Corrosion and Cathodic Protection of Tank Bottom Plates

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Storage tanks are very important equipment in the process of transportation and processing. With the increase of domestic oil storage equipment, the number of storage tanks is also greatly increased. When in use, the storage tank inside is in direct contact with the medium in crude oil, which may lead to corruption. According to the corrosion data of storage tanks, the corrosion area of the bottom panel is the most serious and common, which is close to 80% of the corrosion amount. Corrosion and perforation of storage tanks will lead to leakage, thus causing economic losses and potential safety hazards.

Causes of Corrosion of Crude Oil Tank Plates

1. Electrochemical Corrosion

The crude oil storage tank contains a certain amount of water. The gas-phase water in the storage tank will also become water droplets after condensation sinking to the bottom of the storage tank. Factors such as the uneven bottom of the crude oil tank and liquid flow will lead to increased water in the bottom of the crude oil tank. Generally, the crude oil storage tank has a drainpipe, but the center of the drainpipe is usually about 300 mm higher than the bottom of the crude oil storage tank, resulting in water accumulation at the bottom of the crude oil storage tank of about 300 mm. Now, with the development of science and technology, the research institute invented anti-corrosion coatings, which can separate the water and crude oil in the crude oil storage tank from the steel plate at the bottom of the tank, and effectively play an anti-corrosion role. However, due to the long service time, the water at the bottom of the tank will gradually penetrate into the bottom plate, and the corrosion resistance of the coating will gradually decrease, thus accelerating the corrosion of the bottom plate of the crude oil tank. Once water accumulates too much on the bottom of the crude oil storage tank, most of the bottom contact will cause large-scale corrosion, which will affect the service life of the crude oil storage tank.
 
2. Erosion and Accumulation Corrosion

As the liquid in the crude oil storage tank continuously washes to the surface of the metal, the passivation film will be seriously damaged if it washes to the surface of the metal for a long time, which makes the liquid in the storage tank directly hit the surface of the metal, resulting in the electrochemical reaction, as well as corrosion of the crude oil storage tank. Chinese crude oil is often stored in large crude oil storage tanks. Therefore, the crude oil depot carries out the work of crude oil and discharges in shifts every day. This operation will lead to the flow of the deposited water and crude oil at the bottom of the crude oil storage tank. The crude oil stored in the tank is fluid, especially in the stage of oil delivery from the tank. Long-term cleaning will gradually separate the metal and protective film at the bottom of the crude oil storage tank so that the surface of the metal will come into direct contact with the crude oil and accelerate the corrosion process. If the crude oil contains hard small things, it will also accelerate the process of corrosion of the bottom of the crude oil storage tank. Generally speaking, the faster the oil flows, the harder the particles in the fluid are, and the greater the influence on the bottom of the jar can be. The corrosion of the deposit is mainly due to the fact that the particles and sludge contained in the crude oil have been deposited at the bottom of the crude oil reservoir for a long time, which leads to the inability to effectively diffuse a large amount of electrolytic oxygen and water-soluble oxygen. There is a certain degree of difference between the upper and lower separation surfaces of the deposit. Mainly by changing pH value, potential, dissolved oxygen content, etc., corrosive batteries are formed and electrochemical corrosion is caused.

3. Analysis of Anti-corrosion Measures

First, increase the thickness of the bottom plate, and replace the previous 8mm plate with a 10mm plate to reduce the influence of corrosion. Secondly, the anti-corrosion coating of the crude oil storage tank should play a certain anti-corrosion role, avoiding the corrosive current generated between the poles in the corroded battery. However, anticorrosive coatings usually have problems such as corrosion and degradation, and sometimes there are still some pinholes in the work, which affect the whole protection effect. The failure position of the anti-corrosion layer can easily corrode the metal parts, thus further accelerating the degradation speed of the bottom metal of the crude oil storage tank. Therefore, it is not ideal to use only one anti-corrosion coating for the bottom of the crude oil storage tank. At the same time, if only a single cathode is used for protection and measures, it will increase energy consumption and cost. Therefore, we usually adopt the method of combining cathodic protection and anticorrosive coating protection at the bottom of the crude oil storage tank. Cathodic protection measures shall be taken at the places where the corrosion layer is most damaged. This method has the best effect and the most reasonable cost. The basic principle of cathodic protection technology is to transform the metal material at the bottom of the storage tank into cathodic protection to reduce the corrosion of the metal material stored at the bottom of the crude oil storage area. Generally, cathode maintenance can be roughly divided into two types: anode sacrifice and cathode sacrifice. The second is the protection of current, using a sacrificial anode system. Compared with other auxiliary anode systems using traditional sacrificial anode, the sacrificial anode system has the advantages of more reliability, uniform current dispersion, easy installation and high durability.

Conclusion

With the increasing demand for crude oil in China in recent years, the demand for crude oil storage tanks is also increasing. At present, crude oil is developing towards high sulfur, high chlorine and other heavy gases. The corrosion of these gases to the environment at the bottom of the crude oil storage tank will further deteriorate, and the damage to the bottom of the crude oil storage tank will affect the service life of the crude oil storage tank. Therefore, it is necessary to further study and analyze the causes of corrosion on the bottom of crude oil storage tanks and adopt effective protections schemes to improve their service life and economic benefits.