Self-Healing Effect of Hybrid Sol-Gel Coatings Based on GPTMS, TEOS, SiO₂Nanoparticles and Ce(NO₃)₃Applied on Aluminum Alloy 7075-T6

Abstract

Aluminum alloy AA7075-T6 is most commonly used in the aircraft and automotive industries. Due to the presence of intermetallic particles, AA7075-T6 is susceptible to localized corrosion in chloride solutions. In the present work multilayer hybrid sol-gel coatings, based on silane precursors 3-glycidyloxypropyl(trimethoxysilane) (GPTMS) and tetraethoxysilane (TEOS), were used to protect AA7075-T6 from corrosion. To enrich the barrier properties of the coating, SiO 2 nanoparticles were added to the GPTMS/TEOS sol-gel solution. Inhibition and a self-healing effect were achieved by doping Ce(NO 3 ) 3 in the coating. A multi-layer system was applied on the substrate composed of a first layer doped with Ce(NO 3 ) 3 and the second, undoped sol-gel layer. The addition of cerium increases the lifespan of hybrid sol-gel coating and has a role in self-healing if it is locked within the first layer of the multilayer coating. The self-healing effect was confirmed by immersion of unscribed and scribed coated substrates in 0.1 mol/L NaCl by using the immersion test, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy with chemical analysis. Due to increasing demand for light-weight metals to reduce fuel consumption and, consequently, CO 2 emission, aluminum alloy AA7075-T6 is very attractive in automotive and aircraft industries. In addition to its low density, its advantages are high strength-to-weight ratio and good mechanical properties.1-3 However, due to the presence of zinc and copper intermetallic particles, AA7075-T6 readily undergoes localized corrosion, which limits its use in more aggressive environments. Chromate conversion coatings (CCCs), which were used for decades as providing the most effective corrosion protection of aluminum alloys, are limited, in the European Union. 4 Many surface treatments have been proposed to replace CCCs, the sol-gel process being one of the most promising and environmentally friendly. [5][6][7][8][9][10][11][12] In hybrid sol-gel coatings, which combine inorganic and organic compounds, the inorganic compound provides mechanical support, while the organic compound adds to flexibility and decreases treatment temperature. 8,[12][13][14][15] Hybrid sol-gel coatings achieve protection of underlying substrate by acting as barrier coatings. Addition of silica nanoparticles (SiO 2 ) into sol-gel coating has been reported to additionally increase the barrier effect by increasing the coating thickness, density and reinforcement. [16][17][18][19][20][21][22][23] Still, if these coatings do not contain corrosion inhibitors they lack the ability to inhibit the corrosion process once it commences, i.e. they do not show active inhibition ability resulting in a self-healing of the coating.To obtain an active corrosion protection, corrosion inhibitors can be incorporated into sol-gel coating. According to Hinton et al., 24,25 4 and insoluble CeO 2 , which acts as a protective barrier to oxygen and lowers the cathodic activity by hindering the ...

The Effect of the Methyl and Ethyl Group of the Acrylate Precursor in Hybrid Silane Coatings Used for Corrosion Protection of Aluminium Alloy 7075-T6

“…This indicates that methoxy (O-CH 3 ) groups of MAPTMS and ethoxy (O-CH 2 −CH 3 ) groups of TEOS were replaced by -OH groups during the hydrolysis and condensation reactions. As a result, siloxane bands (Si-O-Si), appeared at the characteristic band at 1055 cm −1 for siloxane-PMMA and siloxane-PEMA, becoming some of the main bands in the spectra for both hybrid sols [9,28,29].…”

“…By increasing the ratio between thermal initiator benzoyl peroxide (BPO) and MMA, the polymerization efficiency can be improved [8]. Some sol-gel coatings were doped in order to increase their cross-linking, increase adhesion to the metal substrate, or to improve scratch resistance and thermal stability [7,[9][10][11][12][13][14].…”

Contemporary Modes of Corrosion Protection and Functionalization of Materials

“…Cerium ions were added also to GTS coatings. 82,83,87 GTS-Ce coatings exhibited enhanced corrosion properties, as shown by reduced current density in polarization curves (Fig. 14a) 82 and excellent protection during longterm immersion up to 50 days in NaCl solution (Fig.…”

“…82 It is noteworthy, however, that an optimal content of cerium should be incorporated since too large concentration may destruct the structural integrity of the coating. 86,87 At the same time, it has to be assured that doping of the coating with cerium does not reduce the barrier properties of the coating, as cerium ions are bigger in size (1.978 Å) compared to silicon atoms (1.068 Å). Larger Ce addition can cause lattice deformation and formation of nanopores and consequently change dramatically the characteristics of the coating.…”

“…86 Self-healing of the coating can be followed electrochemically by measuring the open circuit potential (E oc ) 86 or impedance at low frequency range. 87 Upon immersion of TMZ coating, E oc was cca. −0.58 V and then shifted in more positive direction reaching −0.42 V after approximately 9 days (Fig.…”