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With the recent economic development of my country, the use of various large storage tanks has increased, and the need for corrosion protection of the tank bottom has become increasingly important.
During tank installation, the bottom plate must be welded, which inevitably damages the tank bottom coating. Therefore, coating cannot protect the tank bottom. Therefore, cathodic protection must be installed on the tank bottom. Otherwise, the corrosion rate will be extremely rapid, and pitting corrosion may occur within 1-3 years, rendering the tank inoperable.
Storage tanks are typically located on asphalt sand, and some even have a concrete base underneath the asphalt sand. This design is highly unfavorable for cathodic protection. Because the resistivity of concrete and asphalt sand is very high, it is difficult for the cathodic protection current to be distributed to the tank bottom, or the current received by the tank bottom is very small, making it difficult to meet the cathodic protection requirements. The correct approach is to omit the tank bottom coating (even if coating is applied, the cathodic protection current will be reduced). Instead, fine sand can be used as a bedding layer. This not only saves investment and protects the environment, but also slows corrosion of the tank bottom.
The tank bottom can be protected using impressed current and sacrificial anode cathodic protection. For large storage tanks with high soil resistivity, impressed current cathodic protection is recommended. For small tanks in environments with low soil resistivity, sacrificial anodes are preferred. For impressed current cathodic protection, various anode types can be used, including MMO (mixed metal oxide) strip anodes and MMO flexible anodes. Magnesium and zinc anodes are commonly used for sacrificial anodes, while aluminum anodes are rarely used because they require a certain salinity to activate.
There are many types of cathodic protection beds for tank floors, all designed to ensure uniform distribution of cathodic protection current across the tank bottom. To prevent dissipation of cathodic protection current, insulating membranes are also used in practice, but this increases installation costs. The installation of other materials, such as the cathodic protection power supply, test piles, reference electrodes, and cables, is a matter of detailed design based on actual investigations and will not be discussed in this article.
