Environmental Degradation of a Mg-Rich Primer in Selected Field and Laboratory Environments: Part 2—Primer and Topcoat

Magnesium-rich primer, in a topcoated, scribed condition, was utilized for the corrosion protection of an aluminum alloy (AA)2024-T351 (UNS A92024) substrate. Exposures were conducted in the field at a coastal marine site, Kennedy Space Center (KSC), Florida; at a inland rural site, Birdwood Golf Course in Charlottesville, Virginia; in ASTM B117 with 5% sodium chloride (NaCl); and the same standard test modified with ASTM seawater as well as in full immersion in ambiently aerated 5% NaCl solution. Mg pigment depletion rate, global galvanic protection potential, and coating barrier properties were tracked throughout exposure periods in both field and laboratory environments. Analysis near and far from the scribe was performed. Characterization with scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) was conducted to elucidate coating and scribe morphology, corrosion products present, corrosion of the AA2024-T351 substrate, as well as in an attempt to interrogate the throwing power of the coating system with respect to scratches exposing bare AA2024-T351. The topcoat was observed to strongly mediate the depletion of Mg pigment from the MgRP, due to self-corrosion, in all exposure environments studied as compared to identical environmental exposures of non-topcoated samples. Full immersion in ambiently aerated 5% NaCl solution, ASTM B117 with 5% NaCl, and ASTM B117 with ASTM substitute ocean water (SOW) all resulted in only partial depletion of metallic Mg pigment in the MgRP far from the scribe after 1,000 h. Field exposures in Charlottesville, VA, and KSC, also resulted in similar low levels of Mg pigment depletion far from the scribe after 1 year of exposure. As a result of partial depletion of remote Mg pigment particles, the global galvanic protection potential of the coating system, with respect to remote scratches only, became slightly more positive with exposure time in each environment, from initial values of approximately −1.0 V vs. saturated calomel electrode (SCE) to −0.7 V_SCE after extensive environmental exposure. These values fall between the open-circuit potentials of bare AA2024-T351 (−0.6 V_SCE) and bare Mg (−1.6 V_SCE) and are predicted by mixed potential theory. Barrier properties of the Mg-rich primer coating, as assessed by electrochemical impedance, also slightly degrade with time in each environment but, overall, remain very high (≥10⁹ Ω · cm² at 0.01 Hz) throughout exposure, indicating that significant barrier protection remains after all environmental exposures studied. Characterization after 1,000 h of salt fog exposure in ASTM B117 modified with ASTM SOW indicates a throwing power that extended the entire half-width (≈350 μm) of the AA2024-T351 scribe and a throwing power that extends approximately 200 μm into the scribe after environmental exposure in the field at KSC. Characterization after 1 year of exposure in the field at Charlottesville, VA, and after 1,000 h of exposure in ASTM B117 with 5% NaCl was inconclusive. The uniformity in performance in the lab and field is presumed to be due to the polyurethane topcoat polymer's excellent resistance to UV degradation and electrolyte ingress. No chalking, or any other phenomena signifying significant UV degradation, was observed in lab and field exposures of AA2024-T351 panels coated with MgRP (initial MgPVC = 45%) and advance life polyurethane topcoat reported on in this study.