Hydrophobic coating using sustainable sol-gel process doped with carboxylic acids to protect heritage copper artefacts

Metal corrosion and its mitigation using alkoxysilane based sol-gel coatings presenting specific properties are a matter of interest in many areas of applications, from industrial to cultural heritage materials. Thanks to their adhesive and coupling nature with different molecules and substrates, they show excellent results against aggressive environments. This study focuses on the protection of historical copper-based alloys. A tetramethyl orthosilicate-based (TMOS-based) sol-gel coating was doped with carboxylic acid compounds (heptanoic, octanoic and decanoic acids) recognized for their non-toxicity and their interesting corrosion inhibition properties on ancient corroded copper. The development of the coating and the basic understanding of the fundamental chemistry as well as the protection mechanism on the historical samples were studied. Contact angle measurements allowed to show that the resulting coating displayed a hydrophobic protection at the surface of the samples while its chemical composition was evaluated using thermogravimetric analysis, SEM-EDS and Raman spectroscopy. The porosity of the sol-gel matrix and the distribution of the acids in the porous silica network were then investigated by establishing nitrogen adsorption/desorption isotherms at liquid N2 temperature. Examination of the sample's cross-section by SEM-EDS showed that the TMOS-based coatings doped with carboxylic acids penetrated up to 60 % in depth in the corrosion layer of the ancient copper, while the Raman analyses showed that the acids formed organometallic complexes both at the surface and in the corrosion layer, leading to the protection of the metallic substrate against further corrosion.