Anodic dissolution transpassive potentials

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. 

For metals such as chromium and alloys such as stainless steel, the plot of potential versus corrosion rate above the range is shown in Figure 20.67. Figure 20.68 shows a sudden sharp drop in corrosion above some critical potential. Despite a high level of anode polarization above V, the corrosion rate drops precipitously due to the formation of a thin, protective oxide film as a barrier to the anodic dissolution reaction. Resistance to corrosion above is termed passivity. The drop in corrosion rate above can be as much as 10 to 10 times below the maximum rate in the active state. With increasing corrosion potential, the low corrosion rate remains constant until at a relatively high potential the passive film break down, and the normal increase in corrosion rate resumes in a transpassive region. 

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 ... 

Figure 6.39 Galvanostatic polarization of Ni in the transpassive potential region in sodium nitrate solution (a) anodic polarization curve in the transpassive potential region (b) the current efficiency for metal dissolution and (c) the apparent thickness of the film as measured by coulometry [36].

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 test electrolyte was a deaerated solution of 0.1 M HCl + 0.4 M NaCl. The polarization behavior of the alloy is compared in Figure 6.1 with the alloy constituent metals. The corrosion potential is seen to be more noble than that of Cr and Fe but close to that of Ni and Mo. This is quite typical for the corrosion potential of austenitic alloys. It will be shown that Ni and Mo are enriched on the surface in the metallic state during anodic dissolution. As a consequence, the corrosion potential becomes close to the corrosion potential of these elements. From the polarization data it is suggested that both Cr and Mo are more likely to contribute to passivity, especially the barrier layer, than Fe or Ni and that Mo will contribute only in a narrow range of potential before it undergoes transpassive dissolution. These simple indications will be shown to be only partially correct. In Figure 6.2 are typical XPS spectra of the outer region of the surface films obtained in the same study for the alloy polarized at passive potentials (-100 and 500 mV vs. SCE). 

Tin when made anodic shows passive behaviour as surface films are built up but slow dissolution of tin may persist in some solutions and transpassive dissolution may occur in strongly alkaline solutions. Some details have been published for phosphoric acid with readily obtained passivity, and sulphuric acid " for which activity is more persistent, but most interest has been shown in the effects in alkaline solutions. For galvanostatic polarisation in sodium borate and in sodium carbonate solutions at 1 x 10" -50 X 10" A/cm, simultaneous dissolution of tin as stannite ions and formation of a layer of SnO occurs until a critical potential is reached, at which a different oxide or hydroxide (possibly SnOj) is formed and dissolution ceases. Finally oxygen is evolved from the passive metal. The nature of the surface films formed in KOH solutions up to 7 m and other alkaline solutions has also been examined. 

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... 

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. 

At E > 1.0 V transpassive dissolution as QxCY2 is obtained in 0.5 M H2SO4, with a strong increase of the current density (Fig. 5). At E > 1.7 V oxygen evolution contributes to the large anodic current. These characteristic electrochemical properties indicate the importance of Cr as an alloying additive to obtain corrosion-resistant alloys, especially at negative potentials and in acidic solutions. 

Above the passivity range, that is for potentials above about +600 mV, the steel is brought to conditions known as transpassivity. oxygen may be produced on its surface according to the anodic reaction of oxygen evolution 2H2O —> O2 + 4H + 4e, which produces acidity. Steel reaches these conditions only in the presence of an external polarization (for example in the presence of stray currents). Since the anodic reaction is oxygen evolution, dissolution of iron and consequent corrosion of the steel does not take place (i. e. the passive film is not destroyed). Nevertheless, if these conditions persist until the quantity of acidity produced is sufficient to neutralize the aLkaHnity in the concrete in contact with the steel, the passive film will be destroyed and corrosion will initiate. This aspect wiU be dealt with in Chapter 9. 

While the previous examples were limited in the anodic polarization potential either by transpassive dissolution or by oxygen evolution valve metals can be polarized to potentials of up to 100 V and above. Examples are aluminum, titanium, tantalum, hafnium, and zirconium. Formation characterization and properties of these oxides were treated in Chapter 9. 

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