612 BELL SYSTEM TECHNICAL JOURNAL 



Relatively larger areas of copper deposited upon the cable by re- 

 placement from copper salts have been observed to promote corrosion 

 of the sheathing. Scraps of copper wire corroded in manholes by 

 saline waters are believed to have been the source of the copper com- 

 pounds in such cases. 



Wiping solder in contact with sheathing at the splicing sleeve pro- 

 vides still another example of a corrosion cell originating from the 

 contact of diverse metals. Laboratory measurements of the potential 

 of this couple in dilute chloride, alkali and acid solutions show solder is 

 usually the anodic or corroding electrode. The observed potential 

 differences in hundredth molar solutions at room temperature were as 

 follows: 



Potential Difference 

 Solution in Millivolts 



Potassium chloride 6±3 



Caustic soda 11±3 



Acetic acid 20 ± 8 



Similarly, the 3 per cent tin-lead sheathing in contact with 1 per cent 

 antimony sheath would give rise to a galvanic couple in which the 

 former would be anodic but by smaller values of potential than given 

 above for the solder-sheath couple. Ordinarily in the soil water 

 environments which prevail in the underground plant the potentials of 

 neither of these couples is sufficient to maintain current flow and there 

 is no evidence of attack. The few cases of corrosion of this type which 

 have been observed, and which have been characterized by pitting of 

 the solder and even of the sleeving (where this was 3 per cent tin) are 

 believed to have arisen in electrolytes somewhat alkaline in nature 

 which contained abnormally low concentrations of film-forming con- 

 stituents such as silicates, sulfates or organic colloids. 



In general, the influence of metallic composition upon corrodibility 

 may be readily detected by measuring the rate of sulfation of the 

 metallic material in sulfuric acid.^* This test provides a method of 

 measuring surface activity and affords a means of comparing the 

 relative rates at which similar alloys tend to corrode in corrosive 

 environments or tend to become passive in the presence of film-forming 

 constituents. The sulfation-times measured by means of a recording 

 potentiometer have been determined on specimens of leads of various 

 compositions and for several cable sheath alloys in 7-normal sulfuric 

 acid. The averages of four determinations made on each material 

 bore the following relationship to each other, assuming the sulfation 

 time of spectroscopically pure lead to be one hundred: 



