| Abstract: | A literature survey has lead to the conclusion that a theory which postulates an increased anodic reactivity on a local site in the passive film is very probable. Experiments have been set up to confirm these suggestions. By means of the electron-microanalyser, it is shown that CI?ions are preferentially adsorbed at singular points at the metal surface before the stage that pits can be observed. It has also been demonstrated that pH changes occur at local areas during the induction period. These two observations indicate that corrosion already occurs during the induction period. Induction time measurements have shown that the induction time is not very reproducible. The quantity of transferred charge per initiated pit before the breakdown of the film is redly a better re- producible figure. From this, the quantity of Cl?ions necessary to create an active site is calculated. Experiments with the static potential band method reveal that pits can initiate at any potential higher than the pitting potential. Growing pits can repassivate at any potential. A model for the initiation is given. The pitting corrosion process starts with adsorption of chloride ion at singular points, mainly local stress points. The local anodic current density will be higher and in this way favourable conditions (low pH, high Cl?concentration) are created for the formation of a local site in the metal surface free of a passivating film The creation of those conditions is an autocatalytic process. The time required to form those favourable electrochemical conditions corresponds with the observed induction period. The migration of activating ions and the occurring pH change at a singular point must exceed a critical rate, otherwise passive film stabilizing effects will dominate. This model for the pitting corrosion supports the acid theory and links this theory with the peptization theory. |
