Development of Flower Like Hierarchical Thiourea Loaded Titania–Poly Vinyl Alcohol Nano Composite Coatings for the Corrosion Protection of Mild Steel in Hydrochloric Acid

Abstract

A stable, thiourea doped titania–poly vinyl alcohol (PVA) hybrid nanocomposite anticorrosion coating on mild steel has been developed via sol–gel dip-coating method. For comparative studies, bare titania sol–gel is coated over mild steel and evaluated using electrochemical methods. To investigate structure and morphology of coating, fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectrometry (EDX), X-ray diffraction (XRD), UV absorbance spectra, and atomic force microscopy (AFM) techniques were employed. Effect of drying temperature of coated samples at 100, 200 and 300 °C on the corrosion protection nature was analysed. The coating dried at 200 °C exhibit maximum corrosion protection ability. Thiourea at 1, 1.4, 1.8 wt% were incorporated into titania–PVA hybrid nanocomposite and the corrosion protection ability of these sol–gel coatings were evaluated by weight loss, impedance spectroscopic studies (EIS), and polarization measurement in 1 M HCl solution. FESEM shows that thiourea and the modifier PVA results the cracked bare titania coating to a flower like hierarchical distribution all over the surface of mild steel, AFM analysis revealed that the surface was found to be spiky in nature with surface roughness root mean square (rms) value of 0.33 µm and exhibit better corrosion resistance in 1 M HCl solution. The results of the study reveal that there is an optimum (1.8 wt%) of thiourea loading for beneficial effects on the corrosion resistance and increase the efficiency significantly (97%).This simple, facile and eco-friendly method has great potential to replace environmentally hazardous chromate conversion coatings. Moreover the modified coating on metal offers attractive protection efficiency on extending the exposure period in 1 M HCl to 70 days, which is far superior to the conventional sol–gel coating which losses efficiency day by day. The protective layer consists of a compact structure of hybrid PVA–titania and electron rich thiourea molecules on the surface of the metal offering significant physical barrier against the attack of corrosive ions from acid solution.