Anode reactions active-passive transition

Fig. 1.41 Schematic anodic polarisation curves for a passivatable metal showing the effect of a passivating agent that has no specific cathodic action, but forms a sparingly soluble salt with the metal cation, a without the passivating agent, b with the passivating agent. The passive current density, the active/passive transition and the critical current density are all lowered in b. The effect of the cathodic reaction c, is to render the metal active in case a, and passive...

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

The four types of mixed potential models presented in Figs. 12.6 to 12.9 are simplistic and do not necessarily reflect the complete behavior of carbon steel in Kraft liquors because the models all assume some sort of steady states. Figure 12.10 depicts typical curves from an in situ test in a white liquor clarifier at different scan rates. The passive state does not exist until after the active-passive transition is traversed. Therefore, unless sufficient anodic current density is discharged from carbon steel by a naturally occurring cathodic reaction or an applied anodic protection current, the carbon steel liquor interface remains monostable (active) because the passive film and its low current density properties do not exist. 

The typical features of a metal/solution system that exhibits an active to passive transition is shown in Fig. 1.33, which represents diagrammatically the potentiostatically determined anodic f-log / curve for iron in H2SO4. Initially, the curve conforms to the Tafel equation and curve AB, which is referred to as the active region, corresponds with the reaction Fe-> Fe (aq). At B there is a departure from linearity that becomes more pronounced as the potential is increased, and at a potential C the current decreases to a very small value. The current density and potential at which the transition occurs are referred to as the critical current density i n., and the passivation potential E, respectively. In this connection it should be noted that whereas is determined from the active to passive transition, the Flade potential Ef is determined from the passive to active transition... 

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... 

For passivation, the passivation current density ipjs must be applied either by means of an anodic current (polarisation) or in the reaction vith an oxidant at passivation potential Upas. In the active and passive range, trivalent chromium (Cr ) is dissolved. Above the transpassive breakthrough potential Ua, i.e. after the transition to the transpassive range, the current density, and with it the rate of corrosion, rises once again, since at this high oxidation potential chromium then dissolves in hexa-valent form (Cr ) as chromate. 

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