Transpassive state

In the third case, the transpassive state appears at a more noble potential than the passive state, where the dissolution current that was suppressed at the passive region again increases. The boundary potential... 

Passivation potential, and thermodynamic phase formation, 218 Transition, passive to pit formation, 219 Transpassive state, of metals, 223... 

The state in which the anodic dissolution of metals proceeds from the bare metal stirface at relatively low electrode potentials is called the active state the state in which metal dissolution is inhibited substantially by a superficial oxide film at higher electrode potentials is called the passive state-, the state in which the anodic dissolution of metals increases again at stiU higher (more anodic) potentials is called the transpassive state. 

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 active state, the dissolution of metals proceeds through the anodic transfer of metal ions across the compact electric double layer at the interface between the bare metal and the aqueous solution. In the passive state, the formation of a thin passive oxide film causes the interfadal structure to change from a simple metal/solution interface to a three-phase structure composed of the metal/fUm interface, a thin film layer, and the film/solution interface [Sato, 1976, 1990]. The rate of metal dissolution in the passive state, then, is controlled by the transfer rate of metal ions across the film/solution interface (the dissolution rate of a passive semiconductor oxide film) this rate is a function of the potential across the film/solution interface. Since the potential across the film/solution interface is constant in the stationary state of the passive oxide film (in the state of band edge level pinning), the rate of the film dissolution is independent of the electrode potential in the range of potential of the passive state. In the transpassive state, however, the potential across the film/solution interface becomes dependent on the electrode potential (in the state of Fermi level pinning), and the dissolution of the thin transpassive film depends on the electrode potential as described in Sec. 11.4.2. 

In the stationary state of anodic dissolution of metals in the passive and transpassive states, the anodic transfer of metallic ions metal ion dissolution) takes place across the film/solution interface, but the anodic transfer of o Q en ions across the Qm/solution interface is in the equilibrium state. In other words, the rate of film formation (the anodic transfer oS metal ions across the metal lm interface combined with anodic transfer of osygen ions across the film/solution interface) equals the rate of film dissolution (the anodic transfer of metal ions across the film/solution interface combined with cathodic transfer of oitygen ions across the film/solution interface). 

The behavior of nickel on anodic polarization is matched by the behavior of iron and cobalt on the surface of which oxygen is also liberated at higher current densities. Chromium anode dissolves at low current densities to form bivalent cations. When it becomes passive its potential increases by about 1 Volt. With further inorease of potential chromium enters a state called transpassive state in which instead of bivalent ions hexavalent ions are formed which reaot with the hydroxyl ions present in the electrolyte to form chromate ions according to equation ... 

In the passive state, metal electrodes normally hold extremely small potential-independent dissolution current as shown in Figure 22.7 for metallic iron in acid solution. For some metals such as nickel, however, the passive state changes beyond a certain potential into the transpassive state, where the dissolution current, instead of being potential-independent, increases nearly exponentially with... 

The effect of the oxidizer on the corrosion rate is shown in Fig. 4.13. In regions 1 through 3, both active and passive states are present. At the critical current density (the passivation potential), point 4, the corrosion current drops to passivation current. The stability of the system in this region is controlled by the voltage span of the passive region. In the transpassive state, the corrosion current starts to increase at point 7. 

The barrier layer thickness may be estimated from the measured capacitance at high frequency (e.g. at 5kHz) using the parallel plate formula, = efb/C, with a reasonable estimate for the dielectric constant (in this case, b = 30). The thickness data so calculated are summarized in Figure 4.4.31. As seen, the film thickness in the passive state (V < 0.7 Vjhe) is found to increase hnearly with voltage, in accordance with Eq. (Ill), but in the transpassive state the thickness is found to decrease with increasing voltage. 

I. Betova, M. Bojinov, and T. Tzvetkoff [2004] Oxidative Dissolution and Anion-Assisted Solubilization in the Transpassive State of Nickel-Chromium AUoys, Electwchim. Acta 49,... 

Fig. 1. Schematic polarization diagram explaining the action of the effective cathodic coatings on the steel corrosion ip, icp., ipit, it- respectively currents of initial passivation, complete passivation, pitting formation and corrosion in transpassive state.

Corrosion of filters occurs in the transpassive state. Their cathodic protection is based on the polarization of steel to a potential characteristic of the passive state. Garner (1998) states that over 120 CP installations have been applied, mainly in North America, for the protection against corrosion of equipment made of austenitic stainless steels operating in bleacheries. More information is given by Webster (1989) and Singbeil and Garner (1987). 

The potential range above the ,j, value corresponds to the transpassive state. In the transpassive state, different reactions can take place. For example on the surface of iron and carbon steel in sulfuric acid, oxygen is liberated according to Reaction (8-2), while in the case of chromium or chromium-nickel steel, a transition occurs from the insoluble Cr203 oxide into the soluble dichromate... 

Figure 1. Schematic anodic polarization curve of iron in acid solution between the active and the transpassive states of the metal.

The cathodic protection could also be provided by applying an appropriate potential, and this is common in some industries such as petroleum and natural gas engineering. Another approach is to design a metal alloy and adjust the conditions to push the corrosion potential into the passive region, where the corrosion rate is often at a much more acceptable rate. However, these regions are often metastable, and failure to properly maintain the potential could place corrosion back into the active area or even in the transpassive state. 

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