EVALUATION OF CORROSION INHIBITION OF 2024-T3 BY ZN2+

Zn²⁺ is a cathodic inhibitor for Al alloys whose inhibition characteristics have not been widely reported. In this study, potentiodynamic polarization experiments were carried out on 2024-T3 with varying concentrations of Zn²⁺ from 0.1 to 50 mM, and systematic variation of pH from 4 to 12 in aerated 100 mM NaCl solutions. Strong suppression of the oxygen reduction reaction (ORR) was observed in pH 5 to 8 range and was absent in strongly acidic and alkaline solutions. No anodic inhibition by Zn²⁺ was observed at any Zn²⁺ concentration at any pH. However, anodic polarization carried out after 4 days of pre-exposure in acidic zinc solutions showed significant reduction in anodic kinetics suggesting delayed inhibition. Rotating disk electrode experiments with Cu electrodes showed a two-order of magnitude reduction of cathodic current by the addition of 1.0 mM Zn²⁺ to neutral chloride solutions, suggesting that an important component of Zn²⁺ inhibition is suppression of ORR kinetics at Cu-rich intermetallic particles in Al alloys. SEM and energy dispersive spectroscopic analysis was performed on 2024-T3 coupons subject to free corrosion immersion in acidic, neutral and alkaline inhibited chloride solutions. The window of Zn²⁺ inhibition is wider in free corrosion experiments than in electrochemical experiments. Coupons exposed to acidic solutions under free corrosion conditions showed complete suppression of corrosion and no dissolution of Al₂CuMg particles in solutions whose pH was as low as 4. Speciation of Zn²⁺ in dilute aqueous NaCl solutions was calculated using a thermodynamic modeling to determine the solubility and speciation of Zn²⁺ as a function of pH. Results suggest that Zn²⁺ is a good inhibitor of Al corrosion on the acid side of its solubility minimum at pH 7 to 9 because of Zn²⁺ precipitation triggered by interfacial alkalinity that develops due to the ORR that occurs during free corrosion.