Research On The Corrosion Processes Of The Typical Water And Wastewater Pipelines Made Of A Combination Of Iron/Steel And Cementitious Materials

Corrosion of water supply and drainage pipelines significantly affects the safety of urban water transportation.Among the municipal water supply and drainage pipe networks,pipes that are made of a combination of metal materials such as steel and iron,and cementitious materials such as cement mortar and concrete have been widely used.However,most of the current corrosion studies mainly focus on single metal or non-metal pipes.The corrosion of those pipes that are composed of both iron/steel and cementitious materials has been largely neglected.This study aimed to investigate the corrosion processes of two typical pipelines,i.e.the ductile iron pipes with cement mortar linings(CML)and the reinforced concrete pipes,which are widely applied in conveying reclaimed water and wastewater,respectively.The degradation process of ductile iron with CML in reclaimed water was analyzed by using the electrochemical and microscopic analysis.For reinforced concrete sewers,the corrosion mechanism of concrete and reinforcing steel bars in the corrosion hot spots inside sewers(crown regions and tidal regions)were investigated through both the lab-scale(laboratory corrosion chamber)and pilot-scale(pilot sewer system)experiments.Further,a Hybrid Gaussian Processes Regression(hybrid GPR)model was developed to support the calculation of sewer service life.The major conclusions were as follows.Two corrosion phases of the CML-iron system could be differentiated by electrochemical impedance spectroscopy(EIS)and potentiodynamic polarization(PDP)analysis:(a)passivation phase characterized by the transformation of passive film and enhancement of interfacial resistances,and(b)initiation and propagation of corrosion distinguished by the increase of interfacial capacitance and decrease of interfacial resistances.Three suitable equivalent electrical circuits to model different corrosion behavior were established.The non-destructive electrochemical methods were demonstrated efficient for corrosion monitoring of iron pipes coated with CML,which provided theoretical supports for assessing the protective efficiency of CML and deciding the certain state of ductile iron pipes.Rebar corrosion in sewers depended on the conditions of concrete covers and the development of concrete corrosion.When rebar was covered by concrete or exposed to sewer atmosphere with relatively low levels of gaseous H₂S,H₂S and the biogenic sulfuric acid produced in the superficial corroded concrete layer may not directly participate in corrosion reactions of rebar while oxygen was more likely responsible.When rebar was exposed directly to sewer gas with relatively high levels of gaseous H₂S or covered by poor-conditioned concrete,the corrosion reactions varied in different phases associated with the development of concrete corrosion.Besides,chloride ions played a significant role in initiating rebar corrosion.The interactions between concrete and rebar corrosion were quite intense in sewer systems.The acidic concrete corrosion layer would accelerate rebar corrosion and the dissolution and diffusion of rebar rusts,which in turn could stimulate the concrete cracking in both macro-and micro-levels.Tidal regions were the most susceptible to concrete corrosion in sewers,which should be served as the typical areas for corrosion inspections and monitoring.For the corrosion in tidal regions,the microbial succession occurred,with the coexistence of conventional autotrophic sulfide-oxidizing bacteria(SOB)Acidithiobacillus and acidophilic heterotrophic bacteria Bacillus initially,and the predominant colonization of Mycobacterium bacteria when p H dropped to around 1.The heterotrophic bacteria,i.e.Mycobacterium and Bacillus,might contain species that are capable of sulfuric acid generation.Mycobacterium was proposed for the first time as vital bacteria for concrete corrosion at tidal regions in sewers.Wastewater played a critical role for accelerating tidal corrosion through the effects of consistent hydraulic action,supply of water and substrates and inoculation.The combinations of advanced mineral and microbial analytical techniques were demonstrated effective in investigating reinforced concrete sewer corrosion.Hybrid Gaussian Processes Regression model achieved the best performance for both of extended and limited data in terms of Root Mean Square Error(RMSE)and correlation coefficient compared with Radial Basis Function Neural Network(RBF)and Multivariate Linear Regression(MLR).The derived variances of Hybrid Gaussian Processes Regression model were able to envelope the prediction uncertainties efficiently,thus supporting better decision making of rehabilitation and replacement of damaged sewers.