Synthesis, Characterisation and Corrosion Protection Performance of Hybrid Nanocomposite Coatings

Abstract

The present study is focused on the development of hybrid sol-gel coating as an alternative environmentally-friendly coating for corrosion protection of the automotive AA6111-T4 aluminium alloy. Hybrid Si-Zr coating was successfully prepared by hydrolysis and condensation reactions of 3-glycidoxypropyl trimethoxysilane (GPTMS), tetraethyl orthosilicate (TEOS) with tetra-n propoxyzirconium (TPOZ) in the presence of an acidic catalyst and chelating ligand. It was found that with a ratio 2:1 of silane/zirconia precursors, a single dip procedure using withdrawal speed of 240 mm/min at room temperature and cured at 115°C for 3 hours resulted in dense, uniform and crack-free hybrid coating. Desired adhesion was achieved via the formation of Si-O-Al or Zr-O-Al bonds between aluminium substrate and hybrid film. Further, the corrosion inhibition effectiveness and mechanism of cerium nitrate, benzotriazole, 8-hydroquinoline and lithium carbonate for AA6111-T4 alloy in sodium chloride solution were investigated. Microscopic characterisation shows that cerium nitrate provides corrosion inhibition to the alloy by forming cerium oxide/hydroxide on the intermetallic particles while benzotriazole and 8-hydroquinoline provide corrosion inhibition by the adsorption of the molecules forming a thin organic layer on the alloy surface. For lithium carbonate, a protective layer of insoluble compounds with columnar/petal morphology developed on the surface of the alloy. All these organic and inorganic inhibitors were successfully doped into the hybrid sol-gel coating. The doped coatings are relatively dense, uniform and crack-free. Corrosion resistance and inhibition mechanism of the doped hybrid coatings was evaluated by electrochemical impedance spectroscopy technique. The incorporation of corrosion inhibitors to the hybrid Si-Zr coatings provides additional active corrosion protection when the proper inhibitor was used. Hybrid coatings doped with Ce(NO3)3, BTA and 8-HQ demonstrate high impedance values at low frequencies during immersion in 3.5% NaCl solution suggesting the presence of the inhibitors reinforced the barrier properties of the coatings. It can be suggested that the presence of cerium nitrate followed by benzotriazole and 8-hydroquinoline enhanced the corrosion resistance of the AA6111-T4 alloy. In contrast, hybrid coating doped with Li2CO3 leads to deterioration of the corrosion protection properties and less compatible among other inhibitors to be incorporated into the coating matrix. The work was extended to study the effect of different concentrations of cerium nitrate on the corrosion protection performance of hybrid Si-Zr coating. Addition of lower concentrations of cerium nitrate in the hybrid coating (0.5 and 1.0%) increased the corrosion resistance and confers additional active corrosion protection. However, the addition of high cerium nitrate concentrations (3.0% and 5.0%) in the coating matrix decreased the coating and intermediate oxide resistances and sufficiently decreased the barrier protection properties. Overall, hybrid Si-Zr coatings with the incorporation of corrosion inhibitors offer alternative as a promising approach of environmentally-friendly pre-treatments on automotive AA 6111-T4 aluminium alloy.

Date of Award	31 Dec 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stuart Lyon (Supervisor) & Xiaorong Zhou (Supervisor)