The Role of Al Interstitials on the Passivation of AA2098-T851 Based on the Mixed Potential Model

Abstract In this paper, the electrochemical behavior of the third-generation Al-Li alloy 2098 was investigated with a fundamental approach, where experimental results were theoretically analyzed in terms of the Point Defect Model (PDM). For this purpose, the AA2098-T851 was tested in NaHCO3 solution under a CO2 atmosphere to investigate the kinetics of formation and breakdown of the protective passive film. Experiments were performed at potentials where both metal dissolution and the hydrogen evolution reaction (HER) occur. Electrochemical impedance spectroscopy (EIS) was conducted at potentials obtained by stepping the potential in the anodic direction with the impedance being measured after holding the potential constant after each step for a sufficient time for steady-state to be achieved. The data were interpreted in terms of the Mixed Potential Model (MPM) that describes the passive dissolution of the metal as described by the PDM and uses the Butler-Volmer (B-V) equations to describe the kinetics of the cathodic reaction of hydrogen evolution. Optimization of the MPM on the EIS data showed that the principal point defects in the barrier layer are Al interstitials. Therefore, the diffusivity of these point defects plays a major role in the formation of the passive layer, particularly the outer layer. The barrier layer is formed exclusively via the generation of oxygen vacancies at the metal/barrier layer interface. On the other hand, the passive current density is dominated by the migration of metal interstitials and the hydrolysis of the metal interstitials ejected from the barrier layer at the barrier layer/solution interface. This leads to the formation of the non-defective outer layer. Furthermore, the cathodic Tafel constant (Pc) was obtained by considering quantum mechanical tunneling of the charge carriers produced by HER across the barrier layer on the metal surface, which revealed that the cathodic partial reaction was consistent with the slow discharge of water for the HER.