Changes in protective properties of zirconia and silica sol-gel layers over time

The study investigated the effect of aging time on the protective properties of submicron thickness sol-gel coatings. Coatings were based on zirconia and silica modified with fluoroalkyl substituents that were deposited on P265GH steel and were aged for 3 years. The corrosion tests were carried out in sodium chloride solution and liquid caprolactam. The physicochemical properties of the obtained materials were determined using scanning electron microscopy (SEM), Raman spectrometry, and nuclear magnetic resonance (NMR). The roughness and wettability of the tested samples were also measured with a profilometer and tensiometer. The protective properties were determined by electrochemical and immersion tests. The coating containing ZrO2 was more stable over time. In the case of the ‘SiO2 + F’ fluorine-modified coating, polarization resistance increased by about 3 orders of magnitude compared to the uncoated P265GH steel. However, after 3 years, the protective nature of this type of coating had deteriorated, compared to its initial state. Moreover, it seems that the ‘ZrO2’ coating better covered all valleys in the surface, forming a smoother surface, while the ‘SiO2 + F’ coating better reproduced the surface topography, also depositing on the ridged surface and creating a thin continuous film.