Preparation and properties of polyurethane cold galvanizing coatings with phosphoric acid modified zinc powder

In this study, polyurethane cold galvanizing coatings were prepared using zinc powder that had been modified with varying amounts of phosphoric acid. The effect of different amounts of phosphate (wt%. (1 %, 3 %, and 5 %)) was investigated on the morphology of modified zinc powder. Additionally, the impact of modification on the mechanical properties and corrosion resistance cold galvanizing coatings was thoroughly studied.The results demonstrate that an appropriate amount of phosphoric acid modification led to the formation of a suitable lamellar Zn3(PO4)2·4H2O on the coating's surface. This lamellar structure improved the bond between the coating and the steel substrate, as well as the coating's density, thereby enhancing the material's mechanical properties. At this level, the impact resistance reached 60 cm, the pencil hardness was 5H, the adhesion resistance was 0.8, and the flexibility reached 1 mm. The result shows that the ZnO and Zn(OH)2 increases the density of the coating, thereby providing a shielding effect and slowing down the electrochemical corrosion rate. Additionally, appropriate zinc particles combined with the layered Zn3(PO4)2·4H2O can reduce the chemical reactivity of Zn and play a shielding role, thus slowing the corrosion of the coating. The results of electrochemical tests showed that the corrosion resistance of the coating initially increased with the increase in phosphoric acid content, but then decreased. The optimal combination of mechanical properties and corrosion resistance was achieved when the phosphoric acid content was 3 %. The polarization test results show that after immersion for 168 h, the corrosion potential of the coating modified to 3 % phosphoric acid is up to −0.80 V, and the corrosion current density is 1.334 ± 0.67 × 10−6 A/cm2.