606 BELL SYSTEM TECHNICAL JOURNAL 



greater than that of the metal from which it is derived ^ and it has 

 been suggested that the presence of the oxide provides some sort of 

 leverage which aids embrittlement.® It is possible that dififerential 

 aeration cell action may be involved for it is conceivable that the 

 surface oxide film in the region of the grain boundaries is the most 

 susceptible to rupture, producing thereby areas which are anodic to 

 the adjacent unfractured surfaces. 



Intercrystalline corrosion of lead may be produced in the laboratory 

 merely by immersion of the specimen in a solution of nitric acid and 

 lead acetate.^ The attack in this case, as also in the case of the 

 simultaneous action of tensile stress and corrosion, occurs along the 

 grain boundaries leaving individual grains of lead which retain the 

 characteristics of the original metal.* While intergranular corrosion 

 of this type can be produced in lead of high purity, the rate of attack 

 in a given medium is usually a function of the purity of the metal. 



Exclusion of the atmosphere or the use of coatings of certain oils 

 or grease have been shown to retard the rate at which lead is embrittled 

 by corrosion-fatigue.^ More practical means of minimizing the inter- 

 granular failure of cable sheathing lies in modification of the com- 

 position of the sheathing. 



Alloying with 3 per cent tin, or 1 per cent antimony materially in- 

 creases the resistance of lead to intercrystalline embrittlement.'" 

 The antimony alloy has a considerably greater fatigue resistance than 

 pure lead as measured in a certain type of laboratory fatigue test '^ 

 but decreases in time, or when the alloy is cold worked, owing to an 

 agglomeration of the dispersed antimony particles which occurs par- 

 ticularly in the region near the grain boundaries. ^^ Lead alloyed with 

 0.04 per cent calcium and suitably age-hardened has been shown in 

 laboratory tests to have a much higher resistance to fatigue failure 

 than the 1 per cent antimony alloy. ^^ Certain ternary alloys of lead 

 containing cadmium are said to possess marked resistance to fatigue 

 failure.^* More recently lead containing 0.1 per cent tellurium has 

 been shown to be about 3-fold more resistant than ordinary lead to 

 mechanical vibration. ^^ It should be emphasized that all of these 

 comparisons of fatigue resistance were made in laboratory tests and 

 are not based on field experience. 



From the foregoing it will be seen that for the most part the aerial 

 cable plant does not present a serious corrosion problem. The im- 

 portance of the unavoidable environmental influences on sheath 

 embrittlement is minimized by the use of lead alloy sheathing to- 

 gether with proper methods of cable suspension. Other types of lead 

 corrosion are rare in aerial cables. 



