Polarization active-passive transition

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... 

Figure 5 Schematic Evans diagram and resulting potential-controlled polarization curve for a material that undergoes an active-passive transition and is in a reducing solution. The heavy line represents the applied currents required to polarize the sample.

Figure 6 Schematic Evans diagram and resulting potential-controlled polarization curve for a material that undergoes an active-passive transition and is in an oxidizing solution. The heavy line represents the applied currents required to polarize the sample. If the sample did not undergo an active-passive transition, it would corrode at a much higher rate in this solution, as is indicated by the intersection of the dotted line and the cathodic curve.

For the case shown in Fig. 8, the anodic and cathodic Evans lines intersect at three points. The polarization curve for this situation appears unusual, although it is fairly commonly observed with CRAs. At low potentials, the curve is identical to that shown in Fig. 5. However, just above the active-passive transition, another Ecmi appears followed by a loop and yet a third ECljU before the passive region is observed. The direction (anodic or cathodic) of the applied current density for each region shown in the polarization curve of Fig. 8 is indicated, showing that the loop consists of cathodic current. The origin of the cathodic loop is the... 

Such cathodic loop behavior is often observed on the reverse scans of polarization curves in which pitting does not occur as shown in Fig. 10 (9). During the initial anodic scan, the oxide is thickening and the anodic line is moving to the left. Thus, upon the return scan, the unchanged cathodic line now intersects the anodic line at several places, leading to the appearance of cathodic loops. Cathodic loops do not pose fundamental problems they merely conceal the passive current density at potentials near the active-passive transition. 

Singbeil and Garner (10) showed that the use of anodic protection can prevent stress-corrosion cracking in the pressure vessel steels exposed to alkaline solutions used in digesters in the pulp and paper industry, as shown in Fig. 17. The 200 mV anodic polarization placed the material above the active-passive transition where cracking had been shown to occur (10). 

Based on the same principle, the corrosion resistance of metals and alloys with polarization curves having active-passive transitions can be greatly improved by an impressed anodic current initially equal to or greater than the critical current for passivity. The potential of the metal moves into the passive region... 

Since the anodic protection technique is restricted to materials that exhibit active-passive transitions, it is important to determine the sensitiveness of the polarization behavior of metallic material exposed tosolutions containing contaminants, such as chloride ions, and temperature. 

When the potential of a metallic component is controlled and shifted in the anodic (positive) direction, the current required to cause that shift will vary. If the current required for the shift has the general polarization behavior illustrated in Fig. 5.40, the metal has an active-passive transition and can be anodically protected. Only a few systems exhibit this behavior in an appreciable and usable way. The corrosion rate of a metal with an active-passive behavior can be significantly reduced by shifting the potential of the metal so that it is at a value in the passive range shown in Fig. 5.40. 

In addition to classical methods such as weight loss (via coupons), solution analysis for dissolved metal, monitoring of gas evolution (in the case of H2) and change in metal resistance, a select number of electrochemical techniques are widely accepted. This section briefly describes these methods. Weight-loss coupons are still the traditional, accepted baseline for comparisons electrical resistance (ER) and linear-polarization resistance (LPR) are the most widely used electrical and electrochemical techniques. Electrode potential monitoring is sometimes a valuable way of following active/passive transitions. There continues to be an emphasis on the use of non-destructive testing (NDT) techniques (particularly ultrasonic types). 

Figure 9. Impedance and AC-resolved RRDE of the active-passive transition of iron in 1 M HjSo. Rotation speed 900 rpm. Current-voltage curve of the iron disk. Data at polarization B Z disk impedance, Nj/Fe(II) and Nj/Fe(III) emission efficiencies respectively for Fe(II) and Fe(III) species, ring potential respectively 0.8 and -0.4 V/SSE, r /Fe(II+III) faradaic capacitance of the passivating layer.

Iron and nickel are examples of metals that display an active-passive transition when anodically polarized in many aqueous solutions. Passivity is generally ascribed to the presence of a thin oxide film 1-4 nm thick that isolates the metal surface from the corrosive aqueous environment. The resistance of this anodic oxide film to dissolution is related to its physical and chemical nature, which determines the corrosion resistance of the metal. The other major factor influencing the rate of metallic corrosion is the aggressiveness of the aqueous environment, i.e., the pH, temperature, and anion content of the solution. 

To understand the influence of allo5dng elements on the passivity of stainless steels, researchers have combined electrochemical and siufece analysis. Polarization diagrams provide the first indication of the overall influence of alloy additions on the active-passive transition, passivity, and pitting resistance. However, siuface analysis by X-ray photoelectron spectroscopy (XPS) of prepassivated siufaces provides a direct observation of the location and the chemical state of an alloying element. Such... 

Figure 17.12 Schematic polarization curve for a metal that displays an active-passive transition.

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

Aluminum alloys are an exception. The oxide film formed in air or on immediate contact with an aqueous environment places aluminum in a passive state and an active-to-passive transition is not observed experimentally in the polarization curve. 

Figure 4.4.15. Steady-state polarization curve and complex plane impedance diagrams at selected potentials through the active-to-passive transition for iron in 1M H2SO4 as repated by Keddam, Lizee, Pallotta, and Takenouti [1984]. The arrows indicate the direction of decreasing fret[uency. (From M. Keddam, O. R. Mattos, and H. J. Takenouti, Reaction Model for Iron Dissolution Studied by Electrode Impedance Determination of the Reaction Model, J. Electrochem, Soc., 128, 251—214, [1981]. Reprinted by permission of the publisher, The Electrochemical Society, Inc.)...

Transition metals, such as Fe, Cr, Ni and Ti, demonstrate an active-passive behavior in aqueous solutions. Such metals are called active-passive metals. The above metals exhibit S-shaped polarization curves which are characteristic of such metals. Consider, for instance, the case of 18-8 stainless steel placed in an aqueous solution of H2SO4. If the electrode potential is increased then the current density rises to a maximum, with the accompanying dissolution of the metal taking place in the active state. The current density associated with the dissolution process indicates the magnitude of corrosion. At a certain potential, the current density is drastically reduced as the metal becomes passivated because of the formation of a thick protective film. Iron shows passivity... 

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