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

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 Mg 2+ and OH − can be supplied from either Mg- or MgO-based coatings. The isolated effects of Mg 2+ in the form of aqueous ion ( M g a q 2 + ) and Mg(II) corrosion products (MgO, Mg(OH) 2 , and MgCO 3 ) on the corrosion of AA2024-T351 was subsequently assessed. AA20 2 4-T351, Al 2 CuMg S-phase, Al-4Cu “matrix phase,” and pure Al were investigated in a variety of Mg 2+ -affected solution chemistries. A mechanism is proposed which describes how Mg and Mg-based compounds (MgO, Mg(OH) 2 , MgCO 3 ) may protect the AA2024-T351 substrate by alteration of the solution chemistry (by increasing [Mg 2+ ] 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.