Properties of the Passive Film on Alloy 22 in Chloride Solutions Obtained by Electrochemical Impedance

Impedance measurements were performed for alloy 22 in the passive and transpassive range, in 1 M NaCl at 90°C. A R_Ω-(R//CPE) circuit model was applied in the full passive range, where R was the film resistance. This model also applied for a wide range of chloride concentrations and pH values, at the open circuit potential. Two time constants were observed at the beginning and at the end of the passive range. In these cases, the resistances for the ion transfer might be located at the film interfaces, and not in the film itself. The protective properties of the film improved with polarization time due to the thickening and ageing of the film. The film resistance and the space charge layer thickness increased with the potential. The oxidation of Cr₃₊ to Cr₆₊ occurred in the film at high potentials, followed by the transpassive dissolution. In the pre-transpassive range of potentials, the film showed a p-type electronic character, while the ionic properties were that of a passive film. The passive film of alloy 22 was an n-type semiconductor, which changed to a p-type for the high passive potentials. N_D = 2.7 × 10₂₀ cm₋₃ and E_FB = −0.551 V_SCE were determined.