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In recent years, with the development of China's economy, more and more large-scale storage tanks have been used, and the anti-corrosion demand for the outer bottom of the storage tank has been put on the agenda.
When the tank is installed, because the bottom plate needs to be welded, the welding will inevitably destroy the coating at the bottom of the tank, so the coating method cannot protect the bottom of the tank. All cathodic protection must be installed at the bottom of the tank, otherwise the corrosion rate is very fast, and porosity may occur in 1–3 years, resulting in the storage tank being unable to operate normally.
The storage tank is generally located on top of the tar sand, and some of the tar sand even have a paved concrete base under the tar sand, which is very unfavorable for cathodic protection. Because the resistivity of concrete and tar sands is very high, it is difficult to distribute the cathodic protection current to the bottom of the tank, or the current obtained from the bottom of the tank is so small that it is difficult to reach the cathodic protection guidelines.
The correct approach is to leave the bottom of the tank uncoated (if there is a coating it will also reduce the cathodic protection current), and use fine sand as a bedding layer. This approach not only saves investment and protects the environment, but also slows down the corrosion of the bottom of the tank.
The bottom of the tank can be protected by an impressed current system or by cathodic protection using sacrificial anodes. For large storage tanks, when the soil resistivity is high, it is advisable to use the impressed current cathodic protection method. For small storage tanks in low soil resistivity environments, it is preferable to use the cathodic protection method with sacrificial anodes.
For impressed current cathodic protection, various types of anodes such as MMO (Mixed Metal Oxide) strip anode and MMO flexible anode can be used. For sacrificial anodes, magnesium and zinc anodes are used more commonly, while aluminum anodes are rarely used because they require a certain salinity environment to activate.
There are many kinds of cathodic protection beds on the tank floor, but the purpose is to ensure that the bottom of the tank can evenly obtain the cathodic protection current. To prevent current dissipation, an insulating impermeable membrane is also used in practical installations, though this increases installation costs.
For cathodic protection power supplies, the installation of test piles, reference electrodes, cables, and other materials is designed in detail according to the actual site investigation, which will not be repeated in this article.
