Electrochemical Behavior of Passive Films Formed on the Surface of Coarse-, Fine- and Ultra-fine-Grained AA1050 Based on a Modified PDM

Electrochemical impedance spectroscopy (EIS) and Mott–Schottky analysis were carried out to evaluate the electrochemical behavior of the passive films formed on the surface of coarse-grained (CG), fine-grained (FG) and ultra-fine-grained (UFG) 1050 Al alloy (AA1050) samples in alkaline media (pH value of 8.0) based on a modification of point defect model (PDM). The EIS results revealed that the polarization resistance increased from about 22.71–120.33 kΩ cm2 for UFG sample when compared to CG sample (annealed sample). The semiconductor properties of the passive films formed on CG, FG and UFG AA1050 samples in the test solution were investigated by employing Mott–Schottky analysis in conjunction with PDM. The results indicated that donor densities were in the range of 2.19 × 1021–0.61 × 1021 cm−3 and decreased with grain refinement. Finally, all electrochemical tests showed that the electrochemical behavior of AA1050 alloy was improved by decreasing the grain size, mainly due to the formation of thicker and less defective oxide films.