614 BELL SYSTEM TECHNICAL JOURNAL 



of lead covered with litharge were freshly scratched after being sub- 

 merged in water." It is of significance that although corrosion 

 readily occurred in this experiment, there was no attack if the scratch 

 were exposed to the atmosphere for two hours before submersion of 

 the specimen. In other words, the oxide film on lead is readily self- 

 healing and injury to it is unlikely to cause corrosion. 



Turning to a consideration of corrosion cells originating from the 

 exposure of the sheathing to an inhomogeneous environment, reference 

 has already been made in discussing soil corrosion to the nature and 

 the importance of oxygen concentration cells, and to the protection 

 which the conduit affords against this hazard. Cables in conduit 

 are seldom subject to contact with the character of inert objects which 

 lead to the establishment of oxygen concentration cells. Relatively 

 large hard particles are generally the most effective agents in producing 

 differential aeration. In the laboratory, lead can be pitted by contact 

 with a glass rod when submerged in a dilute sodium chloride solution. 

 There are a few instances in which cables in conduit appear to have 

 corroded by means of oxygen concentration cells. For example, there 

 is evidence that deep pits in sheathing produced by the leakage of 

 stray currents to earth have continued to deepen to the point of 

 perforation of the sheathing, after removal of positive potential con- 

 ditions. The bottoms of such pits are less accessible to oxygen and 

 appear in some cases to function as the anodic elements in differential 

 aeration cells. Cases of this kind are generally diagnosed by the field 

 forces as "old action." 



Another, but rather uncommon, example of oxygen concentration 

 cell has been observed in the use of a porous duct plugging material 

 contaminated with acetic acid. In this case it seems likely that the 

 naturally protective oxide film on the sheathing was destroyed by 

 the acid following which this region, owing to the exclusion or partial 

 exclusion of oxygen, became anodic to the adjacent areas which were 

 freely accessible to air. Contamination with acetic acid does not 

 appear to be essential to this action since other cases have been re- 

 ported in which the duct plugging material was free from acid. 



Finally there exists the possibility of large scale differential aeration 

 cells where one cable of a multiple run is placed, owing to space limita- 

 tions, in a dip under a large sewer, but bonded to the other cables. 

 Such a cable may suffer severe corrosion in the region of the dip as a 

 result of the lower oxygen content of the atmosphere in this duct as 

 compared to that prevailing in the other ducts. 



The discussion of differential environments has related so far only 

 to oxygen concentration. In a similar manner, underground cables 



