Kinetics of Charge and Discharge of the Film on Superpassive Iron

Abstract

In well-inhibited solutions, passivated iron acquires a small charge when anodically polarized some-what above the potential of incipient activation. The rates of charge and discharge have been determined potentiostatically, and have been found to follow a kinetic equation derived on the assumption of an activation-controlled process with an activation energy which varies with the extent of charge or discharge. The results are interpreted by application of the concept that the characteristics of the passive state derive from a small concentration of positive holes injected into the film. When the potential is suddenly lowered, the high mobility of electrons permits them to set up a space charge by which the activation energy for the slower influx of protons is initially diminished.