622 BELL SYSTEM TECHNICAL JOURNAL 



pressures which obtain underground. It is still, however, one of the 

 most important of corrosion inhibitors. 



Mention has already been made of the protective influence under 

 certain circumstances of silt or clay deposits on the surface of the 

 sheath. There are in the underground electrolyte many other sub- 

 stances mainly organic in nature and often colloidal which aid in the 

 preservation of cable sheathing. Whereas the anions, such as silicates, 

 sulfates, and carbonates, induce passivity by a process of anodic 

 polarization of corrosion cells, the inhibitive mechanism of soil colloids 

 and of the organic materials in soil electrolytes is usually one involving 

 cathodic polarization of these cells. 



Stray Current Corrosion 



The most common kind of cable sheath corrosion, the most de- 

 structive and best recognized, is that which occurs when electrical 

 currents flow from the sheath to ground. In this case the portion of 

 cable of higher potential than earth has the general characteristics of an 

 anode, while the cathode is some extraneous structure. The potentials 

 between anode and cathode may be and generally are greater than those 

 which are possible for the electrolytic corrosion cells which have been 

 described at length in this paper. The size of the currents which may 

 flow for a given potential will of course depend upon the resistance of 

 the path, i.e., upon the electrolytic resistance of the soil solution in 

 contact with the cable. The size of the anodic area will depend upon 

 the area of the sheath in contact with the electrolyte. The nature ot 

 the corrosive attack accordingly will depend upon this area and the 

 rate of current flow or, in other words, the current density. In 

 appearance the corroded area may be a clean cut pit or pits, or it may 

 be roughly etched. When the potential is greater than about 2 volts, 

 a brown colored anodic oxidation product, lead peroxide, may be 

 formed. A simple test for this — the blue coloration which develops 

 when a small amount of it is dissolved in a 5 per cent solution of 

 tetramethyldiaminodiphenylmethane containing dilute acetic acid — is 

 a certain indicator of anodic action. A negative result with this test, 

 which is the more common experience, does not, however, exclude the 

 possibility that the attack was anodic in character; the potential to 

 earth may have been too small or the peroxide may have been actually 

 formed but may have been consumed by local action following removal 

 of the positive sheath potential. 



Occasionally lead chloride, a white salt, may be formed in the 

 corroded areas under anodic conditions. Thus, the finding of a 

 relatively greater concentration of chloride in the corrosion product 



