Study On The Passivation And Depassivation Behavior Of Ductile Iron In The Pore Solution Of Simulated Cement Mortar

Due to the gradient difference in calcium ion concentration between the cement mortar pore solution and water,calcium leaching occurred in the cement mortar lining in the ductile iron pipe which is under long-time water conveyance,with the dissolving of calcium hydroxide and CSH,the decrease of the pore solution pH,the increase of the cement mortar porosity,and the acceleration of chloride ion invasion rate.When chloride concentration increases to a certain value,the surface passive film of ductile iron will be destroyed and corrosion will occur.The durability and service life of ductile iron pipe lined with cement mortar will be seriously affected.In this paper,through the design of experiments,combined with electrochemical testing and micro-testing methods,the depassived behavior of ductile iron in cement mortar pore solution with different pH values was studied,and the passive behavior of ductile iron in slag-cement composite slurry pore solution was explored,and the corrosion condition of ductile iron under the action of two kinds of corrosion inhibitor was analyzed.The main research results are as follows:（1）The depassived behavior of ductile iron in simulated cement mortar pore solution was studied by analyzing the changes of the electrochemical impedance spectroscopy（EIS）,tafel polarization curve and other electrochemical parameters with the pH value and chloride concentration of solution.X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy（SEM）and atomic force microscope（AFM）were used to analyze the chemical composition and microstructure of passivation film on ductile iron surface in simulated solution with different chlorine concentration.The results show that,the chloride threshold of ductile iron desensitization increases with the pH in solution and is exponentially related.FeO,Fe₂O₃ and FeOOH are the main components of the oxidation layer on the surface of ductile iron.When the concentration of chloride in solution exceeds the threshold,the Fe²⁺/Fe³⁺rate on the DI surface will reduce greatly.（2）The real slag-cement composite slurry pore solution was prepared by solid-liquid extraction method.The EIS and tafel polarization curve were used to measure the formation process of passive film on the ductile iron in different amount of slag-cement composite slurry pore solution.The SEM,EDS and X-ray diffraction（XRD）were used to observe the microstructure and chemical composition on the ductile iron surface.The results show that with the increase of slag content,the time required for complete passive of ductile iron increases,and the state of passive is not affected by slag content.（3）The inhibition effect of sodium D-gluconate and EDTMP.Na5 on corrosion of ductile iron in simulated cement mortar pore solution was studied.Tafel polarization method and extrapolation method were used.The corrosion process of ductile iron in simulated pore solution containing two kinds of corrosion inhibitors with different mass concentrations was tested and analyzed.The corrosion inhibition effect of two kinds of corrosion inhibitors on ductile iron was further analyzed by combining with the changes of surface morphology and chemical composition of ductile iron after corrosion inhibition.The results show that the corrosion inhibition effect of the two inhibitors increases with the mass concentration,and the inhibition effect of EDTMP.Na5 is better than that of sodium D-gluconate,while the compound inhibition effect of the two inhibitors is better than their single action.