CORROSION OF METALS— II 613 



Spectroscopically pure lead (99.999% lead) 100 



Corroding lead (A.S.T.M.— Grade I, 99.94% lead) 80 



Chemical lead (A.S.T.M.— Grade II, 99.90% lead, contains 0.06% 



copper) 65 



Common lead (A.S.T.M.— Grade III, 99.85% lead, contains 0.13% 



bismuth) 70 



Corroding lead alloyed with 1.5% tin, and 0.25% cadmium 85 



Chemical lead alloyed with 1.5% tin and 0.25% cadmium 75 



Corroding lead alloyed with 3% tin 70 



Chemical lead alloyed with 0.04% calcium 80 



Corroding lead alloyed with 0.04% calcium 75 



Common lead alloyed with 0.04% calcium 55 



Corroding lead alloyed with 0.5% antimony and 0.25% cadmium .... 25 



Chemical lead alloyed with 0.5% antimony and 0.25% cadmium 25 



Chemical lead alloyed with 1.0% antimony 20 



From an inspection of these results it appears that the surface re- 

 activity of lead is markedly increased by the presence of impurities or 

 by alloying with small amounts of other metals. Of the hardening 

 agents chosen for study, tin, tin and cadmium, and calcium exert the 

 smallest influence on rate of sulfation, while antimony, whether used 

 alone or with cadmium, has the most pronounced effect. The presence 

 of small amounts of copper appears to have an accelerating effect 

 upon reactivity as does also the presence of bismuth. This adverse 

 effect of bismuth has been noted in connection with the use of lead in 

 sulfuric acid plants.^^ Since the environment in which cables are 

 used contains both corrosive and film-forming substances this com- 

 parison of rates of sulfation of lead and its alloys is not necessarily a 

 direct indication of the relative rates of corrosion of these materials 

 when used as cable sheathing. 



It is well known that the intensity with which metals tend to ionize 

 is affected by their physical state, small crystals and strained structures 

 possessing higher intensities and therefore more electronegative or 

 anodic potentials than large crystals and annealed structures. In the 

 case, however, of lead and most lead-alloys suitable for cable sheathing, 

 self-annealing occurs at ordinary atmospheric temperatures, and for 

 this reason it is highly improbable that corrosion is ever initiated as a 

 result of physical condition of the metal. ^^ In the laboratory it was 

 found that lead intensively worked at liquid air temperatures, where 

 self-annealing does not occur, was from 2 to 3 millivolts electronegative 

 to annealed lead when measured immediately afterward at 25° C. in 

 0.2 normal lead-acetate solution. This potential difference was 

 reproducible but could not be maintained for more than ninety minutes 

 at room temperature. 



It is conceivable that scratching or mechanical injury of cable 

 sheathing, such as might occur during installations, could give rise to 

 the familiar metal-metal oxide corrosion cell. The operation of this 

 cell has been demonstrated in a laboratory experiment in which pieces 



