Metals active-passive-transpassive

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

The potentiodynamic technique is used to examine the passivation behavior of a metal or alloy in an electrochemical system. During the potentiodynamic scan, the metal surface may undergo several different electrochemical reactions, wherein the anodic current may vary over several orders of magnitude [34]. Generally, analysis of the anodic curve can provide potentials for active, passive, transpassive, and repassive zones a rough estimation of corrosion current and corrosion potential and a measure of the stability of passivity. Moreover, one can determine whether the passivation is spontaneous or needs to be polarized to induce passivation. In addition, one can determine whether the electrochemical system can induce a spontaneous transition from passive... 

Fig. 1. Polarization curve of metals with active, passive and (a) transpassive potential range including oxygen evolution (b) passive potential range going directly to oxygen evolution (c) continuing passivity for valve metals to very positive potentials. Pitting between critical pitting lim and inhibition potential fsj in the presence of aggressive anions and inhibitors.

Both reactive metal components are oxidized at the metal/oxide interface. However, in the passive range Fe(III) ions are dissolved preferentially with a slow, but still larger, rate by at least one order of magnitude. This situation leads to an accumulation of Cr(III) within the passive layer. XPS studies yield a Cr content of >70 at. % [69-72], In the active/passive transition range, Cr is accumulated to 90% and it reaches a plateau of 80% in the passive range. Finally, it decreases for E > 1.0 V in the transpassive range (Fig. 27b). 

Figure 14 Evans diagrams for active-passive metal when coupled to (a) a metal that holds corr in a passive region, (b) a metal that holds Ecm above pitting or transpassive potential, and (c) a metal that causes a passive-active transition.

Fig. 1 Schematic current density potential curve of metals with active, passive, and transpassive potential range and the critical potentials Ep and E restricting the pitting range. Valve metals with insulating passive layers showing neither transpassive metal dissolution nor oxygen evolution.

A metal is passive if it resists corrosion in strong oxidizing solutions or at appUed anodic polarization [4—9]. Active-passive metal passivates through interaction with oxidizing agents or anodic polarization. A metal is defined as active-passive if it possesses three regions in the polarization curve active, passive, and a transpassive region. A typical anodic polarization curve of an active-passive metal is shown in Fig. 1.3. 

The critical current density and passivation potentials are important characteristics that control metal passivation properties. In the transpassive region, E, the current starts to increase due to oxygen evolution or passive film breakdown. In Fig. 1.3, the passive region is more anodic than the active region. This property of the active-passive metal or aUoy is not observed in the case of normal metals and is only used to define passivity. 

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. 

Figure 4.18 illustrates the principles based on an impressed anodic protection system. An active-passive metal possesses three regions in the polarization curve the active, the passive, and the transpassive regions. In the active region, the corrosion potential and corrosion current are controlled by the Tafel kinetics of the individual redox reactions. 

Mixed potential theory is used to estimate the galvanic current and the galvanic potential in an active-passive metal that passivates at potentials less noble than the reversible hydrogen potential. A galvanic couple between titanium and platinum of equal area of 1 cm is exposed to 1 M HCl. The electrochemical parameters for the active-passive alloy are eeq xi = —163 V vs. SHE anodic Tafel, b Ti = 0.1 exchange current density, ixi= 10 A/cm passivation potential, pp= —0.73 V passivation current, 7pass= 10 A/cm transpassive potential, = 0.4 V vs. SHE and activity of dissolved species [Ti ] = 1 M. The exchange current densities, i°, on platinum and titanium... 

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... 

FIGURE 15.3 Schematic Evans diagram for the behavior of an active-passive metal. (m/m )> reversible potential of the couple o(m/m+)> exchange current density Epp, passivation potential trio critical anodic current density ip, passive current density Ef, transpassive potential. 

In contrast to the active and passive regions, the surface state of the metal in the transpassive region is not well defined and an oxide may or may not cover the surface. 

As the potential increases, the material goes through the states active, passive and transpassive. At low potential levels, the corrosion current increases with the potential in the active range, as does metal dissolution according to Faraday s law. 

Figure 1-41. Current density potential curve of metals with active, passive, and transpassive behavior.

Polarisation from an external source may also affect the range of passivity. Cathodic polarisation may depress the potential from the passive to the active region (see Fig. 3.14) and thus care should be taken to avoid contact with any other corroding metal. Anodic polarisation, on the other hand, can stabilise passivity provided that the potential is not increased into the range of transpassivity (see Fig. 3.14) and anodic protection is quite feasible. 

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