Mechanism of LDH Direct Growth on Aluminum Alloy Surface: A Kinetic and Morphological Approach

The growth of ZnAl layered double hydroxide (LDH) on the AA2024 surface was monitored using synchrotron high-resolution X-ray diffraction. Data were analyzed using the Avrami-Erofe'ev kinetic model. Accordingly, the LDH film growth is governed by a two-dimensional (2D) diffusion-controlled reaction with a zero nucleation rate. Additional methods, including ex situ atomic force microscopy/scanning Kelvin probe force microscopy (AFM/SKPFM) supported by in situ open-circuit potential (OCP) measurements together with scanning electron microscopy (SEM) and transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM/EDX) analysis, provided further insight into the different stages of the mechanism of LDH growth. Prior to the conversion coating formation, an intermediate layer is formed as a basis for the establishment of the LDH flakes. Moreover, a Cu-rich layer was revealed, which could contribute to the acceleration of LDH growth. The formed LDH layer does not show any cracks at the interface but presents minor irregularities in the structure, which could favor adhesion to subsequent organic coatings. The findings presented in this work provide an important insight to the corrosion performance of the LDH conversion coatings and the pathway to adopt for further optimization.