Mechanistic Framework for Understanding pH-Induced Electrode Potential Control of AA2024-T351 by Protective Mg-Based Pigmented Coatings

Abstract

The performance of Mg-based coatings for the corrosion protection of AA2024-T351 is a topic of great interest. Findings indicate beneficial corrosion inhibition of AA2024-T351 via dissolution of Mg-based pigments. This functionality is largely source independent, as Mg2+ and OH− can be supplied from either Mg- or MgO-based coatings. The isolated effects of Mg2+ in the form of aqueous ion ( M g a q 2 + ) and Mg(II) corrosion products (MgO, Mg(OH)2, and MgCO3) on the corrosion of AA2024-T351 was subsequently assessed. AA2024-T351, Al2CuMg S-phase, Al-4Cu “matrix phase,” and pure Al were investigated in a variety of Mg2+-affected solution chemistries. A mechanism is proposed which describes how Mg and Mg-based compounds (MgO, Mg(OH)2, MgCO3) may protect the AA2024-T351 substrate by alteration of the solution chemistry (by increasing [Mg2+] and [OH−] during Mg-based dissolution) and subsequent chemical deposition of a Mg(OH)2 film on reactive cathodic corrosion sites triggered by site alkalinity. A framework is developed for exploring the critical pH (9.86) above which the OCP of AA2024-T351 is shifted to more negative values well below the pitting, repassivation, and other critical potentials of various intermetallic phases.