CORROSION OF METALS— II 623 



than in the surrounding en\iroiiment is trustworthy evidence of anodic 

 corrosion. 



The corrosion efficiency of stray current anodic corrosion, i.e., the 

 per cent of the current involved in dissolving lead, will often be 

 appreciably less than 100 per cent. The complementary anodic 

 reaction occurring at voltages greater than approximately 2 volts is the 

 evolution of oxygen. For example, in an extract of a black alkali soil 

 containing high concentrations of sulfates, tests showed that less than 1 

 per cent of the current was consumed in the dissolution of lead. Under 

 the conditions generally prevailing, however, it is likely that the 

 corrosion efficiency is reasonably high and that the amount of corrosion 

 will be nearly proportional to the amount of current which flows from 

 the sheath to ground. 



Cathodic or negative corrosion of cable sheathing, which occurs when 

 current fiows from earth to the sheath, has been described already 

 under the discussion of alkaline corrosion. A not uncommon indica- 

 tion of negative conditions is an encrustation of calcium carbonate on 

 the cable. In this case the sheathing is generally not corroded. It 

 seems likely that the alkali produced by electrolysis of lime salts is 

 carbonated as formed and before reaching sufficiently high concentra- 

 tion to initiate corrosion and that calcium carbonate so formed 

 crystallizes on the surface of the sheathing. 



Instances have been observed in which cable sheathing appeared to 

 have corroded from the inside surface. ^^ It is believed that the action 

 in these cases was preceded by the occurrence of cracks or fissures in 

 the sheath which admitted moisture and provided electrolytic paths by 

 means of which current flowed from the sheathing to the copper 

 conductors within. 



Destruction of cable sheathing by stray electrical currents derived 

 from large-scale galvanic cells has been experienced. In this case, 

 which at first was rather mysterious, it was found that contact of iron 

 pipes with beds of buried cinders set up large iron-carbon couples with 

 potentials of approximately 0.7 volt. The soil at this location was 

 unusually low in resistance and the wood conduit in which the cables 

 were housed was water-logged with the result that the cables picked up 

 current in regions near the iron structures and lost it at other points 

 where the cable passed through the general neighborhood of the 

 cinder beds. The electrical condition of the cables, determined by 

 pulling through an adjacent duct a modified calomel reference elec- 

 trode,*^ showed that the potential of the cable with respect to earth 

 varied sharply from point to point and often reversed itself more than 

 once in a section between two manholes. Removal of corroded cables 



