Study On The Failure Mechanism And Growth Law Of Passivation Film Under The Influence Of Rich Chlorine And Stray Current

In the alkaline environment of concrete,the steel bar can form a dense passivation film to protect the internal steel bar from corrosion.The premise of corrosion of reinforcement is the failure and damage of passive film.Submarine high-speed railway tunnel may be a hot area of infrastructure construction in the future because of its advantages such as fast speed,large transport volume,and little impact from bad weather and sudden war.However,the chloride ion rich in the seabed environment,the stray current generated by the high-speed rail power supply system and the small high temperature caused by it have adverse effects on the passive film,which can significantly increase the risk of corrosion of the reinforcement in the tunnel lining concrete structure,thereby threatening the safety of the entire tunnel structure.Based on the chloride ion environment and stray current environment,the failure process of passive film under the action of external adverse factors is discussed in this paper.By analyzing the growth law and microstructure characteristics of the passive film under the circumstance of stray current and chloride ion,the critical conditions of corrosion of steel reinforcement are studied and determined.The author reveals that the mechanism of corrosion,control of corrosion degree and prevention of corrosion are of great significance.HPB300 and HRB400 are the main load-bearing reinforcements in reinforced concrete structures.Due to the difference of element type and composition proportion,the performance of the passive film is also different,so it is necessary to test the two kinds of steel bars separately.The effects of stray current,chloride ion concentration and temperature on the corrosion of HPB300 reinforcement were tested through electrochemical series of tests,and the influence rules of different factors and combinations on the corrosion of reinforcement were analyzed;By comparing and analyzing the passive film state under different working conditions,the corrosion prediction models of HPB300 reinforcement based on breakdown potential,passive area length and chloride ion concentration in corrosion holes were established respectively.In this study,the concentration of chloride ions significantly affects the length of the passivation region and the breakdown potential position in the polarization curve.However,the structure of passivated membrane is more complete at lower potential.The passive film is not sensitive to chloride ions.The damage degree of chloride ions to the passive film is not high.The influence of temperature on passive film is obvious.The combined action of chloride ion and high temperature will aggravate the damage degree of passive film.High potential can break through the passive film and form corrosion holes,which plays a decisive role in the destruction of the passive film.The failure mechanism of passive film under the action of different factors was confirmed by scanning electron microscope and electrochemical experiments,and the influence degree of various adverse factors in the failure process of passive film was analyzed.It is found that the difference of warpage residual images of passive film after corrosion reflects the different failure modes of passive film under different working conditions;Combined with the experiments of electrochemical scanning electron microscope,dynamic polarization curve and electrochemical noise,the influence of adverse factors on the failure sensitivity of passive film was analyzed from multiple angles,and the deduction diagram of passive film failure process under coupling effect was established.The corrosion prediction models of HRB400 reinforcement were established based on the distribution of breakdown potential and open circuit potential.The corrosion resistance of HRB400 reinforcement is better than that of HPB300 reinforcement.It is recommended to use a single type of HRB400 reinforcement for subsea tunnel.From the microstructure,it is found that the passive film is not a traditional single-layer or inner and outer double-layer structure,and the internal structure of the passive film is not uniform and dense,but there are weak parts.The growth rule of passive film under different electric field intensity,the stability and long-term growth mode of passive film after being separated from solution were analyzed.The results show that the surface of the passive film presents the appearance of high and low undulation and non-uniform growth when tested for a short time.The thickness of the passive film increases slightly with the immersion time,and the surface undulation gradually decreases;In the long-term test,the passive film shows a local concentrated growth mode.The external electric field can instantly change the state of the passive film: the appearance of the passivation potential will rapidly increase the thickness of the passive film.The potential thickening effect(260 nm)of 0.5 h is far more than the thickness(100 nm)grown in the concrete solution for 3 days without potential;If the potential exceeds the breakdown potential,the passive film will be degraded and its thickness will be reduced.When the passive film leaves the alkaline solution environment of concrete for a long time,it not only stops growing,but also degrades to varying degrees.When the solution contains chloride ions,the chloride ions increase the thickness of the passivation film.But the density of passivated film is poor,resulting in easier degradation.High resolution transmission electron microscopy(HRTEM)was used to reveal the fine characteristics and dynamic evolution of the internal structure of the passive film during the growth process.A new mode of chloride ion destroying the structure of the passive film was found,and the mechanical basis of chloride ion passing through the ion channel was given.The results show that the passive film formed in alkaline environment has a large continuous crystal structure;With the support of experimental data,TEM images and physical background,a five-layer precise structure theory of passive film was proposed,and the growth process of multilayer crystals in passive film was defined;The long-term existence of chloride ions can produce a large number of ion channels,which will seriously reduce the compactness of the passive film.It is the main way for chloride ions to invade and destroy the passive film;At the special position of the interface between the ion channel and the crystal,the passive film grows preferentially along the specific crystal direction,which makes the passive film unable to increase the thickness continuously and uniformly.This phenomenon is closely related to the polycrystalline properties of rebar.The rationality of the five-layer structure in the process of passive film growth is verified by the calculation and simulation based on first principles.The simulation of the growth process verifies the growth law and variation characteristics of the passive film.