The Role of Cu-Based Intermetallic on the Direct Growth of a ZnAl LDH Film on AA2024

Abstract

The direct ZnAl layered double hydroxide growth on AA2024 is a fast-occurring reaction, 18 yet is characterized by an inhomogeneous film thickness. It has been shown that at the periphery 19 of Cu-rich intermetallic, the flakes tend to be larger and denser. A combination of in situ and 20 ex situ measurements were used to monitor the changes in the layered double hydroxide film 21 grown on the regions of intermetallics. Immediately after immersion, an activation of the 22 intermetallics is observed due to the dealloying process with an almost immediate film growth. 23 Dealloying is followed by trenching of the adjacent Al matrix leading to an excessive 24 production of large and dense layered double hydroxide flakes at the periphery of the 25 intermetallic. However, the scanning electron microscopy cross-section images revealed that 26 the trenching process leads to defects in the area surrounding the intermetallic. This could 27 weaken the corrosion resistance performance of the layered double hydroxide conversion 28 coating and lead to adhesion failure of consecutive polymer coatings. Nevertheless, this work 29 highlights a few advantages and drawbacks of the layered double hydroxide conversion 30 coatings and pathways to its potential optimization and improvement.