Passivity Flade potential

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... 

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-... 

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.

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. 

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.

Flade, Friedrich — (Sep. 16, 1880, Arolsen, now Bad Arolsen, Germany - Sep. 5, 1916, near Manancourt, France) After studies of chemistry in Halle and Munich, Flade received his PhD in 1906 from the University of Marburg, Germany. There he qualified as University teacher (habilitation) in 1910 [i], Flade observed that iron shows a sudden potential change when it goes from the passive to the active state. Now, the electrode potential of a metal where the current associated with the anodic metal dissolution drops to very small values bears his name (- potential, subentry -> Flade potential). He also showed that loading of the iron surface with oxygen is essential for its -> passivation [ii—vi]. Flade fell in World War I in the Battle of the Somme, and he was buried in Manancourt, France. 

Passivation potential — Figure 2. Evaluation of XPS data on the chemical structure of the passive layer on Fe formed for 300 s in 1M NaOH as a function of potential with a two-layer model Fe(II)/Fe(III). Insert shows the polarization curve with oxidation of Fe(II) to Fe(III) at the Flade potential EP2, indication of soluble corrosion products Fe2+ and Fe3+, and passivation potential EPi in alkaline solution [i, iii]... 

Flade potential electrode potential F of a metal in contact with a corrosive electrolyte solution where the current associated with the anodic metal dissolution (- corrosion, active region) drops to very small values. See also - Flade, and -> passivation. 

Equation (4.7) corresponds to the potential variation of a metal electrode of the second kind as a function of pH. The Flade potential is used to evaluate the conditions for passive film formation and to determine the stabihty of the passive film. The reversible Flade potential of three important engineering materials is approximately +0.63 V for iron, +0.2 V for nickel, and —0.2 V for chromium [7,8]. The negative value of the Flade potential for chromium (—0.2 V) indicates that chromium has favorable Gibbs free-energy for the formation of passive oxide film on its surface. The oxide film is formed at much lower potentials than in other engineering materials. 

The Flade potential for iron (+0.63 V) indicates that only very strong and concentrated oxidizing agents will form passive films on its surface. However, even weak oxidizing agents form thin and very stable corrosion-resistant surface films on chromium. The 12-30% chromium content in stainless steel gives excellent corrosion resistance properties to steel due to formation of a stable chromium oxide passive film on its surface. Figure 4.2 shows the standard Flade potential measured for stainless steels with different chromium contents. 

As the chromium content in the alloy increases from 8% to 13%, the corrosion rate of iron decreases from 0.08 mm/year to very low values [9]. The Flade potentials of chromium-iron alloys in 4% NaCl solutions increases from —0.57 V (vs. SHE) in the absence of chromium to +0.17 V (vs. SHE) for the ahoy with 12% chromium [7,10]. The critical current density for the passivation of Cr-Fe aUoys at pH = 7 reaches a... 

Other metals that have favorable reversible Flade potentials and form passive film on their surfaces include titanium, silicon, aluminum, tantalum, and niobium. Naturally formed aluminum oxide protects the underlying aluminum metal at pH between 4 and 8. Titanium possesses very high oxidizing potentials and is used to manufacture anodes for cathodic protection systems for the chlorine-alkafi process (production of hydrogen, chlorine, and sodium hydroxide) and many other appfications. 

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