CORROSION OF METALS—II 617 



A number of cases of cable sheath corrosion ha\'e been attributed to 

 the presence of nitrates in the duct electrolyte. The nitrate content 

 of most soil waters is very low — a few parts per million ordinarily — but 

 occasionally contamination from industrial plants or from sewers, 

 the organic matter from which may undergo nitrification, has led to a 

 several fold increase. Concentrations of nitrate from 20 to 425 parts 

 per million have been found in the electrolyte from locations where 

 failure of the sheathing occurred. It has been shown that solutions 

 containing a thousand parts of nitrates per million are markedly cor- 

 rosive to lead.^° Another investigator reports that the addition to 

 natural soft waters of nitrates in excess of 50 parts per million increases 

 their corrosiveness about 20 per cent, higher quantities of nitrate being 

 required to produce this effect in hard waters.'^ Often the corroded 

 region of the sheath is black in appearance owing to the presence of 

 loosely adherent, finely divided lead (possibly oxidized) and antimony 

 which accumulates by some sort of undermining action during the 

 rapid attack. The formation of this black coating in the presence of 

 nitrates is reported by others. ^^ 



The mechanism of nitrate action at the cathodes of corrosion cells 

 is similar to that of oxygen and of lower concentrations of oxidants in 

 general, and consists in depolarization. In addition, the high solubility 

 of lead nitrate prevents an appreciable polarization of anodic areas. 

 It is of interest to note that in the presence of nitrates, in the form of 

 nitric acid, oxygen furnishes but little additional acceleration of 

 corrosion. For example, in 30 per cent nitric acid, the ratio of the 

 rate of corrosion in the presence of oxygen to the rate in the absence of 

 oxygen has been shown to be 1.1, while in 20 per cent hydrochloric acid 

 and glacial acetic acid, this ratio is 10.0 and 10.9, respectively.^' 



Cable sheathing is little affected by the chloride content of most 

 ground waters. In tests, for example, in which chlorides were added 

 to natural waters, there was no increase in corrosion when the chloride 

 content was of less than 1000 parts per million, a value seldom attained 

 in soil waters.'^ Even infiltration of sea water does not constitute a 

 corrosion hazard unless the cable is markedly negative to earth. In- 

 deed, in sea water the corrosion of lead may be retarded by an encrusta- 

 tion of lead chloride which forms on the surface of the metal, as well 

 as by the lower prevailing oxygen content. Extruded bars of lead and 

 lead containing 1.6 per cent antimony, 60 cm. in length and 2.87 cm. 

 in diameter exposed for four years to tidal action have shown losses in 

 weight of 0.65 per cent and 0.51 per cent, respectively,'^ but doubtless 

 mechanical erosion was an important factor in this rather drastic test. 

 Laboratory tests made on lead foil in a sodium chloride solution show 



