Effect of pH on the Passive Behavior of 6061 Al Alloy in Borate Buffer Solutions

The passive behavior of 6061 Al alloy in borate buffer solutions of various pH values ranging from 8.2 to 5.0 have been investigated by using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Potentiodynamic polarization curves revealed that 6061 Al alloy show excellent passive behavior in borate buffer solutions of various pH values ranging from 8.2 to 5.0. EIS results showed that the changes in the pH lead to two distinct behaviors in the EIS plots. The pH ranging from 8.2 to 6.4 result in an inductive loop when for smaller pH values (ranging from 6.2 to 5.0) there is not any loop exhibited. The absence of inductive loops and presence of the imperfect semicircle with large diameter when pH is 6.0 and 5.5, are related to formation of stable passive film. Moreover, it can be concluded that the passive film formed on 6061 Al alloy shows its best protective behavior when pH interval is 6.0 to 5.5, and if the pH is higher or lower than this interval the protective properties of passive film will decrease