|dc.description.abstract||This thesis focuses on the structural and compositional characterisation and corrosion performance of porcelain enamel coatings commonly used on metal substrates for protective and decorative purposes. These characteristics need to be better understood so the coatings remain competitive with alternative surface treatments/finishes. Raman and infrared spectroscopic techniques were used together, for the first time, to investigate porcelain enamel coatings. Five commercial porcelain coatings and their component raw materials were examined. In addition, the more conventional techniques for analysing porcelain enamels - EDS, ICP and XRD, were used. This complement of techniques makes this a unique and comprehensive study of enamel coatings. The effect of compositional changes on corrosion characteristics of porcelain enamels was then investigated using model formulations. Composition diagrams were constructed and used to explain acid and/or alkali attack on the commercial coatings.
Spectroscopic investigations showed that porcelain enamels are complex systems incorporating many different amorphous and crystalline phases. The main compounds in the frit samples investigated were silicates and borates. Si-O and Si-O-Si vibrations characteristic of SiO₄ tetrahedra were found both in the Raman and IR spectra. However, only the IR technique was capable of identifying the B-O bonds associated with B₂O₃. Amorphous silica was confirmed by XRD analysis. Amorphous phases and crystalline compounds were found within the fired glass enamel coatings. These were characterised by vibrational spectroscopy and XRD. The dominant compounds in the amorphous phase are borates and silicates as in the case of the frits. These compounds were identified through the presence of B-O-Si, Si-O-Si and B-O stretching modes in the Raman spectra. The ATR-IR spectrum of the coatings, which are surface specific, is dominated by B-O stretching modes suggesting that borates were at the coating surface. Many of the crystalline phases in the fired enamel coating detected by XRD and vibrational spectroscopic analysis including FeCr₂O₄, Cr₂O₃ and SiO₂ matched those in the colouring pigments.
Corrosion of the enamel coatings was examined in terms of their solid matter (destruction of glass network) as well as their heterogeneity (presence and nature of bubbles in coatings). Composition diagrams developed for porcelain enamel coating systems were useful for correlating the phases in the glass matrix with specific properties to help explain coating corrosion performance. Varying the ratio of glass network formers and modifiers changed the structural properties of the coating and hence, its corrosion performance. Enamels with higher SiO₂ content, but similar modifier to R₂O₃ ratios were more resistant to corrosion because more compact silicate tetrahedra were formed. This decreased the number of non-bridging oxygen atoms in the structure, thereby reducing ion exchange between modifier ions in the coating and hydrogen ions in the solution.
There was a general trend for increased corrosion to be correlated with increased bubble area in the top 20 μm of coating. By contrast, there was no correlation between the bubble area over the entire depth of the coatings and weight loss after acid or alkali exposure. This is attributed to the formation of a barrier when the coating is exposed to acids, thus inhibiting further ion diffusion, which consequently retards further corrosion. This indicates that the upper bubble area rather than the total bubble area through the coating depth is an important factor in chemical resistance.
The information from EDS analyses of crystalline phases in the gloss green A coating, including spectra around pinhole defects, were expanded with Raman microscopy data. Raman spectra of a cross-section indicate α-Cr₂O₃ and α-quartz whereas the pinhole defect had only α-Cr₂O₃. Elemental mapping showed that elemental Cr clusters were in small discrete areas, with greater concentrations around the margin of the pinhole defect.
This study demonstrates the usefulness of Raman and IR in extending the understanding of porcelain enamel coatings. Using the set of techniques - Raman, IR, ICP, EDS and XRD - together, provided structural and compositional information, which can help improve the coating performance and the finish quality of enamel coatings. Composition diagrams developed from the ICP analytical information are very useful for developing coatings with better chemical resistance from optimised oxide ratios.||