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Abstract

The subject of the doctoral dissertation is the development of coatings with improved protection of metals against corrosion and antifouling protection on structural steel S355 and aluminum alloy AA7075. S355 is a widely used metal in the construction, while AA7075 is mostly used in the transportation industry. Although they exhibit suitable mechanical properties, both metals are susceptible to corrosion, especially in an environment containing chloride ions, and also to biofouling in the presence of bacteria and microorganisms. The main purpose of the dissertation is to upgrade the well-researched hybrid sol-gel siloxane-polymethylmethacrylate (siloxane-PMMA) coating into a more effective anti-corrosion coating with lower wettability. Such coating should prevent the penetration of the corrosive species to the surface of the metal and reduce the adhesion of microorganisms to the surface of the coating. The topic of the dissertation is therefore focused on (a) upgrading the synthesis of the siloxane-PMMA coating with different monomers, which have a different length of alkyl chains (Cn, n=1-12), (b) understanding the mechanism of synthesis, (c) optimizing the curing and application of the coating (temperature, application, thickness), (d) testing samples in a real environment and monitoring fouling and toxicity of coatings, and (e) combination of sol-gel coating with conversion coating based on zirconium and chromium.
The methodology for the characterization of sols as final coatings was diverse. FTIR spectroscopy and thermogravimetric analysis were used to characterize the sols, and the following techniques were used to characterize the coatings: NMR, XPS, ToF-SIMS, GD-OES, SEM-EDS, PBDS. The wettability of the coatings was also measured. Corrosion properties were evaluated by electrochemical measurements in NaCl solution and in a salt chamber. Monitoring the toxicity of the coatings with microbiological tests was performed with algae (Desmodesmus subspicatus) and water fleas (Daphnia magna Straus), and by testing in a real environment by monitoring the reduction of biofouling.
The starting point for the development of innovative coatings was based on the siloxane-PMMA coating, where the monomer MMA (methyl methacrylate) was replaced with a monomer containing a longer alkyl chain, namely: EMA (ethyl methacrylate), BMA (butyl methacrylate), HMA (hexyl methacrylate), OMA (octyl methacrylate) and DMA (dodecyl methacrylate). The coatings were synthesized under the same synthesis conditions, with the same molar ratio of reagents, resulting in coatings of comparable thickness. Wettability was found to decrease with longer alkyl length until butyl group and then increase again. Coatings with longer monomer length (Cn, n>4) had a more porous structure. The results of the electrochemical measurements and the results of the tests in the salt chamber confirmed that the coatings containing the monomer with ethyl and butyl groups had the best anti-corrosion properties, while the coating with the methyl group was slightly worse. Coatings with longer alkyl chains showed relatively poor anti-corrosion properties, as their structure is not homogeneous. The reason for such properties is the steric effect of longer chains and less efficient synthesis due to differences in the polarity of the reagents.
A siloxane coating with BMA monomer, which had the lowest wettability, was chosen for further study of the optimal system for anti-corrosion protection. The minimum curing temperature (150 °C) and thickness (4 µm) were optimized for long-term corrosion protection. Reduction of coating delamination from metal surface was achieved by primer application of the Zr/Cr conversion coating.
Coatings containing shorter chain monomers (Cn, n4) were evaluated for toxicity when used in a real environment. The results confirmed that siloxane coatings with ethyl and butyl monomers exhibit lower toxicity than hitherto known siloxane-PMMA.
The results in this dissertation showed that it is possible to upgrade the siloxane-PMMA coating by using acrylate monomers with the length of alkyl chain up to butyl and at the same time improve corrosion protection properties, increase wettability and minimise toxicity.