| Chitosan is a natural polymer product which is obtained from the deacetylation of chitin. It has been used in many fields because of its non-toxicity, biocompatibility and multi-functionality. Since there is a lot of hydroxyl and amino groups in the chitosan structure, which has interactions with metallic surfaces by adsorption, chitosan becomes a potential corrosion inhibitor of metal. However, as it is insoluble in water, chitosan can only be dissolved in some diluted acid. Meanwhile, the efficiency is unsatisfactory when chitosan is directly used as a corrosion inhibitor. Therefore, it is significant to product a chitosan derivative with the features of good corrosion inhibition, water solubility and pro-environment. Thiourea and its derivatives have been studied as inhibitors for corrosion especially for iron and steel for a long time. In recent years, many studies have been focused on the modification of chitosan based on introducing thiourea which can offer more -NH2 and enhance active adsorption points. But people pay more attention on their adsorption of heavy metal ions rather than corrosion inhibition performance. Also, the solubility of thiourea-chitosan which was synthesized based on chitosan is not very good.In this study, in order to enhance the solubility, thiourea-chitosan (TUCTS) was synthesized based on degraded chitosan (CTS) and the optimal conditions of the synthesized TUCTS were established. The structure of TUCTS was characterized by FT-IR,1H-NMR, elemental analysis and surface properties measurements. Additionally, the inhibition performance of TUCTS on Q235 steel in 1 mol·L-hydrochloric acid was studied by weight loss method, polarization curve, electrochemical impedance spectroscopy and surface morphology analysis.The optimal conditions of the synthesized TUCTS are as follows:the mole ratio of CTS to epichlorohydrin is 1:4, the volume of acetone and H2O is 250 mL, the first step of the reaction temperature is 30℃ and the second step of the reaction temperature is 60℃, the mole ratio of CTS to thiourea is 1:6. The CMC value of TUCTS is 200mg·L-1. When the concentration of TUCTS increases from 5 to 200 mg-·L-1, the surface tension dropped rapidly to 40.14 mN·m-1The weight-loss methods analysis show that the corrosion of Q235 steel in 1 mol·L-1 hydrochloric acid is significantly inhibited after adding TUCTS at 25℃and 40℃ and the inhibition efficiency increases rapidly when the concentration of TUCTS increases from 5 mg·L-1 to 200 mg·L-1. This result is consistent with the surface tension analysis. When the concentration of TUCTS was increased continuously, the inhibition efficiency trended to smooth rise, and it reached the maximum value (90%) when the concentration was 400 mg-L"1 because the inhibitor adsorption on metal surfaces is approaching saturation. When the temperature rises to 60℃, the corrosion inhibition efficiency of inhibitor declined somewhat. The rate of corrosion inhibition increases with the rise of the concentration of TUCTS, and it still can be 88% when the concentration was 1000 mg·L-1. However, the protective effect of inhibitor decreased as the temperature reaches to 80℃.Polarization measurements reveal that TUCTS is a mixed-type inhibitor and inhibits both of the cathodic reaction and the anode reaction. The inhibition efficiency increases with the concentration of TUCTS The anodic and cathodic Tafel slopes of prepared inhibitors changed slightly, which indicate that these inhibitors affect both anodic and cathodic reaction but don’t change the reaction inhibition mechanism.EIS shows that the charge transfer resistance is proportional to the inhibitor concentration, which indicates the protective film on the Q235 steel surface is formed by the adsorption of the inhibitor molecules. The obtained Nyquist plots of Q235 steel with corrosion inhibitor show the phenomenon of degradation and present a single capacitive loop, which is consistent with the situation without TUCTS, indicating that the presence of inhibitor does not change the mechanism of steel dissolution. The increase of Rt indicates that the charge transfer reactions occurring in the protective film are strongly inhibited by the adsorbed inhibitor molecules. With addition of 200 mg·L-1, TUCTS can protect Q235 steel from corrosion in 168 hours at 25℃.The surface morphology analysis show that the corrosion of Q235 steel in 1 mol·L-1 hydrochloric acid is significantly inhibited after adding TUCTS at 25℃ and 40℃. However, the protective effect of inhibitor decreased as the temperature increased. This result is consistent with the weight loss method. |
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