Experimental study on corrosion of ductile iron water supply pipe
The tap water corrosion of ductile cast iron for water-supply pipe (DCIWP) is studied by means of electrochemical method and mass loss method. Taking into account the effects of initial corrosion on the correctness of the experimental results, a mass loss test including a period of pre-corrosion is conducted to study the main influence factors for the corrosion process through the orthogonal test design. The result turns out that at first the corrosion rate of the iron fluctuates significantly, then declines gradually over time and becomes stable in 48 h finally. The influence of pH of the water and test temperature is roughly equal and far more than the impact of total hardness and total residual chlorine of the water. The main influence factors affecting the corrosion of the ductile cast iron can be ranked as follows: temperature>pH>total residual chlorine>total hardness.
|Key words： ductile cast iron water supply pipe corrosion rate electrochemical method mass loss method|
At present, the average leakage rate of urban water supply pipe network in China is about 20%, while Japan is about 10%, and the United States is 8% . The leakage rate of urban water supply pipe network has been high. Corrosion inside the pipeline is one of the main causes of leakage of urban water supply pipe network . On the one hand, the occurrence of corrosion in the water supply pipe reduces the hydraulic conveying capacity, increases the probability of leakage and squib, and on the other hand causes the deterioration of the tap water quality and affects the normal drinking of tap water [3, 4]. The internal corrosion rate is related to the tap water quality and the operating conditions of the pipe network. Li Yafeng et al  used cast iron test pieces to study the effects of dissolved oxygen, pH value, residual chlorine, flow rate conditions on corrosion in cast iron pipes by weight loss method; Wang Wei  based on the weight loss test of cast iron test pieces, The main order of the influence of pH, dissolved oxygen, residual chlorine, temperature and sodium chloride on the corrosion process in the pipeline was analyzed by grey correlation method; Fan Kangping  was plated with the actual pipe network simulation system. The corrosion of zinc pipes was studied to investigate the main influencing factors of corrosion in pipes.
Although the research on the corrosion rate and its influencing factors in the water supply pipeline has achieved certain results, the influence of the initial corrosion on the corrosion experiment results has not been specially studied. According to Shen Zezhen et al. , the corrosion process and residual layer thickness of cast iron in dilute sulfuric acid The research and the study of the corrosion rate of cast iron in reclaimed water with time in Liu et al.  show that the corrosion rate at the initial stage of corrosion is fluctuating. This has a large impact on the average corrosion rate calculated by the weight loss method. Therefore, the initial corrosion law can provide a theoretical basis for scientific design internal corrosion test, and obtain more accurate and reasonable corrosion test results.
In this paper, the initial corrosion law of ductile iron in tap water environment was studied by electrochemical method and weight loss method. Then the pre-corrosion was introduced into the weight loss test process. Four factors including temperature, pH value, total residual chlorine and total hardness were selected. The experimental design studies the relationship between the corrosion rate and the influencing factors in the ductile iron water supply pipe.
Table 1 Tap water quality during the test
2 Experimental methods
The weight loss method corrosion test piece is processed according to the standard coupon type I specification in HG/T 3523-2008 , and the size is: (50.0±0.1) mm×(25.0±0.1) mm×(2.0±0.1) mm, hanging hole φ (4.0±0.1) mm, the calculated reaction surface area is 28.00 cm2, and the pre-treatment and post-treatment of the corrosion test piece are carried out according to GB/T 18175-2000 . The spheroidal graphite cast iron working electrode used in the electrochemical test is processed by the wire-cut cast iron standard coupon by the WEDM process, and the size is (33±0.1) mm×(9±0.1) mm×(2±0.1) mm. The surface area is 7.44 cm2. Before the reaction, it is ground with 1000#, 1200#, 1500#, 2000# metallographic sandpaper to the surface without scratches. After washing with distilled water, it is degreased with absolute ethanol and anhydrous acetone.
Table 2 Four factors and three levels orthogonal test table
Fig.1 Curves of corrosion rate under three temperature versus time
The corrosive solution used in the experiment was prepared with tap water. The water quality of the tap water during the test is shown in Table 1. The main water quality parameter monitoring methods or instruments are as follows: HACH HQ 30 d water quality analyzer, mercury thermometer, HACH DR890 portable photometer, total hardness measured by GB/T 5750.4-2006 ethylenediaminetetraacetic acid disodium titration method .
The electrochemical experiment was carried out on the CS2350 electrochemical workstation. The three-electrode system was used. The ductile iron electrode was used as the working electrode, the saturated calomel electrode was used as the reference electrode, the large-area platinum electrode was used as the auxiliary electrode, and the etching solution was tap water. The electrochemical workstation is equipped with Corr Text software for testing. The test method is steady-state polarization dynamic potential scanning, setting timing measurement (once per hour), scanning potential: -0.05~0.05 V (relative to open circuit potential), scanning speed: 0.1 mV/s, one test cycle was 72 h, and the experimental temperatures were 15, 20 and 25 °C, respectively. The measured data was fitted and calculated by the three-parameter method to obtain the instantaneous corrosion rate. In addition, three sets of weight loss method corrosion comparison experiments were set up, the temperature was 15, 20 and 25 °C respectively, the corrosion solution was tap water, and the experimental period was 120 h.
The internal corrosion weight loss test was carried out on an RCC-II type rotary coupon corrosion tester. The experiment is divided into two stages: (1) Pre-corrosion stage: Place the dry weighing standard test piece in a beaker, add 2L of tap water, and let it stand for several hours (reaction time is obtained by electrochemical test results, 50% update every 12 h) The volume of tap water), take out the parallel test piece, clean and weigh. (2) Formal experiment stage: Take out the remaining pre-corroded corrosion test piece, place it in the target corrosion solution, update 50% volume of corrosion solution every 12 h, and start operation for 72 h. After the end of the experiment, the corrosion test piece was taken out, washed, and weighed. Finally, the weight loss was measured in combination with the pickling blank and the pre-corrosion stage, and the average corrosion rate was calculated from the difference in mass before and after the corrosion.
The internal corrosion weight loss experiment uses the orthogonal experimental design method to study the influence of selected factors on the internal corrosion rate. The temperature, total hardness, total residual chlorine and pH value are selected for four-factor and three-level orthogonal experiments. The specific arrangement is shown in Table 2.
3 Results and discussion
3.1 Corrosion rate changes with time
The initial corrosion rate was measured by electrochemical method with time. The results are shown in Fig. 1. The corrosion rate of ductile iron in tap water environment with time can be obtained from Fig.1: within 24 h of the initial corrosion, the corrosion rate fluctuates greatly and has a significant downward trend; after 24 h, the corrosion rate tends to be stable. , slow decline; after 48 h, the corrosion rate is basically unchanged, reaching a stable stage.
The initial corrosion rate was measured by time using the weight loss method. The results are shown in Fig. 2. The curve of corrosion weight loss corrosion rate with time at three temperatures in Figure 2 further shows that the initial corrosion stabilization time of ductile iron is about 48 h, and the average corrosion rate values at different temperatures are different, which is also related to electricity. The results of the chemical experiments were consistent.
Fig.2 Curves of corrosion rate in mass loss test underthree temperature versus time
Table 3 Results of the corrosion mass loss orthogonal test
The results of orthogonal experiments show that: (1) The corrosion rate increases with decreasing pH. Combined with the principle of corrosion electrochemistry, as the pH value decreases, the depolarization effect of the cathode H+ on the metal surface is significantly enhanced, and the cathode hydrogen evolution reaction occurs, which accelerates the dissolution of the Fe component in the anode reaction. On the other hand, Safiur et al.  It has been found that the corrosion layer on the surface of the test piece tends to form an anaerobic environment, and the depolarization of microbial metabolites (such as H2S) accelerates the dissolution of metal components. (2) When the temperature is raised from 15 °C to 25 °C, the corrosion rate is significantly increased. In general, temperature accelerates the process of suppressing and accelerating corrosion. The temperature rise accelerates the anode process (iron oxidation and iron ion release) and the cathode process (oxygen and hydrogen diffusion rate), but the carbonate The process of inhibiting corrosion such as sedimentation also has an accelerating effect . It can be inferred from the experimental results that the acceleration effect on corrosion is stronger than the inhibition of corrosion with increasing temperature. (3) The corrosion rate decreases with increasing hardness, but the amplitude is weak. Calcium and magnesium ions in water under high hardness conditions are more likely to form a dense protective film on the metal surface, and combined with the formation of non-conductive products (mainly FeOOH and Fe2O3)  in the corrosion process, it has a certain inhibitory effect on corrosion, but this layer The formation of the membrane is susceptible to the surrounding environment, and its effect is not quite obvious, which is consistent with the experimental results. (4) The influence law between corrosion rate and total residual chlorine is not obvious. It is generally believed that residual chlorine has a bactericidal and disinfecting effect, can inhibit the growth of microorganisms in water, and thus weaken the microbial corrosion effect , so appropriately increasing the residual chlorine concentration can inhibit the corrosion of ductile iron. On the other hand, the decay rate of residual chlorine also increases with the increase of concentration. Therefore, when the residual chlorine concentration is 0.5 mg/L, the effect of reducing microbial corrosion is not as good as that at 0.25 mg/L. However, due to the test period Shorter, the microbial growth is not obvious, so the change of residual chlorine concentration has little effect on the corrosion rate.
In summary, combined with the results of orthogonal experiments, when the tap water temperature in the pipeline is between 15 and 25 °C, the pH is about neutral, medium hardness, and the total residual chlorine does not exceed 0.50 mg/L: temperature and pH. The effect on corrosion is basically the same, which far exceeds the influence of total hardness and total residual chlorine; the influence of various factors on the corrosion rate from large to small is: temperature>pH>total residual chlorine>total hardness.
- (1) Analyze the experimental results of electrochemical test and weight loss method, and find that the corrosion rate of ductile iron in tap water changes with time into two stages. The corrosion rate of the first stage fluctuates greatly and gradually decreases with time. The second stage The corrosion rate gradually stabilized and the stabilization time was about 48 h.
- (2) Comparing the results of orthogonal weight loss experiment with pre-corrosion stage, it can be seen that when the tap water temperature is between 15~25 °C, the pH value is about neutral, medium hardness, and the total residual chlorine does not exceed 0.50 mg/L. The effects of temperature and pH on corrosion are basically the same, far exceeding the influence of total hardness and total residual chlorine; the influence of various factors on corrosion rate from large to small is: temperature>pH>total residual chlorine>total hardness .
Source: China Water Supply Pipes Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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