Publication

Mixed Zn/Li/Al-LDH based coating grown on the surface AA7075-T6 to improve its corrosion resistance

Abstract

Alternative chromium-free conversion coatings are still in focus of interest. In this work, the formation of mixed Zn/Li/Al LDH-CO32-/OH− (layered double hydroxide)-based multilayered CC (conversion coatings) was demonstrated for the first time. It was grown in-situ on the surface of AA7075-T6 aluminium alloy under mild conditions (30 ​°C) in a treatment bath containing 0.1 ​M Li2CO3 at pH ​= ​11.5 and in the presence of NH4OH. The Li ​+ ​ions required for the LDH structure formation came from the treatment bath, while the Zn2+ and Al3+ were provided by the alloy dissolution. The formed Zn/Li/Al LDH-CO32-/OH- CC significantly enhanced the corrosion resistance of the AA7075-T6 alloy. Based on the results of electrochemical impedance spectroscopy (EIS), the total impedance modulus |Z| at 0.01 ​Hz was increased by one order of magnitude compared to the bare substrate, while the salt spray test (SST) revealed no pitting after 816 ​h of exposure. These excellent protective properties were associated with the changes of the AA7075-T6 alloy surface during the LDH growth process, formation of a passive oxide layer with good barrier protection ability and LDH's smart nanocontainer function.