Cette thèse s’inscrit dans une démarche d’optimisation des performances de la PEMFC, à travers le développement d’outils de diagnostic dédiés à la gestion de l’eau. Celle-ci est un des éléments clé de l’augmentation des performances et de la durée de vie des piles à combustible. La technique du bruit électrochimique a été utilisée pour détecter des phénomènes liés au comportement de l’eau lors du fonctionnement de la pile dans différentes conditions opératoires (humidité relative des gaz, température et courant). Le bruit électrochimique enregistré pendant ces expériences a été associé à des mécanismes source grâce à une démarche expérimentale appropriée et à un traitement de signal basé sur l’analyse fréquentielle et temporelle. Les résultats de l’analyse spectrale et statistique ont permis de décrire l’état de santé de la pile à combustible (assèchement et noyage), par la génération des descripteurs dans le domaine fréquentiel et temporel, signatures de chaque état de fonctionnement de la pile à combustible. Enfin, de manière à compléter l’approche effectuée sur la détection de l’assèchement de la membrane et le noyage de la cellule par l’étude des bruits électrochimiques, des capteurs de mesure de pression en amont de la cellule de PEMFC ont été utilisées et comparer au signaux de tension pour détecter le phénomène de noyage d’une mono-cellule de PEMFC. / This thesis deals to optimize the performance of the PEMFC, through the development of diagnostic tools dedicated to water management. This water management is one of the key elements to increase the performance and life of fuel cells. The electrochemical noise technique was used to detect the phenomena related to the water behavior during the operation of the cell, under different operating conditions (relative humidity of the gases, temperature and current). The electrochemical noise recorded during these experiments was associated with mechanisms through an appropriate experimental approach and signal processing based on frequency and time analysis. The results of spectral and statistical analysis made it possible to describe the state of health of fuel cell (drying and flooding), by the generation of the descriptors in the frequency and time domain, signatures of each operating state of the fuel cell. Finally, in order to complete, the approach carried out on detecting the drying of the membrane and flooding of the cell by electrochemical noise analysis, the pressure measurement was combined with voltage signals, to detect the flooding phenomena of fuel cell.

Neste trabalho foi utilizado um método matemático para classificar registros de potencial e corrente de ensaios de corrosão na técnica de amperimetria de resistência nula (ZRA). Foi aplicado o método estatístico de múltiplas variáveis simples chamado Análise dos Componentes Principais (PCA), cujo objetivo principal foi identificar padrões nestes dados de ruído eletroquímico. Foram testados o aço carbono UNS G10200, os aços inoxidáveis austenítico UNS S31600 e o superduplex UNS S32750 em meios de ácido sulfúrico (5% H2SO4), cloreto férrico (0,1 mol/L FeCl3) e hidróxido de sódio (0,1% NaOH). Os ensaios foram replicados com oito repetições para se ter reprodutibilidade e conhecimento dos aspectos estatísticos envolvidos. Os resultados mostraram que a análise de componentes principais pode ser utilizada como uma ferramenta para analisar sinais de ruído eletroquímico, identificando os clusters dos comportamentos potencial-tempo, corrente-tempo e acessoriamente identificar os outliersdos registros temporais. / In this study, a mathematical method was used to classify potential and current records of electrochemical noise tests in zero resistance ammeter (ZRA) configuration. The statistical method of multiple simple variables called Principal Components Analysis (PCA) was applied to identify patterns in these electrochemical noise data. The carbon steel UNS G10200, the austenitic stainless steels UNS S31600 and the super duplex UNS S32750 were tested in the solutions of sulfuric acid (5% H2SO4), ferric chloride (0.1 mol/L FeCl3) and sodium hydroxide (0.1% NaOH). The tests were replicated with eight repetitions to obtain reproducibility of the relatedstatistical aspects. The results showed that the principal components analysis could be used as a tool to analyze electrochemical noise signals, identifying behavior clusters of potential-time, current-time and also identifying the outliers in time domain.

Ce travail s’inscrit dans le contexte d’une meilleure prédiction de la durée de vie de structures soumises au risque d’amorçage et de propagation de fissures multiples de corrosion sous contrainte (CSC). Ainsi, en développant une méthodologie expérimentale originale basée sur des mesures conjointes de corrélation d’images numériques (DIC), d’émission acoustique (EA) et de bruit électrochimique (EN), et leur analyse, ce travail vise à contribuer à l’amélioration de la compréhension des mécanismes impliqués dans le développement de colonies de fissures courtes de CSC qui interagissent entre elles, et à la simulation du comportement de cette colonie. Le choix de conditions optimales de traitement thermique d’un alliage base-Ni et de pH d’une solution de polythionate, a permis la maitrise des paramètres géométriques et morphologiques de colonies de fissures intergranulaires, qui ont été investiguées par DIC grâce à l’optimisation d’une préparation de surface adaptée. Les différentes phases de propagation d’une fissure unique et d’une colonie de fissures ont été identifiées, de même que les mécanismes associés, par des expérimentations et analyses réalisées en 2D et en 3D. Cette approche expérimentale innovante a permis de poser les bases d’une approche numérique puis de la valider. Un focus particulier a été porté sur l’EN au travers d’une analyse critique des perturbations engendrées par le bruit de l’instrumentation et par l’asymétrie du système d’étude. Les limitations de la technique pour son application à l’étude quantitative de la corrosion sous contrainte ont été évaluées sur la base des résultats de l’étude. Une transposition de la démarche expérimentale à hautes pressions et températures est proposée comme perspective à court terme de ce travail, qui permet également d’envisager la prise en compte de différents modes de propagation des fissures en lien avec la microstructure du matériau dans l’approche numérique. / This work concerns with the current needs of enhancing the tools used for predicting the remnant lifetime of structures subjected to the risk of initiation and propagation of multiple stress corrosion cracks (SCC). The approach consists in developing an original experimental methodology based on joint measurements of digital image correlation (DIC), acoustic emission (EA) and electrochemical noise (EN). The final objective is to contribute to both the understanding of the mechanisms involved in the development of interacting short stress corrosion cracks and to the modeling of the colony behavior. The choice of optimal conditions for the heat treatment of a Nickel-base alloy and for the pH of a polythionate solution allowed controlling the morphological parameters of intergranular cracks colonies, which were investigated by DIC owning to an optimized suitable surface treatment. The different propagation stages of a single crack and some colonies were identified, together with the involved mechanisms, through experiments and analyses performed in 2D and 3D. This innovative experimental approach allowed settling the basements of the numerical modeling and validating it. A particular attention was focused on EN measurements through a critical analysis of the perturbations generated by the instrumental noise and the asymmetry of the studied system. The limitations of the technique for its application to the quantitative study of SCC were evaluated on the basis of the present results. A transposal of the experimental approach towards high temperature and pressure conditions of test was finally proposed as a short-term prospect of this work, also allowing considering other modes of crack propagation linked to the material microstructure in the numerical approach.

The corrosion behaviour of cast Al-Mg alloy A535 and its composites containing 10 wt.% and 15 wt.% fly ash, and 10 wt.% hybrid reinforcement (5 wt.% fly ash + 5 wt.% SiC) was investigated using weight-loss and electrochemical corrosion tests, optical microscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). The tests were conducted in fresh water collected from the South Saskatchewan River and 3.5 wt.% NaCl solution at room temperature. The pH of the salt solution varied from 3 to 9. For comparison, two other aluminum alloys, AA2618 and AA5083-H116, were tested in the same electrolytes. The results of the weight-loss corrosion test showed that unreinforced A535 alloy had a lower corrosion rate in fresh water and seawater environments than the composites at all the tested pH values. The corrosion rate of the composites increased with increasing fly ash content. As expected, the corrosion rates of A535 alloy and the composites tested in fresh water were lower than those in salt solution. The results of the potentiodynamic and cyclic polarization electrochemical tests showed that the corrosion potential (Ecorr) and pitting potential (Epit) of the alloy were more positive than those of the composites. The corrosion and pitting potentials of the composites became more negative (active) with increasing fly ash content. The composites showed more positive (noble) repassivation or protection potential (Erp) than the matrix alloy, with the positivity increasing with fly ash content. Analysis of the electrochemical noise data showed that pitting corrosion was the dominant mode of corrosion for the alloy in 3.5 wt.% NaCl solution. Optical microscopy and SEM revealed that Mg2Si phase and Al-Mg intermetallics corroded preferentially to the matrix. The EDS data indicated that the protective oxide film formed on A535 contained Al2O3 and MgO.

The corrosion behaviour of cast Al-Mg alloy A535 and its composites containing 10 wt.% and 15 wt.% fly ash, and 10 wt.% hybrid reinforcement (5 wt.% fly ash + 5 wt.% SiC) was investigated using weight-loss and electrochemical corrosion tests, optical microscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). The tests were conducted in fresh water collected from the South Saskatchewan River and 3.5 wt.% NaCl solution at room temperature. The pH of the salt solution varied from 3 to 9. For comparison, two other aluminum alloys, AA2618 and AA5083-H116, were tested in the same electrolytes. The results of the weight-loss corrosion test showed that unreinforced A535 alloy had a lower corrosion rate in fresh water and seawater environments than the composites at all the tested pH values. The corrosion rate of the composites increased with increasing fly ash content. As expected, the corrosion rates of A535 alloy and the composites tested in fresh water were lower than those in salt solution. The results of the potentiodynamic and cyclic polarization electrochemical tests showed that the corrosion potential (Ecorr) and pitting potential (Epit) of the alloy were more positive than those of the composites. The corrosion and pitting potentials of the composites became more negative (active) with increasing fly ash content. The composites showed more positive (noble) repassivation or protection potential (Erp) than the matrix alloy, with the positivity increasing with fly ash content. Analysis of the electrochemical noise data showed that pitting corrosion was the dominant mode of corrosion for the alloy in 3.5 wt.% NaCl solution. Optical microscopy and SEM revealed that Mg2Si phase and Al-Mg intermetallics corroded preferentially to the matrix. The EDS data indicated that the protective oxide film formed on A535 contained Al2O3 and MgO.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Sinais diversos estão presentes em nosso cotidiano, assim como nas medidas realizadas nas atividades de ciência e tecnologia. Dentre estes sinais, tem grande importância tecnológica aqueles associados à corrosão de estruturas metálicas. Assim, esta tese propõe o estudo de um esquema local de transformada de Fourier janelada, com a janela variando em função da curtose, aplicada a sinais de ruído eletroquímico. A curtose foi avaliada nos domínios do tempo e da frequência e processada pelo programa desenvolvido para esse fim. O esquema foi aplicado a sinais de ruído eletroquímico dos aços UNS S31600, UNS G10200 e UNS S32750 imersos em três soluções: FeCl3 0,1 mol=L (cloreto férrico), H2SO4 5%(ácido sulfúrico) e NaOH 0,1%(hidróxido de sódio). Para os aços inoxidáveis, estas soluções promovem corrosão localizada, uniforme e passivação, respectivamente. Visando testar o desempenho do esquema de Fourier desenvolvido, testes foram realizados utilizando-se inicialmente sinais sintéticos e em seguida sinais de ruído eletroquímico. Notou-se que os sinais têm características de não-estacionaridade e a maior parte da energia está presente em baixa frequência. Os intervalos de tempo e de frequência onde se concentra a maior parte da energia do sinal foram correlacionados. Para os picos máximos dos sinais de potencial e corrente obtidos de amperimetria de resistência nula, a correlação entre eles foi baixa, independente da forma de corrosão presente. Conclui-se que o método se adaptou bastante bem às características locais do sinal eletroquímico permitindo o monitoramento dos espectros tempo-frequência. O fato de ser sensível às características locais do sinal permite analisar aspectos dos sinais que do modo clássico não podem ser diretamente processados. O método da transformada de Fourier janelada variável (Variable Short-Time Fourier Transform - VSTFT) adaptou-se muito bem no monitoramento dos sinais originados de potencial de circuito aberto e amperimetria de resistência nula. / Several signals are present in our daily lives, as well as in the measurements from scientific and technological procedures. Among these signals, it has great technological importance those associated with corrosion of metallic structures. Thus, this thesis considers a local windowed Fourier transform, varying according to the kurtosis of signals, applied to electrochemical noise. The kurtosis was assessed in the kurtosis of the time and frequency domains and evaluated by the software developed for this purpose. The scheme has been applied to electrochemical noise signals of UNS S31600, UNS G10200 and UNS S32750 steels immersed in three solutions: FeCl3 0.1 mol=L (ferric chloride), H2SO4 5 % (sulfuric acid) and NaOH 0.1 % (sodium hydroxide). For stainless steels, these solutions cause localized corrosion, uniform corrosion and passivation, respectively. To test the performance of the proposed Fourier scheme, tests were conducted initially using synthetic signal and then electrochemical noise signals. It is worth noticing that the signals have non-stationary characteristics and most part of its energy is present at low frequency. The time and frequency intervals which concentrate most part of the energy of the signal were correlated. For the maximum peaks of the potential and current signals obtained by zero resistance ammeter, the correlation between them was low, regardless of the form of present corrosion. We conclude that the method is well adapted to the local features of the electrochemical signal allowing the monitoring of time-frequency spectograms. As the method is sensitive to the local features of the sign, it allows to analyse signals that by the classic approach can not be directly processed. The windowed Fourier transform (Variable Short-Time Fourier Transform - VSTFT) adapted very well in monitoring of the signals originated by open circuit potential and zero resistance ammeter.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Sinais diversos estão presentes em nosso cotidiano, assim como nas medidas realizadas nas atividades de ciência e tecnologia. Dentre estes sinais, tem grande importância tecnológica aqueles associados à corrosão de estruturas metálicas. Assim, esta tese propõe o estudo de um esquema local de transformada de Fourier janelada, com a janela variando em função da curtose, aplicada a sinais de ruído eletroquímico. A curtose foi avaliada nos domínios do tempo e da frequência e processada pelo programa desenvolvido para esse fim. O esquema foi aplicado a sinais de ruído eletroquímico dos aços UNS S31600, UNS G10200 e UNS S32750 imersos em três soluções: FeCl3 0,1 mol=L (cloreto férrico), H2SO4 5%(ácido sulfúrico) e NaOH 0,1%(hidróxido de sódio). Para os aços inoxidáveis, estas soluções promovem corrosão localizada, uniforme e passivação, respectivamente. Visando testar o desempenho do esquema de Fourier desenvolvido, testes foram realizados utilizando-se inicialmente sinais sintéticos e em seguida sinais de ruído eletroquímico. Notou-se que os sinais têm características de não-estacionaridade e a maior parte da energia está presente em baixa frequência. Os intervalos de tempo e de frequência onde se concentra a maior parte da energia do sinal foram correlacionados. Para os picos máximos dos sinais de potencial e corrente obtidos de amperimetria de resistência nula, a correlação entre eles foi baixa, independente da forma de corrosão presente. Conclui-se que o método se adaptou bastante bem às características locais do sinal eletroquímico permitindo o monitoramento dos espectros tempo-frequência. O fato de ser sensível às características locais do sinal permite analisar aspectos dos sinais que do modo clássico não podem ser diretamente processados. O método da transformada de Fourier janelada variável (Variable Short-Time Fourier Transform - VSTFT) adaptou-se muito bem no monitoramento dos sinais originados de potencial de circuito aberto e amperimetria de resistência nula. / Several signals are present in our daily lives, as well as in the measurements from scientific and technological procedures. Among these signals, it has great technological importance those associated with corrosion of metallic structures. Thus, this thesis considers a local windowed Fourier transform, varying according to the kurtosis of signals, applied to electrochemical noise. The kurtosis was assessed in the kurtosis of the time and frequency domains and evaluated by the software developed for this purpose. The scheme has been applied to electrochemical noise signals of UNS S31600, UNS G10200 and UNS S32750 steels immersed in three solutions: FeCl3 0.1 mol=L (ferric chloride), H2SO4 5 % (sulfuric acid) and NaOH 0.1 % (sodium hydroxide). For stainless steels, these solutions cause localized corrosion, uniform corrosion and passivation, respectively. To test the performance of the proposed Fourier scheme, tests were conducted initially using synthetic signal and then electrochemical noise signals. It is worth noticing that the signals have non-stationary characteristics and most part of its energy is present at low frequency. The time and frequency intervals which concentrate most part of the energy of the signal were correlated. For the maximum peaks of the potential and current signals obtained by zero resistance ammeter, the correlation between them was low, regardless of the form of present corrosion. We conclude that the method is well adapted to the local features of the electrochemical signal allowing the monitoring of time-frequency spectograms. As the method is sensitive to the local features of the sign, it allows to analyse signals that by the classic approach can not be directly processed. The windowed Fourier transform (Variable Short-Time Fourier Transform - VSTFT) adapted very well in monitoring of the signals originated by open circuit potential and zero resistance ammeter.

Neste trabalho foi utilizado um método matemático para classificar registros de potencial e corrente de ensaios de corrosão na técnica de amperimetria de resistência nula (ZRA). Foi aplicado o método estatístico de múltiplas variáveis simples chamado Análise dos Componentes Principais (PCA), cujo objetivo principal foi identificar padrões nestes dados de ruído eletroquímico. Foram testados o aço carbono UNS G10200, os aços inoxidáveis austenítico UNS S31600 e o superduplex UNS S32750 em meios de ácido sulfúrico (5% H2SO4), cloreto férrico (0,1 mol/L FeCl3) e hidróxido de sódio (0,1% NaOH). Os ensaios foram replicados com oito repetições para se ter reprodutibilidade e conhecimento dos aspectos estatísticos envolvidos. Os resultados mostraram que a análise de componentes principais pode ser utilizada como uma ferramenta para analisar sinais de ruído eletroquímico, identificando os clusters dos comportamentos potencial-tempo, corrente-tempo e acessoriamente identificar os outliersdos registros temporais. / In this study, a mathematical method was used to classify potential and current records of electrochemical noise tests in zero resistance ammeter (ZRA) configuration. The statistical method of multiple simple variables called Principal Components Analysis (PCA) was applied to identify patterns in these electrochemical noise data. The carbon steel UNS G10200, the austenitic stainless steels UNS S31600 and the super duplex UNS S32750 were tested in the solutions of sulfuric acid (5% H2SO4), ferric chloride (0.1 mol/L FeCl3) and sodium hydroxide (0.1% NaOH). The tests were replicated with eight repetitions to obtain reproducibility of the relatedstatistical aspects. The results showed that the principal components analysis could be used as a tool to analyze electrochemical noise signals, identifying behavior clusters of potential-time, current-time and also identifying the outliers in time domain.