THE CORROSION OF METALS 



35 



0.3 



0.2 



0.1 







-0.1 



PH 



Fig. 6- 



-Effect of acidity and of hydrogen peroxide on the equilibrium potential of iron 

 in buffer solutions of varying acidity. 



1. Potential of iron in tenth normal solutions of sodium acetate-acetic acid solutions. 



2. Potential of iron in tenth normal solutions of sodium acetate-acetic acid solutions 



-|- 0.6 per cent hydrogen peroxide. 



hydroxide is precipitated and forms a more or less porous rust film at 

 an appreciable distance from the metal surface. Owing to the mildly 

 amphoteric nature of iron there may exist, especially under alkaline 

 conditions or higher temperatures a considerable concentration of 

 ferrite (Fe02') ions which, upon reacting with ferrous ions, may 

 precipitate ferrous ferrite (Fe304) or black magnetic oxide of iron 

 which, also being precipitated in a somewhat granular form at some 

 distance from the surface of the metal, is non-protective. In contrast 

 to these examples is the highly protective film of silicate which pre- 

 sumably forms upon lead and lead rich alloys when immersed in water 

 or soil solutions containing as little as ten parts per million of silicate. 

 As is well known, distilled water is corrosive to these metallic materials. 

 Were it not for this fortunate effect of silicates upon lead it is doubtful 

 that it or its alloys could be used for cable sheathing in the present 

 type of underground construction which permits exposure to soil and 

 surface waters at times. 



