Preparation, Characterization and Electrochemical Study of Lanthanum-Silica Sol-Gel Thin Films
Authors: García-Galván Federico R. , Carmona Noemi , Jiménez-Morales Antonia , García-Heras Manuel , Galván Juan Carlos , Villegas María Angeles 
Published: February 2020

This work presents a low cost environmentally friendly alternative to corrosion protection of aluminium alloy based on silica sol-gel thin films doped with lanthanum compounds. Sols were prepared from solutions composed of tetraethylorthosilicate and methyl-triethoxysilane as precursors, distilled water to perform the hydrolysis, absolute ethanol as solvent and nitric acid as catalyst. Lanthanum acetate hydrate or lanthanum isopropoxide in three different concentrations were added as doping agents. These lanthanum compounds can play as the well-known Cr (VI) salts offering a suitable protective effect without any environmental negative impact. L-cysteine as green corrosion inhibitor has been also added to a set of sol formulations. Films were deposited upon microscope glass slides and aluminium alloy AA2024-T3 sheets by dip coating at a constant drawing speed (2.0 mm·s−1), in a controlled room under 25°C and 35% relative humidity. For convenience, sol-gel monoliths of 20 mm diameter were also prepared. After coating, monoliths and films were dried and annealed in air at 20, 100 and 500 °C. A model polyvinyl butyral film was applied on a series of these sol-gel coatings to simulate the performance of commercial anticorrosive paints (organic coatings). For this matter poly (vinyl butyral-covinyl alcohol-co-vinyl acetate) solutions (17% w/w) were prepared in ethanol.
Transmission spectra of the as prepared films were recorded with a UV–VIS-NIR double-beam spectrophotometer equipped with an integrating sphere. Optical measurements (from UV-Vis absorption and diffuse reflectance spectroscopies) pointed out that the coatings were colourless and transparent, and they diminish the diffuse reflectance of the metallic surface up to ∼60 %. Scanning electronic microscopy coupled with energy dispersive spectroscopy (SEM-EDS), atomic force microscopy (AFM) and contact angle measurements were also applied for the coatings characterization. As concerns the structural characterization of the lanthanum environment X-ray Absorption Fine Structure (XANES) and X-ray Absorption Near Edge Structure spectroscopy (EXAFS) were also performed. Likewise, the remaining sols were kept to gelify at 60 °C for 4 days and then powdered to obtain suitable samples to be analysed by using other characterisation techniques (Fourier transformed infrared, FTIR, and differential thermal analysis, DTA).
The corrosion resistance of the sol-gel coated AA2024-T3 aluminium alloy was studied by weathering tests according to modified ISO 1776 and 695 Standards. Samples were immersed into an acid solution (HCl 0.1 N at 25 °C) or into a basic one (NaOH 0.1 N at 25°C) for 1 h. In parallel studies, anodic and cathodic delamination tests were also carried out during immersion of coated samples into a 0.6 M NaCl aqueous solution. Just before the experiment was initiated, the centre of the PVB/sol-gel thin film/metal samples was damaged with an intentionally scribed linear defect. Potentiodynamic polarization technique and global and electrochemical impedance spectroscopy were also applied.
The results show that the presence of lanthanum compounds and L-cystiene induces an effective delay of early corrosion of metallic substrates, demonstrated by accelerated corrosion tests in acid and basic media. XAS studies confirmed the incorporation of lanthanum ions into silica sites to be essential for the material anticorrosive properties. Optical and scanning electron microscopies allowed the coated samples texture and microstructure observation, both before and after the corrosion tests were carried out. These last two techniques joined with the electrochemical studies allowed to evaluate not only the inhibitor effect of lanthanum compounds and L-cysteine, but also the self-healing properties of the studied systems.
As concluding remarks, we may indicate the convenience and effectiveness of the prepared sol-gel silica films doped with lanthanum isopropoxide or lanthanum acetate as a protective barrier for the delay of early corrosion of metallic surfaces. These properties are scarcely dependent on the lanthanum precursor used, which could be selected taking in mind the corresponding cost. Depending on the reaction parameters in the sol-gel process, La2O3 or La(OH)3 nanoparticles can be intentionally produced. The combination of these La2O3 nanoparticles with L-cysteine produces a synergic effect which improves considerably the corrosion protection behaviour of sol-gel coated AA2024-T3 aluminium alloy in NaCl aqueous solutions. The uniformly distributed La2O3 or La(OH)3 nanoparticles impregnated with corrosion inhibitor behave as active fillers and nanoreservoirs for the sol-gel thin film, providing a prolonged and smart release of the corrosion inhibitor (L-cysteine) in response to local pH changes during the samples contact with the aqueous solutions. The action of the organic inhibitor combined with the precipitation of insoluble lanthanum products from the inorganic framework at the cathodic and anodic corrosion zones, provides self-healing properties to the protective system, avoiding the corrosion processes as expected.

Acknowledges:

This work was supported by MINECO, Spain (Projects MAT2012-38541-C02-02, MAT2015-65445-C2-1-R and MAT2015-65445-C2-2-R).