Study On The Corrosion Behavior And Influencing Factors Of Iron Pipes In Water Distribution Systems

The drinking water goes through a long way from waterwoks to the customer and the secondary pollution is often ignored. The deterioration during transportation mainly reflect in the turbidity, colourity, total bacteria count and the concentration of iron etc. All these problems are closely related to the corrosion process in the distribution system. The main factors influencing the corrosion process include the water flow velocity, the concentration of residual chlorine and bacteria etc. This paper which aimed at ensuring the water security was carried out through static and dynamic experiments to analyze the mechanism of metal corrosion. It was implemented by cutting down the turbercles from pipe sections in water distribution systems and then isolating iron bacteria and sulfate-reducing bacteria to evaluate the effect of microorganisms on the growth surrounding. The electrochemical methods including Electrochemical Impedance Spectroscopy(EIS) and potentiodynamic polarization curve were applied to analyze the corrosion process. The surface anaysis was implemented by SEM and AFM. The influence of flow condition on corrosion behavior was also studied by dynamic experiments.It was found that the amount of iron bacteria and SRB was the largest on the surface of iron pipes, indicating that the adorption of pipe materials was important and the corrosion of iron pipes was closely related to microorganisms.The results revealed the corelation of flow rate and corrosion rate according to the running condition of water distribution systems. The increase of velocity accelerated the charge transfer and mass transfer process. Moreover, water flow was able to cause erosion-corrosion which was more detrimental to both pipe material and water quality. The retention time of water flow was another reason leading to the increase of corrosion rate. Besides, the water quality deterioration caused by iron corrosion reflected on the variation of turbidity, colourity, total iron and so on. Thus, the corrosion of iron pipes was a critical factor accounting for “ red water”.The relationship of residual chlorine and corrsion was established through measuring the corrosion rate under different conditions. Chlorine provided iron surface more opputunities to adsorb chlorine ions by improving the active spots, thus reducing the charge transfer resistance. Therefore, the active dissolution of anode was accelerated and the corrosion rate was increased. The pit etching was also observed on samples exposed to chlorine by SEM, indicating that the corrosion in the presence of chlorine was penetrated into the interior in the way of pitting corrosion.It was found that the corroion products formed in the presence of chlorine consisted of Fe2O3, Fe3O4, α-Fe OOH and γ-Fe OOH. The structure of corrosion products was coarse and hetergeneous with numerous pores. β-Fe OOH was likely to form with chlorine ions and easily converted into α-Fe OOH and γ-Fe OOH due to the unstability. Hence β-Fe OOH was not detected in XRD analysis. There was more space and pores between the phase of β-Fe OOH, producing the interspace for water to creative an appropriate surrounding to initiate pitting corrosion. Besides, chlorine ions might prevent corrosion products from converting into stable α-Fe OOH, thus inhibiting the formation of protective passive layers.