Research On The Passive Film And Corrosion Layer Structure Of Steel Bars Under Carbonization Condition Of C50 Concret

Microalloying is an important method to improve the strength of steel bar.The popularization and application of microalloying high strength can greatly save the raw materials of reinforced concrete structure.At the same time,the passivation and corrosion process of microalloying high strength steel bar can be accurately understood,which can provide scientific theoretical basis for the design and prediction of service life of reinforced concrete structure.In this paper,scanning electron microscopy,energy dispersive spectroscopy,transmission electron microscopy,X-ray photoelectron spectroscopy,X-ray diffraction spectroscopy,electrochemical testing and other characterization methods were used to study the passivation behavior of HRB500E microalloyed high-strength steel bars in simulated concrete pore solution with p H13.2～11.5,the growth mechanism and structure of passivation film,and the corrosion layer structure of steel bars under carbonation conditions of concrete.The results show that part of Mn S in Mn S inclusions and its composite inclusions is preferentially dissolved.The inclusions and cementite lamellae form a corrosion microcell with the surrounding matrix,respectively,and then a larger area of local corrosion begins until the iron oxide completely covers the surface of the steel bar.In the environment of p H≥12.5,the passivation film is mainly composed of contiguous nano-particles covering the surface of the steel matrix,and the surface is densely distributed with a multilateral passivation layer and calcium-rich precipitates.In the micro-carbonization environment of p H 11.5,the structure of the passivation film is slightly different from that in the high alkaline environment.The surface is scattered with polygonal passivation layers and calcium-rich precipitates.The Cr element accumulates in the passivation film and exhibits p-n junction semiconductor characteristics.In the environment with p H value of 13.2～11.5,the thickness of passivation film increased with the extension of immersion time.After 20 days of immersion in p H 13.5,12.5 and 11.5 environments,the average thickness of the passive film formed on the steel bar is about 2.94 nm,4.59 nm and 3.62 nm,respectively.Under the condition of concrete carbonation and chloride accelerated corrosion,the corrosion products of microalloyed steel bars are mainly siderite（Fe CO₃）,goethite（Fe OOH）,wuestite（Fe O）and brownmillerite（Ca O·Fe₂O₃）.The average rust expansion coefficient of the corroded layer is greater than 12.The samples were immersed in simulated solution for 10 days,the inner layer of the corrosion layer is a dense passivation layer,mainly composed of Fe O,Fe CO₃ and Ca O·Fe₂O₃.After 20 days of immersion,the dense rust layer in the inner layer disappeared,the rust layer stratification was not obvious,and it was transformed into a loose rust layer with many cracks in some areas,and the corrosion rate increased significantly,reaching 0.0715mm·year^-1.