Mixed Zn/Li/Al-LDH based coating grown on the surface AA7075 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 hydroxides) based multilayered CC (conversional coatings) was demonstrated for the first time. It was grown in-situ on the surface of AA7075 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. Li ions for the LDH structure formation came from the treatment bath, while Zn and Al were provided by the alloy dissolution. The formed Zn/Li/Al LDH-CO32-/OH- CC significantly enhanced the corrosion resistance of AA7075 alloy. Based on the results of electrochemical impedance spectroscopy (EIS), the total impedance module |Z| at 0.01 Hz was increased by one order of magnitude compared to the bare substrate, while salt spray test (SST) showed no pitting after 816 h of exposure. Such excellent protective properties were associated with the change of the AA7075 alloy surface in the process of LDH growth, formation of the passive oxide layer with good barrier protection ability and LDH smart nanocontainer function.