Passivity 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... 

Figure 4. Schematic diagram of active, passive, transpassive, and polishing states. M2+ (aq), dissolved metal ion MO, metal oxide or hydroxide M, metal atom.

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. 

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). 

It is a well-known fact that iron dissolution occurs in four different states, namely, the active, passive, transpassive, and brightening states, determined by the nature and kinetics of the reactions involved, which depend in turn on the potential and electrolyte composition. A schematic polarization curve for anodic dissolution of iron in acid solutions is given in Fig. 1. The shape of the curve depends on the nature of the electrolyte, the polarization program, and hydrodynamics. The Fe/Fe, and... 

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. 

Passivation looks different when observed under galvanostatic conditions (Fig. 16.2b). The passive state will be attained after a certain time t when an anodic current which is higher than is applied to an active electrode. As the current is fixed by external conditions, the electrode potential at this point undergoes a discontinuous change from E to Ey, where transpassive dissolution of the metal or oxygen evolution starts. The passivation time t will be shorter the higher the value of i. Often, these parameters are interrelated as... 

Fig. 2 shows a typical polarization curve and one can identify the active, passive and transpassive potential region. In the active region, several peaks can be observed which can be attributed to different oxidation states. 

Molybdenum exhibits unusual polarization behavior. The initial portion of the curve, shown dashed in Fig. 5.20, is very difficult to determine experimentally because it occurs at very low current densities indicating that the passive state is very rapidly established by traces of dissolved oxygen or by very low concentrations of other cathodic reactants. In fact, many of the published curves show only the transpassive range over which the current density rapidly increases. The implication is that as long as the potential is below 200 mV (SHE), the corrosion rate of molybdenum would be very low and this is observed. 

In the back scan in Fig. 4.1 (soHd arrows), the potential is lowered from positive (anodic) to negative (cathodic) values resulting the active-passive metal to shift from the transpassive region to the passive region and finally reaches the active state. The passive film is depassivated by removing the anodic apphed potential or by shifting... 

In the back scan in Fig. 4.13, the corrosion rate decreases from point 8, by decreasing the concentration of the oxidizer. The active-passive metal passes from the transpassive to its passive state. The passivity in the back scan is retained at concentrations 4-3a-2, lower than those necessary for the formation of the passive film. Thus, the region 4-3a-2 represents borderline passivity, where decreasing or increasing the oxidizer concentration results in a transition of the system to the active or passive region, respectively. The passivity decays within a short period of time to the normal active state of the active-passive metal. 

The ennobling of the metal surface is based on the assumption that the conducting polymer, in its oxidized state, will set the metal at a potential within its passive range where the dissolution rate is slow. This generally involves the formation of a thin insulating metallic oxide layer, more or less porous, the effect of which is to protect the metal from a rapid dissolution, and make it behave like a noble metal. To understand this effect it must be recalled that three distinct areas related to active, passive, and transpassive... 

Electrochemical destruction occurs by potential changes from the passive region. This can happen by lowering the potential to the active region, or by increasing it to the transpassive region as illustrated in Fig. 5.1, or possibly only above a critical potential for localized corrosion (see below). In certain cases, e.g. for Fe in a moderately alkaline environment, a passive oxide can be reduced to other oxides or to the metallic state. 

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