Common corrosion parts, forms and corrosion causes of tower equipment

1. HCl-H2O (hydrogen chloride) corrosion
Corrosion part: five-layer tray at the top of the atmospheric tower, tower body, part of the volatilization line and the condensation cooling system at the top of the atmospheric tower; part of the volatilization line and condensation cooling system in the vacuum tower.
Corrosion form: general corrosion and uniform thinning of carbon steel parts; pitting corrosion of Cr13 steel and chloride stress corrosion cracking of 1Cr18Ni9Ti stainless steel.
Causes of corrosion: When the chloride salt contained in crude oil is heated to above 120 ℃, it begins to hydrolyze to form HCl, which forms hydrochloric acid when it encounters water droplets at the low temperature part of the top of the tower, which becomes a highly corrosive dilute hydrochloric acid corrosion environment. Chemical corrosion with the device body. Corrosion is further aggravated in the presence of hydrogen sulfide.
Protective measures: mainly process protection, supplemented by material anti-corrosion.
Process protection is "one desalination and four injections": deep desalination of crude oil, alkali injection of crude oil after desalination, ammonia (or amine) injection of overhead distillation line, corrosion inhibitor injection, and water injection. The principle of this anti-corrosion measure is to remove impurities in crude oil, neutralize the acid corrosive medium that has been generated, change the corrosive environment and form a protective barrier on the surface of the equipment.
Material anti-corrosion is to improve the material grade on the basis of process protection, and choose 20R+0Cr13 composite plate to manufacture the shell of the 5-layer tray part at the top of the atmospheric tower.

2. S-H2S-RSH (mercaptan) corrosion
High temperature sulfur corrosion part: the bottom system of coking fractionation tower is the most serious, followed by the bottom system of distillation and vacuum tower, and the bottom system of catalytic fractionation tower.
Corrosion form: chemical corrosion, uniform thinning.
Causes of corrosion: Hydrogen sulfide, mercaptan and elemental sulfur can directly react with metals at 350~400℃, and the elemental sulfur decomposed by hydrogen sulfide at 340~400℃ has stronger activity, which makes the corrosion more intense.
Protective measures: mainly use corrosion-resistant steel. Such as 20R+0Cr13 composite board.

3. RNH2-CO2-H2S-H2O (wet hydrogen sulfide) corrosion
Corrosion part: bottom of regeneration tower of desulfurization unit, reboiler of regeneration tower and pipeline of rich liquid system, temperature 90~120°C, pressure 0.2MPa.
Corrosion form: Stress corrosion cracking and uniform thinning caused by carbonate and amine in alkaline medium (PH8~10.5).
Causes of corrosion: The polyamine-type substance produced by the irreversible reaction of ethanolamine and carbon dioxide is the most common degradation substance that promotes equipment corrosion, and carbon dioxide is decomposed at 120 degrees. Free or combined CO2 can cause corrosion. Serious corrosion occurs at high temperature (90°C) with water. When the concentration is 20~30%, the corrosion is particularly serious. The corrosion ratio of hydrogen sulfide and carbon dioxide mixture is corresponding The corrosion of carbon dioxide concentration is lighter and decreases with the increase of hydrogen sulfide concentration. That is, hydrogen sulfide has the effect of inhibiting carbon dioxide corrosion.

4. Fe+2CO2+2H2O, Fe(HCO3)2+H2Fe(HCO3), (heating) FeCO3+CO2+H2, Fe+H2CO3, FeCO3+H2 corrosion
Protective measures: After welding equipment and pipelines with operating temperature higher than 90 degrees, perform stress relief heat treatment, control the hardness of welds and heat-affected zones to be less than HB200, use 18-8 steel for heat exchange equipment tube bundles, improve operating conditions, and control operating temperature. Injecting corrosion inhibitors, etc. into the monoethanolamine system.

5. RCOOH (naphthenic acid) corrosion
Corrosion parts: the oil transfer line at the outlet of the decompression furnace and the parts below the feed section of the decompression tower are the most important. The oil transfer line at the outlet of the atmospheric furnace and the feeding section of the atmospheric furnace are second. The oil collecting tank of the coking fractionator is second.
Corrosion form: the surface of the corroded steel is smooth and free of scale, and only sharp holes are left in the parts where the medium flow rate is low.
Corrosion reason: naphthenic acid is not strongly corroded at low temperature. Once boiled, especially in high temperature anhydrous environment, the corrosion is the most intense: 2RCOOH+Fe--Fe(RCOO)2+H2 When the acid value is greater than 0.5mgKOH/g crude oil, the temperature is 270~280℃ and 350~400℃ , naphthenic acid corrosion is the most serious.
Protective measures: mainly use corrosion-resistant steel, such as 316L, etc.; the welding seam on the inner wall of equipment pipes and furnace tube elbows should be smoothed to keep the inner wall smooth to prevent pre-generated eddy currents from aggravating corrosion; appropriately increase the diameter of the oil transfer line at the furnace outlet , reduce the flow rate.

6. Corrosion of hydrofluoric acid
Corrosion parts: mainly the equipment and pipelines in the alkylation unit that are in contact with the medium, mainly the alkylbenzene unit of the washing plant.
Corrosion forms: uniform corrosion; hydrogen bubbling and hydrogen embrittlement; stress corrosion and crevice corrosion.
Causes of corrosion: The corrosion of metal materials by hydrofluoric acid is electrochemical corrosion, and the corrosion is carried out according to the electrochemical process, that is, the anode produces metal dissolution (uniform corrosion) and the cathode releases hydrogen, resulting in hydrogen bubbling, hydrogen embrittlement and stress corrosion cracking.
Protective measures: Material selection: carbon steel below 65 °C, and the oil in the hydrofluoric acid medium with a concentration of more than 75% has better corrosion resistance, but the killed steel plate should be used. When the temperature is higher than 71 ℃ and lower than 136 ℃, it can be applied to Monel alloy in any concentration of hydrofluoric acid medium, but when the medium contains harmful impurities such as oxygen or iron salt, the corrosion resistance will be reduced.
Special requirements for manufacturing: All carbon steel and Monel equipment in contact with hydrofluoric acid medium should undergo stress relief heat treatment after welding.
Weld hardness should not be greater than HB235.