Comprehensive study on the electrochemical, morphological, and adhesion properties of Cr-free thin film: with and without polyurethane coating

Abstract

The deposition process of titanium-based conversion coating (Ti-CC) and the influence of this layer on adhesion strength and anticorrosion performance of polyurethane coatings were investigated. Aluminum 2024-T3 (Al-2024) substrates were treated by Ti-CC at different pH (2.5, 3.5, 4.5, 5.5), Ti concentrations (0.5, 1, 5, 10 g/L), and immersion times (0, 30, 120, 300, 600 s). Electrochemical impedance spectroscopy (EIS) and direct current polarization (DC) were used to evaluate the anticorrosion performance of the different samples. The microstructure, chemical composition, and surface characteristics of the samples were studied by field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and contact angle measuring device. The anticorrosion resistance and adhesion property of the polyurethane coating were examined by salt spray and pull-off tests, respectively. Results demonstrated that the surface treatment of the Al-2024 substrate in titanium bath enhanced the anticorrosion performance of the substrate. According to the results of EIS and DC polarization, the sample treated in (Ti = 1 g/L, t = 2 min and pH = 4.5) showed the highest anticorrosion resistance (22.90 kohm cm2) and lower current density (icorr = 2.05 µA/cm2). The FE-SEM images revealed more uniformity and compact structure at the optimized condition. XPS spectrum demonstrated that the Al-2024 substrate was covered by aluminum and titanium oxide/hydroxide after surface treatment. Contact angle measurement indicated that surface treatment of the substrate by Ti-CC caused an increase in hydrophobicity of the surface. The Ti-CC improved the adhesion strength and anticorrosion performance of the polyurethane coating applied on Al-2024 substrates.