CORROSION OF METALS— II 607 



Corrosion of Lead Directly Buried in Soils 



It is not the practice of the Bell System to bury lead-covered cables 

 directly in the soil without the use of a protective coatinpj. Recogni- 

 tion of the corrosion hazard involved in such construction is one of 

 the considerations which led to the use of conduit for the housing of 

 even the first cables which w^ere placed underground. The more 

 recent actual experience of certain small users with soil corrosion has 

 served to confirm the soundness of this practice. The idea that cable 

 sheathing might be buried safely in direct contact wdth soils was sug- 

 gested by the fact that lead had been widely used as water pipes.^^ 

 Many miles of telephone cables accordingly were laid directly in the 

 earth, notably in Indiana, and frequently with unfortunate results. 



In certain sections where it is considered economical to bury cables 

 in the ground, a coating has been devised for the protection of the 

 sheathing against corrosion. This consists in wrapping the lead-alloy 

 sheathed cable with asphalt-impregnated paper followed by one or 

 more layers of jute impregnated with a preservative compound, and 

 in some cases steel tape armoring over which there is wrapped a final 

 layer of jute. The structure is flooded with asphalt before and after 

 each serving of paper and each layer of jute. The steel tape is em- 

 ployed where there exists any danger of induction from power lines; 

 it may be omitted in locations where there is little likelihood of trouble 

 from this source. 



Before discussing the corrosion of cables in conduit, which is the 

 principal concern of the present paper, it will be of interest to review 

 the results of corrosion studies which have been made on lead and lead- 

 alloy sheathing materials buried directly in soils. In addition to the 

 presence of soluble salts, the underground environment in this case 

 involves direct contact of the metal with relatively large soil particles 

 and aggregates — a markedly heterogeneous condition. These points 

 of contact of soil particles and metal become areas of reduced oxygen 

 concentration as compared with surrounding regions of the metal 

 surface which are more freely accessible to the soil atmosphere. The 

 resulting oxygen concentration cells with a driving force of approxi- 

 mately 100 millivolts provide one of the most important means by 

 which metals corrode in soils. The use of conduit affords an effective 

 barrier against soil action of this character. Silt deposits which some- 

 times occur on cables in conduit do not give rise to differential aeration 

 action probably because under such circumstances cathodic polariza- 

 tion of the corrosion cells is maintained. ^^ This inhibitive function of 

 soil colloids has been observed recently in connection with a study of 



