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Synthesis and characterization of modified nano-TiO2 films for corrosion protection of stainless steel

Corrosion protection of metals has been widely studied all over the world. Various surface engineering methods are used to increase the corrosion resistance of metals. One of the most effective methods is to coat the metal surface with ceramic compounds, such as transition metal oxides, nitrides or silicides (1). Among these ceramic coatings, TiO2 nano-coatings are commonly used for protecting metals due to their excellent anti-corrosion, heat-resistant ability, and low-cost, etc. (2-4). A sol-gel and dip-coating technology is widely used for the preparation of TiO2 films on metal substrates (5). A TiO2 film can offer protection function only for the film surface is defect-free. In order to obtain smooth surface, compact and pore-free structure, it is necessary to modify TiO2 films by ion doping (6). In the present work, a TiO2 film and a B-Fe-Cemodified TiO2 film were synthesized on the surface of 316L stainless steel (316L SS) substrates by a sol-gel and dip-coating method. The corrosion resistance of the films and their protective properties for the stainless steel were investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. Tetra-n-butyl titanate (TBT) was used as titania precursor. 4.8 mL TBT was added to the mixture of 40 mL ethanol and 1.7 mL ethylacetoacetate (EAcAc), then 0.24 mL de-ionized water was slowly dropped into the above mixture under vigorous stirring for 24h. The B-Fe-Ce-modified TiO2 sol was synthesized similar to the above the method of preparing TiO2 sol by adding the appropriate amount of Fe(NO3)3·9H2O, Ce(NO3)3·6H2O and H3BO3. The molar ratio of Ti:Fe:Ce:B was 50:1:1:1. The 316L SS specimens were coated with the above sol solutions by a dip-coating method, respectively, then the samples were calcined at 450 for 2 h. The morphologies of the samples were observed by scanning electron microscopy (SEM). The results showed that the TiO2 film and the B-Fe-Ce-modified TiO2 film were composed of anatase TiO2 nanoparticles with the diameters about 15 nm and 10 nm, respectively. The corrosion protection of the different prepared films on the 316L SS in a 0.5 M NaCl solution was evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. The impedance spectra of the stainless steel with different TiO2 films contained semicircles for the capacitive reactance, but the charge transfer resistance of the B-Fe-Ce-TiO2/316L SS electrode was bigger than that of the TiO2/316L SS electrode, which demonstrated that the electrochemical reaction process of the B-Fe-Ce-TiO2/316L SS electrode surface was repressed. The EIS results suggested that the B-Fe-Ce-TiO2 film could provide greater corrosion protection for 316L SS in the 0.5M NaCl solution. The potentiodynamic anodic polarization curve measurements showed the similar electrochemical behavior analyzed by EIS. Upon increasing the potential above the corrosion potential, a stable passive region was found where the current density remained stable over the entire region. The potentiodynamic anodic polarization curve of the B-Fe-Ce-TiO2/316L SS electrode showed a larger stable passive region and a higher breakdown potential, indicating that the modified film had better corrosion resistance and protective property for 316L SS.