Flade relation

When the applied potential is removed, passivity decays within a short time in the manner shown in Fig. 6.3. The potential first changes quickly to a value still noble on the hydrogen scale, and then it changes slowly for a matter of seconds to several minutes. Finally, it decays rapidly to the normal active potential of iron. The noble potential arrived at just before rapid decay to the active value was found by Flade [6] to be more noble the more acid the solution in which passivity decayed. This characteristic potential, ( )f, was later called the Flade potential, and Franck established it to be a linear function of pH [7], His measurements in acid media, combined with later data by others, provide the relation at 25°C ... 

This reproducible Flade potential and its 0.059 pH dependence is characteristic of the passive film on iron. A similar potential-pH relation is found for the passive film on chromium, for Cr-Fe alloys, and for nickel, for which the stan-... 

When activated cathodically, the Flade potential of chromium and stainless steels follows the relation n(0.059pH), where n may be as high as 2. For self-activation, n is 1 [8],... 

Stability of passivity is related to the Flade potential, assuming the following schematic reaction to take place during anodic passivation ... 

Figure 1-25. Thicknesses of the inner Fe(II), of the outer Fe(III), and d of the total layer of passive iron as a function of the electrode potential E (SHE) (StrehblowandSpeckmann, l984 Hanptetal., 1986). and the related potentiodynanuc polarization curve pi is the passivation potential, f2 the Flade potential insert phase model.

XPS signals, and its close relation to the potentiodynamic polarization curve (Haupt and Strehblow, 1987a Haupt et al., 1986). The decrease of Fe(II) due to its oxidation to Fe(in) and the related peak in the polarization curve meet the extrapolated Flade potential according the relation p =0.58-... 

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