https://www.um.edu.mt/library/oar/handle/123456789/63162 2026-06-28T17:24:22Z 2026-06-28T17:24:22Z https://www.um.edu.mt/library/oar/handle/123456789/64004 2020-11-22T06:11:02Z 2019-01-01T00:00:00Z

Title: Sol-gel coatings for the protection of ferrous heritage metal Abstract: Ferrous Iron corrodes when exposed to indoor uncontrolled climates as is the case with Heritage Metal (HM) being showcased openly in a museum. One way of protecting metals from corrosion is by applying protective coatings. This work proposes a novel way of protecting HM through the application of organic-inorganic hybrid (OIH) silica coatings produced through the sol-gel method as opposed to more conventionally used coatings such as acrylates or nitrocellulose-based lacquers. Several precursors (X–Si(OR)3) were studied for their anti-corrosion properties, namely X= methyl, n-propyl-, n-hexyl-, n-octyl- and phenyl-triethoxysilane with tetraethyl orthosilicate (TEOS) in various mol% ratios (X–Si(OR)3:TEOS). Coating systems involving mixtures of X–Si(OR)3:TEOS displayed similar trends in their corrosion behaviour: the corrosion resistance of the coatings improves with increasing organic content reaching a maximum, the corrosion resistance is then observed to drop again with further organic loading. These observations have been explained through their mechanical properties. Higher silica content leads to harder, more well-adhered coatings; however, if the silica content is too high then brittle coatings form. On the other hand, if the coating contains a large portion of organics, then coatings will be softer and show poor adhesion, even though hydrophobicity is improved. OTES:TEOS 20:80 mol% and MTES:TEOS 60:40 mol% were further considered and applied onto corroded surfaces and their extent of corrosion protection on such a surface was also evaluated. Crack formation was visibly reduced by increasing the silica concentration, thus resulting in thicker coatings and though the increase in XTES:TEOS ratio, both leading to an improvement in corrosion protection. OIH alkoxysilane coatings impregnated with silica nanoparticles and in/organic chemical inhibitors have been considered in this work. Nanoparticles having a diameter of 10-20 nm purchased commercially (Commercial Nanoparticles, CNPs) produced a slight improvement in corrosion protection. Nanoparticles developed through a bottom up approach, the Stöber method (Stöber nanoparticles, SNPs) did not bring about an improvement in corrosion protection. More protective coatings were obtained on clean metal with in/organic inhibitor molecules when added to MTES sols, at a given concentration of inhibitor for all six inhibitors studied. This was not the case with OTES coatings for which only one of the six molecules produced an improvement over the blank coating. A comparative study was drawn between OIH silane coatings and conventionally used lacquers for HM namely, Paraloid B-48, B-72 and Ercalene. Electrochemical testing carried out on dip-coated clean metal coupons revealed OIH silane coatings to be more protective than the lacquers. This was not the case on corroded surfaces where OIH coating were found to be less protective than the lacquers which has been attributed to the thickness of coatings. For accelerated corrosion testing, thicker coatings were applied using both dip and brush application, the latter being a frequently used mode of application by conservators. OIH alkoxysilane coatings were significantly more protective than lacquers using both applications. Finally, a reversibility study on all OIH silane coatings was conducted where it was determined that all coatings can be removed using a mixture of sodium hydroxide solution with isopropyl alcohol solvent. This thesis proves the suitability of OIH silanes as coatings for HM since they are transparent and do not affect the aesthetics of the object, are applied without affecting the microstructure of the metal, provide significant corrosion protection which exceed that offered by conventional lacquers, and can be removed with relative ease. Description: PhD

2019-01-01T00:00:00Z