Magnesium and Magnesium Oxide Primer on AA2024-T351: Assessment of Field Performance

Abstract

A Mg-rich primer (MgRP) consisting of metallic magnesium pigment was developed for the protection of AA2024-T351.1-10 Both a MgRP and a MgO-rich primer (MgORP) with topcoats were tested in laboratory primer cycle test and long-term field exposure conditions to evaluate coating barrier properties, scribe protection, and sacrificial anode-based cathodic protection in the case of MgRP. Results show that detectable amounts of Mg pigment in MgRP is preserved through 4.25 y of field exposure. Furthermore, cathodic protection afforded by these Mg pigments remains a viable form of protection in MgRP for as long as 2.5 y in coastal marine field exposure at Kennedy Space Center. Beyond this, scribe coverage of Mg2+-based products was observed to increase during the course of a 4.25 y coastal marine exposure. Mg2+ repartitioning from the coating into the scribe was also observed for MgORP after 2.5 y of exposure. This observation forms the basis for the explanation of how a MgORP functions as a protective primer. The performance of a MgORP was also studied to assess chemical protection effects of stored and released Mg2+ (both in solution and corrosion product) and to investigate how MgORP functions for corrosion protection. Several observations indicated favorable effects of Mg2+ on the corrosion protection of AA2024-T351 as Mg2+ repartitioning was also observed. Scribe damage was found to be similar for both MgRP and MgORP after 2.5 y of exposure.