Early developed non-bulk Fe-based glasses, e.g. Fe-Cr(-Mo)-metalloid(s), exhibit extraordinary corrosion resistance, but low glass formation ability (GFA). Newly developed bulk glass-forming Fe-based alloys have on the contrary high GFA, but also very different compositions and therefore their corrosion behaviour is expectedly not similar. Fundamental investigations regarding corrosion behaviour were performed for one of the most prominent bulk glassy alloy, namely (Fe44.3Cr5Co5Mo12.8Mn11.2C15.8B5.9)98.5Y1.5. Particularly, the free corrosion and the anodic polarization behaviour, the passivation ability and the pitting susceptibility have been assessed in electrolytes with varying pH values and anion species concentrations. Due to its monolithic single phase microstructure this alloy has a much lower corrosion rate in acids than a two-phase conventional steel (DIN X210Cr12) with much higher content of passivating Cr, i.e. 11.4 at. %. However, the high concentration of electrochemically active Mn and B as well as the unfavourably high Mo to Cr concentration ratio determine a higher corrosion rate of this bulk glassy alloy in strong alkalis and also a very poor passivation ability in acids. On the contrary, the high content of Mo has a positive influence on the pitting resistance by inhibiting very effectively the propagation of pits occurring at Y2O3 inclusions. Detailed microscopic analysis investigations by HRSEM and in-situ AFM revealed the formation of characteristic morphological features at the micro- and nanometre scale on the surface of samples exposed to acidic solutions. These were explained by selective dissolution of active elements, e.g. Mn, B. This study demonstrated the necessity to investigate the corrosion properties of newly developed bulk glass-forming Fe-based alloys – they are not per-se highly corrosion resistant, but their corrosion behaviour depends on their particular chemical composition. / Früh entwickelte, nicht-massive amorphe Eisenbasislegierungen, z.B. Fe-Cr(-Mo)-Metalloid(e), zeigen bemerkenswerte Korrosionsbeständigkeit, aber niedrige Glasbildungsfähigkeit (englisch: glass-forming ability, GFA). Neu entwickelte massiv-glasbildende Eisenbasislegierungen haben im Gegenteil eine höhere GFA, aber auch sehr unterschiedliche Zusammensetzungen und deshalb ist ihr Korrosionverhalten ist wie zu erwarten nicht änlich. Grundlegende Untersuchungen des Korrosionsverhaltens einer der bekanntesten massiven amophen Legierung, nämlich (Fe44.3Cr5Co5Mo12.8Mn11.2C15.8B5.9)98.5Y1.5, wurden vorgenommen. Insbesondere wurde das Augenmerk auf das freie Korrosions- und das anodische Polarisationsverhalten, die Passivierungseigenschaften und die Anfälligkeit gegenüber Lochfraß in Elektrolyten mit verschiedenen pH-Werten und Anionenkonzentrationen gerichtet. Aufgrund ihres einphasig monolitischen Gefüges zeigt diese Legierung in Säuren eine viel niedrigere Korrosionsgeschwindigkeit als die eines zweiphasigen herkömmlichen Stahls (DIN X210Cr12) mit viel höherem Gehalt an passivierendem Cr, d.h. 11.4 at.%. Der höhere Gehalt an electrochemisch aktivem Mn und B sowie das nachteilige Verhältnis von Mo zu Cr Konzentration sind für eine höhere Korrosionsgeschwindigkeit dieser massiven amorphen Legierung in konzentrierten Alkalien sowie eine geringere Passivierungsfähigkeit in Säuren verantwortlich. Der hohe Gehalt an Mo hat jedoch einen positiven Einfluss auf die Lochfraßbeständigkeit – er hindert sehr wirksam das Wachstum der an Y2O3-Einschlüssen gebildeten Löcher. Detaillierte mikroskopische Untersuchungen durch HRSEM und in-situ AFM zeigten die Bildung charakteristischer Morphologien im Mikrometer- und Nanometerbereich auf der Oberfläche von Proben, die starken Säure ausgesetzt waren. Dieses wurde durch selektive Auflösung aktiver Elemente, z.B. Mn, B, erklärt. Diese Arbeit unterstreicht die Notwendigkeit, die Korrosionseigenschaften der neu entwickelten, massivglasbildenden Eisenbasislegierungen zu untersuchen – diese sind nicht per-se „hochkorrosionsbeständig“, stattdessen hängt ihr Korrosionsverhalten vielmehr von ihrer besonderen chemischen Zusammensetzung ab.

Thesis (MSc (Genetics))--University of Stellenbosch, 2010. / Includes bibliography. / ENGLISH ABSTRACT: The first aim of this study was to reliably and rapidly detect Grapevine rupestris stem pittingassociated virus (GRSPaV) in grapevine. This was achieved by screening 94 grapevines using crude plant extracts in both quantitative and conventional reverse transcription polymerase chain reaction (RT-PCR). The second aim was to establish a technique capable of differentiating GRSPaV sequence variants. The application of this technique is for the largescale screening of diseased vines to associate sequence variants of GRSPaV with disease symptoms. Nested quantitative polymerase chain reaction and high resolution melting assays (qPCR-HRM) were developed for three regions of the GRSPaV genome (coat protein, RNAdependant RNA-polymerase and triple gene block movement protein). The qPCR-HRM technique using the high saturation dye, EvaGreen™, and the Rotor-Gene™ 6000 analyzer was validated with a panel of sixteen sequence-characterized viral isolates. Diluted RT-PCR products and cloned cDNA gave the most consistent amplification plots and dissociation profiles. RT-PCR products generated from total RNA extracts were used as template for qPCR-HRM assays and for direct sequencing of sixteen samples in the three aforementioned regions. The average amplification efficiency for qPCR was 1.52±0.04. Auto-calling of userdefine genotypes was performed at a confidence interval of 70%. Phylogenetic analysis of the three regions of the GRSPaV genome was performed with published GenBank sequences to confirm the HRM data. The dominant sequence variants found in the South African sample set radiated with Group II, reference full-length variant GRSPaV-SG1. GRSPaV-infected samples can in future be subjected to qPCR-HRM assays developed during this study. This can be performed to establish similarity to known genotypes and therefore phylogenetic groups. Mixed infection of sequence variants and quasi-species were a common occurrence. The assay will be useful in establishing correlation of specific genotypes to different phenotypical expression of viral disease. This could provide insight into the etiology of diseases associated with GRSPaV. / AFRIKAANSE OPSOMMING: Die eerste doel van hierdie studie was om die virus wat met Rupestris-stamverpitting (Grapevine rupestris stem pitting-associated virus of “GRSPaV”) in wingerd verbind is, vinnig en betroubaar op te spoor. Dit is bereik deur 94 wingerdstokke vir die teenwoordigheid van die virus te toets met beide kwantitatiewe en konvensionele trutranskripsie polimerase kettingreaksies (RT - PCR) vanaf ongesuiwerde plant-ekstraksies. Die tweede doel was die daarstelling van ’n tegniek om onderskeid te tref tussen variante van GRSPaV met verskillende nukleotiedvolgordes. Hierdie tegniek kan op groot skaal gebruik word om ge-affekteerde wingerdstokke te toets om sodoende siektesimptome met spesifieke variante van GRSPaV te verbind. Ge-neste kwantitatiewe polimerase-kettingreaksies (qPCR) en hoë-resolusie smelt-analises (HRM) is ontwikkel vir drie streke van die GRSPaV-genoom (mantelproteïen, RNS-afhanklike RNS-polimerase en trippelgeenblok bewegingsproteïen). Die tegniek van qPCR-HRM met die hoë-versadingingskleurstof EvaGreen™ en die Rotor- Gene™ 6000 ontleder se geldigheid is bevestig deur vergelyking met ’n paneel van sestien virus-isolate waarvan die volgorde reeds bepaal is. Verdunde RT-PCR-produkte en gekloneerde DNS het die mees konsekwente amplifikasie-uitstipping en dissosiasieprofiele opgelewer. RT-PCR-produkte wat vanuit totale RNS-ekstrakte verkry is, is as templaat vir qPCR-HRM-analises gebruik. Dieselfde produkte is ook gebruik, om die volgorde van sestien monsters in drie streke direk te bepaal. Die gemiddelde amplifikasiedoeltreffendheid van die qPCR was 1.52±0.04. Gebruiker-gedefinieerde genotipes is deur middel van outooproeping teen ’n vertroue-interval van 70% uitgevoer. Filogenetiese analises vir drie streke van die GRSPaV-genoom is uitgevoer met gepubliseerde GenBank-volgordes om die HRMdata te bevestig. Die dominante volgorde-variante in die stel Suid-Afrikaanse monsters het ooreengestem met Groep II, vollengte-verwysingsvariant GRSPaV-SG1. Monsters wat met GRSPaV besmet is kan in die toekoms onderwerp word aan die qPCR-HRM-analises wat in hierdie studie ontwikkel is. Dit kan uitgevoer word om ooreenkomste met bekende genotipes te bepaal, en dus ook met filogenetiese groepe. Die besmetting van plante met meer as een volgorde-variant het algemeen voorgekom. Die kwasi-spesies populasie-struktuur van die virus het ook gedurig na vore gekom. Die toets sal nuttig wees in die bepaling van korrelasies tussen spesifieke genotipes en verskillende fenotipiese voorkomste van virussiektes. Dit kan insig verleen in die etiologie van siektes wat met GRSPaV verbind word.

Cette thèse porte sur la fonctionnalisation de fils en acier inoxydable via des revêtements colorés base titane, élaborés par pulvérisation cathodique avec un magnétron cylindrique. Ce travail s’intéresse à la caractérisation chimique, morpho-structurale et à l’évaluation de la durabilité mécano-chimique des fils revêtus.Premièrement, la vitesse de dépôt et la composition chimique des films sont déterminées dans des conditions statiques. Les hétérogénéités du plasma dans la cathode sont démontrées et reliées aux paramètres tels que la puissance, la pression et la polarisation d’anodes auxiliaires.Les dépôts monocouches de Ti, TiN et TiOx sont ensuite élaborés en continu. La relation entre la couleur du TiN et sa composition chimique est établie. La couleur dorée est obtenue pour des films stœchiométriques contenant peu d’oxygène (< 5 %at.). Les microstructures sont caractérisées par MET-ASTAR et des cartographies d’orientation sont dressées à l’échelle nanométrique. Tandis que les dépôts de TiN sont colonnaires avec une texturation selon <111>, les grains des films de Ti sont plutôt équiaxes et orientés selon <0001>. Pour une température d’élaboration de 650 °C, les éléments du substrat diffusent dans les films et mènent à la formation de phases de Laves. Les dépôts de TiOx, élaborés en mode métallique, présentent des couleurs d’interférence et une composition proche du monoxyde. Les surfaces revêtues de TiN ont une résistance à la corrosion élevée semblable à l’acier inoxydable 316L, contrairement aux fils revêtus de Ti et TiOx. La ténacité et l’énergie d’adhérence des revêtements sont déterminées par traction in-situ sous MEB : Ti et TiN sont particulièrement adhérents au substrat contrairement à TiOx.Enfin, les dépôts sont architecturés avec l’ajout d’un dépôt de titane entre le substrat et le revêtement céramique. Ainsi, l’adhérence du film Ti-TiOx est largement augmentée par rapport au monocouche TiOx (5 à 200 J/m2). Enfin, les études microstructurales et électrochimiques montrent qu’un paramètre clef de la résistance à la corrosion est la présence de porosité ouverte dans les revêtements. / This thesis treats of the functionalization of stainless steel wires with colored Ti-based coatings, grown by PVD with a cylindrical magnetron, their chemical and morpho-structural characterization, and the evaluation of the chemico-mechanical durability of the coated wires.First, the deposition rate and the chemical composition of the films are determined under static conditions. Cathode plasma heterogeneities are demonstrated and related to parameters such as power, pressure and polarization of auxiliary anodes.Then, Ti, TiN and TiOx monolayer coatings are grown continuously. The relationship between the color of TiN and its chemical composition is established and golden color is obtained for stoichiometric films with low oxygen content (<5% at.). Microstructures are studied with TEM-ASTAR and orientation maps are obtained with a nanometric resolution. While TiN coatings are columnar with <111> texture, Ti grains are rather equiaxed and <0001> oriented. With a 650 ° C substrate temperature, substrate elements diffuse into the films which results in Laves phase formation. TiOx is grown in metallic mode, presents interference colors and a composition close to monoxide. TiN coated surfaces display high corrosion resistance similar to 316L stainless steel, unlike Ti and TiOx coated wires. The toughness and the adhesion energy of the coatings are determined by SEM in-situ tensile tests: Ti and TiN are particularly adherent to the substrate in contrast to TiOx.Finally, coatings are architectured with the addition of a titanium interlayer between the substrate and the ceramic coating. Thus, Ti-TiOx film adhesion is greatly superior compared to the TiOx monolayer (5 to 200 J/m2). Finally, microstructural and electrochemical studies show that a key parameter of corrosion resistance is the presence of open porosity in the coatings.

Nesta dissertação, é apresentada uma rotina computacional para modelar a corrosão por pites em aço inoxidável baseada em modelos matemáticos existentes que levam em consideração efeitos determinísticos e estocásticos. Como contribuição, modificações baseadas em resultados experimentais foram inseridas no modelo, com o objetivo de avaliar a influência de inclusões de MnS na matriz metálica, locais típicos de maior nucleação de pites. Foi modelado também o efeito do potencial de aplicado verificando-se assim a sua influência na formação de pites e a determinação da faixa possível para o potencial de pite. A modelagem consiste da avaliação espaço-temporal da nucleação, com a possibilidade de visualização da evolução de pites metaestáveis e estáveis, além do transiente de corrente anódica gerado pelo processo. Os resultados são apresentados em termos da avaliação das variáveis de maior influência na suscetibilidade de formação de pites, tais como concentração de espécies agressivas, dano à camada passiva e queda no potencial ôhmico, o que possibilita estudos futuros para se obter formas mais eficientes de combate à corrosão por pites. / In this work is presented a computational routine to model pitting corrosion of stainless steel based on pre-existing mathematical models that take into account deterministics and stochastic effects. As contribution, modifications based on experimental results were inserted, evaluating the influence of MnS inclusions on metal surface, a typical site of major pitting nucleation. Applied potential effect was modeled, thus, it was possible to verify its influence on pitting formation and the possible range of pitting. The model consists on spatio-temporal evaluating of nucleation process, with visualization of metastable and stable pitting evolution and the anodic current transient. The results are presented in terms of variables with greatest influence on the susceptibility of pitting formation, such as concentration of aggressive species, passivity layer damage and ohmic potential drop, allowing future studies to obtains more efficient ways to combat pitting corrosion

Bien que dits « inoxydables », les aciers inox peuvent être sujets à diverses formes decorrosion localisée telles que la piqûration. La formation de piqûres, nuisible à l’aspect et à l’intégritéstructurelle du matériau, est notamment contrôlée par les propriétés du film passif protecteur qui seforme à la surface des inox. De nombreuses études ont été menées sur les mécanismes de piqûration desurfaces modèles préparées au laboratoire. Cette thèse, au contraire, étudie les relations entre lespropriétés physico-chimiques et électroniques, d’une part, et la résistance à la corrosion par piqûres,d’autre part, de films passifs formés sur des inox par des finis de surfaces industriels.Les propriétés des films passifs ont été caractérisées par des méthodes variées, utiliséesclassiquement ou de façon plus originale : multi-piqûres et transitoires électrochimiques pour lapropriété d’usage, XPS et SDL pour la chimie de surface, spectroscopie d’impédance électrochimique,chrono-ampérométrie et photoélectrochimie pour les propriétés semi-conductrices des films.Il a été montré que la résistance à la piqûration des finis industriels pouvait valablement êtredéterminée par la méthode des transitoires électrochimiques et par la multi-piqûres, utiliséesauparavant pour les seules surfaces modèles. Par ailleurs, l’analyse comparée des résultats depiqûration et de ceux d’XPS et SDL a mis en évidence que la composition chimique des films passifsne suffit pas à rendre compte des différences de résistance à la piqûration des différents finisindustriels testés. En revanche, l’analyse des propriétés semi-conductrices des différents films passifs apermis de proposer des explications à ces écarts de comportements en corrosion localisée, confirmantque structure et taux de défauts du film passif sont deux paramètres clef de la résistance à la corrosionpar piqûres des films passifs.Enfin, une amélioration du comportement en piqûration des surfaces industrielles a pu êtremise en évidence, soit après un traitement court de ces dernières en milieu nitrique acide, soit àl’application aux échantillons d’un balayage en potentiel en milieu sulfate neutre. / Stainless steels can suffer localized corrosion like pitting corrosion which can damage thestructural integrity of the material. The pit formation is mainly controlled by properties of theprotective layer formed on the stainless steel surface, which is called passive film. Many studies haveexamined pitting mechanism on model surface prepared in the laboratory. This work is dedicated tostudy the relations between physicochemical and electronic properties of industrially processedpassive films and their pitting corrosion resistance.The properties of passive films have been characterized with several methods which can beconventional or in a more innovative way: multi-pitting statistics and electrochemical noise for the useproperty, XPS and SDL for the surface chemistry, electrochemical impedance spectroscopy,chronoamperometry and photo-electrochemistry for semi-conductive properties of the passive films.Multi-pitting statistics and electrochemical transients which were usually used on modelsurfaces have been shown to be accurate for the evaluation of the pitting resistance of industrialsurface finishing samples. Furthermore, the comparison of the results of pitting resistance and of XPSand SDL analyses has shown that the chemical composition of passive films is not sufficient to explainthe difference of pitting resistance behavior among tested industrial passive films. However, the semiconductiveproperties characterizations are able to explain these differences of localized corrosionresistance. They confirm that structuration and defect level are two key parameters of pitting resistanceproperties of passive films.At last, this study highlighted that the pitting resistance of industrial passive films can beimproved by a short nitric acid treatment or by sweep voltammetry in neutral sulfate medium.

Nesta dissertação, é apresentada uma rotina computacional para modelar a corrosão por pites em aço inoxidável baseada em modelos matemáticos existentes que levam em consideração efeitos determinísticos e estocásticos. Como contribuição, modificações baseadas em resultados experimentais foram inseridas no modelo, com o objetivo de avaliar a influência de inclusões de MnS na matriz metálica, locais típicos de maior nucleação de pites. Foi modelado também o efeito do potencial de aplicado verificando-se assim a sua influência na formação de pites e a determinação da faixa possível para o potencial de pite. A modelagem consiste da avaliação espaço-temporal da nucleação, com a possibilidade de visualização da evolução de pites metaestáveis e estáveis, além do transiente de corrente anódica gerado pelo processo. Os resultados são apresentados em termos da avaliação das variáveis de maior influência na suscetibilidade de formação de pites, tais como concentração de espécies agressivas, dano à camada passiva e queda no potencial ôhmico, o que possibilita estudos futuros para se obter formas mais eficientes de combate à corrosão por pites. / In this work is presented a computational routine to model pitting corrosion of stainless steel based on pre-existing mathematical models that take into account deterministics and stochastic effects. As contribution, modifications based on experimental results were inserted, evaluating the influence of MnS inclusions on metal surface, a typical site of major pitting nucleation. Applied potential effect was modeled, thus, it was possible to verify its influence on pitting formation and the possible range of pitting. The model consists on spatio-temporal evaluating of nucleation process, with visualization of metastable and stable pitting evolution and the anodic current transient. The results are presented in terms of variables with greatest influence on the susceptibility of pitting formation, such as concentration of aggressive species, passivity layer damage and ohmic potential drop, allowing future studies to obtains more efficient ways to combat pitting corrosion

Projetos de instalações industriais com requisitos de assepsia e resistência à corrosão têm os aços inoxidáveis austeníticos como materiais de engenharia, e a liga AISI 316L é amplamente utilizada. A soldagem de chapas espessas é executada por processos MIG e a qualificação do procedimento de soldagem é realizada com base em propriedades mecânicas, avaliação insuficiente para aplicações que necessitam de uma película passiva resistente. A microestrutura da zona fundida da liga AISI 316L exerce influência sobre a resistência à corrosão, e há a necessidade de definir os mecanismos que governam a influência da fase sigma na resistência à corrosão. Inserido neste contexto, o objetivo do presente estudo foi avaliar a influência da fase sigma na resistência à corrosão em microrregiões de juntas soldadas multipasse da liga AISI 316L produzidas pelo processo MIG nos modos de transferência metálica pulsado, curto-circuito e spray. A metodologia consistiu em reproduzir amostras soldadas com parâmetros de soldagem aplicados na indústria para os modos de transferência metálica de interesse, com detalhada caracterização microestrutural da zona fundida de cada condição de soldagem. Em seguida, foram conduzidos ensaios eletroquímicos de corrosão em microrregiões da junta soldada em solução de 3,5% NaCl, e a influência da fase sigma na corrosão por pite foi avaliada por ensaio de imersão em solução de cloreto férrico (6% FeCl3). Caracterizou-se a área exposta à varredura por técnicas de microscopia ótica, microscopia eletrônica de varredura e microanálise química. A soldagem no modo pulsado resultou em uma zona fundida com microestrutura bifásica com a mais baixa fração volumétrica de ferrita delta, de refinada morfologia e isenta de fase sigma, proporcionando o mais nobre desempenho nos ensaios de corrosão, que se manifestou pelo mecanismo de corrosão localizada. A avaliação da área exposta à varredura demonstrou que, previamente a corrosão por pite, a corrosão incia-se de forma localizada, contudo, sem corrosão preferencial de uma das fases, característica que proporcionou parâmetros eletroquímicos mais nobres do que aqueles com corrosão seletiva de fases. Está característica é atribuída à ausência da fase sigma na microestrutura da zona fundida do modo pulsado. No modo curto-circuito ocorreu à decomposição eutetóide da ferrita delta formando a austenita secundária e a fase sigma, sendo está última precipitada principalmente no interior da ferrita delta. A morfologia da corrosão se dá, na fase inicial, como corrosão seletiva de fases, com degradação preferencial da austenita secundária e em direção a ferrita delta, devido à fragilização desta fase pela precipitação de fase sigma em seu interior. A degradação seletiva das fases austenita secundária e ferrita delta causam danos localizados ao filme passivo e, nestas regiões empobrecidas de cromo e molibdênio, ocorre à corrosão por pite. O modo spray com a mais elevada energia de soldagem resultou em uma ferrita delta grosseira e com estreitas bandas de austenita na microrregião de enchimento do chanfro e na raiz da solda, com alto índice de fase sigma nestas localizações. O processo corrosivo da zona fundida caracterizou-se por corrosão seletiva da fase austenita secundária e em direção à matriz austenítica, uma vez que a fase sigma revestiu a ferrita delta, tornando-a a região de comportamento catódico entre o par galvânico formado entre as fases austenita e ferrita delta. A corrosão seletiva da matriz austenítica causa a fragilização localizada do filme passivo com consequente corrosão por pite. O trabalho realizado permite concluir que o potencial de pite foi reduzido com a presença de fase sigma e fases a esta associada, e justamente o modo pulsado obteve destacada resistência à corrosão em função da ausência da fase sigma em sua microestrutura. / Industrial plant designs with asepsis and corrosion resistance requirements have austenitic stainless steels as engineering materials, and the AISI 316L alloy is widely used. The welding of thick plates is performed by MIG processes and the qualification of the welding procedure is carried out based on mechanical properties, insufficient evaluation for applications that require a resistant passive film. The microstructure of the molten zone of the AISI 316L alloy influences the corrosion resistance, and it is necessary to define the mechanisms that govern the influence of the sigma phase on corrosion resistance. In this context, the aim of the present study was to evaluate the influence of the sigma phase on the corrosion resistance in microrregions of multipass welded joints of the AISI 316L alloy produced by MIG process with metal transfer in pulsed, short circuit and spray modes. The methodology consisted in reproducing welded samples with welding parameters applied in the industry for the modes of metallic transfer of interest, with detailed microstructural characterization of the molten zone of each welding condition. Then, electrochemical corrosion tests were carried out in microrregions of the welded joint in 3.5% NaCl solution, and the influence of the sigma phase on pitting corrosion was evaluated by immersion test in ferric chloride solution (6% FeCl3). The area exposed to the scanning was characterized by optical microscopy, scanning electron microscopy and chemical microanalysis. Pulsed mode welding resulted in a molten zone with a biphasic microstructure with the lowest volume fraction of delta ferrite, refined morphology and sigma phase free, providing the noblest performance in the corrosion tests, which occurred in the form of localized corrosion. The evaluation of the area exposed to the scan showed that, prior to pitting corrosion, the corrosion started in a localized manner, however, without preferential corrosion of one of the phases, a characteristic that gave better electrochemical parameters than those with selective corrosion of phases. This characteristic is attributed to the absence of the sigma phase in the microstructure of the molten zone of the pulsed mode. In the short-circuit mode, the eutectoid decomposition of the delta ferrite formed the secondary austenite and the sigma phase, the latter being mainly precipitated inside the delta ferrite. The corrosion morphology occurs in the initial phase as selective corrosion of phases, with preferential degradation of the secondary austenite and towards the ferrite delta, due to the embrittlement of this phase by the precipitation of the sigma phase inside. The selective degradation of the secondary austenite and delta ferrite phases causes localized damage to the passive film and, in these impoverished regions of chromium and molybdenum, occurs to pitting corrosion. The spray mode with the highest welding energy resulted in a coarse delta ferrite with narrow bands of austenite in the chamfer filling microrregion and at the root of the weld, with a high sigma phase index at these locations. The corrosive process of the molten zone was characterized by selective corrosion of the secondary austenite phase and towards the austenitic matrix, since the sigma phase covered the delta ferrite, making it the region of cathodic behavior between the galvanic pair formed between the austenite and ferrite delta phases. Selective corrosion of the austenitic matrix causes localized embrittlement of the passive film with consequent pitting corrosion. The study accomplished allows concluding that the pitting potential was reduced with the presence of sigma phase and phases associated with it, and precisely the pulsed mode obtained outstanding corrosion resistance due to the absence of the sigma phase in its microstructure.

Nos últimos anos houve, no Brasil, um significativo investimento em usinas de álcool para suprir a demanda. Todavia, surgiram problemas de natureza econômica e ambiental, já que a tecnologia atualmente utilizada nas destilarias de álcool produz em média 13 litros de vinhaça para cada litro de álcool. Na busca de uma solução para reduzir o volume deste resíduo, desenvolveu-se um equipamento para concentração de vinhaça, denominado Ecovin, pela empresa Citrotec localizada em Araraquara-SP. Para cada Ecovin são utilizados cerca de 4000 tubos com costura nos trocadores de calor, e para reduzir seu custo de fabricação, avaliou-se como opção o uso de tubos da liga AISI 444. Inserido neste contexto, o presente trabalho procurou caracterizar os mecanismos de corrosão e mostrar de forma comparativa o desempenho dos tubos nas juntas tubo-espelho. Estudo-se, portanto, dois tipos de junta, a do projeto atual, composta por chapa e tubo da liga AISI 316L; e a nova proposta, composta por chapa da liga AISI 316L e tubo da liga AISI 444. Para tanto, iniciou-se com a avaliação da soldabilidade das ligas estudadas, através da caracterização microestrutural, qualificação do procedimento de soldagem da junta tubo-espelho e de ensaios de sensitização nas soldas. Em seguida, ensaios de perda de massa por imersão foram realizados nas soluções de 0,5 M \'H IND.2\'SO IND.4\' e 0,5 M \'H\'CL\', nas temperaturas de 30°C, 50°C, 70°C e 90°C, de acordo com o intervalo de temperatura de operação do Ecovin. Nas mesmas soluções, e na temperatura ambiente, foram realizados ensaios eletroquímicos de polarização potenciodinâmica em amostras que reproduziram o ciclo térmico de soldagem das juntas tubo-espelho estudadas, nas regiões do metal de base, zona afetada pelo calor (ZAC) e no metal de solda. Na junta tubo-espelho a corrosão ocorreu preferencialmente na ZAC formada entre a interface tubo-metal de solda, e no cordão de solda à margem do tubo, com a atuação de mecanismos de corrosão generalizada e localizada. Na junta tubo-espelho dissimilar observou-se que o processo corrosivo foi predominante na superfície do tubo AISI 444, típico do mecanismo de corrosão galvânica, onde o tubo AISI 444 caracterizou-se como a região anódica. O desempenho da liga AISI 316L, assim como a junta soldada composta somente por essa liga, apresentou um melhor desempenho em corrosão, porém, na solução contendo cloreto, a variação da temperatura exerceu uma influência proporcional para ambos os casos avaliados nesse estudo. Destaca-se ainda que a liga AISI 316L sofreu corrosão por pite na ZAC e no metal de solda, em ambas as soluções, com maior severidade do que a liga AISI 444. Os resultados obtidos indicam que para a aplicação requerida, o tubo AISI 444 pode ser utilizado na temperatura de 50°C com satisfatório desempenho e similar à junta composta somente por AISI 316L. / Recently in Brazil there has been constant investments in ethanol plants, to supply the internal and external market, resulting at a high increase of the volume of this product. However, economical and environmental problems arose because of this new demand, whereas the current technologies produce thirteen liters of vinasse for each liter of alcohol. Each ECOVIN uses nearly 4000 welded tubes in the heat exchangers, and there is an option to use welded AISI 444 alloy tubes in order to reduce the manufacture costs of the ECOVIN. In this context, this work determines the corrosion mechanisms and analyzes the performance of welded tubes and the tube-to-tubesheet welded joints. The following types of tube-to-tubesheet were evaluated: current welded joints of this project, with all components manufactured with AISI 316L alloy; and the welded joint suggested for the project, using the welded tubes of AISI 444 alloy and the plate of AISI 316L alloy. For this purpose, the first step was to evaluate the weldability of the studied alloys, through microstructural characterization, welding procedure qualification and the intergranular corrosion test. After that, mass loss tests were conducted in 0,5 M \'H IND.2\'SO IND.4\' and 0,5 M \'H\'CL\' solutions, at 30°C, 50°C, 70°C and 90°C, according to the temperature range of the equipment operating. Electrochemical polarization tests were made in the same solution concentration used in the mass loss tests, but only in room temperature. These tests were made in samples that were welded with the same thermal cycle of the tube-to-tubesheet welded joints, and the evaluations were made on the base metal, heat affected zones and weld metal. The uniform and localized corrosion process occurred preferentially in the interface formed between tube and weld metal, and on weld metal near the tubes. In the tube-to-tubesheet dissimilar welded joints, it was observed that the corrosion process was predominant on the surface of AISI 444 alloy, probably because of the galvanic corrosion process, where the AISI 444 was an anodic region. The best performance on corrosion process was observed in AISI 316L, both the base metal and the similar welded joints. However, for the tests in chloride environment, in this process, the temperature caused a proportional influence in the corrosion rate of AISI 316L and AISI 444. In the AISI 316L alloy it was observed nucleation of pitting in HAZ and in weld metal, in both solutions, and more aggressive than observed in the AISI 444 alloy. The results showed that the AISI 444 alloy can be applied for the initial ranging temperatures of the equipments operation, with satisfactory performance.

Cette étude porte sur la compréhension des mécanismes de la corrosion par piqûres susceptible d’affecter les aciers au carbone prévus pour le stockage des déchets radioactifs. Des modèles de transport/réaction ont été développés pour simuler, par la méthode des éléments finis, la propagation d’une piqûre à la surface du fer en milieu chloruré. Pour les modèles conservatifs, la piqûre est activée par un potentiel imposé au métal. A partir de différentes études paramétriques (géométrie, environnement…), les simulations ont permis d’identifier les facteurs critiques responsables d’un accroissement ou d’une inhibition de la vitesse de corrosion localisée. Par ailleurs, les essais sur des électrodes occluses, dites « lead-in-pencil », confirment que le confinement favorise la précipitation d’un film salin en fond de piqûre. Enfin, la simulation de situations de couplage entre la piqûre et la surface cathodique externe a permis de décrire l’évolution réaliste de la vitesse de corrosion localisée en milieu aéré et en potentiel libre. L’analyse des résultats de ces trois axes démontre que la chute ohmique dans la piqûre est le facteur contrôlant sa progression. Dans la majorité des cas, toute augmentation de cette chute ohmique (due à la précipitation, au confinement ou à l’augmentation du rapport des surfaces cathodique/anodique), conduit à une vitesse de corrosion plus rapide des surfaces externes. Ce phénomène est qualifié « d’évasement ». La croissance en profondeur, pour une vitesse d’oxydation dépendante du pH, reste à confirmer. Pour simuler le comportement à long terme du fer vis-à-vis de la piqûration, dans les conditions de stockage, les nouveaux modèles devront reposer sur la détermination plus précise de l’environnement et des lois de dissolution et de précipitation / This study deals with the understanding of the mechanisms of pitting corrosion susceptible to occur on carbon steel selected for high level nuclear waste containers. Transport/reaction models have been developed in order to simulate, by finite element method, the propagation of a pit on iron in chlorinated medium. For conservative models, pit is activated by imposing a potential on metal. Thanks to different parametric studies (geometry, environment…) simulations have permitted to classify, by severity order, the factors responsible of the increase or the inhibition of the localized corrosion rate. Otherwise, experiments on occluded (called « lead-in-pencil ») electrodes confirm that confinement favors the formation of a salt film at the bottom of the pit. Finally, the simulation of galvanic coupling between the pit and the surrounding cathodic surface has permitted to describe the realistic evolution of the localized corrosion rate in aerated medium and for free corroding conditions. The analysis of the results from these three ways of investigation has demonstrated that the ohmic drop inside the pit is the most important factor controlling his growing. In most study cases, any increase of this ohmic drop (due to precipitation, confinement or augmentation of the cathodic /anodic surfaces ratio), leads to a faster corrosion rate of the surrounding surface. This phenomenon is called “opening” of the pit. The progression at the pit bottom for a pH-dependant oxidation rate is still to confirm. In order to simulate the long term behavior of iron during pitting, in storage application, new models require a more accurate determination of the environment conditions, dissolution and precipitation laws

Lors de l’usinage, les conditions de pression et de température à la surface usinée sont trèsélevées. La microstructure et l’état mécanique du matériau sont impactés, ce qui a desconséquences sur la réactivité chimique de la surface usinée. Dans cette thèse, ons’intéresse au tournage de superfinition de l’acier inoxydable martensitique X4CrNiMo16-5-1traité thermiquement (dénommé APX4 optimisé). Il présente la particularité d’êtremultiphasé à la température ambiante (martensite, ferrite et austénite). L’objectif est dequantifier les relations entre les conditions de coupe, les propriétés d’usage et ladégradation par corrosion des surfaces usinées. Des essais d’usinage en coupe orthogonale(type QST) ont été réalisés afin de comparer directement les résultats expérimentaux auxprédictions numériques 2D des surfaces usinées.Les changements de phase du matériau ont été étudiés par des essais de calorimétriejusqu’à 1200°C. Les cycles thermiques réalisés ont mis en évidence différentestransformations métallurgiques. L’étude bibliographique complémentaire permet deconclure quant aux possibilités de transformation de phase en usinage, qui sont quasiinexistantes du fait des cinétiques de chauffage extrêmement élevées en tournage.Les échantillons usinés ont été caractérisés avec une étude microstructurale parmicrographie optique et par MEB-EBSD. Ces deux techniques mettent en évidence lesdéformations importantes en extrême surface indiquées par l’étirement des îlots de ferriteparallèlement à la surface usinée. Ces résultats microstructuraux ont été mis en relationavec les résultats de microdureté Vickers. Une relation de la dureté superficielle a étéexprimée en fonction des conditions de coupe. La technique EBSD a permis de montrer pourles échantillons usinés dans les conditions les plus sévères la déformation de la ferrite avecapparition de sous-joints de grain, un affinement microstructural de la martensite et uneaccentuation de l’affinement de la matrice martensitique à proximité de la ferrite déformée.Une simulation numérique de prédiction des déformations à l’échelle macroscopiquea été réalisée. Ces résultats numériques ont été comparés aux résultats de déformation etde microdureté issus d’une campagne d’essais. Des essais numériques complémentairesappliqués à l’échelle de la microstructure (matériau considéré biphasé) permettent ded’appréhender la déformation de la phase ferritique et son influence locale sur ladéformation de la matrice martensitique. Ces résultats sont cohérents avec les résultatsexpérimentaux.Les phénomènes de corrosion par piqûres ont été étudiés par le biais d’essais depolarisation à l’aide de la microcellule électrochimique et d’essais spécifiques pulsés. Cesderniers essais ont permis l’analyse des piqûres générées (en diamètre et en densité). Larésistance à la corrosion localisée des surfaces d’acier inoxydable martensitique reste bonneaprès usinage, même améliorée (densité de piqûres plusieurs fois plus faible que l’état deréférence). Ce comportement est corroboré à l’état de compression de la surface.La microdureté de surface, qui est contrôlée majoritairement par l’avance, conditionne lepotentiel de piqûre. L’augmentation du diamètre de piqûre a été reliée à l’affinementmicrostructural (observé à partir d’un certain seuil de microdureté superficielle). / During machining, pressure and temperature conditions at the machined surface are veryintensive. Machined material microstructure and mechanical state are changed, whichimpacts the chemical reactivity of the machined surface. In this PhD study, we focus onsuperfinish turning of heat treated martensitic stainless steel X4CrNiMo16-5-1 (namedoptimized APX4). It has to be mentioned that this material has a multiphased microstructureat ambient temperature (martensite, ferrite and austenite). The aim is then to quantifyrelationships between cutting conditions, properties and the deterioration (corrosion) ofmachined surfaces. Machining trials in orthogonal cutting configuration (QST) have beenrealized in order to compare experimental results directly to 2D numerical forecast ofmachined surfaces.Material phase transformations have been studied by calorimetry tests until 1200°C. Testshave evidenced different metallurgical transformations. The complementary bibliographystudy allows to conclude that phase transformation possibilities during machining are veryquasi nonexistent due to extremely high heating kinetics in turning.Machined samples have been characterized by a microstructural study by opticalmicrography and SEM-EBSD. Both techniques highlight high strains in extreme machinedsurface as indicated by the stretching of ferrite islands in parallel to the machined surface.Microstructural results have been linked to the Vickers microhardness results. Surfacehardness has been expressed as a function of the cutting conditions.EBSD measurements have shown for machined samples in the most severe conditionsstraining of the ferrite with low angle grain boundaries, microstructural refining ofmartensite and heightening of martensitic matrix close to the strained ferrite.A numerical simulation predicting strains at the macroscopic scale has been carried out.These numerical results have been compared to strain and microhardness results arise fromone trials campaign. Further numerical simulations applied at the microstructure level(considered as biphased material) allow understanding of ferritic phase strain and its localinfluence on martensitic matrix strain. These results are consistent with experimentalresults.Pitting corrosion phenomenon has been studied by polarization testing using theelectrochemical microcell and specific pulsed testing. Last used method has been conductedto analyze the generated pits (diameter and density). Localized corrosion resistance ofmachined martensitic stainless steel surfaces remains good, even improved (few times lowerpitting density than the reference state). This behaviour is corroborated to the compressivestate of the surface.Surface microhardness, which is mainly controlled by the feed rate, conditions the criticalpitting potential. Increased pitting diameter has been linked to microstructural refining(observed above a surface microhardness level).