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BELL SYSTEM TECHNICAL JOURNAL 



The manner in which the conductance of the surrounding electrolyte 

 influences the rate of corrosion is illustrated in Fig. 4A in which the 

 upper curve represents the cathodic and the lower the anodic polariza- 

 tion. Assuming equal anodic and cathodic areas the corroding current 

 density for the lower conducting solution is represented by "M" and 

 that for the higher conducting solution by "N." In the actual case 

 where anodes and cathodes are in close juxtaposition, the internal 

 resistance is low and consequently the corroding current density 

 approaches that represented by the intersection of the polarization 

 curves. 



^A 



C I 



\ I 



CURRENT DENSITY 



A 



B 



Fig. 4- 



-Effect of conductance and of electrode area on corrosion current densities. 

 M ■= Lower conducting solutions. 

 N = Higher conducting solutions. 



L = Corrosion current density for cells of equal cathode and anode areas. 

 K = Corrosion current density when ratio anode area to cathode is small. 



Thus far consideration has been confined for the sake of simplicity 

 to corrosion cells in which the anodic and cathodic areas are equal. 

 Usually in actual experience this is not the case. In corrosion charac- 

 terized by pitting, the anodic area is generally small compared to the 

 cathodic areas. This situation is illustrated in Fig. 4B in which it 

 will be seen that under these conditions a high corroding current 

 density corresponding to a rapid rate of attack may occur. Con- 

 versely, in cases where the ratio of anode areas to cathode area is 

 large, the rate of attack will be small, being thus controlled by cathodic 

 polarization. In this connection it may be of interest to consider the 

 effect of impurities upon rate of corrosion. If the contaminating 

 metal is anodic and exists as a separate phase it will tend to dissolve 

 with the formation of small pits which having once formed may possibly 



