Commercial aluminum (Al) alloys are often precipitation strengthened to improve strength and wear resistance. However, localized corrosion due to the galvanic coupling between the precipitates and Al matrix often leads to degraded performance when these alloys are exposed to corrosive environment. In this work, Al-based solid solution was synthesized to simultaneously improve the strength and corrosion resistance of Al alloys, which ultimately led to high tribocorrosion resistance. Specifically, the effects of testing condition (e.g. sliding frequency) and alloying effects (e.g. Mn and Mo) on the corrosion and tribocorrosion behavior of Al-based binary and ternary solid solutions were studied. To understand the effects of wear condition on the depassivation-repassivation kinetics during tribocorrosion, in the first study, the tribocorrosion behaviors of Al-20 at.%Mn alloys were investigated in simulated seawater by changing the sliding frequency from 0.05 to 1 Hz in reciprocal motion. The results show that the depassivation rate of passive film increased with increasing sliding frequency. Mechanical wear also increased with increasing sliding frequency, which was mainly related to the increase of coefficient of friction and real contact area. Chemical wear tended to increase with scratching frequency, most likely due to faster repassivation kinetics at lower frequency. The surface layer was analyzed by cross-sectional transmission electron microscopy, indicating the passive film was primarily consisted of aluminum oxide where manganese was selectively dissolved. Despite extensive past research, the fundamental understanding of the alloying effects on the atomistic structure, composition, and chemical state of the passive layer of Al alloys and their formation mechanism is still not well understood. In the second study, the effects of Mn on the aqueous corrosion of Al-Mn alloys were investigated. It was confirmed that Mn alloying could enhance the corrosion resistance of Al without participating in the surface oxidation. Atom probe tomography analysis confirmed the absence of Mn in the anodized and corroded surface of Al-Mn alloys. The selective dissolution of Mn in these alloys was believed to increase the free volume at the metal/oxide interface to facilitate the formation of a denser, thinner oxide layer with closer to stoichiometry composition, leading to its enhanced corrosion resistance than pure Al. Lastly, to better understand the corrosion and tribocorrosion resistance of Al-based lightweight concentrated alloys and the effects of alloying concentrations on the structure and property of the passive layer, the third study investigated the effects of a passive element (Mo) and non-passive element (Mn) on the corrosion and tribocorrosion behavior of Al-Mn-Mo alloys. Specifically, Al80Mn8Mo12 exhibited higher corrosion resistance than Al80Mn20 due to the formation of a more compact and less defective passive film, as explained by the roles Mo played in both the substrate and the passive film. It was found that the pitting potential and corrosion current density of Al-Mn-Mo increased with Mo%. The effect of Mo alloying concentration on the tribocorrosion behavior of Al-Mn-Mo alloys was investigated as well. Adding Mo to Al-Mn alloys led to a lower wear and tribocorroison resistance of Al-Mn-Mo alloys. In addition, decreasing Mn and Mo concentrations resulted in a reduction of the tribocorrosion resistance in the ternary alloy, which was mainly dominated by the mechanical response under the selected testing conditions. / Doctor of Philosophy / Various critical current and future applications in the fields of aerospace, transportation, energy, and biomedical industries require not only a strong and tough metal, but one that is robust and reliable when interacting with some very corrosive environment. Such corrosive environment is testing the limits of most engineering metals and challenging the current understanding of the underlying degradation mechanism. For example, strength and wear resistance in most precipitation-hardened Al (aluminum) alloys is often achieved at the expense of sacrificed corrosion resistance, mainly due to micro-galvanic coupling between the soft matrix and hard precipitates. In addition, the performance of Al alloys deteriorates dramatically when there is combined wear and corrosion, i.e. tribocorrosion attack at the surface, due to the depassivation on the wear track as a result of mechanical removal of the passive film. Recent study shows that alloying Al with appropriate transition metals in supersaturated solid solution simultaneously improves the corrosion and wear resistance of Al. In this thesis, Al-Mn and Al-Mn-Mo solid solutions was synthesized and studied to understand the effects of testing condition (e.g. sliding frequency) and alloy composition (e.g. Mn and Mo concentration) on the corrosion and tribocorrosion behavior. First, the depassivation mechanism during tribocorrosion of Al-Mn alloys was investigated by performing tribocorrosion test using different sliding frequency from 0.05 to 1 Hz in 0.6 M NaCl aqueous solution. Results showed that both chemical and mechanical wear increased with increasing frequency. The mechanical wear increased with scratching frequency due to faster depassivation rate and increased real contact area, while chemical wear increased with frequency due to higher repassivation kinetics. Secondly, the effects of Mn on the aqueous corrosion and passivation of Al-Mn solid solution alloys were investigated by electrochemical experiments and advanced surface characterization. It was found that Mn addition enhanced the corrosion resistance of Al without participating in the surface oxidation. A denser, thinner oxide layer was formed on Al-Mn due to the increased free volume at the metal/oxide interface as a result of Mn dissolution. Lastly, the effects of alloying concentration on the aqueous corrosion and tribocorrosion of Al-Mn-Mo alloys were studied experimentally. The pitting potential and corrosion current density of Al-Mn-Mo were found to increase with Mo%. The passive film thickness depended on the total alloy concentration, while its compactness and defect density on the individual ones. The tribocorrosion resistance of Al-Mn-Mo alloys decreased with increasing Mn and Mo concentrations. In summary, the results from this thesis develop mechanistic understanding of the corrosion and tribocorrosion mechanisms of Al-based solid solution alloys, which sheds light on a new alloy design strategy for making lightweight, strong, and corrosion-resistant metals.

La composition chimique du film d'oxyde natif formé à l'air ambiant et du film passif formé en milieu acide (0,05 M H2SO4) à 500 mV/ESH sur une surface d'alliage monocristallin Fe-17Cr-14,5Ni-2,3Mo orientée (100) a été étudiée par Spectrométrie de masse d'ions secondaires à temps de vol (ToF-SIMS) et Spectroscopie de photoélectrons induits par rayons X (XPS). Ces films peuvent être considérés comme ayant une structure duplex avec une couche externe enrichie en Fe et une couche interne enrichie en Cr. La passivation provoque un net enrichissement en Mo concentré dans la couche externe des films passifs. Le vieillissement à l'état passif promeut l'enrichissement en molybdène et sa localisation vers l'extrême surface du film passif. Les propriétés électroniques de ces films ont été étudiées par Spectroscopie à effet tunnel (STS). La comparaison des valeurs de Eg mesurées par STS montre que la largeur de la bande interdite devient beaucoup plus élevée après passivation et vieillissement à l'état passif de la surface, en accord avec l'enrichissement en Cr3+, démontré par nos mesures ToF-SIMS et XPS. Les modifications nanostructurales induites par passivation en milieu acide de la surface Fe-17Cr-14,5Ni-2,3Mo(100) couverte par un film d'oxyde natif ont été étudiées par Microscopie à effet tunnel (STM) ex situ et in situ, et par Microscopie à force atomique (AFM). Un effet observé du Mo est de promouvoir l'homogénéité nanostructurale de la surface passivée. Un autre effet du Mo serait de former une couche externe amorphe. / The chemical composition of the air-formed native oxide film and the passive film formed in acidic medium (0.05 M H2SO4) at 500 mV/SHE on the (100)-oriented Fe-17Cr-14.5Ni-2.3Mo single crystal stainless steel surface was studied by Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). These films can be considered as having a duplex structure with an outer layer enriched in Fe and an inner layer enriched in Cr. The passivation causes a net enrichment of Mo concentrated in the outer layer of the passive films. Aging in the passive state promotes the enrichment of molybdenum and its localization to the outermost surface of the passive film. The electronic properties of these films were studied by Scanning tunneling spectroscopy (STS). The comparison of the Eg values measured by STS shows that the width of the band gap becomes much higher after passivation and aging in the passive state of the surface, in agreement with the enrichment of Cr3+, demonstrated by our ToF-SIMS and XPS measurements. The nanostructural modifications induced by passivation in acidic medium of the Fe-17Cr-14.5Ni-2.3Mo(100) surface covered by a native oxide film were studied by Scanning tunneling microscopy (STM) ex situ and in situ, and Atomic force microscopy (AFM). An observed effect of Mo is to promote the nanostructural homogeneity of the passivated surface. Another effect of Mo could be to form an amorphous outer layer.

No description available.

<p>This thesis presents an investigation into the structure and composition of the surface film formed on Mg in aqueous solutions and describes the influence of film stability on corrosion resistance. The composition and structure of the surface film formed on pure Mg in pure H₂O exposed at E_corr for 48 h was investigated using plan-view SEM-EDS imaging and STEM–EDS analysis of a FIB cross-section. The film formed was duplex in nature, consisting of a thinner, more-porous, nano-crystalline MgO-rich inner layer (50-100 nm), and a thicker, less-porous, Mg(OH)₂-rich outer platelet layer (700 nm). The results were consistent with the theory that a chemical breakdown (hydration) of the bulk inner MgO layer (native oxide) is a necessary precursor step to the corrosion process resulting in a significant thickening of the partially protective outer Mg(OH)₂ layer.</p> <p>The surface film formed on pure Mg exposed in 0.01 M NaCl for 24 h at E_corr was found to be a diffuse bilayer structure similar to pure H₂O, including a thin and porous nano-crystalline MgO-rich inner layer (50-100 nm), and a thicker, more porous Mg(OH)₂-rich outer layer (300-600 nm). The un-pitted film region formed in 0.01 M NaCl solution at +0.1 V above E_b for 0.5 h after 24 h aging at E_corr exhibited a similar duplex structure. However, the thickness of both Mg(OH)₂-rich (~450 nm) and MgO-rich layers (~30-60 nm) appeared to be decreased. Furthermore, the pitted region film formed on an anodically polarized Mg sample exhibited thinner MgO-rich layer (~20-30 nm), decorated with macro-pores (~50 nm), and Mg(OH)₂ rich middle layer (~200-300 nm) buried under a thick, more porous needle-like Mg(OH)₂-rich top layer (~0.2-1 µm). EDS analysis confirmed the presence of Cl within the films formed in 0.01 M NaCl, and Fe containing particles within the pitted region anodic film. These results were consistent with an accelerated hydration of MgO to Mg(OH)₂ at lower pH and in presence of Cl^-, formation of more soluble Mg-hydroxy-chlorides, enhanced Mg dissolution due to Cl^- ingress via increased film porosity, and influence of more noble impurities on the pit initiation and growth.</p> <p>The ~50-150 nm film formed spontaneously on Mg exposed to 1 M NaOH was found to consist mainly of a crystalline MgO layer that has been hydrated to Mg(OH)₂ to a variable degree. Although the film exhibited excellent corrosion resistance, it was not stable. The film tended to experience an irregular breakdown/repair process, which was characterized by large irregular potential drops (about 1 V) under E_corr conditions. The breakdown/repair process is believed to involve the hydration-induced stress-rupture of the MgO film at discrete sites and the subsequent formation of a Mg(OH)₂ self-healing corrosion product nodule (~ 500 nm).</p> <p>To understand the corrosion mechanisms and propose surface film evolution models, EIS behavior of Mg in all environments was investigated. The EIS spectra were compared with the proposed Nyquist behavior based on the film structure results. It was confirmed that the diffuse bilayer films formed in pure H₂O and 0.01 M NaCl possessed porosity, in which the electrolyte ingressed, forming the double layer and facilitating Mg faradic dissolution reaction across the Mg/film interface. Decreased porosity and R_ct along with the absence of a C_film/R_por loop, were respectively consistent with lower corrosion resistance of Mg in 0.01 M NaCl and a severely damaged pitted region film. Furthermore, the film compactness and increased R_ct in 1 M NaOH were consistent with a more stable film with excellent corrosion resistance in alkaline environments. In this film, liberated Mg²⁺ ions at the Mg/film interface have to diffuse through the film to facilitate the Mg(OH)₂ precipitation, thus a thicker Mg(OH)₂ film forms.</p> / Doctor of Philosophy (PhD)

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.