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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Zhu, Jingxu

2nd Supervisor

Zhang, Hui

Co-Supervisor

Abstract

Powder coatings are green solutions to replace the environmentally unfriendly liquid paints for anti-corrosive purpose. In this study, UV-resistant polyester/TGIC powder coatings for outdoor application were developed instead of the widely used epoxy-based coatings. Three types of additives and functional filler, namely zinc phosphate with passivation effect, zinc powder with sacrificial anode effect, and two nanoclays with barrier effect were incorporated into the coatings to enhance the anti-corrosive performance. The anti-corrosive properties were evaluated by electrochemical techniques and neutral salt spray tests. The combined effects of the additives with a filler barium sulfate were also investigated. The effect of the film thickness was studied and optimized by the statistical method Mixture Design.

The additives and functional filler significantly improved the anti-corrosive performance of the coatings, e.g., Rp values of the nanoclay-containing coatings had increases of two orders of magnitude from electrochemical measurements. The salt spray resistance of zinc-rich coatings with 80% zinc increased from 500 h to 3000 h. The self-repairing capabilities of the coatings with zinc phosphate and nanoclays were confirmed by electrochemical impedance spectroscopy, and the mechanisms of zinc-rich powder coatings were investigated in detail. Neutral salt spray test gave different results from electrochemical methods for the optimal dosages of zinc phosphate and nanoclays. The discrepancy was caused by different features of the two tests, such that the electrochemical tests probe the function of intact coatings, whereas salt spray measures only the corrosion spreading from the scribe. This finding has theoretical and practical significance in evaluation of anti-corrosive coatings.

The surface qualities and mechanical properties of the coatings were evaluated as per applicable ASTM standards. Other characterization techniques, including SEM/EDS, confocal laser scanning microscopy, Raman spectroscopy, XRD, and microCT, etc., were utilized for analyzing coating structures and identifying the passivation layer, corrosion products, and the underlying mechanisms.

Summary for Lay Audience

Anti-corrosive paints and coatings are widely used to protect metals from corrosion. Most of these coatings are solvent-borne, and the solvents cause pollution and safety issues. Powder coatings are green alternatives to these conventional choices by eliminating the use of harmful solvents.

The performance of coatings is determined by the coating composition, processing parameters, and application methods. The components of coatings include resin, curing agent, pigment, and many types of additives. Unlike epoxy powder coatings commonly used for the anti-corrosive purpose, polyester powder coatings developed in this study can be used outdoors due to their excellent UV resistance. Anti-corrosive additives can be added into coatings to enhance the properties of coatings, and three types of additives were added to the coating composition. The first additive nanoclay had a barrier effect, and the second one, zinc, provided a sacrificial anode effect. The third additive, zinc phosphate, reacted with the coated steel panel to form a protective layer at the damaged spots.

These additives were added into both polyester and epoxy powder coatings to study their effects. The influence of their dosages and the coating film thicknesses were analyzed, and the optimal conditions for different applications and working conditions were determined.

The improved coatings were tested by applicable industrial standards, electrochemical and surface analysis techniques. The results showed that the anti-corrosive properties of the coatings were significantly enhanced. Different test methods gave different results, as they evaluated different aspects of the coatings.

The study contained mechanistic inquiry into details of many aspects of anti-corrosive powder coatings, and provided recommendations and guidance for coating research and development in the industry.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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