Corrosion protection of DC stray current in seawater

The concept of cathodic protection for marine structures is similar to that of soil. It is mainly to take drainage measures and strengthen protection management.

Seawater is an electrolyte with good conductivity. When DC stray current from DC electrical equipment moves in seawater, it is easy to cause DC stray current corrosion of metal structure in seawater. When DC equipment is used on offshore platforms, docks and ships, certain preventive measures must be taken, otherwise the corrosion of DC stray current will be caused.

The DC driven crane used for cargo handling uses rail as the current return conductor, which will produce a lot of stray current. The railway track is laid in parallel with the inner port, reinforced concrete wharf wall or steel pile row wall. These structures of the latter can absorb most of the stray current due to their small longitudinal resistance and can transmit over a long distance. Of course, only in some unexpected cases, there will be obvious stray current interference. In addition, the pipelines and cables on the coast are often faced with a greater risk of stray current corrosion. Stray current drainage should be installed here to protect equipment that may be disturbed.

In some ports and wharves, the central DC power supply system is sometimes used to provide DC power for the welding equipment, mobile DC crane and auxiliary facilities for berthing ships.

Direct drainage: the principle of stray flow drainage of hull in seawater is basically the same as that of buried pipeline. During welding connection, a long cable is directly used as the ground wire to connect the ship which may be disturbed with the negative pole of the welding machine. Therefore, in the welding process, the return current flowing to the negative electrode through the return cable should be basically equal to the working current flowing out from the positive electrode. In order to ensure good electrical connection between ground wire and hull, lug welding shall be adopted for connection between ground wire and hull. Where possible, welding machines shall be placed on deck for direct operation and only cables from shore to AC power supply shall be used.

Sacrificial anode consumption: sacrificial anode can also be used to eliminate the interference of ship hull stray current in seawater. Firstly, the distribution and direction of stray current are determined by detection. The sacrificial anode is then installed at the appropriate position where the current flows out.

Stray current should be guided manually by sacrificial anode instead of directly flowing out of the ship. When there is stray current interference, the function of sacrificial anode is to drain the current; when there is no stray current interference, sacrificial anode can provide cathodic protection for ship hull. Zinc or zinc alloy sacrificial anodes are usually used in seawater.

Strengthen protection management: check whether there is large-scale DC power system near the port terminal, such as power plant, DC electrified railway, etc. We should pay attention to detect the existence of stray current, and judge the strength, direction and position of stray current source by measurement. The ship's power supply system and electrical equipment must be well insulated.

Shandong Linyi Bosi conductive materials Co., Ltd. is a high-tech enterprise specializing in the production of MMO flexible anode, conductive polymer flexible anode and titanium coated anode, mainly providing cathodic protection technology, engineering design, installation and technical services. Tel: 0539 3712117, 15588026668.

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