KINETICS OF PASSIVATION

Assume a defect-free single crystal and a mechanism of oxide film growth by vacancy migration. Thus, the rate of film formation for a single crystal, related to Faraday s law, can be approximated as [13] 

In addition, if the Arrhenius equation applies, then the rate of film growth is described by the forward/reverse current densities at equilibrium 

apply an anodic overpotential for film growth to take place and consequently, an electric field potential gradient (rj/x) develops. If there exists a linear electric field aaoss the film, then the forward (/) and reverse (r) cases become [13], respectively 

The thermodynamics of passivity together with the Nemst equation can be generalized in simple reaction steps for the formation of an oxide film (MO). Hence, 

In addition, the stmcture, composition, and thickness of a passive oxide film may be determined using ex-situ techniques, such as [27-28] 

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Depending upon the mechanism that is employed by the organism to accumulate the solute, internalisation fluxes can vary both in direction and order of magnitude. The kinetics of passive transport will be examined in Section 6.1.1. Trace element internalisation via ion channels or carrier-mediated transport, subsequent to the specific binding of a solute to a transport site, will be addressed in Section 6.1.2. Finally, since several substances (e.g. Na+, Ca2+, Zn2+, some sugars and amino acids) can be concentrated in the cell against their electrochemical gradient (active transport systems), the kinetic implications of an active transport mechanism will be examined in Section 6.1.3. Further explanations of the mechanisms themselves can be obtained in Chapters 6 and 7 of this volume [24,245]. 

The special construction of the spectrometer permits not only a safe specimen transfer without chemical changes, but also a well-defined specimen pre-treatment by sputtering previous to the electrochemical preparation. This is very important in the case of alloys because active dissolution or etching and transpassive corrosion or electropolishing may change the surface by preferential dissolution of one component. The altered surface composition may have an effect on the kinetics of passivation and on the composition of the passive layer, formed subsequently as has been... 

Kinetics of passivation processes Factors Affecting Passivation... 

One of the first mechanistic models of the kinetics of passive film formation was described by Cabrera and Mott [31]. The passive film thickness in their model is controlled by the transport of the metal cations from the underlying metal to the... 

R. Kirchheim [1987] Growth Kinetics of Passive Films, Electrochim. Acta 32, 1619-1629. 

The kinetics of passivation is normally characterized through Faraday s law for determining the rate of film formation in terms of growth of film thickness according to eq. (6.1). As a cmde approximation, the rate of film formation dxjdt) is related to vacancy diffusion and it is assumed to obey the Arrhenius equation (6.2). In fact, dx/dt increases provided that there exists a net anodic current density i and an overpotential r/, at a distance x from the electrode surface. 

R. Kirchheim, Growth Kinetics of passive films, Electrochim. Acta 52 1619 (1987). K. E. Heusler, Untersuchung der Auflosung des passiven Eisen in Schwefelsaure mit der Rotierenden Scheiben-Ring Elektrode, Ber. Bunsenges. Phys. Chem. 72 1197 (1%8). 

R. Kirchheim, The growth kinetics of passive films and the role of defects, Corros. Sci. 29 183 (1989). 

M. Keddam, H. Takenouti, and N. Yu, New data on the kinetics of passive nickel from very low frequency impedance measurements, Corros. Sci. 27 107 (1987). 

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