620 BELL SYSTEM TECHNICAL JOURNAL 



ments revealed the presence of corrosive concentrations of acetic acid. 

 In the investigation made of this trouble it was concluded that the 

 high native acidity of Douglas fir, together with the drastic treatment 

 required to impregnate it with creosote, offered a reasonable expla- 

 nation for the corrosiveness of the conduit.^^ jhe corrosive action was 

 effectively stopped by neutralizing the acid with ammonia gas supplied 

 to the affected conduit in a 2 per cent mixture with air. 



The corrosiveness of air laden with acetic acid vapors lies in the 

 persistence of effective non-polarized corrosion cells of constant voltage. 

 The acid furnishes an abundant and reasonably constant source of 

 replaceable hydrogen ions and the continued precipitation of lead as 

 carbonate by the action of carbon dioxide maintains a low concen- 

 tration of lead ions. Oxygen acts as a cathodic depolarizer. Since the 

 precipitation of lead carbonate or basic carbonate occurs at an appreci- 

 able, although very small, distance from the seat of activity on the 

 metal surface, it offers little or no hindrance to the corrosion action. 



Phenols and other acidic constituents of coal tar pitches have been 

 reported to be corrosive to cable sheathing when in direct contact in 

 the form of protective coatings.^' There is no evidence either from 

 experience with creosoted conduit or from laboratory tests that 

 phenolic vapors from creosote are appreciably corrosive to sheath. 



So much for the corrosive media of the environment of the under- 

 ground cable plant. Of the protective agents, none is more important 

 than soluble silicates. It is well known that lead is markedly corroded 

 in distilled water, and by waters low in hardness and in total solids. 

 Saturation of distilled water with calcium silicate (soluble to the extent 

 of about 100 parts per million), or with silicic acid derived from a 

 suspension of silica flour, will prevent corrosion of lead. The cor- 

 rosiveness of certain natural waters has been greatly reduced by the 

 addition of only 10 parts of sodium silicate (expressed as silicic acid) per 

 million.'^ Analysis of a large number of samples of waters from cable 

 manholes and subways has shown silicate contents of from 2 to 25 parts 

 per million. In concrete conduit values up to 143 parts per million 

 have been found. It is of interest in this connection to note that 

 although silicates appear to protect lead to some extent in all ground 

 waters, their effectiveness is greatest in the range of alkalinity corre- 

 sponding to values of pH between 9 and 11, where pH equals the 

 logarithm of the reciprocal of hydrogen-ion concentration. The 

 resistance of underground cables to corrosion appears to depend chiefly 

 upon the film-forming action of silicates. The minimum concentra- 

 tions required to give protection depend upon the nature and concen- 

 trations of the corroding agents which are also present. 



