| Reinforced concrete is the most widely used bulk building material in the world and has good durability.In the harsh marine environment,the corrosion resistance of steel rebars is one of the important factors that determine the durability of the entire structure.Therefore,enhancing the corrosion resistance of steel rebars is particularly important for the service safety and life extension of concrete structures.In order to improve the corrosion resistance of carbon steel rebar in the marine atmosphere and prolong its service life,this paper envisages the preparation of a passivation film similar to the corrosion resistance of the passive film on the surface of stainless steel on the surface of ordinary carbon steel rebar.Thus,a carbon steel rebar with high corrosion resistance is obtained.Therefore,this thesis carried out a series of work on the preparation of chemical conversion coatings on the surface of ordinary carbon steel rebar,and the study of its structure and corrosion resistance.In this paper,a cerium passivation film(Ce film)was prepared on the surface of mild steel rebar.The effects of different film formation temperature,film formation time and salt solution concentration on the corrosion resistance of the film were studied through orthogonal experiment design.The Ce film obtained by the optimized preparation process has excellent corrosion resistance in the concrete environment containing high concentration of chloride ion(0.6 M NaCl),which is 100 times the charge transfer resistance of the bare steel rebar surface.After immersing for about 1600 h,the corrosion resistance of the film did not decrease like ordinary steel rebars or other protective measures,but increased by about 10 times.Through high-resolution microscope observation,semiconductor performance and interface composition characterization,it is found that the Ce film is about 13 nm thick with a double-layer amorphous structure,a 10 nm CeOx upper and a 3 nm FeOx under layer film.In the simulated environment of chloride ion concrete,the low defect structure of the amorphous film layer has a good blocking effect on chloride ions.The strong oxidizing Ce4+ in the outer layer oxidizes Fe2+ in the lower layer film,making the lower layer film denser,avoiding the direct rapid oxidation of iron on the surface of steel rebar.This self-densification process significantly improves the corrosion resistance of the Ce film.On the basis of the cerium passivation film(Ce-Mo film),the addition of molybdate improves the pitting corrosion resistance of the Ce film in low-concentration sodium chloride(0.1 M NaCl)reinforced concrete environment.A corrosion-resistant cerium molybdate passivation film that can be repaired by self-passivation after being slightly damaged under the condition of applied potential is obtained,and its protection potential is 0.9 V.Long-term immersion in this environment for about 1600 h,its charge transfer resistance has improved three times compared with the initial immersion.After analyzing and characterizing the semiconductor properties and interface components of the film layer,it was found that the film layer has a double-layer film structure,the outer film is CeMoOx,and the inner film is FeOx.The MoO42-in the outer film layer has cation selectivity,which reduces the erosive force of Clto the film and makes the film performance more stable in the service environment.The redox between Ce4+and Mo4+in the outer film increases the Mo6+in the film,enhances the cation selectivity of the film,and improves the pitting resistance of the film.The self-densification process between Ce4+and Fe2+of the inner film further strengthens the corrosion resistance of the film.The corrosion resistance of the prepared Ce-Mo film in the reinforced concrete environment with different chloride ion concentrations was researched,and the chloride ion threshold of the Ce-Mo film in the reinforced concrete environment was 0.5 M.Calculations show that the cerium molybdate passivation film has lower internal stress and stable performance of the passivation film,which improves the re-passivation and self-repair performance in a low-concentration chloride-ion concrete environment.The three elements of molybdenum,chromium and nickel are added to the Ce passivation film.The synergistic effect of multiple elements produces a corrosion-resistant film that is similar to the surface composition of the stainless-steel passivation film(Ce-Mo-Cr-Ni film)and is suitable for concrete environments.The BP artificial neural network model of the CeMo-Cr-Ni film was established based on the results of the orthogonal experiment data.The relationship between the film formation parameters and corrosion performance of the film is predicted,and the process range in which the film is in the best performance is obtained.The study found that the addition of these three elements can not only effectively improve the repassivation self-repairing performance of the film in low-concentration sodium chloride(0.1 M NaCl)concrete environment,but also improve the corrosion resistance in seawater-like concentration sodium chloride concrete environment.When immersed in 0.6 M NaCl concrete environment for about 1600 h,its charge transfer resistance is 148 times that of bare steel rebar.The Ce-Mo-Cr-Ni passivation film is a single-layer film structure.The cerium element not only plays the role of densifying the film layer,but also undertakes the cross-linking interaction between the rebar and the film layer.At the same time,MoO42-improves the cation selectivity of the film,and Ni plays a role in stabilizing the film.The Cr6+in the film oxidizes the Fe in the rebar matrix,repairs the defects in the film,and makes the film denser.This selfdensification process significantly improves the corrosion resistance of the Ce-Mo-Cr-Ni film. |
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