Common corrosion sites in tower equipment

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1. HCl-H2O (hydrochloric acid) corrosion: Affected areas include the five trays at the top of the atmospheric pressure tower, the tower body, certain vapor lines, and the condensation cooling system at the top of the atmospheric pressure tower； Partial vapor line and condensation cooling system of the vacuum tower. Corrosion pattern: general corrosion and uniform thinning of carbon steel components ； Pitting corrosion of Cr13 steel and chloride stress corrosion cracking of 1Cr18Ni9Ti stainless steel. Cause of corrosion: When the chlorides contained in crude oil are heated to above 120°C, they begin to hydrolyze to produce HCl. This acid, combined with water droplets in the cooler areas at the top of the tower, forms dilute hydrochloric acid, creating an environment with extremely high corrosivity. Chemical corrosion occurs with the device itself. The presence of hydrogen sulfide further exacerbates corrosion. Protection measures: Process protection is the primary approach, with material corrosion prevention as a supplementary measure. Process protection refers to the “one desalting and four injections”: deep desalting of crude oil; after desalting, adding alkali to the crude oil, injecting ammonia (or amines) into the overhead distillate line, adding corrosion inhibitors, and injecting water. The principle of this anti-corrosion measure is to remove impurities from crude oil, neutralize the acidic corrosive agents that have formed, alter the corrosive environment, and create a protective barrier on the surface of the equipment. Material corrosion prevention involves improving the grade of the materials, based on process protection, by using composite sheets such as 20R+0Cr13 to manufacture the shells for the five layers of trays at the top of atmospheric pressure towers. 2. S-H2S-RSH (thiol) corrosion: The most severe high-temperature sulfur corrosion occurs in the bottom section of the coking distillation column systems, followed by those in the bottom sections of distillation and vacuum distillation columns, with the bottom sections of catalytic distillation columns being the least affected. Corrosion pattern: chemical corrosion, uniform thinning. Causes of corrosion: Hydrogen sulfide, thiol compounds, and elemental sulfur can all undergo direct chemical reactions with metals at temperatures between 350 and 400°C. Moreover, the elemental sulfur produced by the decomposition of hydrogen sulfide at these temperatures is more reactive, leading to more severe corrosion. Protective measures: mainly the use of corrosion-resistant steel. Such as 20R+0Cr13 composite plate. 3. RNH2-CO2-H2S-H2O (wet hydrogen sulfide) corrosion: Corrosion occurs at the bottom of the desulfurization unit’s regeneration tower, as well as in the reboiler of the regeneration tower and the pipelines of the rich liquid system; the temperature ranges from 90 to 120 degrees, and the pressure is 0.2 MPa. Corrosion morphology: Under alkaline conditions (pH 8–10.5), stress corrosion cracking and uniform thinning caused by carbonates and amines. Cause of corrosion: The polyamine-type substances formed from the irreversible reaction between ethanolamine and carbon dioxide are the most common degradation products that promote equipment corrosion; at 120 degrees Celsius, they decompose and release carbon dioxide. Both free and combined CO2 can cause corrosion; severe corrosion occurs in high-temperature areas with water at temperatures above 90°C. The corrosion is particularly severe when the concentration is between 20% and 30%. The corrosion caused by a mixture of hydrogen sulfide and carbon dioxide is less severe than that caused by carbon dioxide at the same concentrations, and it decreases as the hydrogen sulfide concentration increases. In other words, hydrogen sulfide has an inhibitory effect on carbon dioxide corrosion. 4. Fe + 2CO2 + 2H2O, Fe(HCO3)2 + H2 → Fe(HCO3), (upon heating) FeCO3 + CO2 + H2, Fe + H2CO3, FeCO3 + H2 – corrosion. Protective measures: Conduct stress-relief heat treatment on equipment and pipelines operating at temperatures above 90 degrees; ensure that the hardness of welds and heat-affected areas remains below HB200. Use 18-8 steel for the tubes in heat exchange equipment, improve operating conditions by controlling temperature, and inject corrosion inhibitors into the monoethanolamine system. 5. RCOOH (cycloalkanoic acid): Corrosion occurs mainly in the oil transfer line at the outlet of the vacuum furnace, as well as in the sections below the feed section of the vacuum tower. The oil transfer line at the atmospheric furnace outlet and the feed section of the atmospheric furnace follow. The oil collection tank area of the coking distillation tower is next. Corrosion pattern: The surface of the corroded steel is smooth and free of impurities; in areas where the flow rate of the medium is low, corrosion results only in sharp holes ； Corrosion in areas with high flow rates results in pitting or grooves with sharp edges. Cause of corrosion: Naphthenic acid does not cause strong corrosion at low temperatures. Once boiling occurs, especially in a high-temperature anhydrous environment, corrosion is most severe: 2RCOOH + Fe → Fe(RCOO)2 + H2. When the acid value is greater than 0.5 mgKOH/g of crude oil, and the temperature is between 270–280°C and 350–400°C, naphthenic acid corrosion is most intense. Protection measures: Mainly involve the use of corrosion-resistant steel materials, such as 316L ； The welds on the inner walls of the equipment pipes, furnace tubes, and elbows should be smoothed out to keep the inner surfaces smooth, thereby preventing the formation of eddies that could accelerate corrosion ； Appropriately increase the diameter of the oil transfer line at the furnace outlet to reduce the flow velocity. 6. Corrosion by hydrofluoric acid: The areas affected by corrosion are mainly the equipment and pipelines in the alkylation units that come into contact with the medium, particularly those in alkylbenzene production plants. Corrosion pattern: uniform corrosion ； Hydrogen bubbling and hydrogen embrittlement ； There are 4 types: stress corrosion and crevice corrosion. Cause of corrosion: The corrosion of metal materials by hydrofluoric acid is electrochemical corrosion, which occurs through electrochemical processes; at the anode, the metal dissolves (uniform corrosion), while hydrogen is released at the cathode, leading to hydrogen bubbling, hydrogen embrittlement, and stress corrosion cracking. Protective measures: Material selection: Carbon steel exhibits good corrosion resistance in hydrofluoric acid media at temperatures below 65°C and with a concentration greater than 75%, but normalized steel plates should be used. In temperatures above 71°C and below 136°C, Monel alloy is suitable for use in hydrofluoric acid media of any concentration; however, its corrosion resistance decreases when the medium contains harmful impurities such as oxygen or iron salts. Special requirements for manufacturing: Carbon steel and Monel equipment that come into contact with hydrofluoric acid media must undergo stress-relief heat treatment after welding. The hardness of the weld shall not be greater than HB235.