A Comparative Study on the Anti-Corrosive Performance of Zinc Phosphate in Powder Coatings

Powder coatings are gaining popularity for their economic and environmental benefits. Additives (pigments) such as zinc phosphate enhance the anti-corrosive properties of coatings, but their behavior in powder coatings has not been extensively studied. In this study, zinc phosphate was incorporated into three powder coating systems: polyester clearcoat, polyester and epoxy coatings with filler BaSO4. Neutral salt spray and electrochemical tests (OCP, LPR, and EIS) confirmed that the anti-corrosive performance improved with the addition of zinc phosphate. The optimal additive dosage was determined to be 2% for all of the coating systems studied here, based on salt spray tests. Here, the time until failure increased by 1.5 to 2 times. Using electrochemical tests, an optimal additive dosage of 8% was found for the polyester clearcoat, while the other coating systems maintained an optimal additive dosage of 2%. Performance increased by as much as one order of magnitude based on resistance/impedance measurements. This suggested a synergistic effect between the additive and the filler. The passivation layer was confirmed by both X-ray diffraction and Raman spectroscopy. Based on the results and discussion presented in this article, 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. It is proposed that the two test methods characterize different aspects of the coatings, corresponding to their service conditions. This has theoretical and practical significance in the evaluation of anti-corrosive coatings. Other properties of the coatings, including adhesion, gloss, distinctness-of-image, and pencil hardness, were measured as per applicable standards and the conformance was verified.