THE CORROSION OF METALS 



31 



similar way Cell 3 shows cathode polarization only and Cell 4 both 

 anode and cathode polarization. Since the rate of corrosion is pro- 

 portional to the flow of current per unit area it is obviously limited in 

 the last three cases by the values of current density at which the 

 polarization curves intersect. In the presence of adequate oxygen or 

 in cases where hydrogen is readily discharged corrosion cells are likely 

 to resemble Cell 1. Where this is not the case, as in the absence of 

 oxygen or where the cathodes have high values for hydrogen over- 

 voltage, the result will be as shown for the cathodically polarized Cell 3. 

 The presence of an inhibitor such as a positively charged colloid or 



CURRENT DENSITY CURRENT DENSITY 



Fig. 3 — Types of polarization in corrosion cells. 



the amalgamation of the metal surface with mercury are other con- 

 ditions which promote cathode polarization. On the other hand, the 

 action of passivating agents such as chromates, silicates and in some 

 cases sulfates, carbonates, etc., is to produce anodic polarization as in 

 Cell 2. It will be observed that whereas in the presence of inhibitors 

 of the type mentioned above which polarize the cathode the resulting 

 potential of the corrosion cells and therefore of the metal specimen as a 

 whole should move in the anodic direction as the process of inhibition 

 takes place, in the case of passivating agents (which influence anode 

 processes) the effect of increasing passivation is a trend of potential 

 in the cathodic or noble direction. In both cases corrosion is retarded 

 or prevented entirely. 



