Flade

Initially, the curve conforms to the Tafel equation and curve AB which is referred to as the active region, corresponds with the reaction Fe- Fe (aq). At B there is a departure from linearity that b omes more pronounced ns the potential is increased, and at a potential C the current decreases to a very small value. The current density and potential at which the transition occurs are referred to as the critical current density, and the passivation potential Fpp, respectively. In this connection it should be noted that whereas is determined from the active to passive transition, the Flade potential Ef is determined from the passive to active transition... 

Meta Flade potential (V, M. S.H.E.) Comparison with reversible potential of the eauiUbrium specified... 

The significance of the Flade potential Ef, passivation potential pp, critical current density /pn, passive current density, etc. have been considered in some detail in Sections 1.4 and 1.5 and will not therefore be considered in the present section. It is sufficient to note that in order to produce passivation (a) the critical current density must be exceeded and b) the potential must then be maintained in the passive region and not allowed to fall into the active region or rise into the transpassive region. It follows that although a high current density may be required to cause passivation ) only a small current density is required to maintain it, and that in the passive region the corrosion rate corresponds to the passive current density (/p, ). 

The addition of a more passive metal to a less passive metal normally increases the ease of passivation and lowers the Flade potential, as in the alloying of iron and chromium in 10 wt. % sulphuric acid (Table 10.31) . Tramp copper levels in carbon steels have been found to reduce the corrosion in sulphuric acid. Similarly 0 -1 palladium in titanium was beneficial in pro-... 

Table 10.31 Effect on critical current density and Flade potential of chromium content for iron-chromium alloys in lOwt.% sulphuric acid (after West )...

Chromium (%) Crilical current density ( crii,. Am- ) Flade potential ( f. V)... 

The Flade potential, which is the negative potential limit of stability of the oxide film. At potentials more negative than the Flade potential the oxide film is unstable with respect to its reduction or dissolution, or both, since the rates of these two processes exceed that of film formation. 

All of these three properties of the oxide films on metals are influenced by the anion composition and pH of the solution. In addition the potential of the metal will depend on the presence of oxidising agents in the solution. Inhibition of corrosion by anions thus requires an appropriate combination of anions, pH and oxidising agent in the solution so that the oxide film on the metal is stable (the potential then lying between the Flade potential and the breakdown potential), and protective (the corrosion current through the oxide being low). 

Flade Potential the potential at which a metal which is passive becomes active (see Passivation Potential). 

See also Corrosion Potential, Electrode Potential, Equilibrium Potential, Flade Potential, Open-circuit Potential, Passivation Potential, Protection Potential, Redox Potential.)... 

Figure 3. Current vs. potential curve for iron dissolution in phosphoric acid solution at pH 1,85. Ep, Flade potential Ep, passivation potential Epii- critical pitting potential EiP, transpassivation potential. Solid and broken lines correspond to the cases without and with CF ions, respectively.

Flade potential, 247 Flame-annealed gold surfaces and the work of Kolb, 81 Flat band potential, 483 Fluctuations asymmetrical and unstable systems, 255 controlling progress in pitting, 299 in pitting dissolution, 251 and corrosion processes, 217 during dissolution, 252 at electrodes, theory, 281 during film breakdown, 233 and mathematical expressions thereof, 276... 

When the polarization curve is recorded in the opposite (cathodic) direction, the electrode will regain its active state at a certain potential The activation potential is sometimes called the Flade potential (Flade, 1911). The potentials of activation and passivation as a rule are slightly different. 

Even before the Flade potential the dissolution of iron is a complex process which has been thoroughly investigated by Epelboin and coworkers with AC impedance. The starting point is the same basic scheme as that proposed for chlorine evolution ... 

Fig. 12.64. Spontaneous decay of passivity as a function of time, showing the Flade potential of iron in 0.5 M sulfuric acid.

Fig. 20. Composition (Fe(II) and Fe(III)) of the passive layer formed for 300 s on Fe in 1 M NaOH calculated from XPS measurements on the basis of a bilayer model including the potentiodynamic polarization curve with indication of formation of soluble Fe2+ and Fe3+ species. Hp and Epi are the passivation potentials in alkaline solution and acidic electrolytes (Flade potential) extrapolated to pH 12.9 [12],...

Fig. 21. Composition of the passive layer on Fe formed in 1 M NaOH for 300 s as a function of the electrode potential from XPS results on the basis of a homogeneous layer model, two methods for background correction for data analysis at the extrapolated passivation potential Efj = -0.18 V (Flade potential), the layer composition corresponds to Fe304.

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