Occurrence of Salt Films during Initiation and Growth of Corrosion Pits

The purpose of this investigation is to employ simple t ransport laws to gain a perspective of events which occur dur ing early stages of pit init iation and growth. The init iation of pits on the surface of passive metals undoubtedly occurs at flaws in the passive surface film. At the moment of pit initiation, the ~laws would be expected to be very small, on the order of the passive film thickneses. Calculations indicate that the current density to such small regions would tend.to be exceedingly high and that a salt film is therefore l ikely to fo rm at very ear ly stages of init iation of pit growth. The presence of a high resistance layer at the sur-face of corrosion pits growing on passive metals has been recognized for some time. For example, Franck (1) summarized his research on iron in 1961 and directed attention to electropolishing layers which were thought to be responsible for producing pol-