Corrosion protection of metallic substrates of historical interest by lanthanum-silica sol-gel coatings

The European Corrosion Congress – EUROCORR. Applied Science with constant Awareness, September, 9-13 2018, ICE Krakow, Poland.

Lanthanum compounds as doping agents in silica based sol-gel coatings can play as environmentally friendly corrosion inhibitors of metallic surfaces of interest in automotive and/or aeronautical industries. In the framework of an interdisciplinary research project the application of lanthanum-silica sol-gel coatings to metallic surfaces of historical interest is being explored for the first time. This contribution presents such application as far as four different metallic substrates are concerned: copper, bronze, lead, and steel. Coatings were prepared from tetraethoxysilane (TEOS) as precursor, distilled water for hydrolysis, absolute ethanol as solvent and HNO3 as catalyst of the hydrolysis. Two types of coatings were formulated: one of them using lanthanum nitrate as doping agent, and another one using lanthanum tri-acetate. Both coatings were deposited upon either common glass slides or metallic plates from 1.0 to 1.5 mm in thickness of copper, bronze, lead, and steel by dip coating at a constant drawing speed (2.0 mm s-1), under controlled temperature (25°C) and relative humidity (35%). Partial thermal densification was accomplished in a forced air stove at 60°C for 72 h. Low quality plates of these four metals and alloys were selected since they are among the most common substrates of metallic items of historical and Heritage interest. Once coatings were made the remaining sols were gelled at 35°C for 48 h. After that, sol-gel monoliths were powdered for characterization by Fourier transformed infrared spectroscopy (FTIR), differential thermal analysis (DTA) and thermogravimetry (TG). A set of sol-gel coating/metal systems was coated with transparent polyvinylbutyral films to simulate commercial top-coatings. Coatings thickness was evaluated by the interference fringe method. The corrosion resistance of both types of lanthanum-silica sol-gel coatings was studied through cathodic delamination tests and electrochemical impedance spectroscopy (EIS) measurements carried out during immersion of coated samples into a 0.6 M NaCl aqueous solution. In addition, chemical and optical quality was also evaluated by weathering and resistance tests. Microstructure was studied by field emission scanning electron microscopy (FESEM). The results indicate that a concentration of 1 mol % of La2O3 respect to the sol-gel coating formulation is adequate for La3+-ions to remain entrapped in the pores and interstitial sites of the silica matrix and thus behave as active corrosion inhibitors that are released when the metal/coating system is set in contact with the aqueous solution. Moreover, the precipitation of insoluble lanthanum products provides a self-healing mechanism which later plays as a protective barrier for the delay of early corrosion. The electrochemical study by joined cathodic delamination tests and EIS has proved to be very effective in evaluating not only the inhibitor effect of lanthanum compounds but also their self-healing properties.

Projects ref. MAT2015-65445-C2-1-R and MAT2015-65445-C2-2-R. Program Geomateriales 2 (S2013/MIT 2914 CAM EU) (Dr. J. Peña-Poza contract).