Blistering Phenomena in Early Generation Mg-Rich Primer Coatings on AA2024-T351 and the Effects of CO2

An organic coating system containing a Mg-pigmented organic primer (MgRP) for corrosion protection of Al 2024-T351 has been developed as a candidate to replace chromate-containing surface pretreatments and primers. Corrosion protection mechanisms of the MgRP have been shown by Battocchi and Bierwagen (Corrosion Science, 2006. 48(5): p.1292-1306) to include sacrificial anode based cathodic protection of the aluminum and barrier protection by the primer and topcoat polymers. King (Corrosion, NACE Research Topical Symposium Issue, 2010) has shown this capability is mediated by the percent volume concentration (PVC) of Mg and the conductivities of the organic polymers in the primer and topcoat.

Differences in corrosion behavior of the MgRP between field and laboratory exposures have also been previously reported by Pathak (Corrosion Science, 2010. 52(4): p.1453-1463). In particular, blistering of the coating system often occurs in ASTM B-117 laboratory tests but is not typically seen in samples exposed in the field at sites such as Daytona Beach, FL. To understand the origins of these discrepancies, it is paramount to understand the mechanisms of corrosion protection afforded by the Mg-rich primer system and to develop means to quantify and monitor the depletion of Mg when exposed to various exposure environments.

This talk reports on laboratory full immersion, laboratory cabinet testing and field exposure results aimed at assessing the mechanism of blistering, the sacrificial anode capacity as well as barrier properties before and after MgRP depletion. Acceleration factors were assigned based on changes in anode capacity and barrier properties in each environment. Other environmental factors such as CO₂ were investigated. Preliminary findings suggest time-of-wetness, chloride concentration, and environmental CO₂ concentration to each play a role in corrosion behavior of the MgRP coating system. The results help explain the influence of CO₂ on blister formation and origins of discrepancies between lab and field exposure results.