Corrosion conditions of oil storage tanks storing oils in different media

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Generally, heavy oil tanks are less corroded than light oil tanks, and above-ground tanks are less corroded than underground tanks. The underground oil tanks store light-grade oils, and they suffer from corrosion and perforation caused by impurities in the oils as well as groundwater; in some cases, the diameter of these holes reaches 10–20 mm. The bottom of heavy oil tanks, such as crude oil tanks, sludge oil tanks, and lubricating oil tanks, suffers severe corrosion; followed by corrosion at the water-oil interface. Corrosion at the oil-vapor interface is also significant, while corrosion at the top of the vapor layer is less severe. For crude oil tanks, the main causes of corrosion are that crude oil contains naphthenic acids, inorganic salts, sulfides, and microorganisms, all of which can corrode steel. The most severely corroded area is the aqueous phase at the bottom of the tank, as the water separated from the crude oil at the tank bottom contains large amounts of highly permeable inorganic salts. Some of these salts can hydrolyze to produce acidic substances, which first cause uniform corrosion as well as corrosion in the weld areas ； Secondly, electrochemical corrosion occurs due to the presence of corrosion products and the effect of salts in water ； Furthermore, the anaerobic conditions at the bottom of the tank are highly suitable for the growth of sulfate-reducing bacteria, which can cause severe needle-shaped or filamentous bacterial corrosion; as a result, the heating tubes at the bottom of the crude oil tank are also severely corroded. According to available information, in crude oil tanks suffering from severe corrosion, the bottom of the tank and the heating tubes can become corroded and perforated within just 3 to 4 years. The characteristics of this corrosion include the formation of spots and pitting; the maximum diameter of the perforations can reach 15 mm, while the depth can be 4 to 5 mm. The corrosion rate is generally between 0.4 and 0.8 mm per year, with a maximum of up to 2 mm per year. The corrosion of light oil tanks, such as gasoline tanks, kerosene tanks, naphtha tanks, and diesel tanks, varies depending on the medium contained within them. Tetraethyl lead in gasoline, sulfides and antistatic additives in kerosene, etc., all have a corrosive effect on carbon steel. The tops of gasoline tanks and the fuel surfaces suffer from severe corrosion; in these areas, corrosion caused by kerosene is less severe, while corrosion induced by diesel is even milder ； Naphtha has relatively high levels of sulfur, salt, and chlorine, which can lead to corrosion in an HCl-H2S-H2O system. Whether it’s a light-duty or heavy-duty oil tank, the main cause of corrosion on their surfaces is electrochemical corrosion induced by water vapor, oxygen in the air, and volatile hydrogen sulfide present in the oil ； The corrosion at the alternating vapor-liquid regions of the tank wall is mainly caused by oxygen concentration cells ； The area with high oxygen concentration is the cathode, while the area with low oxygen concentration is the anode ； Bottom corrosion of tanks is mainly caused by water separation from oil, and this is also the most severely corroded area in oil tanks. The physicochemical parameters for oil separating water are as follows: temperature 5–40°C, SO4²⁻ 0.065%–0.2%, pH value 2–8, Na+ 0.2%–0.25%, Cl⁻ 0.2%–2%, Fe²⁺ and Fe³⁺ 0.0035%–0.012%. The corrosion rates of carbon steel in storage tanks containing different types of oil are shown in the table below: Corrosion rates of carbon steel in storage tanks containing different types of oil (mm/year). Phase: Crude oil tank, Gasoline tank, Diesel tank, Paraffin oil tank, Naphtha tank, Tar tank, Sewage oil tank, Aviation kerosene tank. Gas phase, Oil phase, Water phase: 0.06, 0.05, 0.30, 0.25, 0.35, –0.29, 0.20, 0.14, 0.03; 1.00, 0.07, –0.12, –0.30, 0.34, 0.30, 1.00, 0.34, 0.30, 1.00, 0.16, 0.05, 0.30. Note: The data in the table come from materials presented at the National Conference on Tank Corrosion and Protection, specifically from Jinling Refinery. There are three main types of corrosion on the outer walls of storage tanks: corrosion at the welds on the bottom outside the tank, electrochemical corrosion caused by salty groundwater, and corrosion due to stray currents. For the external insulation of certain oil tanks, insulating materials such as perlite or slag wool are used; these materials contain large amounts of soluble alkaline salts. As rainwater seeps in over time, these alkaline substances can cause corrosion of steel, with an annual corrosion rate of over 0.8 mm/year.