Passivity transpassivation 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.

Fig. 11-9. Anodic polarization curve of a metallic electrode for active dissolution, passivation, and transpassivation in aqueous acidic solution > u = anodic current of metal dissolution = passivation potential = transpassivation potential = maximum metal...

The transition from the active state to the passive state is the passivation, and the transition in the reverse direction is the activation or depassivation. The threshold of potential between the active and the passive states is called the passivation potential or the passivation-depassivation potential. Similarly, the transition from the passive state to the transpassive state is the transpassivation, and the critical potential for the transpassivation is called the transpassivation potential. Further, a superficial thin film formed on metals in the passive state is often called the passive film (or passivation film), the thickness of which is in the order of 1 to 5 nm on transition metals such as iron and nickel. 

In the case of nickel electrodes on which the passive film is a p-f pe nickel oxide (NiO), the energy gap ( 0.2 eV) between the valence band edge and the Fermi level at the flat band potential is small so that the transpassivation potential Etp is relatively close to the flat band potential as in Fig. 11-13. 

FIGURE 22.24 Anodic polarization curves for passivation and transpassivation of metallic iron and nickel in 0.5 kmol m-3 sulfuric acid solution with inserted sketches for electronic energy diagrams of passive films [32] /ip = passivation potential, TP = transpassivation potential, fb = flat band potential, /Fe = anodic dissolution current of metallic iron, Nl = anodic dissolution current of metallic nickel, and io2 — anodic oxygen evolution current. 

At the transpassivation potential (Figure 6.3), the properties of the passive film change and one observes a renewed increase in the rate of dissolution. This behavior is referred to as anodic depassivation. It may be the result of film oxidation at high anodic potentials or of film breakdown favored by the presence of certain anions. Generally speaking, in the transpassive potential region, one observes three types of metal dissolution behavior ... 

The zone of stable passivity ranges from the passivation potential to the transpassivation potential (pitting potential) E, . One can therefore reduce the corrosion of a passivating metal to a negligible level by imposing a potential E such that ... 

Fig. 11-13. Anodic polarization curve of a metallic nickel electrode in a sulfuric add solution transpassivation arises at a potential relatively dose to the flat band potential because of p-type nature of the passive oxide film. [From Sato, 1982.]...

FIGURE 22.26 Schematic polarization curves for anodic metal dissolution, passivation, passivity breakdown, pitting corrosion, and transpassivation Eb = film-breakdown potential and Ep]l — pitting potential. 

Electrochemieal eorrosion of an electron conductive material, such as a metal, can be studied by applying a potential and measuring current using a three-electrode electrochemical cell. The typical current density-electric potential curve in such an experiment is shown in Figure 9.3 [1,3]. The curve consists of three parts aetivation, passivation, and transpassivation. Mez+zMe in Figure 9.3 is the electrode potential of the metal ion/metal electrochemical couple. Below this potential, no electroehemieal metal dissolution can take place. In the case of iron, the (reduetion) electroehemieal reaction... 

FIG U RE 9.3 Typical current density-applied potential curve showing three main processes in corrosion (1) activation, (2) passivation, and (3) transpassivation. 

Because the metal dissolution is an anodic process, for example, Fe(s) Fe +(aq) + 2e , the current of the process is assumed to be positive. When potential increases from Mez+zMe lo f (passivation or Flade potential), the current is increasing exponentially due to the electron transfer reaction, for example, Fe(s) -> Fe +(aq) + 2e", and can be described using Tafel s equation. At a E the formation of an oxide layer (passive film) starts. When the metal surface is covered by a metal oxide passive film (an insulator or a semiconductor), the resistivity is sharply increasing, and the current density drops down to the rest current density, 7r. This low current corresponds to a slow growth of the oxide layer, and possible dissolution of the metal oxide into solution. In the region of transpassivation, another electrochemical reaction can take place, for example, H20(l) (l/2)02(g) + 2H+(aq) + 2e, or the passive film can be broken down due to a chemical interaction with environment and mechanical instability. Clearly, a three-electrode cell and a potentiostat should be used to obtain the current density-potential curve shown in Figure 9.3. 

Electrochemistry of the metal corrosion (dissolution) under an applied potential is presented in the form of a typical polarization curve. The origin of the common three regions (activation, passivation, and transpassivation) is explained. 

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