Ion Transfer through a Protective Film

The first assumption of this treatment is metal ion dissolution through the protective film, as shown in Fig. 34, where the potential difference between both sides of the film is defined as y, t). The potential 

According to the discussion at the end of Section III.5, at the steady state, the total overpotential of anodic dissolution is determined by the ohmic drop, so that the A mo/s(jc, y, t) is assumed to be approximately zero at steady state  

Therefore, from Eq. (75a), we can write the following equations concerning the fluctuations and average values  

After arriving at the film surface, the metal ion Mz + forms an adsorbed complex (MJQ with the aggressive anion X. Then the complex quickly dissociates into the metal ion and aggressive ions in the solution. This is the second and the most important assumption. The reaction mechanism is described as 

From the first reaction, assuming that the anion adsorption process is rate determining, and neglecting the potential difference as shown in Eq. (75a), the average dissolution current density is written as a function of the surface concentration of the aggressive ions, (CJx, y, 0, /)), i.e., 

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