Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2007-2008. / The processes of generalized and localized corrosion induced at a cobalt electrode polarized in slightly alkaline aqueous media (pH 7-10) have been investigated using electrochemical methods and surface analysis techniques. In buffer solutions of H2C03/HCO,/CO^ at near neutral pH (7-8), particularly aggressive corrosion phenomena have been observed. The presence of carbonic acid in solution, as well as the high solubility of the CoO oxide, were identified as being responsible for the accelerated metal dissolution. However, the addition of a low concentration of phosphate species to the corrosive environment allows an important inhibition of cobalt dissolution under the form of the Co(CO,)2_ soluble complex. Potentiodynamic experiments have shown the efficiency of benzotriazole to inhibit the active dissolution of cobalt in slightly alkaline media. Using a novel approach to obtain the SERS effect (Surface-Enhanced Raman Scattering) directly from a massive cobalt electrode, the interaction mode of benzotriazole with the metal surface has been determined. Neutral benzotriazole molecules, first chemisorbed under cathodic polarization, are thereafter deprotonated at the zero charge potential of the System. As a result, a thin polymer-like [Co(II)(BTA)2-H20]„ film, insoluble and highly protective, is found on the electrode surface. This passive film restrains the access of chemical species known to be aggressive to the active sites of the electrode. In a situation of incomplete surface coverage by benzotriazole, cobalt species which originate from further dissolution processes, are preferentially complexed to deprotonated benzotriazole species present in the bulk solution. Such a mechanism contributes to the formation of the [Co(II)(BTA)2H20]„ film. To avoid degradation of cobalt through the initiation of localized corrosion induced by chloride ions, a protection method based on the passivation of the metal has been developed. It has been demonstrated that when cobalt is submitted to specific potentiostatic conditions, a passive bilayer film composed of CoO and C03O4 is electrogenerated on the metal surface. The uniform color of that film, attributed to iridescence, shows that the layer has a constant thickness ail over the electrode surface. A complete characterization of the optical and physical properties of that film is presented. / The processes of generalized and localized corrosion induced at a cobalt electrode polarized in slightly alkaline aqueous media (pH 7-10) have been investigated using electrochemical methods and surface analysis techniques. In buffer solutions of H2C03/HCO,/CO^ at near neutral pH (7-8), particularly aggressive corrosion phenomena have been observed. The presence of carbonic acid in solution, as well as the high solubility of the CoO oxide, were identified as being responsible for the accelerated metal dissolution. However, the addition of a low concentration of phosphate species to the corrosive environment allows an important inhibition of cobalt dissolution under the form of the Co(CO,)2_ soluble complex. Potentiodynamic experiments have shown the efficiency of benzotriazole to inhibit the active dissolution of cobalt in slightly alkaline media. Using a novel approach to obtain the SERS effect (Surface-Enhanced Raman Scattering) directly from a massive cobalt electrode, the interaction mode of benzotriazole with the metal surface has been determined. Neutral benzotriazole molecules, first chemisorbed under cathodic polarization, are thereafter deprotonated at the zero charge potential of the System. As a result, a thin polymer-like [Co(II)(BTA)2-H20]„ film, insoluble and highly protective, is found on the electrode surface. This passive film restrains the access of chemical species known to be aggressive to the active sites of the electrode. In a situation of incomplete surface coverage by benzotriazole, cobalt species which originate from further dissolution processes, are preferentially complexed to deprotonated benzotriazole species present in the bulk solution. Such a mechanism contributes to the formation of the [Co(II)(BTA)2H20]„ film. To avoid degradation of cobalt through the initiation of localized corrosion induced by chloride ions, a protection method based on the passivation of the metal has been developed. It has been demonstrated that when cobalt is submitted to specific potentiostatic conditions, a passive bilayer film composed of CoO and C03O4 is electrogenerated on the metal surface. The uniform color of that film, attributed to iridescence, shows that the layer has a constant thickness ail over the electrode surface. A complete characterization of the optical and physical properties of that film is presented. / Les processus de corrosion généralisée et localisée induits à une électrode de cobalt polarisée en milieux aqueux faiblement alcalins (pH 7-10) ont été étudiés à l'aide de méthodes électrochimiques et de techniques d'analyses de surface. Dans des solutions tampons H^CO^/HCOj/CO, de pH quasi-neutres (7-8), des phénomènes de corrosion particulièrement agressifs ont été observés. La présence d'acide carbonique dans la solution, de même que la grande solubilité de l'oxyde CoO dans ces conditions de pH, ont été identifiées comme étant responsables de la dissolution accélérée du métal. Toutefois, l'ajout d'une faible concentration de phosphates dans le milieu corrosif permet d'inhiber considérablement la mise en solution du cobalt sous la forme de l'ion soluble complexe Co(C03)*\ Des expériences potentiodynamiques ont démontré l'efficacité du benzotriazole à inhiber la dissolution active du cobalt en milieux faiblement alcalins. En développant une nouvelle voie d'obtention de l'effet SERS (Surface-Enhanced Raman Scattering) directement à partir d'une électrode de cobalt, le mode d'interaction du benzotriazole avec la surface métallique a été déterminé. Les molécules neutres de benzotriazole, d'abord chimisorbées sous polarisation cathodique, sont déprotonnées au potentiel de charge nulle du système. 11 en résulte la formation d'un mince film polymérique du type [Co(Il)(BTA)2-H20]„, insoluble et hautement protecteur, qui bloque l'accès des espèces réputées agressives aux sites actifs de l'électrode. Dans une situation de recouvrement de surface incomplet par le benzotriazole, les espèces de cobalt issues des processus de corrosion subséquents sont préférentiellement complexées par les espèces de benzotriazole déprotonnées et présentes en solution, consolidant ainsi la formation du film de [Co(II)(BTA)2H20]„. Afin de lutter contre la détérioration du cobalt via l'initiation de la corrosion localisée induite par les ions chlorures, une voie de protection par passivation du métal a été mise en place. En soumettant le cobalt à des conditions potentiostatiques particulières, un film passivant composé d'une bicouche de CoO et de C03O4 a pu être électrogénéré à la surface du métal. La coloration uniforme de ce film, attribuée à un phénomène d'iridescence, montre que la couche est d'épaisseur constante sur toute la surface de l'électrode. Une caractérisation complète des propriétés optiques et physiques de ce film est présentée.
| Identifer | oai:union.ndltd.org:LAVAL/oai:corpus.ulaval.ca:20.500.11794/19283 |
| Date | 12 April 2018 |
| Creators | Gallant, Danick |
| Contributors | Pézolet, Michel, Simard, Stéphan |
| Source Sets | Université Laval |
| Language | English |
| Detected Language | English |
| Type | thèse de doctorat, COAR1_1::Texte::Thèse::Thèse de doctorat |
| Format | xxix, 307 f., application/pdf |
| Rights | http://purl.org/coar/access_right/c_abf2 |
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