Performance of a magnesium-rich primer on pretreated AA2024-T351 in selected laboratory and field environments: Conversion coating pretreatments
摘要：The effect of conversion coatings on the corrosion protection of AA2024-T351 by magnesium-rich primer (MgRP) was evaluated in topcoated and non-topcoated, scribed conditions. Protection of remote scratches and global protection by the coating after exposure in selected laboratory and field environments was investigated. Exposure studies focused on chromate conversion coating, trivalent chromium pretreatment (TCP), and non-chromium pretreatment, and compared to non-film forming (NFF) surface pretreatment. Exposures were conducted in the field under two different environments: at a coastal marine site and at an inland rural site. ASTM B117 with 5 wt% NaCl, modified ASTM B117 with acidified ASTM substitute ocean water and UV light, as well as full immersion in ambiently aerated 5 wt% NaCl solution were compared to field environments. Mg pigment depletion rate, global galvanic protection potential, coating barrier properties, and scribe protection were investigated. In systems without a topcoat, full immersion studies resulted in significant depletion of Mg, and all other environments led to depletion of Mg at different rates. In contrast, a polyurethane topcoat limited the Mg metallic pigment depletion, resulting in only partial Mg depletion in all chosen environments. In NFF pretreated AA2024-T351 with MgRP, magnesium was galvanically coupled to AA2024-T351 immediately and was available for cathodic protection from the beginning of exposure. This is indicated by a shift in global galvanic protection potential from -1.4 VSCE to more positive potentials with increasing exposure time. In the case of conversion coating pretreated AA2024-T351, there was limited galvanic coupling with the MgRP. Upon prolonged exposure in full immersion, the global galvanic protection potential decreased to more negative potentials below the open-circuit potential of AA2024-T351, indicative of galvanic coupling. In the case of systems with topcoat, the global galvanic protection potential was heavily regulated by the polyurethane topcoat and there was no significant global galvanic coupling between AA2024-T351 and Mg in the timeframe over which experiments were conducted. Mg was preserved and available for any future sacrificial anode based cathodic protection and local protection. The barrier properties of the MgRP pigmented coating also degraded with time at a higher rate in systems in the absence of topcoat. This was attributed to UV degradation of the pigmented coating resin and could be reduced with the polyurethane topcoat. Scanning electron microscopy/energy dispersive spectroscopy characterization of the scribe after different B117/field exposure times indicated that the protective throwing power increased as a function of exposure time in both AA2024-T351/NFF/MgRP and AA2024-T351/TCP/ MgRP systems. Moreover, a secondary protection mode was identified.