| During the service of reinforced concrete, penetration of chloride ions and concrete carbonation lead to damage of the passive film of the steel bars in concrete, which speeds up the corrosion process of the steel bars. To address this problem, micro/nano Al2O3-TiO2 ceramic coating was plasma sprayed on the surface of the steel bars so as to improve the corrosion resistance. In this paper, the bonding property of micro/nano Al2O3-TiO2 ceramic coating bars with concrete, which has an alkaline environment inside, and the corrosion resistance of the coatings in the alkaline and salt-alkaline environment are studied. The experimental results are expected to provide theoretical and technological basis for the engineering applications of such kind of steel bars.Three different types of coatings, i.e., micro Al2O3-13wt.%TiO2(AT13), micro Al2O3-40wt.%TiO2(AT40) and nano-structured Al2O3-13wt.%TiO2(nAT13), were used on steel bars by plasma-sprayed technology. Firstly, the phase evolution from powders to coatings were analyzed by XRD. The toughness of the coatings, the bonding properties of the coatings with the steel bars as well as with the concrete were tested. Secondly, the corrosion resistance of the coatings immersed in alkaline and salt-alkaline solutions was evaluated respectively by electrochemistry methods, and after the immersion, in order to study the underlying mechanisms contributing to corrosion of coated steel bars, the microstructure and constituents of the coatings were examined by SEM-EDS and XRD, respectively. Finally, considering that the bonding strength between the coatings and the steel bar was lower than expected, and the defects in the coatings significantly contributes to diffusion of chloride ions, the use of organic sealant was preliminarily studied to improve the properties of the coatings.Based on the experimental results, the following conclusions can be drawn:The surface morphology of AT13 and AT40 exhibits laminar structure, and micro-cracks can be observed on the surface of AT13. The fully-melted agglomerates of nAT13 has a compact bond with the steel bars and appears homogeneous at its surface. The porosity is high and the pore size is big for both AT13 and AT40 coatings,and AT 13 has a higher porosity than AT40. In contrast, nAT13 has a much lower porosity and smaller pore size than the other two coatings. The main phase evolution during spraying is the transformation of ?-Al2O3 to ?-Al2O3.In the three-point-bending tests, the structure of AT13 and AT40 were destroyed at a small angle, and AT40 was peeled off the steel bars, while nAT13 kept intact after bending even at an angle of 60°. The bonding strengths of AT13 and AT40 with their steel substrates are 28.60MPa and 29.62MPa, respectively. nAT13 has a bonding strength of 39.96MPa, which was about 30% higher than that of AT13. Since the ?-Al2O3 present in all the coatings can react with alkalis in the concrete to form a loose product, the bonding strength between the coating bars and concrete is lower than that between Q235 steel bar and concrete. However, the relactive bonding strength of among AT13, AT40 and nAT13 ceramic-coated steel bars and the Q235 steel bar with concrete by post treatment were 0.9,1.78 and 0.89, respectively, which meets the standard of the relactive bonding strength of epoxy-coated steel bars which should be above 0.8.Solution can move into the surface of the substrates through the defects of the coatings such as pores and cracks. When the pH value of the solution is 13.5, the OH- in the simulated concrete pore solution can react with ?-Al2O3 and Al in the coatings. The solid precipitates and reaction products can fill the pores leading to the occurrence of filling effect that make the coatings even denser. With the pH value of the solution decreasing, the corrosion of the reactants in AT13 and AT40 is decreased and the filling effect is weakened, while passivating effect of the permeated alkaline solution on the substrates is increased. As the nano particles has a high specific area and high reactivity, corrosion is observed on nAT13 after immersion in the solutions of different pH values. However, as the porosity of nAT13 is low, significant filling effect is observed. Besides, the erosion mainly occurs on the surface layer of the coating, and the internal structure is not affected.As all coatings exhibits significant filling effect in simulated pore solution, a dense coating can prohibit the penetration of Cl-. The naked Q235 steel bars corroded in the solution of 0.25 M Cl-. While introduction of 0.25M Cl- in the solution compacts all the coatings, which further protects the substrates. With the concentration of Cl- increased further, the corrosion resistance of all the steel bars under the coatings was decreased, and for AT13, pitting corrosion is observed in 1.0M Cl- solution, while AT40 and nAT13 have no corrosion tendency. The failure mode of ceramic-coated steel bars is that when the generation rate of large volume of corrosion products at the substrates is higher than the leaching-out rate of the corrosion products, the pores in the coatings will be enlarged and the coatings will even be peeled off. Moreover, the ceramic coatings are treated by dipping with a sealant of nano-SiO2 modified polyurethane to fill the pores and thus the corrosion resistance of the coated steel bars can be improved. |
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