Dans le cadre de ces travaux, il a clairement été établi que l’hydrogène piégé ou diffusible pouvait avoir une forte influence sur les propriétés mécaniques des matériaux. Cependant, cet effet varie de façon importante en fonction de leur microstructure, leur composition chimique et leur traitement thermique. En effet, les aciers martensitiques trempés/revenus dédiés à des tubes pour des milieux sous-service présentent, de par leur structure, différents types de pièges tels que les dislocations, les joints de grains, les précipités, les inclusions, les lacunes et d’autres interfaces qui jouent un rôle important dans les mécanismes endommageants. Ces aciers de haute résistance mécanique, lorsqu’ils sont soumis à des contraintes mécaniques et à un environnement agressif (qui dépend de la pression en H2S et du pH de la solution) peuvent rompre à cause du phénomène de Sulphide Stress Cracking (SSC). Ce dernier est une forme de fragilisation par l’hydrogène (FPH) qui inclut un amorçage de fissure suivi d’une étape de propagation conduisant à la rupture, dont la contribution de l’hydrogène reste encore mal comprise. En parallèle de l’impact de la microstructure de l’acier, les champs de contrainte et déformation subis par le matériau modifient les effets induits par l’hydrogène. C’est pourquoi un montage de perméation sous contrainte a été utilisé afin de pouvoir réaliser des essais mécaniques jusqu’à rupture sous flux d’hydrogène et les comparer au comportement du matériau lorsque celui-ci est sollicité à l’air ou dans un environnement H2S. Ainsi, l’impact sur le comportement mécanique du flux d’hydrogène mais également de son piégeage peut être étudié. Dans ce cadre, des éprouvettes plates et axisymétriques, lisses et entaillées ont été employées. Les informations expérimentales obtenues dans ce travail ont servi à alimenter un modèle numérique qui a permis de caractériser localement l’état mécanique et les concentrations d’hydrogène piégé et diffusible dans le matériau. Ceci a rendu possible la définition d’un critère local de rupture. / The findings of this work established that the diffusible and trapped hydrogen could have a strong influence on the mechanical properties of materials. However, this effect varies significantly with the materials’ microstructure, chemical composition, and heat treatment. Due to their structure, quenched and tempered martensitic steels (developed for tubes suitable for sour service environments) have different types of traps such as dislocations, grain boundaries, precipitates, inclusions, vacancies and other interfaces that play an important role in the damage mechanisms. These high strength steels may break due to Sulphide Stress Cracking (SSC) if subjected to mechanical stress and an aggressive environment (which depends on the H2S partial pressure and pH solution). This phenomenon is a form of hydrogen embrittlement (HE) that includes a crack initiation followed by a propagation step leading to failure. However the hydrogen contribution is still insufficiently understood. In addition to the impact of the microstructure on the steel, the stress and the deformation fields in the material also modify the effects induced by hydrogen. To investigate this event, electrochemical permeation tests under stress were used to perform mechanical tests under hydrogen flux until failure is reached. The results were compared to those mechanically loaded in air or in a H2S environment. This enabled the examination of the impact of the hydrogen flux and trapping on the mechanical behavior of martensitic steel. In this framework, flat and axisymmetric, smooth and notched specimens were employed. Experimental data obtained in this work were used to provide a numerical model that enables the locally characterization of the mechanical condition and the concentrations of trapped and diffusible hydrogen in the material. These outcomes enabled us to determine a local failure criterion.

Foi avaliada a influência de microrganismos na corrosão e na reação de absorção ou liberação de hidrogênio em amostras de ferro Armco e do aço SAE 4140. Utilizou-se um consórcio microbiano, no qual estava presente a bactéria redutora de sulfato (BRS) Desulfovíbrio desulfuricans que, por meio de reações metabólicas, produz gás sulfídrico (H2S), conhecido por catalisar a reação de hidrogênio. Através de ensaios de permeação de hidrogênio em amostras de ferro Armco determinou-se a corrente de permeação e a concentração superficial de hidrogênio desenvolvida, utilizando-se uma célula de Devanathan-Stachurski. Primeiramente determinou-se a corrente em meio bacteriológico Postgate C abiótico, substituindo-o então pelo meio bacteriológico inoculado com o consórcio de microrganismos. Os conseqüentes transientes de corrente foram determinados em diferentes valores de potenciais catódicos, entre –1500 mV(ENH) e o potencial de corrosão do ferro. Observou-se um aumento na corrente de permeação após a colocação dos meios inoculados o que significa um maior fluxo de hidrogênio se difundindo através da amostra. Porém, após um determinado período de tempo (em torno de 30 horas) a corrente de permeação tende a se estabilizar em decorrência de alguns fatores como os depósitos biológicos formados na superfície metálica (biofilme), a formação de filmes de precipitação de sais em função da polarização catódica aplicada e também devido ao filme de sulfeto de ferro, dificultando desta forma a passagem do hidrogênio. Ensaios de tração de baixa taxa de deformação (BTD) com corpos de prova de aço SAE 4140, foram utilizados nos mesmos meios citados, ao potencial de corrosão e com aplicação de potencial catódico de –750 e –1000 mV(ENH), para avaliar a morfologia da fratura. Em meios com H2S produzido metabolicamente, observou-se fratura frágil, associada à fragilização pelo hidrogênio, ao contrário dos resultados obtidos em meios sem o H2S, que promoveram fratura dúctil. Ensaios potenciostáticos com as amostras de ferro Armco e o aço SAE 4140 foram realizados em condições semelhantes aos ensaios de permeação e de baixa taxa de deformação. Os corpos de prova foram analisados por microscopia eletrônica de varredura para observar o biofilme, bem como a morfologia da corrosão. Nos ensaios de potencial a circuito aberto intensa colonização da superfície por bactérias foi observada após duas horas, porém com aplicação de potenciais catódicos (–1000 mV(ENH)), observou-se um número menor de células bacterianas aderidas à superfície metálica. Os produtos metabólicos, principalmente o H2S, provenientes do metabolismo das bactérias redutoras de sulfato (BRS), aceleram o processo de corrosão, principalmente quando o meio em questão propicia zonas de anaerobiose como a formação de biofilme e a formação de sulfeto de ferro (FeS) sobre a superfície metálica. / The present investigation was undertaken to evaluate the influence of microorganisms on the corrosion and on the hydrogen absortion and evolution reaction of Armco iron and SAE 4140 steel samples. A mixed group of microbes was used, in which was present the Sulfate-Reducing Bacteria (SRB) Desulfovibrio desulfuricans that, through metabolic reactions, produces hydrogen sulfide gas (H2S), recognized for catalyzing the hydrogen reaction. The permeation current of hydrogen and its concentration on the surface were determined on Armco iron samples, using a Devanathan-Stachurski cell. The experiments were carried out in an abiotic Postgate C bacterial growth medium and after the current stabilization, which lasted 24 hours, this medium was replaced by on environment inoculated with microorganisms. The consequent current transients were determined for different cathodic potentials, from –1500 mV(ENH) to the iron corrosion potential. An increase in the permeation current was observed after the addition of the inoculated environment, which means a higher flux of hydrogen diffusing through the sample. However, after a certain period of time (around 30 hours), the permeation current tended stabilize, due to the biologic deposits formed on the metallic surface and to the films precipitated due to the cathodic polarization, which reduce hydrogen ion migration. Low-strain rate tests with SAE 4140 steel samples were used in the same environments, at the corrosion potential and with the application of cathodic potentials of –750 and –1000 mV(ENH) to evaluate the fracture morphology. In environments with metabolically produced H2S, a fragile fracture was observed, associated with hydrogen embrittlement, while in H2S-free environments a ductile fracture was produced. Potentiostatic tests with Armco iron and SAE 4140 steel samples were performed in similar conditions to permeation and low-strain rate tests. The samples were analyzed by Scanning Electron Microscopy (SEM) in order to observe the biofilm, as well as the corrosion morphology. In the samples without applied potential, a heavy surface colonization by bacteria was observed after 2 hours, while in the samples under cathodic potential (–1000 mV(ENH)) only few bacterial cells adhered to the metallic surface. It was observed that Sulfate-Reducing Bacteria (SRB) accelerate the corrosion process, mainly when the environment is able to provide anaerobeose zones such as the biofilm and the iron sulfide (FeS) on the metallic surface.

Foi avaliada a influência de microrganismos na corrosão e na reação de absorção ou liberação de hidrogênio em amostras de ferro Armco e do aço SAE 4140. Utilizou-se um consórcio microbiano, no qual estava presente a bactéria redutora de sulfato (BRS) Desulfovíbrio desulfuricans que, por meio de reações metabólicas, produz gás sulfídrico (H2S), conhecido por catalisar a reação de hidrogênio. Através de ensaios de permeação de hidrogênio em amostras de ferro Armco determinou-se a corrente de permeação e a concentração superficial de hidrogênio desenvolvida, utilizando-se uma célula de Devanathan-Stachurski. Primeiramente determinou-se a corrente em meio bacteriológico Postgate C abiótico, substituindo-o então pelo meio bacteriológico inoculado com o consórcio de microrganismos. Os conseqüentes transientes de corrente foram determinados em diferentes valores de potenciais catódicos, entre –1500 mV(ENH) e o potencial de corrosão do ferro. Observou-se um aumento na corrente de permeação após a colocação dos meios inoculados o que significa um maior fluxo de hidrogênio se difundindo através da amostra. Porém, após um determinado período de tempo (em torno de 30 horas) a corrente de permeação tende a se estabilizar em decorrência de alguns fatores como os depósitos biológicos formados na superfície metálica (biofilme), a formação de filmes de precipitação de sais em função da polarização catódica aplicada e também devido ao filme de sulfeto de ferro, dificultando desta forma a passagem do hidrogênio. Ensaios de tração de baixa taxa de deformação (BTD) com corpos de prova de aço SAE 4140, foram utilizados nos mesmos meios citados, ao potencial de corrosão e com aplicação de potencial catódico de –750 e –1000 mV(ENH), para avaliar a morfologia da fratura. Em meios com H2S produzido metabolicamente, observou-se fratura frágil, associada à fragilização pelo hidrogênio, ao contrário dos resultados obtidos em meios sem o H2S, que promoveram fratura dúctil. Ensaios potenciostáticos com as amostras de ferro Armco e o aço SAE 4140 foram realizados em condições semelhantes aos ensaios de permeação e de baixa taxa de deformação. Os corpos de prova foram analisados por microscopia eletrônica de varredura para observar o biofilme, bem como a morfologia da corrosão. Nos ensaios de potencial a circuito aberto intensa colonização da superfície por bactérias foi observada após duas horas, porém com aplicação de potenciais catódicos (–1000 mV(ENH)), observou-se um número menor de células bacterianas aderidas à superfície metálica. Os produtos metabólicos, principalmente o H2S, provenientes do metabolismo das bactérias redutoras de sulfato (BRS), aceleram o processo de corrosão, principalmente quando o meio em questão propicia zonas de anaerobiose como a formação de biofilme e a formação de sulfeto de ferro (FeS) sobre a superfície metálica. / The present investigation was undertaken to evaluate the influence of microorganisms on the corrosion and on the hydrogen absortion and evolution reaction of Armco iron and SAE 4140 steel samples. A mixed group of microbes was used, in which was present the Sulfate-Reducing Bacteria (SRB) Desulfovibrio desulfuricans that, through metabolic reactions, produces hydrogen sulfide gas (H2S), recognized for catalyzing the hydrogen reaction. The permeation current of hydrogen and its concentration on the surface were determined on Armco iron samples, using a Devanathan-Stachurski cell. The experiments were carried out in an abiotic Postgate C bacterial growth medium and after the current stabilization, which lasted 24 hours, this medium was replaced by on environment inoculated with microorganisms. The consequent current transients were determined for different cathodic potentials, from –1500 mV(ENH) to the iron corrosion potential. An increase in the permeation current was observed after the addition of the inoculated environment, which means a higher flux of hydrogen diffusing through the sample. However, after a certain period of time (around 30 hours), the permeation current tended stabilize, due to the biologic deposits formed on the metallic surface and to the films precipitated due to the cathodic polarization, which reduce hydrogen ion migration. Low-strain rate tests with SAE 4140 steel samples were used in the same environments, at the corrosion potential and with the application of cathodic potentials of –750 and –1000 mV(ENH) to evaluate the fracture morphology. In environments with metabolically produced H2S, a fragile fracture was observed, associated with hydrogen embrittlement, while in H2S-free environments a ductile fracture was produced. Potentiostatic tests with Armco iron and SAE 4140 steel samples were performed in similar conditions to permeation and low-strain rate tests. The samples were analyzed by Scanning Electron Microscopy (SEM) in order to observe the biofilm, as well as the corrosion morphology. In the samples without applied potential, a heavy surface colonization by bacteria was observed after 2 hours, while in the samples under cathodic potential (–1000 mV(ENH)) only few bacterial cells adhered to the metallic surface. It was observed that Sulfate-Reducing Bacteria (SRB) accelerate the corrosion process, mainly when the environment is able to provide anaerobeose zones such as the biofilm and the iron sulfide (FeS) on the metallic surface.

Les aciers THLE ont la particularité de posséder à la fois une bonne ductilité et de hautes caractéristiques mécaniques. Ceci les rend particulièrement adaptés pour l'industrie automobile, dont les principales exigences sont l'allègement du véhicule et la sécurité des passagers. Toutefois, il est bien connu que l'augmentation des caractéristiques mécaniques accroît la susceptibilité à la fragilisation par l'hydrogène. Ce travail de thèse est consacré à l'étude de la susceptibilité vis-à-vis de la fragilisation par l'hydrogène de quatre aciers THLE : un DP, un TRIP, un CP et le BAS 100, un acier enrichi en vanadium et chrome. Un acier aux propriétés mécaniques plus modestes, dénommé HE (Haute Elasticité) a servi de référence. Les caractéristiques de transport de l'hydrogène dans ces aciers ont été étudiées, grâce à des essais de perméation électrochimique avec chargement en milieu acide, éventuellement additionné d'un promoteur d'hydrogénation (l'arsenic). Comme observé sur d'autres aciers, il faut souligner l'absence de conditions d'entrée stationnaires, dont il faut tenir compte dans l'évaluation des caractéristiques de diffusion. La diffusivité à température ambiante est apparue élevée pour tous les aciers, et une corrélation a été établie entre la microstructure et la diffusivité de l'hydrogène dans le matériau : plus la microstructure est fine et complexe, moins la diffusivité est élevée. De plus, l'évaluation des concentrations subsurfaciques sur les courbes en présence d'arsenic a révélé des valeurs relativement élevées pour les trois aciers aux caractéristiques mécaniques les plus élevées (TRIP 800, CP 800 et BAS 100). Ces valeurs sont conformes avec les teneurs en hydrogène diffusible mesurées par dosage juste après la perméation. Les dosages d'hydrogène résiduel, réalisés par désorption thermique sous vide après perméation, ont par ailleurs indiqué que le piégeage profond dans ces aciers était peu important, même après chargement sous polarisation et en présence d'arsenic. Ceci peut s'expliquer par des structures très bien élaborées, très fines et comportant peu de défauts. Des essais de traction ont montré qu'une hydrogénation sévère (en présence d'un promoteur) était nécessaire pour obtenir une fragilisation notable des aciers THLE. Hormis les cas extrêmes de dégradation spontanée par HIC (cloquage, fissuration), la fragilisation est imputable à l'hydrogène diffusible ou faiblement piégé car les teneurs en hydrogène piégé profondément restent négligeables. Dans les conditions industrielles, en décapage acide HCl en présence d'inhibiteurs, les résultats de perméation, de dosage et de traction s'accordent à montrer l'absence de fragilisation sur ce type d'acier. Les inhibiteurs testés semblent jouer un rôle de barrière physique, par adsorption sur le métal nu, limitant ainsi tant l'entrée d'hydrogène que la corrosion. / The distinctive feature of Very High Strength Steels (VHSS) is to present a good combination of ductility and high strength. This makes them particularly interesting for the automotive industry because of the increasing demand for the reduction of car weight and the improvement of passengers security. However, it is known that increasing mechanical characteristics enhances susceptibility to hydrogen embrittlement. The aim of this doctoral thesis work is to study the susceptibility to hydrogen embrittlement of four very high strength steels : a DP (Dual Phase), a TRIP (Transformation Induced Plasticity), a CP (Complex Phase) and BAS, a Cr-V enriched high strength steel. Low alloyed steel with lower mechanical properties, HE (high Elasticity) has been used as a reference. Hydrogen transport characteristics in these steels were investigated thanks to electrochemical permeation tests including charging in acid solution, possibly with the addition of a hydrogenation promoter (arsenic). As already observed on other steels, the absence of stationary entry conditions is to be underlined due to its necessity for the evaluation of diffusion characteristics. Diffusivity at room temperature has appeared to be very high in each of the five steels and a correlation between hydrogen diffusion coefficient and microstructure has been drawn : the finer and the more complex it is, the lower is the apparent diffusion coefficient. Moreover, sub-surface concentrations calculated on the permeation transient in the presence of arsenic have revealed relatively high values for the three steels with the higher mechanical properties (TRIP 800, CP 800 and BAS 100). These values comply with the diffusible hydrogen content measured by thermal desorption technique just after permeation. Otherwise, residual hydrogen dosage tests by thermal desorption under vacuum, have indicated that deep trapping is very low for these steels, even after charging under polarisation and in the presence of arsenic. These results can be explained by fine and homogeneous microstructures that are very well developed with few lattice defects. Ordinary tensile tests have shown the necessity of extreme charging conditions (in the presence of a promoter) for VHS steel embrittlement. With an exception in the case of extreme spontaneaous damages created by HIC (blistering, cracking), embrittlement is imputable to diffusible and weakly trapped hydrogen because deeply trapped hydrogen content is very low. In industrial conditions, during acid pickling while in the presence of inhibitors, permeation, dosage and tensile tests results suggest the absence of embrittlement for the steels. Tested inhibitors seem to act as a physical barrier, by adsorption on the bare steel surface, and limit that way hydrogen absorption and corrosion.

Foi avaliada a influência de microrganismos na corrosão e na reação de absorção ou liberação de hidrogênio em amostras de ferro Armco e do aço SAE 4140. Utilizou-se um consórcio microbiano, no qual estava presente a bactéria redutora de sulfato (BRS) Desulfovíbrio desulfuricans que, por meio de reações metabólicas, produz gás sulfídrico (H2S), conhecido por catalisar a reação de hidrogênio. Através de ensaios de permeação de hidrogênio em amostras de ferro Armco determinou-se a corrente de permeação e a concentração superficial de hidrogênio desenvolvida, utilizando-se uma célula de Devanathan-Stachurski. Primeiramente determinou-se a corrente em meio bacteriológico Postgate C abiótico, substituindo-o então pelo meio bacteriológico inoculado com o consórcio de microrganismos. Os conseqüentes transientes de corrente foram determinados em diferentes valores de potenciais catódicos, entre –1500 mV(ENH) e o potencial de corrosão do ferro. Observou-se um aumento na corrente de permeação após a colocação dos meios inoculados o que significa um maior fluxo de hidrogênio se difundindo através da amostra. Porém, após um determinado período de tempo (em torno de 30 horas) a corrente de permeação tende a se estabilizar em decorrência de alguns fatores como os depósitos biológicos formados na superfície metálica (biofilme), a formação de filmes de precipitação de sais em função da polarização catódica aplicada e também devido ao filme de sulfeto de ferro, dificultando desta forma a passagem do hidrogênio. Ensaios de tração de baixa taxa de deformação (BTD) com corpos de prova de aço SAE 4140, foram utilizados nos mesmos meios citados, ao potencial de corrosão e com aplicação de potencial catódico de –750 e –1000 mV(ENH), para avaliar a morfologia da fratura. Em meios com H2S produzido metabolicamente, observou-se fratura frágil, associada à fragilização pelo hidrogênio, ao contrário dos resultados obtidos em meios sem o H2S, que promoveram fratura dúctil. Ensaios potenciostáticos com as amostras de ferro Armco e o aço SAE 4140 foram realizados em condições semelhantes aos ensaios de permeação e de baixa taxa de deformação. Os corpos de prova foram analisados por microscopia eletrônica de varredura para observar o biofilme, bem como a morfologia da corrosão. Nos ensaios de potencial a circuito aberto intensa colonização da superfície por bactérias foi observada após duas horas, porém com aplicação de potenciais catódicos (–1000 mV(ENH)), observou-se um número menor de células bacterianas aderidas à superfície metálica. Os produtos metabólicos, principalmente o H2S, provenientes do metabolismo das bactérias redutoras de sulfato (BRS), aceleram o processo de corrosão, principalmente quando o meio em questão propicia zonas de anaerobiose como a formação de biofilme e a formação de sulfeto de ferro (FeS) sobre a superfície metálica. / The present investigation was undertaken to evaluate the influence of microorganisms on the corrosion and on the hydrogen absortion and evolution reaction of Armco iron and SAE 4140 steel samples. A mixed group of microbes was used, in which was present the Sulfate-Reducing Bacteria (SRB) Desulfovibrio desulfuricans that, through metabolic reactions, produces hydrogen sulfide gas (H2S), recognized for catalyzing the hydrogen reaction. The permeation current of hydrogen and its concentration on the surface were determined on Armco iron samples, using a Devanathan-Stachurski cell. The experiments were carried out in an abiotic Postgate C bacterial growth medium and after the current stabilization, which lasted 24 hours, this medium was replaced by on environment inoculated with microorganisms. The consequent current transients were determined for different cathodic potentials, from –1500 mV(ENH) to the iron corrosion potential. An increase in the permeation current was observed after the addition of the inoculated environment, which means a higher flux of hydrogen diffusing through the sample. However, after a certain period of time (around 30 hours), the permeation current tended stabilize, due to the biologic deposits formed on the metallic surface and to the films precipitated due to the cathodic polarization, which reduce hydrogen ion migration. Low-strain rate tests with SAE 4140 steel samples were used in the same environments, at the corrosion potential and with the application of cathodic potentials of –750 and –1000 mV(ENH) to evaluate the fracture morphology. In environments with metabolically produced H2S, a fragile fracture was observed, associated with hydrogen embrittlement, while in H2S-free environments a ductile fracture was produced. Potentiostatic tests with Armco iron and SAE 4140 steel samples were performed in similar conditions to permeation and low-strain rate tests. The samples were analyzed by Scanning Electron Microscopy (SEM) in order to observe the biofilm, as well as the corrosion morphology. In the samples without applied potential, a heavy surface colonization by bacteria was observed after 2 hours, while in the samples under cathodic potential (–1000 mV(ENH)) only few bacterial cells adhered to the metallic surface. It was observed that Sulfate-Reducing Bacteria (SRB) accelerate the corrosion process, mainly when the environment is able to provide anaerobeose zones such as the biofilm and the iron sulfide (FeS) on the metallic surface.