Mott‐Schottky Plot of the Passive Film Formed on Iron in Neutral Borate and Phosphate Solutions

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Abstract
The semiconductive properties of the passive film formed on iron were investigated by measuring the impedance of passivated iron electrodes in neutral borate and phosphate solutions. Mott‐Schottky type of plot was applied to the impedance data to obtain the donor density and the flatband potential as a function of film formation potential, oxidation time, and ion species present in the solution. The value of obtained for the passive film is in the range of and decreases with increasing formation potential and oxidation time, indicating that the structural and electronic defects in the passive film decrease with increasing film thickness. The value of of the passive film formed in phosphate solution is greater than that in borate solution. From the fact that when the electrolyte was changed from borate to phosphate solution increased from the value in borate solution to the value in phosphate solution in a time period of about 103s, it is evident that phosphate ions penetrate into the passive film producing a large number of defects.

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