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% relative humidity test, were carried out. The salt spray and 100% 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 μm with a low surface roughness (~ 0.1 μm) 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%), 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%) 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.