CORROSION OF METALS— II 621 



The effectiveness of silicates in passivatintj lead lies in the extremely 

 low solubility of lead silicate. Consequently silicate ions are precipi- 

 tated as lead silicate in close contact with the sheath at the anodic 

 areas of the corrosion cells. As the more anodic regions become 

 polarized in this fashion other areas tend to function as anodes but with 

 the same result until the surface of the sheath becomes entirely covered 

 with an insoluble coating of lead silicate which is impervious to the 

 corrosive elements of the environment. 



Chromates and phosphates stand ne.xt to silicates in ability to 

 passivate lead, but do not occur in the electrolytes in contact with 

 underground cables. Sulfates, however, are a common constituent of 

 these environments and in laboratory studies have been shown to be as 

 effective as phosphates.'*^ The passivating effect of sulfates is directly 

 proportional to concentration, 2500 parts per million reducing the rate 

 of corrosion of distilled water about 50 per cent.^*^ Electrolytes from 

 the cable plant seldom contain as much as 10 per cent of this amount 

 of sulfate and so the specific contribution of sulfates alone is not large; 

 however, added to that of various other film-formers it is of importance. 



Carbonates exert a marked retarding influence on the corrosion of 

 lead. The water which comes in contact with underground cables 

 always contains carbonate ions derived either from soluble carbonates 

 from the soil or from carbon dioxide of the soil atmosphere. Numerous 

 analyses of the air in cable ducts has shown it to run from 0.1 per cent 

 to 10 per cent of carbon dioxide, usually averaging about 1.5 per cent or 

 0.015 atmospheres pressure. Pressures of carbon dioxide within this 

 range reduce the rate of corrosion of lead in distilled water about 50 

 per cent. It is claimed that high pressures of carbon dioxide, e.g., 6 

 atmospheres, increases the solvent action of water on lead.*^ Carbon- 

 ate equilibria calculations of the system lead carbonate-carbon 

 dioxide-water show that the film of corrosion products which forms on 

 lead in aerated distilled water is a hydrated oxide of lead when the 

 partial pressure of carbon dioxide is less than lO^''* atmospheres. 

 Above this value for carbon dioxide and up to a pressure of about 10 

 atmospheres, the film should consist of lead carbonate. Basic carbon- 

 ate, if a true solid phase, should also be found within this range. The 

 bicarbonate of lead would appear to be stable at still higher carbon 

 dioxide pressures. It is of interest that there is a minimum in the 

 calculated solubility curve for lead carbonate in the region of 10~® 

 atmospheres of carbon dioxide. Increasing solubility at pressures 

 greater than this is due to the increasing concentration of bicarbonate 

 ions. This means that the effectiveness of soil carbonates in passivating 

 cable sheathing is somewhat reduced by the higher carbon dioxide 



