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Introduction to Corrosion Control in Some Units of Petrochemical Refineries 1. Corrosion in Low-Temperature Areas 1.1 Corrosion in Low-Temperature HCl-H2S-H2O Systems (1) Corrosion Mechanism The corrosive agents in the low-temperature areas at the top of distillation towers are mainly the HCl-H2S-H2O mixture. During the processing of crude oil, H2S generated from various sulfides and HCl produced by the hydrolysis of salts volatilize along with the light components and water present in the crude oil; they then condense and accumulate in the low-temperature areas of the light oil zone at the top of the distillation unit, especially at the points where the gas-liquid phase transition occurs. When water vapor condenses, H2S and HCl dissolve in the condensed water; once their relative concentration reaches around 100 ppm, the pH value drops to 2–3, creating a severe electrochemical corrosion environment. The resulting hydrochloric acid can not only react directly with iron, but also destroy the ferrous sulfide corrosion product film that forms as a result of the reaction between H2S and iron; this film provides some protective effect, and its destruction leads to pitting and underscale corrosion. Generally speaking, corrosion in the HCl-H2S-H2O system is quite severe; corrosion is relatively mild in the gas phase while it is more severe in the liquid phase, with the most severe conditions occurring at the phase transition zone between gas and liquid, namely the so-called \"dew point\" area. Carbon steel primarily suffers from uniform corrosion and pitting, producing large amounts of corrosion product FeS, while austenitic stainless steels experience pitting and stress corrosion cracking (SCC). This type of corrosion mainly occurs in the top trays of atmospheric and vacuum distillation units, as well as in the condensation and cooling systems, the top system of the viscosity-reduction distillation column, and the oil-water separation tanks. ⑵To address the low-temperature corrosion in the overhead system of distillation units, our company has adopted anti-corrosion measures such as the \"one removal and one injection\" process (namely, electrical desalination of crude oil and injection of neutralizing and corrosion-inhibiting agents at the overhead), material upgrades, and anti-corrosion coating applications. These measures have yielded excellent results, effectively controlling the corrosion in the overhead system of distillation units. 1. Corrosion in the 1.2H2S－HCN－NH3－H2O system ⑴ Corrosion mechanism: In catalytic absorption and stabilization systems, H2S and HCN cause corrosion of metals. When CN- is present in ferrous sulfide formed from H2S and iron, it can dissolve the protective film of FeS, forming the complex ion Fe(CN)4-, thereby accelerating corrosion. FeS + 6CN- → Fe(CN)4- + S2-. The complex ion Fe(CN)4- further reacts with Fe to form ferrocyanide, Fe2[Fe(CN)6]. This type of corrosion occurs primarily in the heat-affected zones of welds in catalytic desulfurization systems, absorption and stabilization systems, as well as in the feed water lines, feed water tanks, ammonia tanks, and heat exchangers. The manifestations of such corrosion are mainly stress corrosion cracking and perforation. ⑵Corrosion condition a) Catalyst unit ①: Top of the distillation tower – The medium inside the tower is oil-gas and water vapor, with the temperature at the top of the tower being around 110–120°C. ②Distillation tower top oil-gas to deoxygenated water heat exchanger: The shell side suffers from severe corrosion, with a lot of rust and scale, which often causes the baffle plates to stick to the shell, making it difficult to remove them. The anti-corrosion measure is to replace it with an aluminum-coated tube bundle. ③Light diesel – deoxygenated water heat exchanger in the fractionation tower: This heat exchanger has had severe leakage issues, with pitting corrosion being the characteristic form of corrosion. The 09Cr2ALMoRe tube bundles are currently in trial use, with good results. 1.3 Corrosion mechanism of furoic acid: Furfural is a heterocyclic furan aldehyde with the molecular formula C5H4O2; it is a colorless, transparent liquid at room temperature and has low corrosivity. However, it oxidizes easily in the presence of air, light, and moisture. The areas prone to corrosion in furfural-activated clay systems are mainly found in the wet furfural system. When the temperature of furfural exceeds 230°C, tar-like substances are formed. If these conditions are not properly managed, furfural gets oxidized into furoic acid, C4H3COOH, which possesses strong corrosive properties. The reaction is as follows: C5H4O2 + O2 → C4H3OCOOH. The corrosion problems in furfural processing systems are essentially due to the corrosion caused by furoic acid. One effect is concentration cell corrosion: Since some carbonaceous substances are inevitably generated during the production of the equipment, carbon deposits form on it. Due to the narrow gaps between the metal and these carbon deposits, concentration cell corrosion cells are created, with these gaps serving as the anodes. Because these gaps are small, they act as regions where oxides accumulate; once the oxides are used up, they cannot be replenished, resulting in a lower potential ； Outside the gap, there is an abundance of oxidizing substances; the electrons generated by the metal dissolution are quickly captured at the microcathodes. Electrons do not accumulate easily at the microanodes, resulting in a higher potential. Thus, concentration cell corrosion occurs between the inside and outside of the gap. This type of corrosion starts slowly, but as more metal dissolves, its rate of occurrence **increases**. 2. Corrosion in high-temperature areas 2.1 Corrosion by high-temperature naphthenic acids (1) Corrosion mechanism Naphthenic acids are a general term for very complex mixtures of high-boiling-point carboxylic acids, with the general formula CnH2n-1COOH. Naphthenic acid causes little corrosion at low temperatures; however, once it boils, corrosion becomes severe in high-temperature, anhydrous environments, especially at the gas-liquid phase transition zone. The reaction is as follows: 2RCOOH + Fe → Fe(RCOO)2 + H2↑. The corrosion rate of naphthenic acid ranges from 220°C to 400°C. Within the temperature range of 220°C to 270°C, corrosion gradually increases, reaching its peak between 270°C and 280°C. When the temperature exceeds 280°C, the corrosion rate begins to decline, but a second peak in corrosion occurs when the temperature reaches 350°C to 400°C. The iron naphthenate formed as a result of naphthenic acid corrosion is oil-soluble, and can be carried away by the oil flow; it does not easily form a protective layer on the surface of metal equipment. The metal surface affected by naphthenic acid corrosion remains smooth and free of defects. In areas with high flow rates, linear grooves appear in the direction of the flow, while at low flow rates sharp pits form. In areas subjected to severe erosion, the surface thickness decreases uniformly. In high-temperature systems, in addition to reacting directly with iron, naphthenic acids can also react with the corrosion product FeS, destroying the protective film formed by FeS on the metal surface and thus creating a corrosion cycle. 2RCOOH + FeS → Fe(RCOO)2 + H2S; H2S + Fe → FeS + H2. In addition to temperature, factors that affect naphthenic acid corrosion also include the flow rate of the medium, the acid value, and the sulfur content in crude oil. At present, it is not yet clear what the specific critical temperature, critical acid value, and critical flow rate for naphthenic acid corrosion are, nor which of these factors – temperature, acid value, or flow rate – plays a dominant role in the corrosion process under different operating conditions; further research is needed on this topic. ⑵Protection measures ① Application of aluminized steel: Through technical improvements, aluminized tube heat exchanger tubes have been successfully used in areas subject to naphthenic acid corrosion. ②Chemically deposited Ni-P alloy coating: The heat transfer coefficient of the chemically deposited Ni-P alloy coating is similar to that of carbon steel, so it does not reduce heat transfer efficiency. In contrast, the heat transfer coefficient of 18-8 material is only 2/3 of that of carbon steel, which affects the heat transfer performance.