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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 cannot be used 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), use fine sand as a bedding layer, which not only saves investment, 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 and a cathodic with a sacrificial anode. For large storage tanks, when the soil resistivity is high, it is advisable to use the impressed current cathodic protection method, and for small storage tanks in the environment of low soil resistivity, it is advisable to use the cathodic protection of sacrificial anode. For impressed current cathodic protection, various types of anodes such as MMO (Mixed Metal Oxide) strip anode, MMO flexible anode and so on can be used. For sacrificial anodes, magnesium anodes and zinc anodes are used more, and aluminum anodes are rarely used because they require a certain salinity environment to activate.
There are many kinds of cathodic protection bed on the tank floor, but the purpose is to make the bottom of the tank can evenly obtain the cathodic protection current, in order to prevent the dissipation of the cathodic protection current, the insulating impermeable membrane is also used in practical use, but the installation cost will be increased. For cathodic protection power supplies, the installation of test piles, reference electrodes and cables and other materials is designed in detail according to the actual investigation, which will not be repeated in this article.
