Studies on Pitting Corrosion of Al-Cu-Li Alloys Part III: Passivation Kinetics of AA2098-T851 Based on the Point Defect Model

Elmira Ghanbari; Alireza Saatchi; Xiaowei Lei; Digby D Macdonald

doi:10.3390/ma12121912

Studies on Pitting Corrosion of Al-Cu-Li Alloys Part III: Passivation Kinetics of AA2098-T851 Based on the Point Defect Model

Materials (Basel). 2019 Jun 13;12(12):1912. doi: 10.3390/ma12121912.

Authors

Elmira Ghanbari¹, Alireza Saatchi², Xiaowei Lei³, Digby D Macdonald⁴

Affiliations

¹ Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, CA 94720, USA. elmira.ghanbari@berkeley.edu.
² Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, CA 94720, USA. alireza.saatchi@berkeley.edu.
³ Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, CA 94720, USA. leixw1987@gmail.com.
⁴ Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, CA 94720, USA. macdonald@berkeley.edu.

Abstract

In this paper, the passivation kinetics of AA2098-T851 was investigated by a fundamental theoretical interpretation of experimental results based on the mixed potential model (MPM). The steady state passive layer formed on the AA2098-T851 in NaHCO₃ solution in a CO₂ atmosphere upon potentiostatic stepping in the anodic direction followed by stepping in the opposite direction was explored. Potentials were selected in a way that both anodic passive dissolution of the metal and hydrogen evolution reaction (HER) occur, thereby requiring the MPM for interpretation. Optimization of the MPM on the experimental electrochemical impedance spectroscopy (EIS) data measured after each potentiostatic step revealed the important role of the migration of Al interstitials in determining the kinetics of passive layer formation and dissolution. More importantly, it is shown that the inequalities of the kinetics of formation and dissolution of the passive layer as observed in opposite potential stepping directions lead to the irreversibility of the passivation process. Finally, by considering the Butler-Volmer (B-V) equation for the cathodic reaction (HER) in the MPM, and assuming the quantum mechanical tunneling of the charge carriers across the barrier layer of the passive film, it was shown that the HER was primarily controlled by the slow electrochemical discharge of protons at the barrier layer/solution (outer layer) interface.

Keywords: Al–Cu–Li alloys; mixed potential model; passivation kinetics; point defect model.

Grants and funding

FA7000-14-2-0017/U.S. Air Force Academy