Corrosion and Cathodic Protection for Bottom Plates of Tanks

Storage tanks play a crucial role in the transportation and processing of oil. The rising prevalence of domestic oil storage facilities has led to a significant increase in the number of storage tanks. During operation, the inner surface of the storage tank comes into direct contact with the crude oil medium, making it susceptible to corrosion. Based on corrosion data, the bottom panel is the most commonly and severely affected area, accounting for nearly 80% of the total corrosion. Corrosion and perforation of storage tanks can result in leakage, causing both economic losses and potential safety hazards.

Causes of Corrosion in Crude Oil Tank Plates

1. Electrochemical Corrosion:
Crude oil storage tanks, containing a certain amount of water, face challenges such as gas-phase water condensation, resulting in water droplets at the tank's bottom. Factors like uneven tank bottoms and liquid flow contribute to increased water accumulation. Although there are drainpipes, their placement often leads to water accumulating about 300 mm above the tank bottom. Anti-corrosion coatings have been developed to separate water and crude oil from the steel plate, effectively preventing corrosion. However, over time, water may penetrate the coating, diminishing its corrosion resistance and accelerating bottom plate corrosion. Excessive water accumulation on the tank bottom can lead to widespread corrosion, impacting the tank's service life.

2. Erosion and Accumulation Corrosion:
Continuous washing of the tank's metal surface by crude oil can damage the passivation film, exposing the metal to direct contact with the liquid and initiating electrochemical reactions. Chinese crude oil storage tanks often undergo daily crude oil discharge and work shifts, causing deposited water and crude oil to flow at the tank bottom. The fluidity of stored crude oil, especially during delivery, gradually separates the protective film, facilitating direct contact between the metal surface and crude oil. The presence of hard particles in crude oil further accelerates bottom corrosion. Long-term deposition of particles and sludge leads to the formation of corrosive batteries through changes in pH value, potential, and dissolved oxygen content, causing electrochemical corrosion.

3. Analysis of Anti-corrosion Measures:
Firstly, increasing the thickness of the bottom plate from 8mm to 10mm can mitigate the impact of corrosion. Secondly, while anti-corrosion coatings play a role, relying solely on them may lead to issues such as corrosion, degradation, and incomplete protection. Combining cathodic protection and anti-corrosive coatings is a more effective approach. Cathodic protection is applied at areas most damaged by corrosion, involving sacrificial anode systems. This method offers reliable, uniform current dispersion, easy installation, and high durability compared to traditional sacrificial anode systems. Combining these methods provides optimal effectiveness and cost efficiency in protecting the bottom of crude oil storage tanks.

Conclusion

In recent years, China has experienced a growing demand for crude oil, leading to an increased need for crude oil storage tanks. The evolving nature of crude oil, with a trend toward high sulfur, high chlorine, and other heavy gases, poses environmental challenges as the corrosion of these gases at the bottom of crude oil storage tanks worsens. This corrosion can significantly impact the integrity of the tank's bottom, ultimately affecting the overall service life of the crude oil storage tank. Therefore, it becomes imperative to conduct further research and analysis into the causes of corrosion at the tank bottom and implement effective protective measures to enhance the service life and economic benefits of crude oil storage tanks.