CORROSION OF METALS— II 611 



In addition to chemical influences, the electrical condition of the 

 cable with respect to earth or to other adjacent metallic structures has 

 an important bearing on corrosion. Where it can be done without 

 jeopardy to other structures, it is desirable to maintain the cable net- 

 work very slightly (of the order of 0.2 volt) negative or cathodic to 

 earth. There is evidence that under this condition the sheathing is 

 less readily corroded by couple action of a miscellaneous character. 

 At appreciably higher negative potentials alkali or lime salts may be 

 electrolyzed producing thereby caustic alkali or free lime which are 

 corrosive to sheathing. On the other hand, an electrically positive 

 condition of the cable may be conducive to the ordinary "stray-cur- 

 rent" or anodic corrosion. 



The Origin and Nature of Corrosion Cells on Cable Sheathing 

 The mechanism by which cable sheathing corrodes in conduit in- 

 volves the replacement by the metal of hydrogen or another metal in 

 compounds present in the surrounding environment — a process which 

 has been described in some detail in a previous paper.^^ Most com- 

 monly, the dissolving lead replaces hydrogen from water. The areas 

 or points on the sheathing at which lead dissolves are the anodes of 

 small corrosion cells, the cathodes of which are the regions at which 

 hydrogen is deposited. The driving force of these cells arises either 

 from some chemical or physical inhomogeneity of the metal, or some 

 inhomogeneity of the environment. Their electrolytic operation is 

 influenced by the conductance and chemical nature of the environment, 

 and by the size and distribution of the anodic and cathodic areas. 



Corrosion cells owing their origin to sheath composition are exem- 

 plified by the presence of two metallic phases, one of which is lead and 

 the other either an impurity, such as copper, bismuth, nickel, zinc, 

 etc., or a hardening agent such as tin, calcium, cadmium, or antimony. 

 Copper and antimony, for example, are cathodic to lead under the 

 prevailing conditions and facilitate the discharge of hydrogen. The 

 small proportion of cathodic area on the metal surface in both cases, 

 however, will result in high cathodic current densities inducing 

 polarization, and a low rate of corrosion except perhaps in acid solu- 

 tions where the potential of the lead-hydrogen cell will be increased. 

 This acceleration in acid solution is borne out by laboratory corrosion 

 tests which show that the rate of corrosion of lead containing 3 per cent 

 tin is about 50 per cent greater and lead containing 1 per cent antimony 

 is about 10 per cent greater than that of soft lead in dilute (0.001 

 molar) acetic acid. Antimonial lead is said to corrode more rapidly 

 than pure lead in humic acids.^^ 



