618 BELL SYSTEM TECHNICAL JOURNAL 



that the rate of corrosion increases with increasing salt concentration 

 up to a maximum at 1 per cent and that at concentrations of 3 per cent, 

 which corresponds roughly to that of sea water, the rate is markedly 

 less.^* The favorable experience with cables submerged in sea water 

 or mixtures of sea water and soil waters indicates that corrosion 

 inhibitive agents exert under these exposures a predominating in- 

 fluence. The effect of the chloride content of the duct electrolyte must 

 be mainly one of increasing the conductivity, there being insufficient 

 concentrations in relation to the concentration of film-forming sub- 

 stances to produce even pitting or local attack. 



A type of cable sheath corrosion of considerable importance is that 

 which is fostered by alkalies. It is characterized usually by the 

 formation in the region of attack of deep red crystals of lead monoxide 

 or litharge. Occasionally the yellow form of litharge or a greenish 

 hydrated lead monoxide may appear, but in one case where the strength 

 of caustic was so great as to cause discomfort upon handling the cable, 

 no colored compounds developed. The red monoxide crystallizes out 

 of saturated solutions of alkali plumbites w^hich are formed by the 

 solution of lead in alkalies.'^ It can be produced in the laboratory by 

 immersing specimens of lead in saturated lime water and aerating the 

 solution for several days with carbon dioxide-free air. The appearance 

 of this red oxide on cable sheathing is a certain indicator of alkali 

 attack. If detected before failure of the cable, the action can be 

 stopped usually by removing the source of the alkali and thoroughly 

 flushing the cable conduit with water. 



A source of alkali affecting underground cables is concrete conduit 

 and occasionally other concrete structures. Free lime in the surface 

 layers of fresh concrete is usually converted by the action of carbon 

 dioxide into calcium carbonate within a few weeks and this is less 

 alkaline in nature. Seepage of moisture through concrete which may 

 occur in less dense grades of this product may leach free lime from 

 within. At the Panama Canal water seeping through the concrete 

 floors and walls of lock chambers caused serious corrosion of the 

 sheathing of power cables in a short time. Analysis of the seepage 

 water disclosed high alkalinity. In the same locality a telephone 

 cable in vitrified clay conduit was corroded by the seepage of water 

 through cement sacks used for wrapping the conduit joints.'^ Greater 

 attention is being given recently to the production of concrete conduit 

 of greater impermeability, lower alkalinity and to "curing" methods. 

 The use of high alumina cement, which is much less corrosive to cable 

 sheathing,''-* has been proposed. 



