A Simple Model of the Galvanically Coupled Electrode

As a simple model to illustrate the above variables, consider a surface as shown in Fig. 4.2 consisting of alternate anodic and cathodic strips (e.g., uniform scratches through the oxide coating of a hot-rolled steel 

The figures span the distance from the center of an anodic strip (0.5 cm wide) to the center of an adjacent cathodic strip 1.5 cm wide (i.e., the center-to-center distance for the strips is 1.0 cm). It is assumed that the anodic and cathodic reactions are confined to the respective areas, as stated above. Current flows in the solution as positive ions from the anodic area where the reaction, M — Mm+ + me, occurs to the cathodic 

Another governing relationship, however, is Ohm s law, which leads to a dependency of the corrosion current on both the polarization characteristics of the anodic and cathodic reactions and on the total electrical resistance of the system, Rtotal. Rtotal includes the resistance in the metal between anodic and cathodic areas, RM a metal junction resistance if different metals are associated with the two areas, Rac any anode- or cathode-solution interface resistance, Rai and Rci and the solution resistance, Rs. The latter depends on the specific resistivity or conductivity of the solution and the geometry of the anode-solution-cathode system. 

Since a major variable governing corrosion is frequently the solution resistivity, emphasis is placed on analyzing qualitatively how this can be an important factor. The flux of current from anode to cathode will follow approximately semicircular channels, perpendicular to the isopotential surfaces, for the simple geometry shown in Fig. 4.3(a) and (b). The current-channel boundary surfaces have been drawn so as to define channels of fluid extending from the anode to the cathode with a 

The distribution of potential in the solution along the solution/metal interface is shown in Fig. 4.6. If the anode and cathode areas are not connected, they will exhibit their thermodynamic or open circuit potentials, with the potentials in the solution at the anode and cathode being equal to +1000 mV and 0 mV, respectively. When the anode and cathode areas are in contact, current will pass causing polarization of the interface reactions. With a solution-specific resistivity of 1000 ohm-cm, the solution potential at the center of the anode is decreased 

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