REPOSITORY > RESULTS

Abstract

Sustainability principles, such as reducing environmental impact, limiting the use of hazardous chemicals, improving energy and material efficiency, and extending product lifetime, are increasingly driving the development of advanced materials for industrial applications. In this context, this doctoral dissertation addresses the corrosion protection of aluminium alloy AA3003 using Cr(VI)-free conversion coatings, with a focus on systems based on trivalent chromium and zirconium (TCP), zirconium (ZrCC), and titanium (Zr/TiCC), which represent more environmentally acceptable alternatives to traditional chromate treatments. The study aimed to investigate the formation mechanisms of these coatings and the influence of processing parameters on their corrosion protection performance.
Conversion coatings were prepared by diluting concentrated solutions in aqueous baths according to the manufacturer’s recommendations, with solution concentration varied within a selected range. The coating formation process was monitored by open circuit potential measurements, providing insight into its multi-stage nature, including the initial activation of the aluminium surface, growth of the conversion layer, and subsequent densification. Based on the potential evolution, characteristic conversion times were determined and used for further characterisation of the coatings.
The corrosion protection properties of the prepared coatings were evaluated using potentiodynamic polarisation measurements performed after air drying and after immersion in 3.5% NaCl solution and simulated acid rain, as well as by long-term exposure tests in a salt spray chamber. The results showed that conversion time significantly influences the electrochemical behaviour of the coatings, while longer treatment times do not necessarily lead to improved protection. Differences in coating stability were observed depending on their chemical composition and exposure conditions.
To explain the observed differences in protective performance, the morphology and chemical composition of selected coatings were analysed using surface analytical techniques, enabling correlation between structural characteristics and electrochemical behaviour. It was found that trivalent chromium-based coatings (SurTec® 650) form relatively compact, uniform, and multilayer structures with a thickness of approximately 40 nm, providing effective barrier protection and stable performance in chloride environments.
In contrast, zirconium-based coatings (MC1700) are thinner (approximately 20 nm – 50 nm) and exhibit a nodular morphology, forming on the matrix and intermetallic particles, resulting in relatively uniform coverage and good long-term stability. Conversely, the Zr/Cr(III)-based coating (MC1300) exhibits a coarser and more heterogeneous structure with pronounced localised growth primarily on intermetallic particles, resulting in less uniform surface protection.
Comparative analysis of the protective performance, particularly based on long-term salt spray exposure, showed that, in aggressive chloride environments on aluminium alloy AA3003, , a general trend TCP > ZrCC > Zr/TiCC can be identified. TCP coatings (especially ST650 and B30002) exhibit the highest protection efficiency, followed by zirconium-based systems, while Zr/Ti-based coatings show the lowest long-term stability. The results of this doctoral dissertation contribute to a better understanding of the formation and protection mechanisms of Cr(VI)-free conversion coatings on aluminium alloy AA3003 and provide guidelines for the optimisation of industrial surface protection processes. The research, therefore, has both scientific and practical relevance.