In this work, the passivity of AZ31B alloy in NaOH solutions was studied by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and MotteSchottky analysis. Potentiodynamic polarization results indicated that decreasing NaOH concentration leads to decrease the corrosion rate of this alloy. EIS results showed that the reciprocal capacitance (1/C ) of the passive film is directly proportional to its thickness which increases with decreasing NaOH concentration. Therefore, it is clear that dilute NaOH solutions offer better conditions for forming the passive films with higher protection behaviour, due to the growth of a much thicker and less defective films. The MotteSchottky analysis revealed that the passive films displayed n-type semiconductive characteristics, where the oxygen vacancies and interstitials (over the cation vacancies) preponderated. Also, MotteSchottky results showed that the donor densities evaluated from MotteSchottky plots are in the range of 1020 cm3 and decreased with decreasing NaOH concentration.
