Electrical Behavior of Passive Iron

From a study of the steady-state and of galvanostatic transients, it is con-cluded, in agreement with Vetter and Well, (i) that there exists a layer on iron when it is passive, (ii) that an electric field drives ions through this layer, and (iii) that the resistance to the flow of these ions is nonohmic. In the steady state, the thickness of this layer varies l inearly with passivating potential from about 8A to about 68A. Transition from the steady passive state to the active state can occur slowly (probably by uniform thinning) for anodic current constant and less than the steady-state value, or rapidly (probably without thinning) for cathodic currents. The results indicate that, when the system is in a nonsteady state, there is a nonzero charge distribution within the layer. Even if the steady-state current be restored, it may take a long time for this charge distribution to relax back to the neutral steady state. The nature of passivity, as exemplified by the elec-trical behavior of a passive iron electrode, has bee