CORROSION OF METALS— II 619 



Another important source of alkali is the electrolysis of sodium 

 chloride or common salt by electrical currents flowing to the cables. 

 Under these conditions caustic soda is produced at the sheath which is 

 cathodic or negative to earth ; hence the terms ' ' cathodic " or " negative 

 corrosion. The salt usually comes from that used in the winter to 

 thaw out street car switches, although in one case it has been traced to 

 the drippage from salt-ice mixtures of ice-cream delivery trucks. In 

 still another case, a power cable, negative to earth, suffered alkaline 

 attack as the result of the electrolysis of alkali salts concentrated at a 

 low point in the cable run by heat dissipation of the cable. Finally it 

 should be mentioned that the use of calcium chloride on streets for 

 melting snow or laying dust would lead undoubtedly to its coming in 

 contact with the underground cable plant and being converted into 

 corrosive free lime in areas negative to earth. 



It has been known since the Middle Ages that lead is corroded by 

 acetic acid. In the presence of the carbon dioxide of the atmosphere, 

 the corrosion product is the pigment, white lead. The attack manifests 

 itself by the formation of a white encasement around the globules of 

 moisture on the sheath; at first a mottled effect is produced which in 

 time develops into a heavy white encrustation of the carbonate or basic 

 carbonate of lead. The early use of wood conduit was attended with 

 occasional cases of acetic acid corrosion and it was found that the wood 

 tar creosote used as the preservative contained this acid. Since that 

 time coal tar creosotes have been specified for the preservation of w'ood 

 conduit. The conduit most widely used in this country is yellow pine. 

 Properly creosoted this product has not been known in Bell System 

 experience to cause corrosion except when used under such unusual 

 circumstances as close proximity with steam pipes or exposed on 

 viaducts over railroad yards to the heat of locomotive stacks. In these 

 cases acetic acid was liberated as a product of the slow decomposition 

 of the wood. A recent instance of acetic acid attack in creosoted 

 conduit manufactured from southern yellow pine has been reported and 

 attributed to acid liberated by the destructive decomposition of the 

 wood by the Kansas sun.'*" 



The most serious corrosion of cables by acetic acid on record is that 

 which occurred on the Pacific Coast in creosoted Douglas fir conduit a 

 few years ago.'*^ Following the initial satisfactory use of this product 

 for subsidiary cables it was employed extensively for main com- 

 munication subways. With the expansion of the cable plant into this 

 newly constructed duct system several cases of acetic acid corrosion 

 occurred — most of them within the first 15 months in conduit of recent 

 installation. Analysis of the atmosphere within the cable compart- 



