The Effect of Pore Sealing in a Multilayer Si–O–Zr/Al2O3 Coating Designed to Protect Aluminium from Corrosion

Abstract

This study deals with the combination of two corrosion protection strategies for aluminium: barrier protection (provided by a 3.8 μm thick hybrid sol–gel coating) and aluminium pore sealing via the use of a 100 nm thick layer of aluminium oxide. A Si–O–Zr hybrid sol–gel coating (TMZ) was synthesised by combining two separately prepared sols (i) tetraethyl orthosilicate and 3-methacryloxypropyl trimethoxysilane and (ii) zirconium(IV) n-propoxide chelated with methacrylic acid. The synthesis of the Si–O–Zr hybrid sol–gel was evaluated at various stages using real-time infrared spectroscopy. A 100 nm thick Al2O3 film was prepared via thermal atomic layer deposition at 160 °C using trimethyl aluminium and water as precursors. The coating and film properties were assessed via focused ion beam/scanning electron microscopy coupled with energy-dispersive X-ray spectrometry. Sealing with the Al2O3 film did not affect the microstructure and composition of the underlying sol–gel coating. The coating’s corrosion performance in 0.1 M NaCl solution was evaluated using electrochemical impedance spectroscopy. Compared to individual coatings, the multilayer TMZ/Al2O3 coating ensured prolonged (more than three weeks) durable corrosion protection for the aluminium. The impedance magnitude increased by two orders compared to the uncoated substrate (|Z|10 mHz from 16 kΩ cm2 to almost 830 MΩ cm2). Thus, the pore sealing of the sol–gel coating using an ALD alumina film produced a protective multilayer coating system, with |Z|10 mHz remaining above 5 MΩ cm2 after four weeks in NaCl solution.

Investigations on the Impact of a Series of Alkoxysilane Precursors on the Structure, Morphology and Wettability of an Established Zirconium-Modified Hybrid Anticorrosion Sol–Gel Coating

“…Gels 2024, 10, 315 2 of 18 For anticorrosion application, in addition to the chemical resistance, mechanical strength and adhesion to metal substrates, coatings require hydrophobicity to reduce interactions between the metal surface and corrosive agents, usually entrapped in water [15][16][17]. Amongst the numerous anticorrosion sol-gel coatings reported in the state of the art, a formulation based on the combination of 3-trimethoxysilylpropylmethacrylate and a zirconium complex has been intensively investigated and has shown exceptional performances for the protection of aluminium alloys against corrosion [18][19][20][21][22][23][24][25][26][27]. Different strategies have been employed to improve the anticorrosion barrier properties of this formulation, including varying the ratio of the Si/Zr [18,19], the hydrolysis conditions [20], the nature of the chelating agent [21,22], the incorporation of anticorrosion compounds [23], and silanes such as TEOS [24,26] and diphenylsilanediol [27].…”