Application of aluminized steel in processing high-sulfur crude oil

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Application of aluminized steel in units for processing imported high-sulfur crude oil Abstract: Through an analysis of sulfur corrosion during crude oil processing, the sulfur-corrosion resistant material—aluminized steel—was specifically employed, resulting in significant economic and social benefits. Industrial applications have shown that aluminized steel exhibits better resistance to sulfur corrosion than 18-8 stainless steel, owing to the high resistance of the passivation film formed by aluminum-iron alloys at high temperatures to hydrogen sulfide. Furthermore, after aluminizing, the surface microhardness is 2 to 3 times higher than that of the base material, resulting in better erosion resistance. In line with market demands and resource policy guidelines, China has increased its crude oil imports in recent years, with the majority of this oil coming from the Middle East and Russia; the sulfur content in this oil is nearly ten to dozens of times higher than that of Daqing crude oil. At the same time, the sulfur content in crude oil extracted domestically is also on the rise. Therefore, the corrosion problem in the processing of high-sulfur crude oil needs to be addressed urgently. 1 Corrosion caused by sulfur during crude oil processing 1.1 Forms of sulfur presence and its distribution The sulfur content in crude oil ranges from 0.05% to 14%, with the sulfur content in most crude oils being less than 4%. Sulfides in crude oil can generally be divided into five categories: thiols, thioethers, disulfides, sulfones, and thiophenes. The first four can be further divided into cyclic and acyclic categories. Thiophenes are further classified according to the number of aromatic rings into benzothiophenes, dibenzothiophenes, naphthothiophenes, and other polycyclic thiophenes. In terms of its corrosivity, sulfur in crude oil is generally divided into active sulfur and inactive sulfur. Active sulfur includes elemental sulfur, hydrogen sulfide, thiols, and disulfides, while the remaining sulfides that cannot react directly with metals are collectively referred to as inactive sulfur. Crude oil contains sulfur; during its processing, as inert sulfur is continuously converted into active sulfur, sulfur corrosion occurs not only in the primary processing units but also in the secondary processing units, and even extends to downstream chemical processing facilities. 1.2 Types of sulfur corrosion and corrosion sites (1) Corrosion in low-temperature light oil areas: H2S present in crude oil, as well as H2S generated from the decomposition of organic sulfides, together with corrosive substances produced during crude oil processing (such as HCl and NH3) and substances added intentionally (such as ethanolamine and water), create a corrosive environment that causes severe corrosion in the low-temperature areas of the facilities. Typical examples include the HCl-H2S-H2O corrosion environment at the top of distillation units; the HCN-H2S-H2O corrosion environment at the top of fractional distillation units in catalytic cracking units; the H2S-NH3-H2-H2O corrosion environment in the vent gas coolers of hydrocracking and hydrorefining units; and the RHN2 (ethanolamine)-CO2-H2S-H2O corrosion environment in the regeneration towers and gas absorption towers of dry gas desulfurization units, among others. (2) Wet hydrogen sulfide stress corrosion cracking: Wet hydrogen sulfide environments are commonly found in the light oil sections of secondary processing units in refineries, such as catalytic corrosion-resistant materials in cracking units, the absorption and stabilization sections of product refining units, the desulfurization sections for dry gas and liquefied petroleum gas in those units, the stripping towers of acid water stripping units, the coolers in hydrocracking and hydrodesulfurization units, high-pressure separators, and the process equipment downstream of them. Wet hydrogen sulfide can cause three types of corrosion in carbon steel equipment: uniform corrosion, hydrogen damage, and wet hydrogen sulfide stress corrosion cracking. (3) High-temperature sulfur corrosion: The corrosive environment caused by sulfides at high temperatures refers to the corrosive condition resulting from sulfur, hydrogen sulfide, and thiols in heavy oil at temperatures above 240°C. Typical high-temperature sulfide corrosion environments are found in the lower sections of atmospheric and vacuum distillation columns as well as in the pipelines at the column bottoms, in heat exchangers for atmospheric heavy oil and vacuum residue, etc.; as well as in the lower sections of the main fractionation column in catalytic cracking units. (4) Polythiic acid stress corrosion cracking: If the equipment is subjected to sulfur-induced corrosion during operation, sulfides are formed on its surface. When oxygen and water penetrate the equipment during shutdown periods, they react with these sulfides on the surface to form polythiic acid. Under the combined effect of polythiic acid and tensile stress, stress corrosion cracking due to polythiic acid can occur. Polyoxysulfate stress corrosion cracking is often associated with intergranular corrosion in austenitic steels; intergranular corrosion caused by polyoxysulfates occurs first, which then leads to polyoxysulfate stress corrosion cracking. (5) Sulfuric acid dew point corrosion: During combustion in the heating furnace, fuel oil generates high-temperature flue gases containing SO2 and SO3. In the cooler sections of the heating furnace, SO2 and SO3 combine with moisture in the air to condense at temperatures below the dew point, resulting in sulfuric acid dew point corrosion. (6) NOx-SOx-H2O type corrosion: This corrosion mechanism causes stress corrosion cracking in equipment such as the regenerators and three-spinners of catalytic units. 2 In terms of the problem of corrosion caused by sulfides, refineries generally use highly alloyed chromium-nickel steels as corrosion-resistant materials. These materials are not only expensive but also suffer from stress corrosion issues. Studies both domestically and internationally have shown that aluminized steel exhibits excellent resistance to sulfide corrosion, as it is capable of forming a dense alumina protective layer during the corrosion process. The aluminum elements in the aluminized layer also help to suppress the thermal decomposition of thiol compounds. Regarding corrosion caused by certain low-temperature sulfides, aluminized steel again shows good corrosion resistance. Therefore, aluminized steel, as a material that is inexpensive and possesses good corrosion resistance, is widely used in equipment for processing high-sulfur crude oil. The aluminized layer, dominated by FeAl alloy phases, is continuous, uniform, and dense, with no brittle regions present. The surface aluminum concentration of the aluminized layer is an important indicator for evaluating its performance. Quantitative analysis of the aluminum content in the case layer was carried out using an electron probe analyzer; it was found that the aluminum content (in terms of mass fraction) in the case layer ranged from 20% to 35%. Moreover, the gradient of aluminum content decreasing with depth within the case layer was relatively gentle, which ensures good corrosion resistance for the case layer. The microhardness of the aluminized layer is another important indicator of its performance. Under normal conditions, the higher the aluminum infiltration layer concentration, the higher the microhardness as well. By controlling the aluminizing process, the aluminum concentration in the coated layer can be effectively regulated, thereby achieving microhardness levels that meet various requirements. 2.1 Laboratory high-temperature hydrogen sulfide corrosion test: The sulfur corrosion resistance of materials such as aluminized steel was evaluated in the laboratory, and the experimental results are shown in Tables 2–4. Table 2 Relationship between material corrosion rate (μm/a) and corrosion temperature. Temperature/°C: 240, 270, 300, 370, 430, 480. 316L steel: 1.61, 1.83, 2.32, 2.38, 2.03, 2.02; 1Gr18Ni9Ti: 1.86, 2.10, 2.94, 6.58, 4.46, 3.96; Aluminum-doped steel: 1.02, 1.13, 1.36, 2.85, 2.20, 2.01; 0Cr13 steel: 2.14, 2.66, 3.38, 6.67, 8.23, 8.3; 12AlMoV steel: 3.28, 3.62, 4.5, 4.9, 8.45; Carbon steel: 3.48, 5.8, 16, 75.6, 13.56, 11.23. Note: Hydrogen sulfide concentration is 0.02 mol/L. Table 3 Relationship between material corrosion rate (μm/a) and hydrogen sulfide concentration. Concentration/mol•L-1: 0.02, 0.04, 0.06, 0.08, 0.1. 316L steel: 2.38, 2.42, 2.58, 2.72, 3.0. 1Gr18Ni9Ti: 6.58, 6.9, 7.4, 7.5, 7.5. Aluminum-doped steel: 2.85, 3.1, 3.25, 3.7, 3.8. 0Cr13 steel: 6.76, 4.97, 5.5, 5.8, 6. 12AlMoV steel: 4.92, 11.7, 16, 27.4, 46. Carbon steel: 75.6, 45, 32, 32, 24. Note: Experimental temperature: 370°C. Table 4 Relationship between material corrosion rate and time (μm/a) Time/h 50 100 150 200 316L steel 1.8 2.38 2.40 2.40 1Gr18Ni9Ti 3.50 6.58 7.82 7.86 Aluminum-impregnated steel 1.82 2.85 3.25 3.26 0Cr13 steel 2.96 4.67 4.84 4.9 12AlMoV steel 2.76 4.92 5.48 5.52 Carbon steel 26.4 75.6 76.2 77.2 Note: Corrosion temperature: 370°C; hydrogen sulfide concentration: 0.02 mol/L. As can be seen from Table 2, for 316L steel, 1Gr18Ni9Ti, and aluminized steel, the corrosion rate increases gradually as the temperature rises, reaching its maximum around 370°C. This is because at this temperature most organic sulfides decompose, resulting in almost the highest concentration of hydrogen sulfide. As can be seen from the data in Tables 2, 3, and 4, the corrosion resistance of aluminized steel is more than twice that of 1Gr18Ni9Ti steel, indicating that the dense and continuous alumina film on the surface of aluminized steel maintains a stable passive state, thereby suppressing corrosion by hydrogen sulfide. 2.2 Laboratory low-temperature hydrogen sulfide corrosion tests (see Table 5): Materials – 316L steel, 18-8 steel, aluminized steel, 0Cr13 carbon steel. Corrosion rate/μm•a-1: 2.1, 4.4, 1.5, 2.2, 15. As can be seen from Table 5, the corrosion resistance of aluminized steel in a low-temperature hydrogen sulfide corrosion environment is 10 times that of carbon steel, and more than twice that of 1Gr18Ni9Ti steel. This shows that aluminized steel can be used in low-temperature hydrogen sulfide environments as a substitute for carbon steel. 2.3 Industrial Applications In 1999, the Harbin Petrochemical Branch used aluminum-impregnated steel in the 15–30th trays of the catalytic fractionation tower, as well as in the primary cooler and the top circulation cooler (see Table 6). It is currently operating well; the trays of the fractionation tower are black, the sealing layer remains intact, and there is no significant change in its thickness. The aluminum-coated heat exchanger tube bundle appears black-gray in color, with no signs of corrosion on its surface; there is also no corrosion at the joints where the tubes are welded to the tube sheet. Table 6 Application of Aluminized Steel at Harbin Petrochemical Branch Equipment Name Medium Temperature/°C Pipe Side Shell Side Pipe Side Shell Side Corrosive Medium Service Duration First intermediate cooler Intermediate-stage oil circulation water 60 350 Sulfides 1999 to present Top circulation cooler Top oil and gas circulation water 60 90 Sulfides, chloride ions 1999 to present 15th tray of the distillation tower Oil and gas 100–310 100–310 Sulfides, chloride ions 1999 to present 3. Results and Discussion Under conditions of high-temperature sulfur corrosion, after 2 years of use, the microstructure of the aluminized steel remained essentially unchanged. Both laboratory test results and industrial applications have shown that the corrosion resistance of aluminized steel is superior to that of 1Gr18Ni9Ti stainless steel. Since aluminum is a highly self-passivating metal, a strong and dense alumina protective film forms spontaneously on the surface of aluminized steel, preventing the intrusion of reactive sulfur. The presence of aluminum effectively reduces the catalytic effect of steel on the thermal decomposition of organic sulfides, thereby lowering its corrosion rate. Relevant domestic departments conducted high-temperature sulfur corrosion tests on aluminum-iron alloys and iron-chromium alloys, and the results showed that the alloying element Al has a greater inhibitory effect on corrosion than Cr. Foreign studies have shown that although 18-8 chromium-nickel stainless steel is also resistant to high-temperature sulfur corrosion, high-nickel alloys tend to form low-melting eutectics of nickel and nickel sulfide, which affects their resistance to high-temperature hydrogen sulfide corrosion; whereas the sulfide layer formed by aluminum-iron alloys at high temperatures exhibits greater resistance to hydrogen sulfide. The surface microhardness of aluminized steel is 2 to 3 times higher than that of the base material, and its erosion resistance is better than that of 18-8 stainless steel. Aluminized steel also exhibits good corrosion resistance in low-temperature hydrogen sulfide environments, as the continuous and dense oxide film on its surface is in a passive state, which prevents corrosive agents from penetrating into the matrix. Aluminized steel is less expensive than stainless steel and has a longer service life; using aluminized steel can bring significant economic and social benefits.