The Mixed Potential Model for the Passive State and Hydrogen Evolution Reaction of AA2098-T851

Abstract

Theoretical and experimental investigations for fundamental understanding of the passivation process and its kinetics could lead to the prediction of the electrochemical behavior of the third-generation Al-Li alloy 2098. In this paper, the kinetics of the passive film formation on AA2098-T851 in 0.1M NaHCO3 in a CO2 atmosphere at 25°C, at potentials with both metal dissolution and hydrogen evolution reaction (HER) was investigated. This was done by performing electrochemical impedance spectroscopy (EIS) on the passive layer after each potentiostic steps in the anodic direction that followed by stepping in the opposite direction. The optimization of the EIS data was done by applying the previously developed Mixed Potential Model (MPM) that considers the Point Defect Model (PDM) for dissolution and passivation of the metal along with the Butler-Volmer equation for the cathodic reaction. It was shown that the inequality of certain kinetic processes of passive layer formation leads to the irreversibility of the metallic passivation in opposite potential scanning directions. Also, considering that the hydrogen evolution reaction incorporates quantum mechanical tunneling of the charge carriers through the barrier oxide layer on the passive metal surface, a cathodic Tafel constant (βc ) was calculated. The estimated βc was in accord with the predicted value for the HER according to the slow discharge of protons mechanism.