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

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.

Mixed potential model for passivity characters of hyper-duplex stainless steel 2707 in ammonium carbonate solution containing chloride

“…which is in accordance with our previous predictions [34,35]. The coefficient ∂L ss /∂V = 2.91 nm/V in Fig.…”

“…It is also found that, in all cases, k 2 is higher than k 3 , manifesting that the cation interstitial (Cr 3 + i ) rather than the oxygen vacancy (V ⋅⋅ o ) is the dominant point defect, and that the interstitial is responsible for the n-type semiconductor characteristic of the bl [24,26,56]. This is probably due to the larger radius of oxygen vacancy (V ⋅⋅ o ) as compared to Cr, Fe and Ni interstitial, which requires a higher vacancy formation energy [34]. The parameter k 1 controls the annihilation of the minority defect, the cation vacancy at m/bl interface (Fig.…”

“…Thus, it provides the desired electrochemical information about the corrosion kinetics of passive film formed on HDSS 2707 surface with minimal change of the system [32,33]. An innovative mixed potential model (MPM) combining the PDM to describe the partial anodic reactions of the metal dissolution at the atomic level, and the generalized Butler-Volmer equation (GBVE) to describe the cathodic process has been successfully developed and optimized on the experimental EIS data, which allows us to interpret the passivation properties of the oxide film [34,35]. The optimized parameters include the concentrations of the vacancies (C 0 i , C 0 V0 ), the bl and ol resistance (Ω⋅cm 2 ), the net anodic current density (i ss ), cathode current density (i c ), the film thickness (L ss ) and the rate constants (k i ) of the defect reactions as mentioned above.…”

TSA anodising voltage effects on the near-surface coarse intermetallic particles in the AA2024-T3 and AA2198-T8 alloys

“…In the last years, many works were carried out in order to understand the corrosion mechanism and the corrosion resistance of the AA2024 alloy [18][19][20][21][22][23][24][25][26][27][28]12] and also of the Al-Cu-Li alloys [4,11,[29][30][31][32][33][34][35][36][37]. Recently, some researchers reported the effect of microstructure on the corrosion initiation in Al-Cu and Al-Cu-Li alloys [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54]. All these works demonstrated the higher susceptibility of these alloys to localised corrosion and the need for corrosion protection.…”

Salt Spray Corrosion of 2060-T8 Al–Li Alloy in an Aggressive Environment

“…ese alloys displayed crystallographic corrosion morphology, selective attack of grain boundaries, and grain interiors [34,35]. Macdonald et al studied the pitting corrosion of 2098 Al-Li alloy, they thought the Al interstitial comprise the majority of the point defects in the barrier layer of this alloy, the formation of a passive layer had lower capacitance, which is associated with the point defect structure of the barrier layer [38]. Queiroz reported the EIS behavior of anodized and primer coated 2198 AL-Li alloy, they found the uncoated surface of this alloy showed higher pitting susceptibility, larger amounts of corrosion products formed on the anodized and primer coated surface of this alloy, and the diffusion was associated with large amounts of porous corrosion products on the exposed surface of this alloy to the corrosive environment [14].…”