26 



BELL SYSTEM TECHNICAL JOURNAL 



The potential differences which are responsible for the existence of 

 corrosion cells arise either from some chemical or physical inhomo- 

 geneity of the metal or from some inhomogeneity of the environment 

 at the iTietal surface. These cells or galvanic couples provide the 

 means whereby the natural tendency of metals to corrode may express 

 itself. The nature of the electrodes composing some of these elec- 

 trolytic cells is given in the following table : 



Example of Cell 

 Two-phase alloys, metal containing 

 metallic impurities, two metals in con- 

 tact or metal with porous metal coat- 

 ing. 



Iron with porous oxide scale. 

 Iron with carbonaceous dust particles 

 on surface. 



Exclusion of air at point of contact of 

 metal and inert material or decreased 

 oxygen concentration at bottom of pits 

 on surface of metal. 



Metal in contact with solution of two 

 different concentrations or two differ- 

 ent solutions. 



Metal which has been subject to non- 

 uniform heat treatment or cold work- 

 ing. 



It will be observed that there are ample possibilities for a metal of even 

 high purity to corrode. The existence of more than one metallic 

 phase in metals in industrial use often does not have a significant 

 bearing upon their corrosion behavior. For example, studies con- 

 ducted by these Laboratories have shown that high purity lead, lead 

 hardened with 1 per cent antimony, 3 per cent tin or 0.03 per cent 

 calcium, when used as cable sheathing show approximately the same 

 resistance to corrosion. While one of these materials may be some- 

 what more corrodible than others in a given natural environment, 

 the reverse will be true for another set of conditions. The environment 

 is of far greater importance than the composition of the sheathing and 

 consequently the control or avoidance of corrosion is attained by 

 maintaining the cable plant in non-corrosive surroundings. 



Of the types of corrosion cells listed above, those due to concentration 

 differences in the surrounding medium are among the most prevalent. 

 Differential aeration is one of the most common causes of corrosion. 

 Lead is corroded beneath the point of contact with a large grain of 

 sand when in an atmosphere containing oxygen and water vapor.^^ 

 The point of contact is less accessible to oxygen than surrounding 

 parts. The potential of this cell is somewhat less than 0.1 volt, the 

 value determined in these Laboratories for the difference between the 



