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BELL SYSTEM TECHNICAL JOURNAL 



ably less than when taken at points more widely separated. Figure 2 

 gives a schematic representation of this experiment. The change in 

 potential is the result of current fiow through the electrolyte from zinc 

 to iron. The current densities are highest in the region of the interface, 

 the metal ion concentration becoming increased at the anode area and 

 decreased at the cathode area, producing thereby anodic and cathodic 

 polarization, respectively. 



This polarization behavior of corrosion cells largely determines the 

 rate of corrosion. It is obvious that the effective potentials of corro- 

 sion cells may be reduced by polarization to zero, in which case the 

 rate of corrosion is limited to that required to maintain this polariza- 



CURRENT DENSITY 



Fig. 2 — Illustration of galvanic polarization. 



tion. In other words, the progress of corrosion may be controlled by 

 the extent either of anode polarization or cathode polarization or both, 

 that is, either one may determine the final result. Figure 3 represents 

 the variety of current density-potential relationships which may exist 

 in corrosion cells. In Cell 1, in which there is no appreciable polariza- 

 tion of either anodic or cathodic areas (as indicated by the small 

 change of potential with current), corrosion current flow is limited by 

 the resistance of the electrolytic paths between anodes and cathodes 

 and since this may be small if these areas are contiguous the corrosion 

 rate may be high. In Cell 2 the anode is highly polarized as repre- 

 sented by the solid line or progressively less polarized as the point of 

 intersection with the non-polarized cathode occurs at higher and 

 higher current densities as represented by the dotted lines. In a 



