Corrosion Process Analysis And Life Evaluation Of Cement Mortar - Ductile Iron Based On Electrochemical Method

The cement mortar lining of ductile iron pipe which is in hydrodynamic enviorment for a long time, lead to cement mortar demineralization caused by calcium hydroxide dissolution or reduction of CSH gel decalcification of pore solution, there is little amount of calcium ions and a certain amount of chloride ions in the low pH water, further, it will increase the chloride ion concentration of the pore solution of the surface of the of ductile iron. Gradually, ductile iron surface solution alkalinity will be reduced or chloride ion concentration will increase.It will lead deassivation corrosion damage ductile iron surface gradually, it will reduce the durability and service life of the ductile iron pipe.Therefore, to study ductile iron critical chloride ion concentration and analysize ductile iron pipe lined with cement mortar in the process of water corrosion process then we can build the chloride ion transmission model under calcium leaching, all the above are very important to predict service life of the structure of cement mortar- ductile iron pipe.In this paper, by designing the experiment, using electrochemical testing methods,we studied relationship between the ductile iron critical chloride ion concentration, ductile iron alkalinity and the corrosion process of cement mortar lining ductile iron pipe in the water. we also analysized the influence of calcium leaching on chloride ions in cement base material during the transmission process. The main research content and results are as follows:(1)to carry out simulation of ductile iron surface passivation membrane characteristic test of pore solution, It configurated pore solution by using Ca(OH)2, NaOH and KOH.Through using the calcium hydrogen carbonate and sodium chloride solution to adjust alkalinity and chloride ions.By using electrochemical testing method, it made analysis of the corrosion potential of nodular cast iron electrode and the electrochemical impedance spectrum changing with the solution alkalinity and chloride ion content, at last, it got the results of Passivation membrane formation on the surface of nodular cast iron and depassivation process and established the ductile iron surface chloride depassivation threshold equation related to alkalinity of pore solution and chloride ion concentration. The results showed that:time of forming stable passivation membrane on ductile iron surface to decreases with the increase of the solution alkalinity; chloride ion depassivation concentration of ductile iron increases with the increase of solution alkalinity.(2)By using the method of accelerated calcium leaching, it made low flow rate of circulation water erosion tests and immerse erosion experiment.With electrochemical test method, it studied three kinds of liner accelerated corrosion process of nodular cast iron tube specimen and analysized the corrosion time, corrosion solution and influence of kinds of lining on corrosion degree of corrosion current density and electrochemical impedance spectroscopy in the ductile iron pipes lined with cement mortar. The results showed that: three kinds of lining of ductile iron pipe corrosion degree increase with the extension of corrosion time. lined with ordinary Portland cement grinding and water-based epoxy sealing layer of ductile iron pipes have maximum corrosion resistance.(3)the influence of calcium leaching on the chloride ions transmission process among cement mortar was studied, then established ion transport model under the condition of chloride, then it carried out the cement mortar lining of the layers of free chlorine ion concentration test experiment, and analyzed the test values and the model results. Then conducted analysis of the numerical simulation in 6m ammonium chloride accelerated corrosion case. The results show that the calculated value is consistent with the results of the experiment; when 6 m ammonium chloride corroside for 117 days, face free chloride ion concentration in the cast iron table solution get to ductile iron critical chloride ion concentration.(4)Based on electrochemical test methods and calcium corrosion of chlorine ion transport model, it made the lining life assessment of cement mortar ductile iron pipe. The results show that when deliver 0.4,0.5, and 0.6 mol/L chloride ion water quality, service life Of ordinary portland cement mortar lining of ductile iron pipe are 5910 days,5650 days and 3750 days’ respectively.