| Popis: |
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. |
