When constructing underground pipelines, attention must be paid to the impact of stray currents

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Example: A company’s underground stainless steel pipeline, which was originally connected using flanges, decided to switch to a fully welded connection due to leaks at the joints. During the construction process, it rained heavily at night, causing water to accumulate in the trench and submerging the completed pipe sections. Construction continued on the second day, and as a result, the stainless steel pipes in the water were drilled with thousands of holes. The pipeline had to be constructed again. Analysis: The culprit here is the stray current generated by the welding machine. Direct current leaking into soil and water can cause severe corrosion damage to steel equipment underground and underwater. Current enters the steel equipment from one place and exits from another. The area from which the current leaves becomes the anode region, where electrolytic corrosion occurs. The amount of metal corroded is proportional to the amount of electric charge that flows, and can be calculated using Faraday’s law. For example, 1 ampere-hour of charge corresponds to the corrosion of 1.04 grams of iron. In this case, it is a stainless steel pipe; the water contains chloride ions, and the area where stray current flows causes the passivation film to be broken down, resulting in pitting corrosion of the stainless steel pipe. There are many sources of direct current, including electric locomotives, trams, DC welding machines, electrolyzers, and cathodic protection systems. To prevent stray currents from leaking into the soil and water, on the one hand, the DC system must form a complete closed circuit; it is strictly prohibited to use soil and water as part of this circuit. For example, in electric railways, the conductivity must be continuous between the rails in order to reduce resistance and allow current to flow smoothly; on the other hand, the conductive lines need to be well insulated from the soil and water to prevent current from leaking in.