Corrosion region

Constraints can limit the degree of part load operation of a fuel cell. For example, a PAFC is limited to operation below approximately 0.85 volts because of entering into a corrosion region. 

Figure 22 Schematic anodic polarization curve indicating active and passive corrosion regions.

Fig. ISM The effect of changing the rate of the cathodic reaction on the corrosion potential and the corrosion current in a system undergoing passivation. Inhibiting the cathodic current can have the adverse effect of shifting the corrosion potential from the passive region (point E on line 3) to the active corrosion region (jtoint A on line I).

The trajectory of the potential of steel reinforcement in a concrete stracture exposed to chlorides and then protected by a cathodic protection system is shown in Figure 20.4. The initial condition is represented by point 1 where the chloride content is zero and the steel is passive. By increasing the chloride content, the working point shifts to 4, within the corrosion region. Corrosion of the steel occurs rapidly by pitting or macrocell mechanisms. Applying cathodic protection leads to 5 so that the passivity is restored or to 6 without fuUy restoring passivity. 

Pourbaix diagram for Fe shows that the corrosion region extends into mid-pH values at moderate potentials. The phase field for Cr203 overlays much of this territory, allowing for an enhancement in the passivity of Fe alloys by the addition of Cr. 

The following regions are clearly outHned in F. 2.16 (i) immunity, in which the metal is considered to be immune from corrosion attack (ii) corrosion region, in which the metal corrodes and forms soluble species and (iii) passive region, in which the metal is coated with oxide or hydroxide. By decreasing the potential (cathodic protection), the metal can move from the active corrosion region to the immunity region. Zinc, because of its equiHbrium potential, is used as a sacrificial anode to protect the iron from corrosion. 

Stage n is the stress corrosion region where the rate of corrosion is similar to the rate of crack propagation. Therefore, the crack remains sharp and propagates into weakened glass. 

As shown above, oxidation occurs when the electrode potential is higher, and reduction when it is lower, than the equilibrium potential. Hence, when we have, as in corrosion, two simultaneous reactions, of which one is oxidation (anodic reaction) and the other one is reduction (cathodic reaction), the real potential must lie between the equilibrium potentials of the two reactions. If we consider corrosion of iron in aerated water, with reduction of oxygen as the cathodic reaction, the potential has to be somewhere between the lines a and e in Figure 3.10. In acid (and usually in neutral) solutions the potential will lie in the corrosion region, in alkaline solutions in the passive region. With efficient oxygen supply, which for instance can be promoted by heavy convection of the solution, passivity may also be achieved in neutral water. 

In some instances, increasing the velocity of the corrodent over the surface of the metal will increase the corrosion rates when concentration polarization occurs. However, with passive metals, increasing the velocity can actually result in lower concentration rates because the increased velocity shifts the cathodic polarization curve so that it no longer intersects the anodic polarization ciu e in the active corrosion region. 

Corrosion region Thermodynamic calculations indicate that, in such region of an E-pH diagram, a metal is stable as an ionic (soluble) product and therefore susceptible to corrosion attack. Experience is required to find out the extent and form of the corrosion attack that may occur in the corrosion region(s) of a Pourbaix diagram. 

The passive region in the potential-pH diagram is clearly shovm in the location between the two corrosion regions. In the diagram, the concentration of ionic species ranging from saturated solution (10°) to very dilute solution (10 °), are shown. Roughly Ippm of ionic species is... 

The potential-pH diagram suggests that platinum is fairly stable thermodynamically there is only a small corrosion region around 1V at pH -2 to 0 (we note that the potential-pH diagram is valid only in the absence of ligands with which platinum can form soluble complexes or insoluble compounds). However, as described below, platinum actually dissolves under fuel cell operating conditions, which is unsatisfactory especially for its usage in the cathode of fuel cells for automotives. 

FIGURE 14.14 (a) Optical micrograph of the localized corrosion region on the 6092-T6 Al/B4C/20p MMC after 1 day of immersion in ASTM seawater (note hydrogen bubble at the corrosion site), (b) SVET map of the area shown in (a), units in pA/cm. (c) SIET map of the area shown in (a), units in pH. (Reprinted from Ding, H. et al, J. Electrochem. Soc., 156, C352 (color online), 2009. Reproduced with permission of The Electrochemical Society.)... 

Optical and SEM images of the metal dusting of 35/45 alloy after 160 hours of corrosion at 650°C showing carbon bundles on the surface, localized corrosion regions filled with carbon deposit, alloy surface protected by Cr-rich oxide film, and filamentous carbon in the carbon bundles. 

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