CORROSION OF METALS— II 615 



may be exposed to different hydrogen ion, metal ion or salt concentra- 

 tions and where the demarcation between concentration zones is 

 sufficiently pronounced may give rise to differential concentration 

 cells, the driving forces of which have theoretical values of 29.5 milli- 

 volts per ten-fold difference in ion concentration. An examination of 

 the Rhinelaijd cable which connects Berlin and Cologne has shown 

 that the most extensive corrosion occurred at points where there was 

 an abrupt change in the character of the soil or geological structure.^* 

 There is usually sufficient diffusion and circulation of underground 

 waters to equalize ionic concentrations and prevent the development 

 of cells of this type assuming serious proportions. 



Effect of Environment on Operation of Corrosion Cells 

 The nature of the more common electrolytic cells by means of which 

 cable sheathing corrodes has been discussed at some length. Con- 

 sideration will now be given to the manner in which various environ- 

 mental factors influence the operation of these cells. In general, 

 these factors may be classified either as corroding or film-forming 

 agents, although their influence will depend quite as much upon their 

 concentration as upon their specific nature. It is meaningless to 

 report that a metal corrodes or does not corrode in this or that electro- 

 lyte unless full experimental details are given. Only with a complete 

 knowledge of the condition of a metal surface and of the nature and 

 concentrations of the components of its environment can the resulting 

 behavior be predicted. For example, it has been shown that it is 

 often the ratio of the concentrations of film-forming to corroding sub- 

 stances which determines the character of attack.^^ For high values 

 of this ratio, the metal will be protected; for low values it will be uni- 

 formly corroded, but for intermediate values of this ratio, the surface 

 will be only partly protected with the result that corrosion will be 

 localized in the form of destructive pitting. With these limitations in 

 mind, some of the principal constituents of the environment which 

 affect the behavior of cable sheathing may be classified as follows: 



Corroding Protective 



Oxygen Silicates 



Nitrates Sulfates 



Chlorides Carbonates 



Alkalies Colloidal substances 



Organic acids Certain organic compounds 



Of the corroding elements, oxygen is the most important in its 

 effect upon the operation of corrosion cells. Owing to the high po- 

 tential required to discharge hydrogen on pure lead (i.e., its high 

 hydrogen over-voltage) these cells tend to cease functioning owing to 



