En general, un material tenderá a corroerse espontáneamente en un determinado medio si los productos de corrosión son más estables que el material de partida, o si bajo las condiciones de exposición la corrosión resulta favorecida. En la práctica, lo que se busca es extender la vida útil de estructuras, equipos o dispositivos, es decir, mejorar su durabilidad. / In general, a material will tend to corrode in a specific environment when the corrosion products are more stable than the starting material or, when exposed to certain conditions, a corrosion process results favored. In general practice, the goal is to extend the service life of structures, equipment or devices, that is, to improve their durability.

Les alliages à base nickel sont utilisés dans les centrales nucléaires pour la fabrication des tubes générateurs de vapeur, parce qu’ils présentent une bonne résistance aux sollicitations mécaniques élevées dans un milieu corrosif à haute température. La résistance au milieu corrosif est attribuée à une couche protectrice très fine de Cr2O3 qui se forme à la surface de l’alliage. Les tubes générateurs de vapeur comptent parmi les composants les plus délicats à entretenir, d’une part, du fait de leur importance pour la sûreté et, d’autre part, parce que les tubes d’échange sont soumis à de nombreux mécanismes de dégradation à cause de conditions sévères de fonctionnement.. L’objectif de cette étude a été d’évaluer et de comparer le comportement en tribocorrosion de deux alliages modèles Ni – 15%Cr et Ni – 30%Cr. Ces alliages ont été utilisés pour mettre en évidence, évaluer et comparer l’influence de la teneur en chrome sur la formation de la couche d'oxyde superficielle et le rôle de cette dernière sur les mécanismes et cinétiques d'usure par tribocorrosion. L’étude de la tribocorrosion a été effectuée à l’aide d’un tribomètre pion – disque par application d’un frottement unidirectionnel, en régime continu et intermittent en milieu aqueux boré – lithié à la température ambiante. La dégradation de la couche protectrice formée à la surface des deux alliages a été suivie en mettant en œuvre un protocole expérimental en trois étapes. Dans la première étape du protocole, le suivi de l’évolution du potentiel libre et l’enregistrement des diagrammes d’impédance électrochimique ont été utilisés pour analyser le comportement électrochimique des deux alliages en absence de frottement. Lors de la deuxième étape, les mêmes techniques électrochimiques ont été employées en présence d’un frottement continu et pour différentes pressions de contact. Durant l’étape trois du protocole, des essais de tribocorrosion en régime de frottement intermittent ont été réalisés pour mettre en évidence la capacité d’auto cicatrisation de la surface des deux alliages modèles. Pour les deux alliages Ni - Cr, les essais de tribocorrosion ont mis en évidence une augmentation de l’usure totale avec la pression de contact appliquée. Quel que soit la force normale appliquée, la principale composante de l’usure totale est l’usure mécanique du substrat mis à nu. L’usure purement corrosive du substrat diminue avec l’augmentation de la teneur en chrome. Du point de vue qualitatif, le principal mécanisme d’usure identifié a été l’usure abrasive par micro - labourage. Dans cette étude a été effectuée une étude de l’usure d’alliages modèles Ni – Cr, dans les conditions de tribocorosion, ainsi que une analyse du mécanisme et la cinétique d’usure. Le comportement de la couche superficielle d’oxyde de chrome et son effet sur la résistance à la tribocorrosion ont été évalués. Ces résultats peuvent servir de base à la compréhension de l’origine des problèmes qui peuvent apparaître pendant la vie d’un alliage inoxydable à base nickel soumis à des efforts mécaniques en milieu agressifs et permettre une sélection plus pertinente des matériaux métalliques pour diverses applications industrielles, particulièrement dans le domaine nucléaire. / Some components of nuclear power plants, as steam generator tubes are made from Ni base alloys. These components are exposed to severe environment of high temperature and high pressure and submitted to contact mechanical stresses. These Ni – based alloys properties are determined by their ability to form on their surface an inner protective barrier film mainly composed of Cr2O3. The steam generator tubes are among the most difficult components to maintain, on the hand, because of their safety importance and secondly, the exchange tubes are subject to various degradation mechanisms, because of the harsh conditions of work. Wear by tribocorrosion is a physicochemical aging mechanism which occurs in the management of the nuclear power plants life time.Tribocorrosion is an irreversible process which involves mechanical and chemical / electrochemical interactions between surfaces in relative motion, in the presence of a corrosive environment. The goal of this study was to quantify in terms of quantity and quality the wear generated by tribocorrosion process on Ni – Cr model alloys. Two model alloys: Ni -15Cr and Ni -30Cr were used to highlight, evaluate and compare the influence of the chromium content on the formation of the protective oxide layer and the role played by the latter one on the kinetics and mechanisms of wear by tribocorrosion. The tribocorrosion experiments were performed by using a pin -on–disc tribometer under controlled electrochemical conditions in LiOH – H3BO3 solution. The corrosion – wear degradation of the protective layer during continuous and intermittent unidirectional sliding tests was investigated by a three-stage tribocorrosion protocol. In the first stage, electrochemical techniques (open circuit potential measurements and electrochemical impedance measurements) were used without applying unidirectional sliding to monitor and evaluate the characteristics of protective oxide layer formed on the surface of the two model alloys. In the second stage, the same electrochemical techniques were employed during the application of the unidirectional sliding test to evaluate the total material loss for different applied contact pressures. In the last stage of the experimental procedure the self healing process was investigated by intermittent unidirectional sliding tests.The total material loss after a tribocorrosion experiment, determined by profilometric measurements, is the sum of two components: the material loss due to corrosion of active material in the wear track, which can be calculated fro, the corrosion current values given by impedance measurements; applying Faraday’s law, and the material loss due to mechanical wear, the difference between total material loss and the material loss due to corrosive wear. Experiments proved that the total material loss increases with the increase of the contact pressure in both cases of Ni – Cr model alloys. The prevailing contribution in the total volumetric material loss is the material loss due to mechanical wear of active material in the sliding track. Nevertheless, the increase of the chromium content improves the resistance to the corrosive wear of active material in the sliding track. From qualitative viewpoint, a mechanism of abrasive wear by micro-ploughing was revealed in the sliding tracks.

O presente trabalho teve como objetivos determinar a resistência à corrosão e as propriedades mecânicas da liga Cu10Ni-3Al-1,3Fe, nas condições solubilizada e envelhecida a 550°C. Foi também objetivo do trabalho identificar a microestrutura nestas condições de tratamento térmico. As propriedades mecânicas foram avaliadas a partir de ensaios de tração e medidas de microdureza. Para caracterizar a resistência à corrosão e corrosão-erosão, foram realizados ensaios de polarização potenciodinâmica na condição estagnada e associada à erosão em 0,01M NaCl, 0,01M Na2SO4 e 0,0001M Na2S.9H2O. Para tanto, foi necessário elaborar a liga Cu10Ni-3Al-1,3Fe, por fundição em forno do tipo mufla. O lingote obtido foi solubilizado a 900ºC e laminado a frio. Posteriormente a chapa obtida foi novamente tratada a 900ºC por 1 h para garantir uma microestrutura homogênea de fase a. A partir desta chapa foram retiradas amostras para os tratamentos térmicos de envelhecimento por até 1.032 h a 550ºC. Os exames metalográficos mostraram intensa precipitação nas condições envelhecidas. Através dos ensaios de tração e medidas de microdureza, verificou-se que a presença dos precipitados melhora significativamente as propriedades mecânicas, sendo que os valores máximos de dureza, limite de escoamento e de resistência ocorrem para o tempo de 16 h de envelhecimento. Este efeito é decorrente de duas parcelas, uma dos precipitados intergranulares que se formam num processo de precipitação celular e a outra das partículas finamente dispersas (precipitados intragranulares). Em relação a resistência à corrosão, constatou-se que a liga, em todas as condições de tratamento térmico, apresenta um potencial de quebra (Eq) quando polarizada em eletrólitos que contém cloreto, sulfato ou sulfeto. Em 0,01M NaCl, o potencial Eq está relacionado com o processo de corrosão seletiva do níquel (desniquelação). Observou-se ainda que o envelhecimento provocou um aumento nos valores de Eq, sendo mais evidente para as amostras envelhecidas por 2 e 1.032 h. Nos ensaios de corrosão-erosão (realizados apenas nas condições solubilizada e envelhecida por 16 h), também ocorre o potencial Eq, mas os valores são mais baixos. Nestes casos foram observadas cavidades, que não estão relacionadas com o processo de corrosão seletiva, mas sim com o processo de erosão. Nos eletrólitos de 0,01M Na2SO4 e 0,0001M Na2S.9H2O o potencial Eq está relacionado com a formação de cavidades (pites), e não com o processo de corrosão seletiva. Nestes eletrólitos, não foi observada qualquer dependência entre Eq e o envelhecimento da liga, diferentemente do verificado em 0,01M NaCl, onde o envelhecimento proporcionou um efeito benéfico. Igualmente, não houve diferenças significativas nos valores de Eq quando o material foi submetido à corrosão-erosão. / The present work evaluated the corrosion resistance and mechanical properties of Cu10Ni-3Al-1.3Fe cupronickel alloy, in the solution-treated and 550°C aged conditions. It was also objective of the work to identify the microstructural changes in these conditions of aging treatment. Mechanical properties were evaluated through microhardness and tension tests; to evaluate the corrosion and corrosion-erosion resistance, potenciodynamic polarization tests were done in the stagnated condition and polarization associated to erosion in 0.01M NaCl, 0.01M Na2SO4 or 0.0001M Na2S.9H2O. The alloy had been casting and solution treated at 900°C, and after that cold rolled. From cold rolled sheet, specimens were solution treated at 900°C for 1 h, and aged at 550°C until 1,032 hours. The microstructural exams showed precipitation in the aged samples. The microhardness and tension tests showed that the presence of precipitates improve the mechanical properties, and the maximum value of hardness was obtained after 16 h of aging at 550°C. This effect is related to two microstructural aspects, one regarding intergranular precipitates that were formed by cellular precipitation and other related to the presence of finely dispersed intragranular precipitation. Concerning corrosion resistance, it was verified that the alloy, in all conditions, presented a break potential (Eq) when polarized in the tested solutions. In 0.01M NaCl, Eq is related with the process of selective corrosion of the nickel (denickelification). It was observed although that aging increased the values of Eq, being more evident for the aged samples for 2 and 1,032 h. The corrosion-erosion tests (just accomplished in the conditions solution-treated and aged by 16 h), also presented Eq, but the values were lower. In these cases cavities were observed, however not related to the process of selective corrosion, but to the erosion process. The tests in 0.01M Na2SO4 or 0.0001M Na2S.9H2O showed that Eq is related with the formation of cavities (pits), and not with the process of selective corrosion. In these solutions was not observed dependence between Eq and the aging time, differently of the verified in 0.01M NaCl, where the aging provided a beneficial effect. There were not significant differences in the values of Eq when the material was submitted to the corrosion-erosion in the solutions (0.01M Na2SO4 and 0.0001M Na2S.9H2O).

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / O cromato à amplamente utilizado como inibidor de corrosÃo. No entanto, à altamente tÃxico e carcinogÃnico, o que tem levado pesquisadores a buscarem alternativas ecologicamente amigÃveis a este inibidor. Dentre estes, molibdato, tungstato, silicato e fosfato surgem como potenciais candidatos por apresentarem propriedades inibidoras e serem atÃxicos. Assim, este trabalho tem por objetivo desenvolver um estudo sistemÃtico dos Ãons como inibidores da corrosÃo do Zn. Foram utilizadas soluÃÃes aquosas de NaCl 10-1 mol dm-3 com concentraÃÃes dos inibidores variando de 10-4 a 10-1 mol dm-3 . A eficiÃncia destes com relaÃÃo à corrosÃo do Zn foi avaliada empregando-se medidas do potencial a circuito aberto, curvas de polarizaÃÃo potenciodinÃmica e espectroscopia de impedÃncia eletroquÃmica. As tÃcnicas de Microscopia EletrÃnica de Varredura (MEV), Energia Dispersiva de Raios-X (EDX), DifraÃÃo de Raios-X (DRX) e Espectroscopia FotoeletrÃnica de Raios-X (XPS) foram usadas para caracterizar o Zn apÃs os ensaios de corrosÃo. Todos os Ãnions estudados inibiram a corrosÃo do Zn. Com as tÃcnicas de caracterizaÃÃo foi possÃvel identificar a presenÃa de filmes superficiais de silicato, tungstato, molibdato e fosfato na superfÃcie do Zn apÃs os ensaios de corrosÃo nas respectivas soluÃÃes contendo os inibidores / The chromate is widely used as a corrosion inhibitor. However, it is highly toxic and carcinogenic, which has done researchers to study environmentally friendly alternatives to this inhibitor. Among these, molybdate, tungstate, silicate and phosphate anions appear as potential candidates because they are non toxic. Thus, this work aims to develop a systematic study to investigate molybdate, tungstate, silicate and phosphate as corrosion inhibitors of zinc. Aqueous solutions of 10-1 mol dm-3 NaCl were used and the concentrations of the inhibitors ranging from 10-4 to 10-1 mol dm-3. The corrosion studies were carried out by open circuit potential measurements, potentiodynamic linear polarization and electrochemical impedance spectroscopy. The Scanning Electron Microscopy (SEM), X-Ray Dispersive Energy (XDE), X-Ray diffraction (XRD) and X-Ray Photoelectronic Spectroscopy (XPS) techniques were used to characterize the Zn after the corrosion tests. All the studied anions inhibited the zinc corrosion. The characterization techniques made possible to detect the presence of molybdate, silicate, tungstate and phosphate on the zn surface after the corrosion tests in the solution containing the corresponding inhibitors

Cette thèse s'inscrit dans un programme de développement technologique de la Société LEONI. Ce programme de recherche a pour principal objectif de disposer, à l'issue des trois années de l'étude, d'un panel de connaissances scientifiques le plus pertinent possible sur la durabilité des alliages d'aluminium dans des environnements caractéristiques d'un faisceau électrique automobile. Il s'agit, à terme, d'intégrer, à l'échelle industrielle, l'aluminium ou l'un de ses alliages dans les câblages électriques automobiles. Après une première étape de sélection des matériaux, l'alliage retenu est un alliage d'aluminium 6101. La fabrication des faisceaux électriques automobiles nécessite l'utilisation de brins de petits diamètres qui sont obtenus, à partir d'une ébauche, par un procédé de mise en forme associant plusieurs étapes de tréfilage et de revenu. Chacune de ces étapes conditionne les propriétés de l'alliage. Les travaux réalisés dans le cadre de cette thèse ont eu pour objectif d'analyser et de quantifier l'influence des différentes étapes de ce procédé de mise en forme sur la durabilité de l'alliage 6101. Compte tenu de l'environnement en service des faisceaux électriques, deux types de sollicitation ont été considérés ; ces sollicitations sont associées à un endommagement en corrosion d'une part et à un endommagement en fatigue-corrosion d'autre part. La première partie de ces travaux est donc consacrée à une étude du comportement en corrosion en milieu contenant des ions chlorures de l'alliage d'aluminium AA 6101 à l'état métallurgique T4, correspondant à l'état microstructural du matériau d'ébauche. L'influence des traitements thermomécaniques liés au procédé de mise en forme des fils fins sur la microstructure, les propriétés mécaniques et le comportement en corrosion de l'alliage AA 6101 fait l'objet de la seconde partie des travaux. Enfin, l'influence de contraintes mécaniques cycliques sur le comportement en corrosion de l'alliage AA 6101 en milieu NaCl 0,5M a été étudiée. L'ensemble de ces résultats met en évidence un couplage environnementmicrostructure-état de contraintes avec un fort impact des traitements thermo-mécaniques sur les microstructures et donc sur les propriétés mécaniques et le comportement en corrosion du matériau / This work is in the framework of a technological development program with LEONI. The aim of this program research is to bring technical skills and scientific knowledge, the most pertinent as possible on the durability of aluminum alloys in automotive harness environment. The purpose is to integrate aluminum alloys in automotive wiring systems. After a first step of materials selection, the AA 6101 aluminum alloy has been selected. Automotive wiring harnesses need small diameters of strands which are obtained by a forming process combining several cold-drawing steps and aging heat treatments. Each step of this forming process influences the alloy properties. The aim of this work is to analyze and quantify the influence of each step of the forming process on the durability of AA 6101 aluminum alloy. Considering the automotive harness environment, two kinds of damages have been studied: corrosion damage in one hand and fatigue-corrosion damages in the other hand. The first part of this work was dedicated to the study of the corrosion behavior of AA 6101 aluminum alloy at T4 metallurgical state in chloride media. The influence of thermo-mechanical treatments, due to the process forming of the strands on the microstructure, the mechanical properties and the corrosion behavior of the AA 6101 has been studied in the second part. Finally, the influence of cyclic mechanical stresses on the corrosion behavior, i.e. fatigue-corrosion phenomena, of the AA 6101 in 0.5M NaCl has been considered. The whole results highlight a coupling between environment-microstructure-stress states with a severe influence of the thermo-mechanical treatments on the microstructures, the mechanical properties and on the corrosion behavior

Cette thèse s'inscrit dans un programme de développement technologique de la Société LEONI. Ce programme de recherche a pour principal objectif de disposer, à l'issue des trois années de l'étude, d'un panel de connaissances scientifiques le plus pertinent possible relatif à la durabilité des faisceaux électriques, et plus particulièrement aux cinétiques de corrosion associées aux assemblages entre des câbles électriques en alliage d'aluminium AA1370 et des pièces de connectique en cuivre pur. La société LEONI souhaite également maîtriser les différentes étapes de fabrication de ces composants. En effet, les propriétés du câble, qui est constitué de fils en alliage d’aluminium AA1370, sont conditionnées par chacune des étapes du procédé de mise en forme de ces fils. Ainsi les travaux réalisés dans le cadre de cette thèse ont eu pour premier objectif d’analyser et de quantifier l’influence des différentes étapes de ce procédé de mise en forme sur le comportement en corrosion de l’alliage AA1370 sur la base d’une caractérisation précise de la microstructure du matériau aux différentes étapes du procédé. Les résultats mettent en évidence un couplage « résistance à la corrosion / paramètres microstructuraux critiques » à prendre en considération dans chacune des étapes de fabrication d’un câble en alliage AA1370. Ensuite, le comportement en corrosion d’un câble électrique a été étudié : les processus de corrosion mis en jeu au sein d’un câble constitué d’un nombre important de fils ont ainsi été identifiés, avec en particulier la problématique du confinement de l’électrolyte entre les différents fils du câble. Ces résultats montrent que la connaissance des processus électrochimiques se produisant sur un fil en alliage AA1370 ne suffit pas pour comprendre le comportement en corrosion d’un câble constitué de ces mêmes fils. La géométrie du produit final est un paramètre du 1er ordre. Enfin, le comportement en corrosion d’un assemblage entre un câble en alliage AA1370 et une pièce de connectique en cuivre pur a été étudié à l’aide de techniques de microscopie et d’électrochimie locale afin d’identifier les processus de corrosion mis en jeu lors du couplage galvanique Al/Cu en prenant en compte la géométrie particulière de l’assemblage. L’influence de l’agressivité du milieu sur la durabilité de ces assemblages a été étudiée en combinant l’ensemble des résultats avec ceux obtenus lors d’essais de corrosion industriels. L’étude dans son ensemble a permis de générer des avancées significatives dans la compréhension des mécanismes de corrosion à l’échelle de l’alliage AA1370 sélectionné mais également à l’échelle de l’assemblage. / This work was performed in the framework of a technological development program with LEONI. The aim of this research program was to provide technical skills and scientific knowledge on the durability in automotive harness environment of a wiring harness; it was focused on the corrosion behavior of an assembly between a AA1370 wire and pure Cu connector. Automotive wires are constituted of small diameter strands which are manufactured following a multistep forming process. Thus, the first objective of this work was to analyze the influence of each step of the forming process on the corrosion behavior of AA1370 alloy. The results show that a « microstructural parameters – corrosion resistance » relationship can be established and must be taken into account in each step of the forming process of a AA1370 strand arm. Then, the electrochemical processes occurring when the wire was exposed to an aqueous environment were investigated using electrochemical impedance measurements. The results show that the corrosion behavior of the wire was controlled by the formation of a confined electrolyte between the strand arms. Therefore the knowledge of the electrochemical processes occurring for AA1370 alloy strands was not sufficient to explain the corrosion behavior of a wire made of the same strands. The investigation of wire specific corrosion behavior showed the benefits to work with a representative geometry of the final product instead of testing single strand alone. Then, the corrosion behavior of the assembly between a AA1370 wire and pure Cu connector was also investigated coupling electrochemical measurements and microscope observations, to identify the corrosion processes involved during Al/Cu galvanic coupling, considering a representative geometry of the assembly. Finally, the results from laboratory tests were compared to those from industrials validation tests in order to study the influence of the environment aggressiveness on the assembly durability. The study led to significant advances in the understanding of the corrosion mechanisms at the alloy scale and also at the assembly scale.

This study investigated localized corrosion behavior of austenitic stainless steels under stressed and unstressed conditions, as well as corrosion of metallic thin films. While austenitic stainless steels are widely used in corrosive environments, they are vulnerable to pitting and stress corrosion cracking (SCC), particularly in chloride-containing environments. The corrosion resistance of austenitic stainless steels is closely tied to the alloying elements chromium, nickel, and molybdenum. Polarization curves were measured for five commercially available austenitic stainless steels of varying chromium, nickel, and molybdenum content in 3.5 wt.% and 25 wt.% NaCl solutions. The alloys were also tested in tension at slow strain rates in air and in a chloride environment under different polarization conditions to explore the relationship between the extent of pitting corrosion and SCC over a range of alloy content and environment. The influence of alloy composition on corrosion resistance was found to be consistent with the pitting resistance equivalent number (PREN) under some conditions, but there were also conditions under which the model did not hold for certain commercial alloy compositions. Monotonic loading was used to generate SCC in in 300 series stainless steels, and it was possible to control the failure mode through adjusting environmental and polarization conditions. Metallic thin film systems of thickness 10-200 nm are being investigated for use as corrosion sensors and protective coatings, however the corrosion properties of ferrous thin films have not been widely studied. The effects of film thickness and substrate conductivity were examined using potentiodynamic polarization and scanning vibrating electrode technique (SVET) on iron thin films. Thicker films undergo more corrosion than thinner films in the same environment, though the corrosion mechanism is the same. Conductive substrates encourage general corrosion, similar to that of bulk iron, while insulating substrates supported only localized corrosion.

La protection contre la corrosion par des films ultramince (≤50 nm) d'alumine déposées par ALD sur le cuivre et l'aluminium à 250°C a été étudiée dans une solution aqueuse 0,5 M de NaCl en combinant méthodes d'analyse électrochimique et de surface. L'étude de l'alumine ALD sur un substrat Cu comprend l'effet de l'épaisseur du revêtement, l'effet de l'oxyde interfacial, l'effet de la préparation de la surface et la durabilité du revêtement. Pour le substrat Al, le travail a porté sur l'examen de l'effet de l'épaisseur du revêtement. Les revêtements ont montré d'excellentes propriétés de corrosion sur des substrats Cu électropoli, tandis qu'ils ont échoué à protéger le substrat recuit, de fait d'une mauvaise adhérence à une surface lissée. L'amélioration de la résistance à la corrosion d'alumine ALD sur le substrat Cu est obtenue en l'absence de vieillissement de l'oxyde natif interfacial, et sa modification par un prétraitement. En dépit de remarquables propriétés d'étanchéité sur un substrat Cu électropoli, la protection contre la corrosion de l'alumine ALD n'est pas durable. Le revêtement du substrat Al avec l'alumine ALD conduit à l'augmentation significative de la résistance à la corrosion. Le potentiel de piqûration est augmenté en présence des revêtements l'épaisseur de 20 et 50 nm, ce qui n'a pas été obtenus avec 10 nm en raison de sa faible épaisseur. Cette étude est une étude préliminaire pour l'application de revêtements d'alumine ALD pour la protection contre la corrosion des alliages Al-Cu en combinaison avec d'autres compositions ALD. / Corrosion protection by ultrathin (≤ 50 nm) alumina films deposited by atomic layer deposition (ALD) on copper and aluminium at 250°C was studied in 0.5 M NaCl aqueous solution by combining electrochemical and surface analytical methods. The study of ALD Al2O3 on Cu substrate included investigation of the effect of the coating thickness, the effect of an interfacial oxide, the effect of surface preparation and the durability of the coating. For ALD Al2O3 on Al substrate, the work focused on the examination of the effect of the deposited coating thickness. ALD alumina coatings showed excellent corrosion properties on electropolished copper substrates, while they failed to protect the annealed substrate, as a result of poor adhesion to a smoothened surface. Modification of interfacial native copper oxide by its pre-treatment led to better corrosion protection of ALD alumina on copper substrate. Despite its remarkable sealing properties on electropolished Cu substrate, corrosion protection of ALD alumina was not durable. Coating of Al substrate with ALD Al2O3 led to significant increase of polarization resistance. Better performance was obtained for 10 and 20 nm coatings on Al than on Cu. Apart from significant decrease of current, the pitting potential was increased in presence of 20 and 50 nm coatings, which was not achieved with 10 nm due to its low thickness. This study was a preliminary study for application of ALD alumina coatings for corrosion protection of Al-Cu alloys in combination with other ALD compositions.

The interactions between engineering materials and their environment which give rise to stress corrosion cracking are reviewed and industrial examples from the petrochemical industry are described. In one of the examples, cracking took place in carbon steel exposed to pressurised gas containing carbon monoxide, carbon dioxide and water. The crack morphology in this system was studied by metallography of samples from industrial gas processing plants and the crack growth rates were determined using precracked specimens. Constant extension rate tests, U-bend specimens and potentiodynamic studies were used to evaluate alternative materials and inhibitor additions in CO-C02-H20 environments. Electrochemical noise was accessed as a technique to monitor sec on line. It was found that the CO-C02-H20 system was characterised by a time dependent adsorption of carbon monoxide at anodic and cathodic sites. The adsorption produced a critical balance between crack tip corrosion rate and the repassivation process comparable to the behaviour at the active-passive transition zone in more conventional systems. The anodic passivation exhibited a breakdown potential near to -400 mV (Ag/ AgCI) that defined the zone of sec susceptibility. Inhibition by CO and hence sec was virtually independent of CO partial pressure provided there was a sufficient reservoir of CO. The addition of commercial film forming inhibitors did not greatly influence the system and sec was still observed in CO-C02-H20 environments to which inhibitors had been added. Steels containing alloy additions of more than 9 % chromium were found to be resistant to sec but austenitic-ferritic weld joints cracked. A low alloy 3% nickel steel performed well in the constant extension rate tests but was not wholly resistant to sec. The morphology of stress corrosion cracks in CO-C02-H20 mixtures was influenced by carbon monoxide partial pressure. More corrosion was observed on the crack walls at low carbon monoxide partial pressure and widened cracks resembling •mesa• corrosion were common. This increased corrosion was probably due to difficulty in maintaining passivity in the crevice formed by the growing crack. Similarly, crevicing in precracked specimens appeared to inhibit sec and no crack extension was observed. Electrochemical noise proved to be a useful tool for monitoring. / Thesis (PhD)--University of Pretoria, 1994. / gm2013 / Materials Science and Metallurgical Engineering / PhD

Corrosion and fatigue have been dominant degradation mechanisms in offshore structures, with the combination of the two, known as corrosion fatigue, having amplified effects in structures in the harsh marine environments. Newer types of structure are now being developed for use in highly dynamic, harsh marine environments, particularly for renewable energy applications. However, they have significantly different structural details and design requirements compared to oil and gas structures, due to the magnitude and frequency of operational and environmental loadings acting on the support structures. Therefore, the extent of corrosion assisted fatigue crack growth in these structures needs to be better understood. In this research, fatigue crack growth in S355J2+N steel used for offshore wind monopile fabrications was investigated in air and free corrosion conditions. Tests were conducted on parent, HAZ and weld materials at cyclic load frequencies similar to what is experienced by offshore wind monopile support structures. The seawater used for testing was prepared according to ASTM D1141 specifications and was circulated past the specimens through a purpose designed and built corrosion rig at a rate of 3 l/min, at a temperature of 8-100C and at a pH of 7.78-8.1. A new crack propagation method accompanied by constant amplitude loading was used. Crack growth rates in parent, HAZ and weld materials were significantly accelerated under free corrosion conditions, at all the stress ratios used compared to in air environment. However, in free corrosion conditions, crack growth rates in the parent, HAZ and weld materials were similar, particularly at a lower stress ratio. The results are explained with respect to the interaction of the loading condition, environment and the rate of material removal by corrosion in the weldments. A new model was developed to account for mean stress effects on crack growth rates in air and in seawater, and was found to correlate well with experimental data as well as with the other mean stress models tested.