THE CORROSION OF METALS 33 



continue to function as the anodes of oxygen concentration cells. 

 If, on the other hand, the metallic impurity is cathodic and present as 

 a separate phase, corrosion will be rather more uniform in character 

 and its rate will be controlled in the absence of oxygen by the ability 

 of the impurity to discharge hydrogen. Unless its overvoltage is 

 low, that is, unless it discharges hydrogen readily, the rate of corrosion 

 will be slow, the corrosion cells being polarized cathodically. The 

 presence of oxygen or oxidizing agents under these conditions will 

 depolarize these cathodic areas and accelerate corrosion. 



From the foregoing it is apparent that a knowledge of the anodic 

 and cathodic current density potential relationships which are estab- 

 lished on the surface of a metal in a given environment would make 

 possible an understanding of the processes which are taking place 

 and lead to a prediction of corrosion behavior. It is generally im- 

 possible to measure these quantities as they relate to individual 

 corrosion cells owing to a lack of knowledge of the electrode areas 

 involved. Probably these comprise a wide range of sizes and change 

 in size with the progress of corrosion. Sometimes the nature of the 

 cathodes is also uncertain. Practically, however, it is a simple matter 

 to determine a composite of the resultant potentials and their change 

 with time. These time-potential measurements indicate whether the 

 process is anodically or cathodically controlled and in some cases 

 furnish information as to the rate at which it is proceeding, experi- 

 mental facts which are of value in predicting corrodibility. A record- 

 ing potentiometer is of considerable assistance in this connection. 



Figure 5 illustrates schematically the correlation between these 

 time-potential relationships and the anodic and cathodic polarizations 

 which determine their positions. It will be seen that the potential 

 of iron in a solution of potassium sulfate (represented by the lower 

 solid curve) is mainly determined by the anodic potential of iron in the 

 solution, the cathodic areas being polarized. When potassium chro- 

 mate is added to the solution the resultant potential of iron is shifted 

 markedly in the cathodic direction, the position being determined by 

 anodic polarization. The actual values of the potential of iron in these 

 cases are of the same order as that of iron alloyed and rendered 

 passive by the addition of chromium and nickel .^^ 



In a solution containing hydrogen peroxide, iron is passive even in 

 the acid range as shown in Fig. 6. The abrupt cathodic shift in the 

 potential of iron in the region of pH 6.5-6.8 also shown in Fig. 6 is 

 due evidently to a film which affects both anodic and cathodic areas 

 and which judged from this position would be expected to be less 

 protective than the more pronouncedly passive films (indicated by 



