132 TRANSMISSIVITY IN PASSIVE IRON WIRES 



sharply defined. According to this hypothesis, the p.d. across the 

 active-passive boundary decreases progressively as the activation 

 wave passes along a region of decrement. When the rate of this 

 decrease is gradual, there will be transmission through a relatively 

 great distance before the wave comes to rest; when it is rapid, trans- 

 mission will be correspondingly limited. 



This hypothesis is also consistent with the fact that the return of 

 transmissivity is more rapid in weak than in strong acid, its rate being 

 approximately inversely proportional to the excess of concentration 

 above a certain critical level (about 53 to 54 volumes per cent).' 

 Two chief factors may be distinguished in the chemical part of the 

 passivating process, i.e. in the reaction which reforms the passivating 

 surface layer of oxide: (1) the general oxidative action of the solution, 

 which is the more intense the higher the concentration of HNO3; 

 and (2) the electrochemical oxidative action at the anodal region of 

 the local active-passive circuit. This anodal area, i.e. the active 

 region of the metal, is automatically subjected to the oxidizing — and 

 hence passivating^ — influence resident at every anode. The difference 

 in the rate of repassivation is an index of the energy of the local 

 oxidizing process; in acid of 50 volumes per cent (about 7.5 n) or lower 

 this is not sufficient to rebuild a stable surface film, hence the metal 

 continues to dissolve in this solution; in 55 per cent acid the oxidative 

 action is intense enough to deposit a permanent film in 5 or 6 seconds 

 (at 20°) ; in 60 per cent acid the rate of deposit {i.e. of repassivation) 

 is several times more rapid ; and at higher concentrations still more 

 rapid. The low degree of transmissivity found immediately after 

 repassivation suggests that the passivating film when first deposited 

 is relatively thick (or relatively dense or otherwise resistant to alter- 

 ation) in comparison with what it becomes later, when the state of 

 transmissivity without decrement is reached. This view would imply 

 that the quantity of passivating material deposited during the reaction 



C — 54 

 ' The formula — = constant expresses fairly well the relation of the total 



recovery time to the concentration of the acid, where C is the concentration and 

 t the recovery time. 53 to 54 per cent is the critical concentration; and the time 

 required for the return of complete transmissivity is about proportional to the 

 excess of concentration above this value. 



