Corrosion After The Partial Spalling Of Cement Mortar Linings In Ductile Cast Iron Reclaimed Water Pipelines

The corrosion of metal pipes and cement mortar lining in the reclaimed water pipe network would form leak in the pipe network,affect water quality and threaten the safety of water supply network,and be hence harmful to the sustainable development of society and economy.However,the researches on the corrosion of cement mortar lining and metal pipe in reclaimed water pipe network are scarce either at home or abroad.In view of the quality of reclaimed water,the study of the co-corrosion mechanism of cement mortar lining and iron pipe in reclaimed water pipelines is aimed at achieving the control of reclaimed water pipe network corrosion,which is of great significance and value to ensure the quality of reclaimed water and the safe operation of pipe network.The purpose of this research is to study the corrosion behavior of ductile cast iron pipes and the cement mortar lining in the reclaimed water pipe network and the possible interaction and connection between them through the dynamic simulator.By using the water quality test,electrochemical technique and SEM analysis,the corrosion process and the distribution of corrosion products at the interfacial boundary of ductile iron,cement mortar and water,as well as the pollution of water quality,were studied at different degrees of spalling of inner lining in the simulation of reclaimed water pipelines under the flow system.The main research results were as follows:(1)There were differences in corrosion process and water quality under different levels of the inner lining spalling state.The ductile cast iron specimen without cement mortar lining spalling had the best effect on metal and water quality protection.As the spalling degree increases,the impact of corrosion process on water quality became more serious.However,in the later stage of reaction,as the film of metal corrosion product gradually densified,the influence of specimen corrosion on water quality was significantly reduced.(2)Whether the relationship existed between the ductile iron pipe corrosion and the cement mortar corrosion in the reclaimed water pipe network was related to the degree of spalling of the cement mortar lining.When most of the cement mortar lining was peeled off,there was an obvious mutual promotion between ductile cast iron corrosion and cement mortar corrosion at the two interfaces.(3)No obvious relation between the ductile iron pipe corrosion and cement mortar corrosion was found in the specimens with local cement mortar spalling,and the corrosion current density was generally declining.The surface of partially exposed ductile iron was quickly covered by gradually dense corrosion product film,forming a barrier with non-spalling cement mortar,inhibiting the anodic further dissolution and decreasing its corrosion rate.(4)The overall corrosion rates order of the four kinds of specimens were as follows: ductile cast iron specimens with most of the cement mortar spalling > cement mortar uncoated ductile cast specimens > ductile cast iron specimens with local cement mortar spalling > ductile cast iron specimen without cement mortar lining spalling.(5)As to the ductile cast iron specimen without cement mortar lining spalling,the corrosion current density was low and remained basically unchanged,and the diffusion process was the control step of the corrosion process;as to the corrosion process of the specimen with local cement mortar spalling,the transition from corrosion product film control formed on electrode surface to diffusion control was occurred;corrosion control step of the other two kinds of specimens both changed from the corrosion product film control formed on electrode surface to the charge transfer process control.