Selection and application methods of anti-corrosion coatings for oil storage tanks?
The corrosion differences of oil tanks caused by different types of storage media
I. Crude Oil
Crude oil from different origins has different physical properties and corrosiveness. The corrosiveness of crude oil can be evaluated from five indicators: namely, the acid value of crude oil, the sulfur content in crude oil, the salt content in crude oil, the nitrogen content in crude oil, and the water content. Generally, the sulfur content of crude oil from China and Central Asia is relatively low, while that of crude oil imported from the Middle East is very high.
Crude oil is relatively thick, and the lower the temperature, the greater the viscosity. To facilitate transportation, heating and insulation devices are usually installed in low-temperature regions and crude oil tanks that hold high-viscosity crude oil.
Crude oil contains a relatively high amount of impurities, which accumulate and precipitate water and impurities at the bottom of the tank. The precipitated solution is acidic and highly corrosive, causing severe corrosion of steel, mainly ulcerated pitting corrosion, which may lead to perforation. Therefore, crude oil has a strong corrosive effect on the upper surface of the tank bottom plate. Crude oil with a high sulfur content is more corrosive.
Thicker crude oil can adhere to the tank wall and has a certain anti-rust effect. Therefore, the middle part of the crude oil tank wall is often not painted or has a thin coating nowadays. Most crude oil tanks are large external floating roof tanks, with the largest reaching up to 200,000 cubic meters.
Ii. Fuel Oil
Fuel Oil, as a part of refined oil products, is the heavier residual product separated from crude oil during the petroleum processing after gasoline, coal and diesel. Therefore, it is called heavy oil or residue oil.
Fuel oil is relatively thick, and the lower the temperature, the greater the viscosity. To facilitate transportation, fuel oil tanks are usually equipped with heating and insulation devices. The fuel oil contains a relatively high amount of impurities, which accumulate and precipitate water and impurities at the bottom of the tank. The precipitated solution is acidic and highly corrosive, causing severe corrosion of steel, mainly ulcerated pitting corrosion, which may lead to perforation. Therefore, crude oil has a strong corrosive effect on the upper surface of the tank bottom plate. Thicker fuel oil can adhere to the tank wall and has a certain anti-rust effect. Therefore, the middle part of the fuel oil tank wall now does not need to be painted or the coating is relatively thin.
Iii. Intermediate Product Oils
Intermediate product tanks refer to storage tanks for various intermediate products such as naphtha, crude gasoline, crude diesel, wax oil, residue oil, and hydrocracking feedstocks. The corrosiveness of intermediate product tanks is stronger than that of refined oil products but lower than that of crude oil.
Some intermediate products have their particularities. For instance, naphtha usually contains a relatively high content of aromatic hydrocarbons (solvents with good solubility), and the inner wall coating is required to have strong solvent resistance. Currently, epoxy phenolic paint (also known as phenolic epoxy paint) is widely used. It is a type of coating with Norflak epoxy resin and amine curing agent as the film-forming substances, featuring a high degree of crosslinking. Intermediate product tanks with better solvent resistance than bisphenol A epoxy paint only exist in oil refining enterprises.
Iv. Refined oil Products
Finished oil tanks refer to storage tanks for various types of refined oil products such as gasoline, kerosene, diesel, and aviation kerosene. They are mostly fixed dome tanks or internal floating roof tanks. The refined oil has a relatively high purity, causes less corrosion, and does not require heating or insulation.
It should be noted that there are special regulations for the coating on the inner walls of aviation kerosene (also known as jet fuel) tanks. According to the provisions of MHJ5008-10.2 Civil Airport Fuel Supply Engineering Construction Code, the coating of jet fuel tanks shall not use anti-static coatings containing zinc, copper, cadmium components or zinc-rich coatings. It is advisable to use white or light multi-color anti-static coatings.
Five Solvent
Solvents include benzene, toluene, xylene, heavy aromatic hydrocarbons, ketones, alcohols, esters, etc., which have strong solubility. Generally, organic coatings will soften and fall off after long-term immersion in them. Therefore, at present, the inner walls of such tanks are mostly protected by thermal spraying of metals (zinc, aluminum, alloys, stainless steel, etc.), and inorganic zinc-rich paint can also be used for anti-corrosion.
Six Other products
Usually, the storage tank area also has other supporting storage tanks, such as fire water tanks, fire foam tanks, and tanks for oily and sulfur-containing wastewater, etc. Fire water tanks and some projects also require that they can hold drinking water, and coatings that meet the technical conditions for drinking water tank coatings should be used.
I. Crude Oil
Crude oil from different origins has different physical properties and corrosiveness. The corrosiveness of crude oil can be evaluated from five indicators: namely, the acid value of crude oil, the sulfur content in crude oil, the salt content in crude oil, the nitrogen content in crude oil, and the water content. Generally, the sulfur content of crude oil from China and Central Asia is relatively low, while that of crude oil imported from the Middle East is very high.
Crude oil is relatively thick, and the lower the temperature, the greater the viscosity. To facilitate transportation, heating and insulation devices are usually installed in low-temperature regions and crude oil tanks that hold high-viscosity crude oil.
Crude oil contains a relatively high amount of impurities, which accumulate and precipitate water and impurities at the bottom of the tank. The precipitated solution is acidic and highly corrosive, causing severe corrosion of steel, mainly ulcerated pitting corrosion, which may lead to perforation. Therefore, crude oil has a strong corrosive effect on the upper surface of the tank bottom plate. Crude oil with a high sulfur content is more corrosive.
Thicker crude oil can adhere to the tank wall and has a certain anti-rust effect. Therefore, the middle part of the crude oil tank wall is often not painted or has a thin coating nowadays. Most crude oil tanks are large external floating roof tanks, with the largest reaching up to 200,000 cubic meters.
Ii. Fuel Oil
Fuel Oil, as a part of refined oil products, is the heavier residual product separated from crude oil during the petroleum processing after gasoline, coal and diesel. Therefore, it is called heavy oil or residue oil.
Fuel oil is relatively thick, and the lower the temperature, the greater the viscosity. To facilitate transportation, fuel oil tanks are usually equipped with heating and insulation devices. The fuel oil contains a relatively high amount of impurities, which accumulate and precipitate water and impurities at the bottom of the tank. The precipitated solution is acidic and highly corrosive, causing severe corrosion of steel, mainly ulcerated pitting corrosion, which may lead to perforation. Therefore, crude oil has a strong corrosive effect on the upper surface of the tank bottom plate. Thicker fuel oil can adhere to the tank wall and has a certain anti-rust effect. Therefore, the middle part of the fuel oil tank wall now does not need to be painted or the coating is relatively thin.
Iii. Intermediate Product Oils
Intermediate product tanks refer to storage tanks for various intermediate products such as naphtha, crude gasoline, crude diesel, wax oil, residue oil, and hydrocracking feedstocks. The corrosiveness of intermediate product tanks is stronger than that of refined oil products but lower than that of crude oil.
Some intermediate products have their particularities. For instance, naphtha usually contains a relatively high content of aromatic hydrocarbons (solvents with good solubility), and the inner wall coating is required to have strong solvent resistance. Currently, epoxy phenolic paint (also known as phenolic epoxy paint) is widely used. It is a type of coating with Norflak epoxy resin and amine curing agent as the film-forming substances, featuring a high degree of crosslinking. Intermediate product tanks with better solvent resistance than bisphenol A epoxy paint only exist in oil refining enterprises.
Iv. Refined oil Products
Finished oil tanks refer to storage tanks for various types of refined oil products such as gasoline, kerosene, diesel, and aviation kerosene. They are mostly fixed dome tanks or internal floating roof tanks. The refined oil has a relatively high purity, causes less corrosion, and does not require heating or insulation.
It should be noted that there are special regulations for the coating on the inner walls of aviation kerosene (also known as jet fuel) tanks. According to the provisions of MHJ5008-10.2 Civil Airport Fuel Supply Engineering Construction Code, the coating of jet fuel tanks shall not use anti-static coatings containing zinc, copper, cadmium components or zinc-rich coatings. It is advisable to use white or light multi-color anti-static coatings.
Five Solvent
Solvents include benzene, toluene, xylene, heavy aromatic hydrocarbons, ketones, alcohols, esters, etc., which have strong solubility. Generally, organic coatings will soften and fall off after long-term immersion in them. Therefore, at present, the inner walls of such tanks are mostly protected by thermal spraying of metals (zinc, aluminum, alloys, stainless steel, etc.), and inorganic zinc-rich paint can also be used for anti-corrosion.
Six Other products
Usually, the storage tank area also has other supporting storage tanks, such as fire water tanks, fire foam tanks, and tanks for oily and sulfur-containing wastewater, etc. Fire water tanks and some projects also require that they can hold drinking water, and coatings that meet the technical conditions for drinking water tank coatings should be used.