Electrochemical Corrosion Of Typical Water Supply Metal Pipe And The Impact On Water Quality

Internal corrosion of domestic water supply pipes often leads to water quality deterioration problems. Corrosion properties of two typical metal pipes used in water supply networks and the effect of corrosion on water quality in the pipe were researched from the electrochemical point of view. After that, some practical anti-corrosion measures were put forward.Ductile iron pipe and stainless steel pipe are commonly used in new water supply networks, while it is rare about the research on corrosion of the two pipes under domestic water. And the experiment condition of some previous tests is quite different from actual situation. Some experiments, for example, were conducted under high concentration of chlorine, or under different concentration of inorganic acid, organic acid or alkali. It has been revealed that biocorrosion and electrochemical corrosion are common in the inner wall of water supply pipes. Research on biocorrosion is mainly about sulfate-reducing bacteria and iron bacteria, while it is seldom seen that the papers on electrochemical corrosion under domestic water condition. Electrochemical corrosion is one of the main types in respect to water supply pipe corrosion, especially for the new networks, of which corrosion is characterized by electrochemical factor and biocorrosion is a contributing factor. So, research about electrochemical corrosion and comprehensive corrosion of water supply pipes under domestic water condition is both necessary and practically significant.Corrosion characteristics of ductile iron coupon, 304 stainless steel coupon and ductile iron pipe under domestic water condition were studied in an electrochemistry perspective. Sterile condition was set as a reference to study impacts of microorganism on corrosion.In the third and the fourth chapter, corrosion characteristics of two metal pipes were examined in the municipal water environment. A three-electrode cell was developed to study the corrosion behavior of the two types of pipe in static domestic water. Free-corrosion potential, free-corrosioncurrent density and corrosion rate of the metal were obtained through Tafel extrapolation methods. Polarization resistance in the process of electrochemical corrosion of the metal and the changes of corrosion environments were analyzed by electrochemical impedance spectroscopy(EIS). Results showed that corrosion resistance of ductile iron in static domestic water is poor, and the metal was prone to comprehensive corrosion that was based on electrochemical corrosion. Static corrosion experiments showed that the mean free-corrosion potential(relatively saturated calomel electrode, the same below) during 118 days of the ductile iron coupon under non-sterile water environment was-0.793 V, and-0.763 under sterileenvironment(the reference group). Both were significantly lower than the free-corrosion potential in soil(about-0.5 V), which reflected the great corrosion tendency of ductile iron.The corrosion rate of ductile coupon in the microorganism involved environment was about 0.8 mm/a, which was nearly four times of that of the reference group. The result indicated that microorganisms can significantly affect the process of ductile iron corrosion. EIS showed that polarization resistance of electrochemical corrosion of ductile iron coupon in static domestic water was small, which was less than 2 kΩ·cm2.It was found that the corrosion resistance of stainless steel is so strong under the experiment condition that there was almost no corrosion. The free-corrosion potential of stainless steel in the non-sterile domestic water was-0.326 V, and the free-corrosion rate less than 0.02 mm/a.The two values of reference group were lower,-0.285 V and 0.01mm/a, respectively. And the impact of microorganism on corrosion of stainless steel was very small.In the fifth chapter, corrosion properties of ductile iron pipe and the effects of corrosion on water quality inside the pipe were studied under domestic water environments.The free-corrosion potential and corrosion rate of pipe were determined by polarization curves method. And the water quality during different corrosion stages was also analysised. Static corrosion tests showed that inner wall corrosion of ductile iron pipe where there was non-sterile water was faster and the corrosion tendency was great with free-corrosion potential reaching-0.83 V during 69 days.Water in the ductile pipe deteriorated rapidly. Value of color, turbidity and total iron of the water inside raised quickly. A water quality model was established using total iron and total number of colonies as indicators. And the model could reproduced the experimental data well.The free-corrosion rate measured in the test was smaller than real value due to the three-electrode electrochemical cell which used ductile iron pipe as its main body. The free-corrosion rate of non-sterile group was about 0.04 mm/a, and the reference group less than 0.02 mm/a, which also showed the influence of microorganism on the pipe corrosion.