The physical properties of the protective passive films formed on the surface of stainless steels under electrochemical polarization in different electrolytes were studied. The structure of these films was analyzed as a function of depth using photoelectron spectroscopy (PES). Depth profiling (using PES) of the surface layer was achieved by either changing the angle of incidence to achieve different analysis depths (ARXPS), by argon ion etching, or by varying the energy of the incoming x-rays by the use of synchrotron radiation. The use of hard x-rays with high resolution (HAXPES) provided novel quantified information about the nickel content underneath the passive films. A complex environment was found in these surface layers composed of an outermost monolayer of iron on top of a layer of chromium hydroxides covering an underlayer of chromium oxides. Molybdenum was enriched in the interface between the metal and oxide. Nickel is enriched underneath the passive film and therefore nickeloxides are only present in the surface layer in low concentrations. A comparison was performed on austenitic and duplex stainless prepared by hot isostatically pressed (HIP) or cast and forged processes. HIP stainless steel was produced using the burgeoning technique of pressing gas atomized powders together. The structure of these steels is far more homogenous with a lower porosity than that of the conventionally prepared equivalents. It was shown that hot HIP austenitic steel had better pitting corrosion resistance than its conventional counterpart. Finally, the duplex steel was cycled in a Li-ion battery to explore its potential application as a current collector. It was shown that the passive film formed in the organic solvents is similar in composition and thickness to the films formed in aqueous solutions. However, it is doubtful if steel could be used as current collector in batteries due to its high reactivity with lithium.

In this work, I aim to clarify the mechanism that allows steel to attain higher chloride threshold as it is cathodically polarized. Specifically, I seek to provide empirical information on whether an intrinsic (predominantly interfacial effects of polarization) or an extrinsic (predominantly concentration changes due to polarization) mechanism may be dominant in the beneficial effect of polarization. I carried out this experiment with 12 identical concrete specimens, each with a cast-in steel plate, constantly exposed them to high-chloride environment. The specimens were divided into 4 triplicates and polarized at 4 different level from OCP, -200, -300 to -400 mVSCE The specimens were closely monitored for signs of corrosion. When corrosion was detected in a specimen, it was demolished to gain access to steel-concrete interface. Measurements of pH using a novel procedure and chloride ion concentration were done on the interface using an adapted in-situ pH measurement and a Florida Department of Transportation procedure respectively. The pH and chloride ion concentrations obtained in this study favor to some extent a dominant intrinsic mechanism interpretation, while the evidence in support of a dominant extrinsic mechanism interpretation remains elusive.

The excellent corrosion resistance of stainless steels arises from the presence of a passive film on its surface. Above 10.5wt% Cr a chromium oxide of 1-3 nm is formed on the surface of the metal that in case of damage will reform and hinder further dissolution of the metal. However, the passivity of the stainless steel can be altered by material factors and external factors; such as the composition of the underlying phases, external loads or thermal treatments. In this work the local electrochemical techniques Scanning Vibrating Electrode Technique (SVET) and Scanning Kelvin Probe Force Microscopy (SKPFM) and the local characterization techniques X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) have been used to investigate corrosion phenomena of stainless alloys based on measurements of corrosion current density, work function, thickness and composition of the oxide. The effect on work function of the thickness of the passive film and composition of the underlying phases was investigated for 301LN austenitic stainless steel (Paper I) and a heat treated superduplex 25Cr7Ni type stainless steel (Paper II). It was shown that the work function can be an indicator of corrosion resistance of the phases in the microstructure, and that the composition of the underlying phases had a greater effect on the work function than the thickness of the passive film. External factors such mechanical deformation (Paper I) and welding (Paper III) altered the passivity of the steel and work function. It was found that plastic deformation decreased irreversibly the work function, whereas elastic deformation did not have any permanent effect. Thermal oxides affected the passivity of stainless steels welded joints and were detrimental for its corrosion resistance. Anodic activity, observed with SVET, and pitting corrosion were detected at the heat tint and attributed to the interaction between the composition and the thickness of the oxide. Brushing combined with pickling was recommended for recovering the passivity of stainless steels. / <p>QC 20160516</p>

Turbulent fluid flow at high Reynolds numbers presents significant degradation risks to active-passive metals due to enhanced localized degradation phenomena. A multidisciplinary experiment was proposed to study the relationship between hydrodynamics in fully-developed pipe flow and both the growth and performance characteristics of passive films. Preliminary work was performed to set up (i) an environmental chamber for the experiment, (ii) design a custom wall shear stress sensor and constant temperature anemometer traverse system to monitor hydrodynamic conditions in-situ, (iii) monitor in-situ degradation through an array of ultrasonic thickness transducers, and (iv) acquire data and control the environment via a LabVIEW routine. A validation experiment was conducted on a 1220 mm long experimental section of 45.7 mm inner diameter AA2024-T3 tubing in simulated seawater. Extensive degradation was observed in-situ and confirmed with ex-situ techniques after sequential exposure to fully-developed turbulent flow at an expected wall shear stress of 10 Pa for 180ks (Reynolds number of 122,000) and then at 40 Pa for 630ks (Reynolds number of 262,000). No typical erosion-corrosion hydrological features were observed, however significant pitting and intergranular corrosion were observed with corrosion product caps covering 47% of the total ultrasonic transducers' measurement area. Passive film and pit growth were recorded via ultrasonic thickness measurements with an observed simultaneous decrease in dissolved oxygen content. The validation experiment successfully demonstrated the capability of the designed and constructed sensors for the proposed experiment. Numerous areas of suggested development and research were identified to ensure accuracy and improve interpretation of future experiments. / Master of Science

Although the passive films that form on stainless steels have been extensively studied, the concentration depth profiles are not fully understood. Their thinness makes passive films hard to study, but angle-resolved X-ray photoelectron spectroscopy (ARXPS) is a non-destructive technique that can be used to obtain depth information. An iterative approach to deconvolute ARXPS measurements into depth profiles is discussed, and the chemistry of passive films on a molybdenum-containing 316L stainless steel is investigated. Bipolar electrochemistry, in which the sample is placed along an electric field created by two driving electrodes in an electrolyte, is investigated as a screening tool. It is shown that the method is useful to create corrosion gradients on 304 stainless steel, both under pitting and non-pitting conditions. Chromium carbide thin films were deposited by magnetron sputtering with a variety of deposition parameters on stainless steel, and subsequently analyzed. It is shown that these films present a promising material system for protective coatings to improve the corrosion resistance of stainless steels while also maintaining other useful properties, such as low interfacial contact resistance. Particular attention is given to the electrochemical evaluation of the films, whose high carbon concentrations necessitates different interpretations of the electrochemical results compared to for stainless steels.

Ce travail concerne l'étude du comportement vis-à-vis de la corrosion de deux alliage d'aluminium : l'alliage 2024 (AA 2024 T351) et l'alliage 7075 (AA7075 T7351) et de l'aluminium pur, utilisé comme référence, dans une solution de Na2SO4 0,1 M à l'aide de mesures électrochimiques (courbes de polarisation, courbes de Levich et spectroscopie d'impédance) avec des électrodes à disque tournant. Comparativement aux travaux de la littérature, des données quantitatives sur les processus anodique et cathodique qui se produisent sur les deux alliages ont été obtenues. La première partie de la thèse est consacrée à la caractérisation microstructurale des deux alliages : taille, composition chimique des précipités et fraction surfacique occupée par les différentes phases. La seconde partie présente les résultats d'impédance obtenus au potentiel de corrosion pour différents temps d'immersion et différentes vitesses de rotation. A ce potentiel, le comportement des matériaux est essentiellement contrôlé par le film passif. Les diagrammes d'impédance présentent une dispersion en fréquence, exprimée en termes de « constant phase element (CPE) ». Ce comportement a été analysé à l'aide d'un modèle physique qui permet de montrer une distribution de résistivité dans l'épaisseur des films d'oxyde. Dans la dernière partie, une attention particulière a été portée à l'analyse de la réaction cathodique à la surface des deux alliages qui est à l'origine de leur dégradation importante. La réduction de l'oxygène se produit principalement sur les particules intermétalliques. De façon surprenante, la densité de courant cathodique est nettement plus faible pour l'alliage 7075 qui présente une plus grande surface couverte par les particules. Il a été montré que pour l'alliage 2024, la réaction cathodique est contrôlée par le transport de matière par diffusion convective sur de petites électrodes alors que pour l'alliage AA 7075, la majeure partie des particules, de très petite taille, se comporte comme des microélectrodes pour lesquelles le courant est fixé par la diffusion sphérique, indépendante de la convection. / This work concerns the study of behavior the corrosion of two aluminum alloy: the alloy 2024 (AA 2024 T351) and 7075 (AA7075 T7351) and pure aluminum, used as reference, in a solution of 0.1 M Na2SO4 using electrochemical measurements (polarization curves, curves Levich and impedance spectroscopy) with rotating disk electrodes. Compared to published studies, quantitative data on the anodic and cathodic processes occurring on the two alloys were obtained. The first part of the thesis is devoted to the microstructural characterization of two alloys: size, chemical composition of precipitates and surface fraction occupied by the different phases. The second part presents the results of impedance obtained at the corrosion potential for different immersion times and different speeds. At this potential, the behavior of materials is mainly controlled by the passive film. The impedance diagrams exhibit a frequency dispersion, expressed in terms of "constant stage element (CPE)." This behavior was analyzed using a physical model which allows to show a distribution of resistivity in the thickness of oxide films. In the last part, special attention was paid to the analysis of the cathodic reaction on the surface of the two alloys that is causing their degradation. The oxygen reduction occurs mainly on the intermetallic particles. Surprisingly, the cathode current density is significantly lower for the alloy 7075 which has a greater surface area covered by the particles. It was shown that for 2024 alloy, the cathodic reaction is controlled by material transport by convective diffusion of small electrodes, while for the alloy AA 7075, most of the particles, very small, behaves as microelectrodes for which the current is set by the spherical diffusion, independent of the convection.

Les aciers inoxydables martensitiques sont utilisés dans l'industrie aéronautique où de hautes propriétés mécaniques sont requises. Cependant, dû à leur faible teneur en chrome, ils sont relativement sensibles à la corrosion localisée et particulièrement à la corrosion caverneuse qui se développe en milieu confiné. Tout d'abord, le comportement électrochimique de l'acier inoxydable martensitique X12CrNiMoV12-3 a été étudié dans une solution neutre et chlorurée (NaCl 0,1 M + Na2SO4 0,04 M) en plein bain. Des mesures électrochimiques (courbes de polarisation et mesures d'impédance) couplées à des analyses de surface par XPS ont permis de caractériser les films passifs formés pour différentes conditions. Les résultats obtenus ont permis de montrer le rôle important joué par l'oxygène dissous sur la formation et/ou la modification du film passif pendant l'immersion dans l'électrolyte. Les diagrammes d'impédance obtenus au potentiel de corrosion et en milieu aéré sont caractérisés par deux constantes de temps qui ont été attribuées au film passif (hautes fréquences) et au transfert de charges (basses fréquences). L'analyse de la partie hautes fréquences des diagrammes d'impédance électrochimique à l'aide du modèle en loi de puissance a permis de montrer de faibles variations de l'épaisseur des films pendant l'immersion. Des mesures électrochimiques ont ensuite été réalisées à l'aide du montage de la cellule à couche mince qui permet de travailler avec des épaisseurs d'électrolyte rigoureusement contrôlées. Les essais réalisés ont montré l'aptitude à la repassivation de l'acier inoxydable martensitique dès qu'il est en contact avec l'oxygène dissous en particulier pour des faibles épaisseurs d'électrolyte (inférieur à 100 µm). Lorsque le milieu est confiné entre deux parois en acier afin de reproduire une situation de corrosion caverneuse, il a été montré la corrosion est fortement accélérée lorsque l'épaisseur d'électrolyte est faible (inférieur à 500 µm). / Martensitic stainless steels are mainly used for applications where high mechanical performance is required. However, due to the low chromium content, they are relatively sensitive to localised corrosion, and particularly, to crevice corrosion encountered in confined environments. First, the electrochemical behavior of X12CrNiMoV12-3 martensitic stainless steel has been studied in a bulk neutral chloride solution (0.1 M NaCl + 0.04 M Na2SO4). Electrochemical measurements (polarisation curves and impedance measures) and XPS surface analysis were performed in order to characterise the passive films formed under different experimental conditions. The results showed the important role of dissolved oxygen to form and/or modify the passive film during immersion in electrolyte. The impedance diagrams are characterised by two time constants wich are attributed to passive film response (high frequency range) and to charge transfert resistance (low frequency range). The analyse of the high frequencies part of the diagrams by using the "power law model" showed low evolution of passive films thickness during immersion. Then, electrochemical measurements were perfomed in confined environments by using a thin layer cell where the electrolyte thickness were rigourosly adjusted. The measurements showed that the martensitic stainless steel is in passive state even for low electrolyte thickness (inferior in 100 µm). When the electrolyte is confined between two stainless electrodes in order to reproduce the same conditions find during crevice corrosion, the corrosion is sharply accelerated when the electrolyte thickness is above 500 µm

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Abstract Austenitic high-Mn (15–30 wt.%) based twinning-induced plasticity (TWIP) steels provide great potential in applications for structural components in the automotive industry, owing to their excellent tensile strength-ductility property combination. In certain cases, these steels might also substitute austenitic Cr-Ni stainless steels. The aim of this present work is to investigate the high-temperature flow resistance, recrystallisation and the evolution of microstructure of high-Mn steels by compression testing on a Gleeble simulator. The influence of Al alloying (0–8 wt.%) in the hot rolling temperature range (800°C–1100°C) is studied in particular, but also some observations are made regarding the influence of Cr alloying. Microstructures are examined in optical and electron microscopes. The results are compared with corresponding properties of carbon and austenitic stainless steels. In addition, the mechanical properties are studied briefly, using tension tests over the temperature range from -80°C to 200°C. Finally, a preliminary study is conducted on the corrosion behaviour of TWIP steels in two media, using the potentiodynamic polarization technique. The results show that the flow stress level of high-Mn TWIP steels is considerably higher than that of low-carbon steels and depends on the Al concentration up to 6 wt.%, while the structure is fully austenitic at hot rolling temperatures. At higher Al contents, the flow stress level is reduced, due to the presence of ferrite. The static recrystallisation kinetics is slower compared to that of carbon steels, but it is faster than is typical of Nb-microalloyed or austenitic stainless steels. The high Mn content is one reason for high flow stress as well as for slow softening. Al plays a minor role only; but in the case of austenitic-ferritic structure, the softening of the ferrite phase occurs very rapidly, contributing to overall faster softening. The high Mn content also retards considerably the onset of dynamic recrystallisation, but the influence of Al is minor. Similarly, the contribution of Cr to the hot deformation resistance and static and dynamic recrystallisation, is insignificant. The grain size effectively becomes refined by the dynamic and static recrystallisation processes. The tensile testing of TWIP steels revealed that the Al alloying and temperature have drastic effects on the yield strength, tensile strength and elongation. The higher Al raises the yield strength because of the solid solution strengthening. However, Al tends to increase the stacking fault energy that affects strongly the deformation mechanism. In small concentrations, Al suppresses martensite formation and enhances deformation twinning, leading to high tensile strength and good ductility. However, with an increasing temperature, SFE increases, and consequently, the density of deformation twins decreases and mechanical properties are impaired. Corrosion testing indicated that Al alloying improves the corrosion resistance of high-Mn TWIP steels. The addition of Cr is a further benefit for the passivation of these steels. The passive film that formed on 8wt.% Al-6wt.%Cr steel was found to be even more stable than that on Type 304 steel in 5–50% HNO3 solutions. A prolonged pre-treatment of the steel in the anodic passive regime created a thick, protective and stable passive film that enhanced the corrosion resistance also in 3.5% NaCl solution.