On the Mechanism of Pitting of Aluminum

The Engell‐Stolica potentiostatic method has been used for the kinetic study of the mechanism of pitting initiation of Al 1199. A statistical treatment of the induction time has been developed in order to get meaningful data. Pitting initiation time has been found to be potential‐independent and the linear increase in the induction time for pitting with the increase of the potential applied in the passive range is due to the growth of the oxide film layer on the Al surface. The value of the critical potential obtained by extrapolation of the induction time vs. potential plot to a zero induction time correlates with the value given in the literature. The trend toward a decrease in the kinetic order of the pitting reaction with an increase in the pH value of the aggressive solution has been confirmed. The independency of the kinetic order on the electrode potential of the solution indicates that the pitting initiation is controlled by a chemical process with the formation of an intermediate complex and therefore is consistent with the complex ion theory of corrosion.