CORROSION OF METALS— II 60S 



sioii, corrosion resistance has been a large factor undoubtedly in its 

 successful use. It has long been recognized that lead is one of the least 

 corrodible of metals. Its dull unreactive character is synonymous 

 with inertness. Widely used in ancient times for water pipes, roofings, 

 caskets, linings for public baths, etc., many specimens have come down 

 to us in nearly perfect states of preservation. The Romans, for 

 example, employed lead water pipes in fifteen standard sizes usually 

 ten feet in length ^ and some of these pipes are said to be in use today. 

 In the form of roofings many examples exist which are five centuries 

 old. It seems likely that corrosion has been less destructive than 

 war to the original lead roofs of medieval cathedrals and buildings. 

 Once a protective film has formed on lead the metal may be preserved 

 indefinitely if not physically disturbed. In the air this film is usually 

 an oxide while in the case of underground burial the film which forms 

 on lead may be a silicate or in some cases merely a film of hydrogen 

 shielded by the presence of soil colloids. In other instances sulfates 

 and carbonates exert a retarding influence. Whether or not a pro- 

 tective film forms depends largely upon the character of the environ- 

 ment to which the metal is exposed. Under unfavorable conditions, 

 such as exposure to acetic acid vapors, strong alkalies or contact with 

 large soil particles, lead may be readily corroded. Purity of the metal 

 plays a minor role in corrodibility in the atmosphere although it may 

 aff'ect its behavior in soil waters and other electrolytes. 



The widely different conditions of exposure which prevail in the 

 aerial and underground cable plants make it desirable to consider 

 them separately. Corrosion caused by stray electrical currents, 

 since it occurs mainly in the underground plant, will be discussed under 

 that heading. 



Corrosion of Aerial Cables 



Corrosion is not a primary factor in the life of aerial cables. Failure 

 of these cables is usually due to cracking and confined to sections 

 which are subjected to repeated stresses or in some cases to prolonged 

 mechanical vibration.^ It is now recognized that the nature of the 

 environment affects the endurance of metals to such stressing and 

 vibration, and the term "corrosion-fatigue" has been applied to the 

 embrittlement and cracking which result from the simultaneous ap- 

 plication of tensile and compressive stresses and corrosive media. 



The resistance of lead to corrosion-fatigue is lowered, for example, by 

 exposure to the atmosphere.^ Evidently the protective oxide coating 

 which forms on lead in the air * is not only ineffective in preventing 

 intercrystalline fracture under repeated stressing, but actually con- 

 stitutes an accelerating factor. The specific volume of lead oxide is 



