Surface, Chemical, and Mechanical Properties of Polyurethane-Coated Galvanized Steel Sheets

The current study deals with the surface, chemical, and mechanical properties of polyurethane-coated galvanized steel sheets. To report the corrosion resistance in various corrosive environments, various corrosion tests, including the salt spray test and the 100 percent relative humidity test, were carried out. The salt spray and 100 percent humidity tests show that the polyurethane-coated samples remain almost intact in all the corrosive environments and reveal higher corrosion resistance. A high-resolution AFM study reveals the surface topographic information of the salt spray tested samples for a depth of 6-50 ?m with a low surface roughness (~ 0.1 ?m)and reports an almost uniform coating over the substrate. UV resistance was studied through different tests where UV-vis and UVCON results reported better UV-resistant properties for the polyurethane-coated samples. The gloss test shows better gloss retention (60-80 percent), whereas both the flexibility and adhesion test and the crosscut tape test results, with the highest rating of 5B, show better adhesiveness of the coatings. Scratch hardness test (<1.2 kg load)depicts enhanced mechanical performance for both the samples, whereas the tensile study reveals higher YS (310 MPa), comparable UTS (344 MPa), and higher TEL (30.48 percent)values for the white sample. FTIR study uncovers the improved corrosion resistance and adhesiveness of polyurethane coating by showing multiple bands at 3445.07, 2931.46, 1724.9, and 1577.2 cm-1, which are attributed to the absorption of O-H and N-H, NCO or CH2 group, C=O stretching, and CH presence, respectively. The corrosion mechanisms of uncorroded coated samples, polyurethane-coated corroded samples, zinc-coated corroded samples, and bare metal corrosion have been elucidated.