Application en cinétique électrochimique des concepts d’états stationnaires multiples et de structures dissipatives: Electrocristallisation et passivation des métaux
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The state of an electrochemical system, to which electric parameters (current and electrode potential) allow an access, is generally governed by nonlinear equations for mass balances and electron flux through the metal-electrolyte interface. Two examples of electrochemical systems will be presented, in which these equations can lead to multisteady states (characterized by three current values for a specified potential) and to a spatial self-organization of the electrode surface.
Iron passivation is described by a model involving a dissolution process coupled to a passivation process by the diffusion of the reactant from the bulk of the electrolyte. Such a coupling can give rise to a Z-shaped current-potential curve and a nonuniform anodic dissolution occurs when the electrochemical state is stabilized in the intermediate region between the active and passive states by means of an appropriate control device.
The S-shaped current-potential curve obtained during zinc electrocrystalliza-tion is explained by a coupling of interfacial reactions including an autocatalytic adsorption process. The interactions between these reactions and surface diffusion of adsorbed intermediates Znads I can give rise to large variations of current density, periodically located on the electrode, which become stable and initiate the formation of spongy deposits on a compact substrate.
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