Kinetic Model of Iron Corrosion and Passivity

A minimal model of iron corrosion and passivity that predicts periodic current-time behavior is presented. Existing models are linearly stable and lead to steady-state current-voltage profiles that require xternal fluctuations to induce passivity. A model in which the activation energy for dissolution of a surface oxide increases with increasing coverage gives a collapse in the current-voltage curve at passivation and predicts limit cycle behavior in the current with time. In attempts to devise a plausible mechanism for the anodic oxidation of iron, one approach has been to simulate current vs. voltage behavior given a suitable kinetic model. In this let-ter we rely on the work of the late Professor Epelboin and his group whose use of ac impedance techniques has given excel-lent numerical data (1,2). In recent years increasing interest has been shown in chemical oscillations, and several workers have demonstrated their existence in the anodic dissolution of iron (3-5). However, no detailed mechanism has been pub-lished that shows both passivation and the possibility of peri-odic current-time behavior. Indeed it is generally held that os-cillations only come about through negative resistance or coupling between diffusion and chemical reaction (6). Here we attempt o develop a model from which periodic behavior may spontaneously arise. Furthermore we suggest that limit cycle or chaotic behavior can be used as a kinetic test of mechan-ism. Mechanisms that successfully predict steady-state behav-ior should also give rise to periodic or chaotic kinetics if these are seen experimentally. Epelboin has shown that a mechanism involving two surface oxides can lead to a current that is three valued as the voltage becomes more positive if diffusion of OH- is taken into ac-count Fe + OH--- ~ [Fe(OH)]ads + e [Fe(OH)]ads ~ Fe(OH) § +