AgCl. From this initial test only five steels (AISI 1008, AISI 4140, AISI 4145H, pipeline X-65 and X-70) did not show Stress Corrosion Cracking features. These five steel were tested in accordance with the Linearly Increased Stress Test (LIST) in the dilute pH 2.1 sulphate solution at different electronegative applied potential to minimum value of -1500mV. The experimental procedure tried to reproduce the Stress Corrosion Cracking condition to identify the most aggressive condition the steel is able to support before failing due to Stress Corrosion Cracking to then compare the theory of SCC and HE in low carbon and low alloy steel with the obtained experimental results. The investigation compared the well-known theory of SCC and HE in low carbon and low alloy steel with the obtained experimental results. Surprisingly, the experimental result did not always agree with the theory.

Ce travail de thèse a porté sur l’impact de traces d’oxygène sur la corrosion de fer pur ou d’acier en milieu H 2 S, ainsi que sur le chargement en hydrogène. Trois axes de recherche ont été suivis.Une première partie est consacrée à la chimie de la réaction H 2 S+O 2 . Nous avons d’abord identifié dans la littérature les chemins réactionnels possibles de la réaction H2S+O2 , et les méthodes d’analyse des espèces S-O en solution. Nous avons ensuite utilisé des méthodes thermodynamiques pour prédire les espèces les plus stables, qui ont été validées par des essais expérimentaux. Il ressort de cette analyse que les principaux produits solubles de la réaction H2S+O2 sont les ions sulfate, thiosulfate etsulfite, ainsi que les ions H + . La décroissance continue du pH de la solution d’essai dans un système corrosif en milieu H2S peut être un indicateur d’une pollution par l’oxygène.La seconde partie utilise des méthodes électrochimiques pour étudier les phénomènes de corrosion et de formation de dépôts en présence ou en absence d’oxygène. La vitesse de corrosion est systématiquement plus grande en présence de O2 . Un modèle d’impédance a été developpé pour l’étude de l’évolution de l’interface métal/électrolyte, et ce modèle est en accord avec la littérature et est validé par l’évaluation de la vitesse de corrosion par perte de masse.Enfin, la troisième partie est consacrée au chargement en hydrogène dans le fer pur ou dans un acier pétrolier de type X65. En absence de pollution par l’oxygène, on observe des rendements très proches de 100 % entre le courant de perméation et le courant de corrosion estimé à partir des mesures d’impédance, et ce sur plusieurs semaines. En présence d’O2 , ce rendement est systématiquement beaucoup plus faible. Par ailleurs, quelques tests menés sur l’acier pétrolier montrent également un effet important de l’oxygène sur le caractère plus ou moins protecteur du dépôt, qui n’avait pas été observé sur système modèle utilisant du fer pur / The work in this thesis presents the effect of trace concentrations of oxygen (O2 ) on corrosion and hydrogen uptake of pure iron/steel in H2S-media. Three main avenues of research were conducted.The first part is concerned with the reaction chemistry between dissolved H2S and O2. A literature review is presented, identifying a number of reaction pathways associated with the dissolved H2S and O2 reaction as well as analytical methods used to measure the resulting S-O reaction products. Subsequently, thermodynamic calculations allow for the prediction of the most stable species indissolved H2S -O2 solutions, later confirmed using spectroscopic and chromatographic methods. Such techniques reveal that the principal soluble reaction products are sulfate, thiosulfate and sulfite anions,as well as hydronium ions. From pH monitoring, a continuous rise in acidity of an H2S solution can be an indication of O2 ingress.Secondly, electrochemical methods are used to explore corrosion phenomena and deposit formation in the presence and absence of O2 pollution. Corrosion rates in the presence of O2 are reproducibly much higher if compared against those in the system lacking O2 . An electrochemical impedance spectroscopy model has been developed to study the evolution of the metal / electrolyte interface over time, which is validated using weight loss corrosion rate measurements, and represents impedance data reported in the literature on similar systems.Finally, the third part is devoted to hydrogen uptake in pure iron and petroleum industry sour-grade steel (X65). In the absence of O2 pollution, permeation efficiency yields close to 100% are observed between the permeation current and the corrosion current estimated from the impedance measurements over a period of several weeks. In the presence of O2 , however, this yield is reproducibly far lower. Moreover, some tests conducted on X65 also show a significant effect of O2 on the quite protective nature of the deposit, not observed on the model system using pure iron