Cathodic delamination distributed

The electrochemical mechanism of filiform corrosion is described by a differential aeration cell between the front (low oxygen concentration) and the back (open to air through the cracked/porous tail of dry corrosion products) of the filament s active head . Therefore, the head is the local anode and the tail the local cathode, the opposite to the cathodic delamination described earlier. This is shown in Fig. 7-67 where an optical photograph of the filiform head is compared to the potential distribution in air (center) and in nitrogen (right) (Schmidt and Stratmann, 1998). 

AC impedance studies have indicated that DMFC performance loss due to interfacial failure can be linked to (1) increased ohmic resistance of the cell, (2) enhanced electrode overpotential, and (3) electrode flooding. Ohmic loss due to the interfacial resistance buildup can contribute several tens of milliohm square centimeters to the total ceU resistance. An increase in the overpotential is caused by the loss of contact between the membrane and the catalyst, which renders part of the catalyst layer unusable (a possible major performance loss). Electrode flooding can be attributed to the nonuniform current distribution, which is more significant when membrane-cathode delamination occurs. 

In more detail, the model of cathodic delamination can be described and understood as follows [3,57] the delamination starts with randomly distributed anodes and cathodes in a defect or at a dalaminated area. When iron is taken as an example, the dissolved Fe " at the anode is further oxidized to Fe + by oxygen and forms insoluble corrosion products in the defect that often adhere to the polymer (in the case of a blister) and at the edges of the defect (where oxygen enters the defect). In what way the defect is blocked and a cap of corrosion products is formed on the top of the blister see Figure 20.7. In this cap oxygen... 

Obviously, a large potential difference between the active metal surface in the defect (ca. —O.SVshe) and the intact zinc-polymer interface is observed. Between these areas, a steep potential increase marks the location of the delamination front. The potential maps indicate that, as for polymer-coated iron, a cathodic reaction leads to the delamination of the coating. Fiirbeth and Stratmann proved the cathodic mechanism by small spot XPS analysis of the delaminated surface [83]. While no chloride was detected in the delaminated region, the amount of sodium decreased from a high value near the defect to a small value at the front of the delamination. The distribution of sodium ions was in total agreement with the potential maps. 

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