IntroductionChloride induced corrosion damage of the embedded reinforcement is one of primary reasons for the deterioration of concrete structures in civil engineering. Admixing organic corrosion inhibitors are considered as an effective way to halt the corrosion damage of the embedded reinforcement for its convenient use and high cost-efficiency. However, the corrosion inhibition efficiency of conventional corrosion inhibitors exists due to the limited amount of adsorption functional groups in molecular structures. It is thus essential to develop novel corrosion inhibitors with a great adsorption capacity to satisfy the efficient corrosion protection of reinforcement concrete in harsh marine conditions. As typical crystalline hybrid materials, metal-organic frameworks (MOFs) are self-assembled via coordination between metal nodes and organic ligands. Zeolitic imidazolate frameworks (ZIF-8, a representative MOF material) exhibit many heteroatoms and π-electron system, thus enabling its interaction and high corrosion inhibition potential for reinforced concrete. In this study, ZIF-8 corrosion inhibitors were synthesized and its inhibition effect for reinforced mortar was investigated. In addition, the characteristics of the reinforcement/mortar interface were also evaluated by image analysis based on scanning electron microspcopy (SEM) images and chloride titration.MethodsIn the experiments, 0.60 g Zn(NO₃)₂⋅6H₂O was dissolved in 4.5 mL deionized water. 11.50 g 2-methylimidazole was dissolved in 40.0 mL deionized water. The zinc nitrate solution above was then added into 2-MeIm solution in dropwise under stirring. After stirring for 24 h, white ZIF-8 corrosion inhibitors were collected via centrifugation at 10 000 r/min and then cleaned with deionized water for three times. The water to cement (w/c) ratio was 0.5 and the sand to cement (s/c) ratio was 3. The dosage of ZIF-8 corrosion inhibitors was 0% (blank), 0.2%, 0.4% and 0.8% per dry cement weight in mortar specimen, respectively. The reinforcement electrode was centrally embedded in cylindrical mortar specimen (Φ50×100 mm). The reinforced mortar specimens were then half-immersed in 3.5% (mass fraction) NaCl solution (i.e., 25±1 ℃ and 60±5% relative humidity). The chemical composition, thermal stability, morphology, and specific surface area of ZIF-8 corrosion inhibitors were determined by Fourier transform infrared spectroscopy, thermal gravimetric analysis (TG), field emission scanning electron microscopy (FE-SEM) and N₂ physisorption, respectively. Electrochemical workstation was used to evaluate the electrochemical behavior of the reinforced mortar during 210 d immersion with three electrode systems. The morphologies and chemical elements of the reinforcement were characterized by FE-SEM combined with energy dispersive spectroscopy (EDS). The compositions of product film on the reinforcement surface were evaluated by X-ray photoelectron spectroscopy. The total porosity at the reinforcement/mortar interface was obtained by image analysis based on the FE-SEM images acquired in backscattered electrons (BSE) mode. Chloride content in mortar matrix within 2 mm from the embedded reinforcement surface was measured by a potentiometric titrator.Results and discussionZIF-8 is prepared by a facile and eco-friendly solvent method. The negative OCP shift of the embedded reinforcement is dramatically prolonged by the admixed ZIF-8 corrosion inhibitors. The more pronounced beneficial effect is relevant to a higher dosage of ZIF-8 corrosion inhibitors. Corrosion initiation time for the embedded reinforcement in mortar specimens is delayed by 8.3%-43.3% at 0.2%-0.8% (mass fraction) ZIF-8 corrosion inhibitors. The corrosion damage of the reinforcement embedded in mortar specimens with ZIF-8 corrosion inhibitors merely occurs after 180 d immersion. ZIF-8 corrosion inhibitors can fill in the pore space in cement matrix due to small particle size (i.e., 75 nm), and the bulk mortar resistance of the reinforced mortar are continuously reduced with immersion age. Compared to the blank specimen, a higher resistance of the formed product layer and a lower capacitance of the formed product layer are relevant to the specimens with ZIF-8 corrosion inhibitors at the same immersion stage. Meanwhile, a higher charge transfer resistance and a lower electric double-layer capacitance are also related to the specimens with ZIF-8 corrosion inhibitors at the same immersion stage. Besides, a lower anodic current density appears for the embedded reinforcement in a mortar with ZIF-8 corrosion inhibitors, and this beneficial effect is more pronounced for a higher dosage of ZIF-8 corrosion inhibitors. The inhibition efficiency is 62.1% in a reinforced mortar in the presence of 0.8% (mass fraction) ZIF-8 corrosion inhibitors after 180 d immersion. The reduced dimensions and increased packing density of the corrosion products occur for the specimen with a higher dosage of ZIF-8 corrosion inhibitors. The chemisorption of ZIF-8 corrosion inhibitors is confirmed by the formation of coordination bond between nitrogen atoms in ZIF-8 corrosion inhibitors and iron atoms on the reinforcement surface. Furthermore, the enhanced peak intensity of Fe₂O₃ and FeOOH, together with the reduced peak intensity of FeO appear for the specimen with 0.8% ZIF-8 corrosion inhibitors. The porosity of cement matrix at the reinforcement/mortar interface is reduced by 9.8 % and the chloride content in the vicinity of the embedded reinforcement is efficiently reduced by 28.6% with the admixed ZIF-8 corrosion inhibitors in mortar specimen.ConclusionsThe corrosion initiation time of the embedded reinforcement in mortar was efficiently prolonged by 8.3%-43.3% in the presence of 0.2%-0.8% ZIF-8 corrosion inhibitor. ZIF-8 corrosion inhibitors could reduce the corrosion current density and increase the charge transfer resistance and polarization resistance of the embedded reinforcement after corrosion initiation. The inhibition efficiency of 0.8% ZIF-8 corrosion inhibitors was maintained at 62.1% after 180 d immersion in 3.5% NaCl solutions. ZIF-8 corrosion inhibitors exhibited an effective adsorption on the reinforcement surface, thus reducing chloride content on the reinforcement surface. Also, more highly protective products (FeOOH and Fe₂O₃) could form on the reinforcement surface. ZIF-8 corrosion inhibitors refined the pore structure at the reinforcement/mortar interface and reduced chloride content at the vicinity of the reinforcement. For specimen with 0.8% ZIF-8 corrosion inhibitors, the porosity of cement matrix at the reinforcement/mortar interface was reduced by 9.8%, and the chloride content of cement matrix in the vicinity of reinforcement was declined by 28.6%.