Protection Against Stray Current Corrosion Of Buried Metals Pipeline

The current deviated from its intended path and escaped to the electrolyte (the soil, concrete or seawater) is called leakage current or stray current. Stray current may originate from the electrical railways, cathodic protection systems, electrolysis plants, electric cables and large electric plants. Stray current corrosion process in the metallic structure is totally similar to the classic one: current flows from anode to cathode through the electrolyte by ionic conductivity and from cathode to anode through the metal by electric conductivity. At the anode the metal oxidation occurs, and a total metal loss can be visible as the result of anodic dissolution. Whilst at the cathode hydrogen or oxygen reduction occurs, avoiding corrosion hazard to the cathode. However, organic coating at the cathodic region will be delaminated by stray current. Moreover, stray current density in the points where the current enters and leaves the structure is extremely high, a threshold ranging between 2 and 20mA/cm2 has been proposed. Stray current corrosion may influence the lifetime of the buried metallic structures, and in a few extreme cases, it can also induce big accident such as gas explosion, soil pollution, and so on.In this paper, the impacts factors of stray current in the buried metallic pipeline, such as the soil resistivity, the depth of the buried metallic pipeline and the pipeline dilapidation ratio, were studied by the laboratory simulation system. Pertinent experiment data are provided for the buried metallic pipeline in design, construction and the protection of corrosion.To analyze and estimate stray current corrosion hazard of the buried metallic pipeline, according to the BP neural network method and the part of experimental data, two predictive models for stray current of the buried metallic pipeline were built. Artificial neural network (ANN) has powerful, nonlinear, mapping ability, and the modeling process is easier and more direct than for empirical model. Feed-forward multilayer network with back propagation error is called BP network, which is a kind of ANN used the most widely, it provides a new method to solve the prediction and control problem for a non-linear system, and to be of widespread utility in engineering and is steadily advancing into new areas. The accuracy of BP neural network predictive model was tested by the test sample, and the feasibility of the BP neural network to forecast stray current of the buried metallic pipeline was testified, too. When the constant current flows through the stationary resistive wire connected to DC Power Supply, the inside inducted electric field driving the conduction electrons against the resistive friction of the wire is due to the free charges distributed along the surface of the wire, and generate as well an electric field outside the wire. The distribution model of stray current was discussed by the inducted electric field theory, which in accordance with the experiment results.Stray current corrosion behavior of steel in soil environments was investigated by using potentiodynamic polarization and constant current polarization methods in laboratory, the corrosion products and surface passive films morphology were observed with digital camera and scanning electron microscope. Stray current corrosion critical current density and potential and the weld corrosion were studied.In order to prevent aggressive species from direct contact with the metallic structure surface, organic coating is extensively applied to prevent corrosion of the metallic structure buried in soil or immersed in seawater because of the high corrosion resistance. To protect the metallic structure from corrosion, organic coating must maintain adequate adhesion to the metallic structure surface during environmental exposure. However, if organic coating has a defective region, the delamination of organic coating is easy to occur by stray current, which will lead to the adhesion degradation, and the delaminated rate is extremely rapid. Stray current delamination of organic coating on the metallic structure is caused by hydrogen and hydroxyl ions, which degrade the adhesive strength between the coating and the metal and lead up to organic coating delamination.In the end, the infection of cathodic protection by stray current in the soil and the means protecting from corrosion of stray current were studied.