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20 juillet : conférence de M. Urquidi-Macdonald : corrosion et passivation

Mme Mirna Urquidi-Macdonald, Professeur au Departement of Engineering Science and Mechanics à The Pennsylvania State University aux États-Unis présentera ses travaux ’Extended Point Defect Model’ concernant la passivation et la corrosion des alliages de nickel.

- La conférence aura lieu le 20 Juillet à 10 heures dans la salle de thèse.

corrosion, passivation corrosion, science des matériaux, Point Defect Model, alliages de nickel

In mechanistic analysis of oxide passive films, the Faradic impedance is commonly calculated for the passive layer and in more recent papers ; it is calculated for the bilayer film when a re-precipitate layer exist. More frequently, the overall Faradaic impedance (calculated from the mechanism adopted) of the inner layer is then couple in series with to the impedance of the outer layer, double layer capacitance and the electrolyte resistance. Those "artificial" or "parasitic" passive-electrical elements that are added to the Faradaic impedance (calculated from reaction mechanisms) and used to describe the outer film and the double layer capacitance can "mask" or "overwhelm" the effect of the Faradaic capacitance on the overall calculated impedance. Accordingly, the impact from a "mechanist analysis" portion of the impedance to the overall impedance becomes irrelevant. Is that happens ; the purpose of mechanistic analysis is lost. We developed a theory to link the Faradic from the mechanistic analysis to the electrolyte double layer and the composition and characteristics of the electrolyte. We extended the Point Defect Model (PDM) and we introduced a complete mechanistic description of the inner (passive), outer layer (re-precipitated porous layer that may or not be present), the Helmholtz double layer, and the electrolyte. This new formulation does not require the use of passive electrical elements to account for the overall measured impedance. The expanded PDM model is developed for a specific application (Nickel oxide passive film formed on a Ni-14%Cr alloy). We measured DC and AC impedance data on Nickel samples containing 14% percentile of Chromium as a minor alloy on a 1 M H2SO4 electrolyte at room temperature. Our experimental results (Mott-Schottky) plots confirm that the inner layer for Nickel-Chromium alloys containing less than 16% is a rich Nickel Oxide film. The inner layer behave as a p-type Nickel Oxide and the outer layer, if exist, is a mixture of mainly Ni(OH)2 and a small percentile of Cr(OH)3. The inner film is of the form MxOy, the outer porous layer is of the form Mz(OH)w-M’t(OH)s. M represents the metal base and M’ the most important minor alloy element (the subscripts x, y, z, w, t, and s indicate the oxidation states). In this presentation, we introduce the theory of the extend PDM, the results obtained by fitting the theoretical impedance bases on the present extended PDM and the measured impedance are discussed.


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