| The mathematical models of AC interference are important to explain AC corrosion mechanism. The circuit model which includes positive and negative reaction process is established. Based on the analysis of two reaction processes the factors are given. The irreversible electrochemical reaction, frequency and current density are the main three factors of AC corrosion. The ideal state of reversibility is impossible. If the transfer resistance is very small and diffusion is relatively easy, the anode will dissolve under the positive alternating current. There are a lot of oxygen and hydroxyl in soil. Dissolved iron ions will combine with oxygen or hydroxyl and become iron oxide or ferric hydroxide mixture. Under the reverse of the AC voltage partial iron oxide or ferric hydroxide mixture can be reduced, but some of mixture may be into soil, so the reaction is irreversible. Slight corrosion will occur. The corrosion rate will increase as the the increase of frequency under low frequency, but the corrosion rate is stable under high frequency. If the current through the grounding resistance become larger, the current of Faraday reaction is greater too. The polarization trend will become greater and the irreversible anodic reaction will accelerate.The formulas of electric field and finite element method modeling are used to analyze the different circuit models of pipelines. Firstly, the electric field equations are used to establish boundary conditions of mathematical models. Secondly finite element method is used to simulate the electric field distribution in the whole field in detail. A lot of voltage curves of different pipes under different conditions are gave. These curves are very important to the study for patterns of AC interference. If the pipe itself and the coating are uniform the partial differential equation can be used to plot all kinds of AC voltage curves. If the coating is not uniform the finite element method can be used to plot all kinds of AC voltage curves. A new concept of opening boundary is given. The polarization curve is not boundary condition of numerical simulation. The connecting line of all ends of polarization curves of is the final boundary condition. A new device is designed to give the final boundary condition.AC interferences of different pipelines in different regions are surveyed. Many monitoring experiment about AC interference are launched. A new monitoring instrument for AC interference is used to replace traditional instrument. Based on the analysis and test, the rules of AC/DC hybrid interference under different conditions are found. From analysis for AC interferences, these data provide a basis for the further study for governance of AC interference. The maximum AC voltage is 1341.8V and the maximum AC current density is 170A/m2. A big current density may exist when there is only a little AC voltage. There may have AC corrosion when the AC voltage is less than 100A/m2.Relationships between damage of coating and AC interference are studied. AC interference is used to find damaged points of coating. To verify test results about damage of coating, this study does a lot of excavation working and finds some dangerous points under different cases. AC interference may easily exist when the coating is damaged or there are external circuits. When the coating is not damaged there may be AC signal in the pipelines from external enough large interference.The study developed a new type of intelligent drainage instrument and formulated some process of drainage management means. Based on these processes, two practice of governance for AC interference are carried out and these governance means achieved good effects. The AC/DC current in pipelines about 100 m A can be reduced to 5mA instantaneously. |
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