The Relation between the Microstructure and Corrosion Behavior of Aluminum Alloy AA2024-T3

In light of high strength-to-weight ratio and superior mechanical properties, aluminum alloy 2024-T3 has been widely used in aerospace applications. However, the various intermetallic(IM) particles formed by the addition of copper and magnesium lead to localized corrosion, which causes degradation and deterioration of aluminum alloys. Chromate conversion coatings (CCCs) have been used to passivate the surface of high strength aluminum alloys. Recently, hexavalent chromium, which is toxic and carcinogenic to human health and environment, is prohibited by the WEEEs and RoHS regulations.

There are many alternatives aiming to replace toxic CCCs, for example, trivalent chromium process (TCP), cerium conversion coating, and phosphate conversion coating. However, corrosion inhibition of those conversion systems is still inferior to the CCCs. The relation between microstructure of aluminum alloy 2024-T3 and its corrosion behavior is essential to enhance the anti-corrosion performance of conversion coatings. Heterogeneous phases in the matrix of aluminum alloy generally affect the morphology and anti-corrosion properties of conversion coatings.

S-phase ,which constitutes about 60% of the IM particles, is one of the main IM particles in AA2024-T3 alloy. Several researches have shown that the S-phase dealloys under anodic polarization, leaving behind a porous Cu-rich remnant. This fine and high surface energy structure then coarsens and metallic copper is detached from the particles and dissolve into the solution. Isolated Cu ions in the solution are then reduced back onto the cathodic region of the alloy surface. The plating of metallic Cu will cause secondary pitting on the aluminum alloy.

The heterogeneous structure of AA2024-T3 alloy also plays an important role during film formation of conversion coatings. Cathodic IM particles serve as preferential sites of conversion layer formation and Al matrix dissolution takes place at the periphery areas of those IM particles. Meanwhile, some Cu deposits are created above the Al matrix by dealloyng of the S-phase. Thus, the nucleation of conversion coatings also preciptates on those Cu particles. The nucleation and growth of conversion coatings affected by IM particles is thus discussed in this study.

Trivalent chromium process(TCP) was applied on AA2024-T3 alloy. The gradients in coating formation rate occur locally at the interface between the IM particles and the Al matrix. Also, the galvanic coupling between Cu deposits created from dealloying of the S-phase and the Al matrix affects the formation rate of conversion coatings. These phenomena might cause the conversion coatings to be discontinuous in parts. These phenomena cause the conversion coatings to be discontinuous in parts. In corrosive environments, discontinuities of the conversion coatings deteriorate the anti-corrosion properties of the coatings and make AA2024-T3 alloy susceptible to localized corrosion.

SEM micrograph (Figure 1) shows a region of alloy immersed in 0.5M NaCl solution for 90 min. A trench appears around the large particle in upper right of the picture. Corroded pits are located in the boundary of the Al matrix and IM particles. Figure 2(a) and (b) show the EDX elemental mapping of the polished AA2024-T3 alloy before and after 90 min of immersion in 0.5M NaCl. It is observed that the Cu has migrated to a cathodic IM particle.