Corrosion of Metals — II. Lead and Lead-Alloy Cable 



Sheathing 



By R. M. BURNS 



This paper discusses the corrosion of cable sheathing in the aerial and 

 underground cable plants. Corrosion does not appear to be a primary fac- 

 tor affecting the life of aerial cables; failure of these cables occurs usually 

 from intergranular embrittlement and is minimized by the use of alloy 

 sheathing. It is shown that corrosion of cable sheathing in conduit occurs 

 by means of the operation of small corrosion cells on the surface of the 

 sheath or by the leakage of current from the sheath to ground. The driving 

 force of these corrosion cells arises from some chemical inhomogeneity in 

 either the metal or the surrounding environment. The course and the 

 character of corrosion is determined chiefly by the influence of the constit- 

 uents of the environment on the operation of these ceils. These constit- 

 uents may be classed as corroding or protective; — the corroding including 

 oxygen, nitrates, alkalies and organic acids, while the protective are silicates, 

 sulfates, carbonates, soil colloids and certain organic compounds. Cable 

 sheathing buried directly in soils is seriously corroded by differential aera- 

 tion-cell action resulting from physical contact of relatively large soil 

 particles and metal. In general it is concluded that corrosion of cable 

 sheathing is influenced more by the nature of the environment than by the 

 chemical composition of the metallic material. The incidence of corrosion 

 of cable sheathing is small owing to the maintenance of non-corrosive chem- 

 ical and electrical environments in the cable plant. 



THE intricate cable network of the telephone system offers numer- 

 ous opportunities for the occurrence of corrosion. The property 

 damage resulting from perforation of the sheathing by corrosion and 

 the attending costly interruption of service have served to make the 

 prevention of cable failure a matter of primary concern. The rela- 

 tively low incidence of actual corrosion failures can be attributed 

 largely to the vigilance of the electrolysis engineers and the plant 

 forces. 



Cable sheathing is one of the largest single uses of metallic lead. 

 In 1929 it exceeded even that employed in the manufacture of storage 

 batteries and constituted about 27 per cent of the entire consumption 

 in this country. In the past fifteen years over two million tons of 

 lead have gone into the communications and power cable plants. 

 In the Bell System alone there are about 180,000 miles of lead alloy 

 covered cables, about forty per cent of which are underground. 

 About 95 per cent of the total mileage of telephone wires is in cable, 

 the proportion of open wire construction decreasing each year. 



The earliest telephone cables were of the type employed in telegraph 

 practice, the individual wires being insulated with rubber or gutta 



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