Chelating agent stimulated LDH post-treatment of PEO coated AA2024

Abstract

Layered double hydroxides (LDHs) are well known and broadly investigated on bare aluminum alloys, but not on PEO treated aluminum where source of aluminum ions is restricted. The additions of the chelating agents were tested in this work to stimulate the LDH formation on PEO (plasma electrolytic oxidation) coated aluminum alloy AA2024. 1,3-diamino-2-hydroxipropane-N,N,N’,N’-tetraacetic acid (DHPTA) was identified as the most effective one among tested additives. Its performance in terms of LDH growth stimulation was investigated. The DHPTA additive provides an increase of the pH of the LDH synthesis compared to previously known methods to achieve hydration of aluminum oxide from PEO as the initial stage of PEO etching, as well as the formation of soluble complex compounds of Zn-DHPTA, necessary for the formation of ZnAl LDH. The process of LDH formation was also followed by intercalation of DHPTA ions into LDH galleries. It was shown that LDH was formed both on the surface and insight the pores of parental PEO coating. Moreover, it imparted lubricating properties to the hybrid coating. Finally, PEO-LDH coating demonstrated higher corrosion resistance and long-term stability comparing to the PEO coated aluminum alloy.