Magnesium and Magnesium-Oxide Primer on 2024-T351: Assessment and Characterization of Mg-Based Protection Mechanisms

Aluminum alloy 2024-T351 (UNS A92024) is a naturally-aged precipitation-hardened light-weight high-strength aerospace alloy commonly employed in aircraft airframes for commercial and military application that is susceptible to localized corrosion in the form of pitting, intergranular corrosion, and trenching. Corrosion induced damage must be prevented with an actively protective coating system that can protect macro coating defects that expose this structural alloy to corrosive environments. For this reason, a Mg-rich primer (MgRP) coating was developed for the protection of AA2024-T351. MgRP protects the AA2024-T351 substrate primarily via a sacrificial anode-based cathodic protection process where AA2024-T351 is protected from corrosion via Mg oxidation.

Additional protection mechanisms may operate as well and are investigated herein, as observations made in the field indicate that Mg(II) corrosion products can chemically deposit onto remote scribe, or macro defect, exposing the substrate. The effect is to suppress corrosion damage at the macro defect but the mechanism is heretofore unknown. For this reason, the performance of a MgO-Rich Primer (MgORP) was also studied to assess chemical protection effects of Mg2+ (both in solution and in the form of corrosion products deposited on AA2024-T351) and to investigate its own merit as a reliable coating for corrosion protection.

Existing approaches and frameworks for evaluating the performance of MgRP were applied to the evaluation of MgORP performance as a corrosion preventing coating for AA2024-T351. All coating systems were tested in a variety of exposure environments ranging from standard and modified B-117 constant salt spray, marine and inland field, and diagnostic electrochemical cycle testing in full immersion. The effect of Mg-based dissolution on solution chemistry (by releasing Mg2+ and OH-) was assessed via chemical stability diagrams, which were developed here to include chemical/electrochemical dissolution trajectories superimposed on chemical equilibrium phase boundaries for metal-based precipitation reactions. AA2024-T351 microconstituent phases were synthesized, isolated, and tested in each Mg2+-affected solution chemistry to evaluate the effect of soluble Mg2+ and solid Mg-based compounds on the corrosion kinetics of the alloy. The performance of each coating system with and without topcoat, specifically with respect to Mg2+ release rates, will be discussed. Generally, the topcoat acts to inhibit Mg-based dissolution protection mechanisms (via galvanic coupling and pH-induced electrode potential control) but preserves the polymer matrix from degradation, bolstering barrier protection and promoting longer service lifetimes. Insight will be provided on how coating design impacts corrosion protection performance. The coated interface is a complex system that must be optimized for best protection performance.

Newly considered corrosion protection mechanisms available for AA2024-T351 via Mg2+ storage, release, and redeposition will be elaborated. Briefly, OH- and Mg2+ are commonly produced as products of Mg-based dissolution. The increase in pH causes a change in the corrosion morphology of AA2024-T351 from localized corrosion attack to passive dissolution. The critical pH at which this transition from pitting to passive dissolution occurs is 9.8-9.9 (for AA2024-T351 in 0.9 M NaCl), which is accompanied by a decrease in the OCP from about -0.6 VSCE to -1.1 VSCE. Additional benefits of Mg2+ include precipitation of Mg-based corrosion products at site of locally concentrated cathodic activity which prevent excessive increases in the pH to the point where aluminum actively dissolves.

This thesis elucidates a new scientific mechanism by which metal rich primers operate. A holistic mechanism for pH induced electrode potential control of aluminum alloy substrates brought about by the oxide/compound-pigmented primer is developed, tested, and modelled. Technologically, these conclusions motivate a paradigm shift in the way corrosion preventing coatings for aluminum alloys should be designed, assessed, and monitored. The utility of chemical stability diagrams towards electrochemical sciences is also highlighted.