34 BELL SYSTEM TECHNICAL JOURNAL 



their cathodic positions) produced by hydrogen peroxide and chromate 

 ions. A trend of potential in the anodic direction with time, while it 

 may suggest the breakdown of a passive film, does not necessarily 

 indicate the onset of a corrosive attack, for should there also be present 

 substances such as positively charged colloids or other products ^^ 

 which tend to raise and maintain hydrogen polarization at the cathodic 

 areas, the metal may suffer little or no attack. 



Whether the films which form in corrosion processes are protective 

 in character depends to a considerable extent upon their position or 

 location with reference to the surface of the metal and this in turn 



TIME (solid curves) 

 CURRENT DENSITY (DOTTED CURVES) 



Fig. 5 — Time-potential relationshi]) of iron in K2SO4 as affected by addition of K2Cr04. 



depends upon the solubility of the corrosion products in the medium 

 adjacent to the surface. When iron corrodes in the presence of 

 moisture, ferrous ions are produced at anodic areas and hydroxyl ions 

 at cathodic areas, the process continuing until the solubility limit of 

 ferrous hydroxide is attained, whereupon this compound begins to be 

 precipitated as a gelatinous film over the surface of the metal. In- 

 creasing the alkalinity of the environment naturally represses the 

 solubility of this compound, precipitating it with less solution of iron. 

 In the absence of oxygen the ferrous hydroxide film tends to inhibit 

 corrosion by maintaining hydrogen polarization. When, on the other 

 hand, oxygen is accessible to the system, ferrous ions are oxidized 

 with the result that ferric hydroxide being less soluble than ferrous 



