Performance of a Magnesium-Rich Primer on Pretreated AA2024-T351 in Selected Laboratory and Field Environments: Anodization Pretreatment

The effect of anodization on the corrosion protection of AA2024-T351 by magnesium-rich primer (MgRP) was evaluated in topcoated (TC) and non-TC MgRP systems with and without defects in the form of scribes. Protection of remote scratches and global protection by the coating after exposure in selected laboratory and field environments was investigated. Exposure studies focused on the following pretreatments: anodization without sealing (ANS), anodization with hexavalent chromium sealing (ACS), and anodization with trivalent chromium pretreatment sealing (ATS). Mg pigment depletion rate, global galvanic protection potential of intact coating, and coating barrier properties were investigated. In systems without TC, all chosen environments except for full immersion resulted in complete depletion of Mg pigment resulting from high self-corrosion rates. Polyurethane TC limited the Mg metallic pigment depletion resulting in only partial Mg depletion. In the case of ANS and ATS pretreated AA2024-T351 with MgRP, there was no initial galvanic coupling as inferred by a more positive global galvanic protection potential and predicted by high-pretreatment resistances. Upon prolonged exposure in full immersion, the global galvanic protection potential decreased to more negative potentials with time indicative of gradual galvanic coupling of the AA2024-T351 substrate with the MgRP. This was interpreted as resulting from degradation of the buried anodized layer. In the case of ACS pretreated AA2024-T351 with the MgRP, there was no initial galvanic coupling. After initial lowering of global galvanic protection potential indicating active coupling, there was a shift back to more positive potential. The global galvanic protection potential was heavily mediated by the TC polymer and there was no significant galvanic coupling between AA2024-T351 and Mg in the time frame over which experiments were conducted for TC systems. Mg was preserved and available for any future sacrificial anode-based cathodic protection. The barrier properties of the MgRP pigmented coating also degraded with time at a higher rate in systems in the absence of TC. This was correlated to UV degradation of the pigmented coating resin and could be reduced with the polyurethane TC. Scanning electron microscopy/energy dispersive spectroscopy characterization after different ASTM B117 and field exposure times indicated that the Mg²⁺ redeposition increased as a function of exposure time in AA2024-T351/ACS/MgRP systems. TC limited the Mg²⁺ release by mediating both the global galvanic protection potential and Mg self-corrosion. Corrosion volume analysis by optical profilometry indicated that the scribe protection was better for sealed anodization pretreatments compared to the case of anodized without sealing-based systems.