Surface Instabilities during Pit Initiation

The passive film contains or is susceptible to formation of a distribution of flaws that are potential sites for pitting, depending on the environment and the potential. The dynamic character of this surface under conditions conducive to pitting is illustrated in Fig. 7.8 (Ref 24) for a stainless steel in 0.4 M FeCl3. In Chapter 4, it is shown that by scanning near the surface of a corroding metal with a reference electrode, the positions of local anodic and cathodic sites can be determined. In Fig. 7.8, 

Instability also is observed in the measurement of Ecorr as a function of time in pitting environments, as shown in Fig. 7.11 for type 304 stainless steel in 0.4 M FeCl3 (Ref 26). The surface is initially passivated, and Ecorr remains essentially constant until rapid oscillations in poten- 

Corrosion potential versus time during exposure of type 304 stainless steel at 25 °C to 0.4 M FeCI3. Source Ref 26 

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