An introduction to the current status of corrosion of oil tanks caused by high-sulfur crude oil
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The highest sulfur content is found in crude oil (2.65%)； The acid value was the highest (3.44 mgKOH/g), and the processing of sulfur-containing oils exacerbated corrosion in the storage tank system. 1 Crude oil tanks: Among all oil tanks, crude oil tanks suffer from more severe corrosion. Water sample analysis of the crude oil tank revealed high levels of H2S, SO2, and Cl- in the accumulated water. 2. Semi-finished oil tanks: These tanks are also highly corrosive; the corrosion in coker crude gasoline tanks is particularly severe. At the bottom of the tanks (in the aqueous phase), uniform general corrosion and corrosion beneath scale occur. The main corrosive agents are dissolved oxygen, H2S, and inorganic salts in water. In the upper part of the crude gasoline tanks and at the gas-liquid interface at the top, a thin layer of water forms on the tank walls. H2S emitted from the oil and O2 from the air dissolve in this water layer, creating highly corrosive substances, which results in uniform general corrosion in those areas. This corrosion often appears in the form of pits, with the maximum corrosion rate ranging from 0.5 to 1 mm/year. Furthermore, wet H2S has caused hydrogen-induced cracks in the welds and heat-affected zones of the head-pulling oil sphere tank. 3 Sludge and wastewater tanks and gas holders: Although the levels of H2S and organic sulfur in sludge and wastewater tanks as well as gas holders are much higher than those in finished and semi-finished product tanks and crude oil tanks, heavy anti-corrosion measures were taken from the beginning; as a result, the anti-corrosion coatings are in good condition and have not caused any corrosion to the tanks.
Corrosion mechanism: Sulfide corrosion. Sulfide corrosion is one of the main mechanisms by which high-sulfur crude oil causes corrosion in oil tanks. Sulfides in crude oil decompose at high temperatures to produce H2S and elemental sulfur, which are highly chemically reactive and can react directly with metals. For example, in atmospheric and vacuum distillation units, high-temperature sulfur corrosion environments are primarily found in towers, heat exchangers, vessels, furnace tubes of heating furnaces, and related pipelines where the medium temperature is ≥220°C. Furthermore, in the presence of water, H2S causes H2S-H2O type corrosion, leading to the formation of an iron sulfide (FeS) film on the metal surface. However, this film provides poor protection and can be easily penetrated by reactive sulfides, thereby accelerating the corrosion process. Naphthenic acid corrosion: Naphthenic acids in high-sulfur crude oils are highly corrosive to metals at high temperatures. Naphthenic acid corrosion mainly occurs in the heater tubes of atmospheric and vacuum distillation units, oil transfer lines, bottom lines of towers, as well as in equipment and related pipelines with temperatures above 240°C. Naphthenic acid corrosion intensifies as the temperature rises; generally, when the temperature reaches around 400°C, the naphthenic acid begins to decompose, and its corrosiveness decreases. Chloride corrosion: The chlorides present in crude oil undergo hydrolysis during processing to produce hydrogen chloride (HCl). HCl reacts with water to form hydrochloric acid, which causes severe corrosion of metals. This type of corrosion mainly occurs in the overhead oil and gas coolers, vessels, and their associated pipelines. Microbial corrosion: Microbial activity in crude oil can also cause corrosion, especially by sulfate-reducing bacteria (SRB). Under anaerobic conditions, these microorganisms use sulfate in the solution to reproduce; sulfate is reduced by bacteria into corrosive sulfides and organic acids.