Process Design: 【One Question per Week】Week 45, 2011: Common Corrosions in Petrochemical Production Processes and Solutions.

Common types of corrosion. 1. Based on the mechanism of occurrence, corrosion can be classified into three main categories: chemical corrosion, electrochemical corrosion, and physical corrosion. (l) Chemical corrosion: Chemical corrosion refers to the damage caused by a purely chemical reaction between the metal surface and a non-electrolyte. It is characterized by a redox reaction occurring directly between the atoms on the metal surface and the oxidants in the non-electrolyte, resulting in the formation of corrosion products, without any electric current being generated. Sulfur corrosion of metals in high-temperature gases, as well as the high-temperature oxidation of metals, both fall under chemical corrosion. (2) Electrochemical corrosion: Electrochemical corrosion refers to the degradation that occurs as a result of an electrochemical reaction between the metal surface and an ion-conducting medium. Its characteristic is that it includes at least one anodic reaction and one cathodic reaction. During the corrosion reaction, electrons flow from the anode area to the cathode area through the metal, which inevitably results in the generation of an electric current. Electrochemical corrosion is the most common and prevalent type of corrosion. For example, the corrosion of metals in the atmosphere, seawater, soil, and various electrolyte solutions falls into this category. Under normal circumstances, stress corrosion, fatigue corrosion, wear corrosion, biological corrosion, etc., are all forms of electrochemical corrosion that occur in conjunction with other conditions. (3) Physical corrosion: It refers to the degradation of a metal caused by simple physical dissolution. This type of corrosion is not common in engineering. But recently, this type of corrosion has occurred on the tubes of our pyrolyzer. 2. Classified by the form (characteristics) of corrosion, it can be divided into three main categories: uniform corrosion, local corrosion (crevice corrosion, intergranular corrosion), and stress corrosion. (l) Uniform corrosion: It is characterized by corrosion occurring uniformly over the entire metal surface. Most chemical corrosions fall into this category. Uniform corrosion is the least hazardous type of corrosion; in engineering, providing an adequate corrosion allowance is usually sufficient to ensure the mechanical strength and service life of the material. Uniform corrosion is commonly evaluated by the depth of corrosion of the metal material by the corrosive agent per unit time, or the amount of thinning of the wall thickness of the metal component (referred to as the corrosion rate). The SH 3059 standard specifies that materials with a corrosion rate of no more than 0.05 mm/year are considered fully corrosion-resistant; materials with a corrosion rate of 0.05–0.1 mm/year are considered corrosion-resistant ； Materials with a corrosion rate of 0.1–0.5 mm/year are considered moderately corrosion-resistant materials ； Materials with an annual corrosion rate of over 0.5 mm/year are considered non-corrosion-resistant materials. (2) Local corrosion: Local corrosion, also known as non-uniform corrosion, is characterized by the occurrence of corrosion in specific areas of the metal material. Although local corrosion does not result in as much metal loss as uniform corrosion, its harmfulness is far greater than that of uniform corrosion. This is because uniform corrosion is easy to detect and prevent, whereas local corrosion is difficult to predict and stop; it often causes sudden failure of metal components without any warning, leading to serious accidents. When selecting materials, efforts are made to avoid the occurrence of localized corrosion. Depending on the conditions and patterns under which corrosion occurs, local corrosion can be classified into the following types: 1) Galvanic corrosion – When two metals or alloys with different electrode potentials come into contact in an electrolytic solution, the metal with the more negative potential will corrode more rapidly, while the metal with the more positive potential will experience slower corrosion; this type of corrosion is known as galvanic corrosion. When carbon steel comes into contact with stainless steel in an electrolyte environment, the carbon steel undergoes accelerated corrosion, while the stainless steel remains protected. In engineering, contact between metals with different potentials in a corrosive environment should be avoided as much as possible. 2) Pitting corrosion: In metals with a passive or protective film on their surface, when defects such as mechanical cracks, scratches, or inclusions cause uneven thickness of the film—or even expose the base metal—an active-passive corrosion cell is formed, leading to localized corrosion. This type of corrosion generally progresses vertically, resulting in pits or corrosion holes. Such metals include aluminum and aluminum alloys, stainless steel, heat-resistant steel, titanium alloys, etc. Such corrosive environments are often media containing chloride ions or chlorides.

Between 2006 and 2007, the four crude oil/pre-flash drum heat exchangers in the atmospheric and vacuum distillation unit of my workshop experienced internal leaks multiple times; at its worst, this occurred three times within a single month. It severely affected the normal production in my workshop, resulting in high maintenance costs. After research in the workshop, it was decided to establish a dedicated anti-corrosion task force, and measures such as enhanced one removal and three injections were adopted. Meanwhile, specific individuals were assigned responsibility, with one person from each team tasked with anti-corrosion work to ensure proper injection of corrosion inhibitors. After some time of effort, the pH value of the three cutwater units stabilized, and no internal leaks occurred in the four normal-top/crude oil heat exchangers within half a year

Common metal corrosion types include general corrosion. Anti-corrosion measures involve applying protective treatments to the metal surface and designing the structure with a margin for corrosion. Localized corrosion includes intergranular corrosion, which occurs particularly often in austenitic stainless steels; anti-corrosion measures for this include solution treatment to reduce the carbon content to below 0.03%. Gap corrosion can be mitigated by eliminating gaps as much as possible. Stress corrosion can be addressed through heat treatment and by designing the structure to prevent stress concentration. Corrosion fatigue is a concern in applications such as bellows compensators, and such components should be used with caution in environments with flammable, explosive, or toxic substances. Hydrogen corrosion can be prevented by avoiding the use of high-strength carbon steel; NiCr alloy steel can be used instead. Wear corrosion can be prevented by ensuring that pumps have sufficient NPSH to avoid cavitation. Additionally, electrochemical corrosion can be controlled using methods such as cathodic protection

1. Based on the mechanism of corrosion, it can be classified into three main categories: chemical corrosion, electrochemical corrosion, and physical corrosion. 2. Classified by the form (characteristics) of corrosion, it can be divided into three main categories: uniform corrosion, local corrosion (crevice corrosion, intergranular corrosion), and stress corrosion.

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Corrosion of coolers and countermeasures 1. Overview: Coolers are one of the key pieces of equipment in production facilities. A large number of daily malfunctions and emergency repairs are caused by corrosion and leakage in the tube bundles of heat exchange equipment; this accounts for approximately 60% of all such incidents. It has severely affected the safe, stable, and full-load operation of the production facilities. Additionally, when cooling water exchanges heat with a medium at a higher temperature (in most cases, water flows through the tube side), it is prone to scaling, forming a layer of rust and scale. This increases the thermal resistance, significantly reducing the heat exchange efficiency and failing to meet production requirements. 2 Causes of scaling and corrosion 2.1 Analysis of the causes of scaling and corrosion on the inner walls of tube bundles In most coolers, water flows through the tubes; this is because cooling water contains calcium and magnesium ions as well as bicarbonates. When the cooling water flows over the metal surfaces involved in heat transfer, the following reactions occur: Mg2+ + HCO3- + H2O → MgCO3↓ + Mg(OH)2; 3MgCO3 + CO2. For Ca2+, the reaction is: Ca2+ + 2HCO3- → H2O + CO2 + CaCO3↓. When polyphosphates are added to the water as corrosion inhibitors, the following reaction takes place: 3Ca2+ + 2PO43- → Ca3(PO4)2↓. In addition, oxygen dissolved in the cooling water can also cause metal corrosion, leading to the formation of rust; the reaction is as follows: 2Fe + 2H2O + O2 → 2Fe(OH)2↓. As a result of these reactions, scale gradually forms on the heat transfer surfaces, along with the formation of rust. When the cooler is in operation, the heat transfer efficiency is significantly reduced due to the effect of scale. In some individual tubes, the heat exchange tubes became clogged within less than a year of use. Furthermore, the presence of scale can lead to under-scale corrosion on the inner wall of the tubes, reducing the service life of the tube bundle. The corrosion of metal surfaces by water is mainly electrochemical corrosion. In the corrosion cell, the cathodic reaction is primarily the reduction of oxygen, while the anodic reaction is the dissolution of iron. The corrosion reaction of carbon steel in water is as follows: Anodic reaction: 2Fe → 2Fe²⁺ + 4e⁻; Cathodic reaction: O₂ + 2H₂O + 4e⁻ → 4OH⁻; Overall reaction: 2Fe + 2H₂O + O₂ → 2Fe(OH)₂↓. During corrosion, iron forms iron hydroxide, which precipitates out of the solution. Since this ferrous compound is unstable in oxygen-containing water, it will further react with oxygen to form iron hydroxide. 2Fe(OH)2 + 2H2O + 1/2 O2 → 2Fe(OH)3↓; thereafter, iron hydroxide loses water to form rust. 2Fe(OH)3 → FeOOH ↓ + H2O Therefore, in the case of metal corrosion beneath scale, the inherent electrochemical corrosion process exhibits a self-catalytic effect, which accelerates the corrosion of the metal. 2.2 Causes of corrosion of the oil phase on the outer wall of the tube bundle Corrosion of the outer wall of the tube bundle in heat exchangers is a common problem in the operation of petroleum refining plants; particularly, corrosion in the low-temperature areas at the tops of towers, as well as in the cooling systems used in units for primary and secondary processing such as atmospheric and vacuum distillation, catalytic cracking, and delayed coking, is quite severe. Since the medium flowing in the shell side of the cooler is mostly oil or steam, and its operating temperature ranges from 100 to 160°C, both the corrosion patterns and the results of metallographic analysis indicate that it is electrochemical corrosion. The corrosion agents and operating conditions vary, which leads to some differences in the characteristics of corrosion, but the general principles of corrosion remain the same. Generally, corrosion is mild in the gas phase region, more severe in the liquid phase region, and most severe at the gas-liquid phase transition zone. The corrosion pattern consists of both general corrosion and local corrosion, with pitting and perforation being particularly prominent. The maximum rate of local corrosion can exceed 6 millimeters per year, while the average rate ranges from 1.2 to 5 millimeters per year. Due to varying degrees of presence of HCl, H2S, HCN, NH3, and H2O in the oil phase system, these substances volatilize along with the light components. When in gaseous form, their corrosive effect is generally minimal. However, after condensation and heat exchange, when the temperature drops by more than 100°C and liquid water appears in the condensation zone, corrosion caused by the HCl-H2S-H2O and HCN-NH3-H2O systems occurs within the cooler shell. The severe corrosion damage to primary processing units is caused by a cyclic corrosion process in which HCl and H2S interact with each other. The reactions involved are: Fe + 2HCl → FeCl2 + H2↑; FeCl2 + H2S → FeS↓ + 2HCl; Fe + H2S → FeS + H2↑; FeS + 2HCl → FeCl2 + H2S. The causes of corrosion in the condensation systems of secondary processing units differ from those in primary processing units; however, the degree of corrosion at the gas-liquid phase transition zones in the condensation areas remains the same. Judging from the appearance of the corroded tube bundle, the spaces between the tubes are blocked by loose corrosion products and debris, and pits appear on the metal surface. 3 Methods to address tube bundle corrosion. Generally, for the tube bundles of heat exchangers, it is not advisable to use tubes made of highly corrosion-resistant alloys. Because, first, the cost is high, and second, the heat transfer efficiency is poor. Therefore, over the past decade or so, our factory has adopted different anti-corrosion methods according to various corrosive environments, achieving very good results. The situation is as follows: 3.1 Addressing corrosion and scaling on the inner wall of the tube bundle 3.1.1 To deal with corrosion on the inner wall of the tube bundle, 7910 coating is used for anti-corrosion protection. The main component of 7910 coating is a synthesis of epoxy and amino resins; this material is of the thermosetting type and is composed of polymers. It has excellent resistance to weak acids, strong alkalis, and oxidizing agents in water. The anti-corrosion-treated tube bundle can be used when the shell-side inlet temperature is below 160°C, effectively solving the problem of corrosion on the inner metal surface of the tube bundle. 3.1.2 Addressing tube bundle scaling: Due to the varying operating conditions of production units, when water exchanges heat with media at higher temperatures, a considerable amount of scale forms on the inner walls of the tube bundles, which reduces the efficiency of the heat exchange equipment and affects its heat exchange performance. Based on the actual situation, the following measures were taken: (1) Application of high-pressure water jet technology – While maintaining the production equipment, high-pressure water jet technology was used to clean the rust layer from the inner walls of the tube bundles. This technology is suitable for equipment under mechanical cleaning conditions. It has strong cleaning power and a wide range of applications. When the nozzle pressure is 70 MPa or higher, water is ejected from a specialized nozzle at supersonic speed toward the object to be cleaned, removing various types of scale as if with a sharp blade. Its cleaning efficiency is also beyond comparison to manual cleaning. Applying this technology to heat exchange equipment to remove scale, rust, and certain types of fouling from metal surfaces is an effective method. Taking 1994 as an example, a total of 154 units were cleaned that year, covering an area of 31,491 square meters; calculations show that this resulted in benefits amounting to 6.44 million yuan. (2) Chemical cleaning technology is employed; the tube side of heat exchangers is mostly filled with circulating water. When exchanging heat with a medium at a higher temperature, scaling is likely to form on the inner wall of the tube, creating a scale layer that reduces the cooling efficiency. Some coolers cannot be stopped while in use, and forcing them to stop results in significant losses. We employed chemical cleaning to avoid shutting down the equipment. The situation is as follows: cooling water usually contains calcium and magnesium ions as well as bicarbonates. When cooling water flows over the metal surface, carbonates are formed. Additionally, oxygen dissolved in the cooling water can also cause metal corrosion, leading to rust formation. The heat exchange efficiency decreases due to the formation of rust scale. In severe cases, it is necessary to spray cooling water outside the casing; severe scaling can block the pipes, rendering the heat exchange ineffective. Through experimental screening, nitric acid has been identified as a suitable solvent for removing scale from pickling water. This is because nitric acid reacts with the scale (calcium carbonate, magnesium carbonate) as follows: 2HNO3 + CaCO3 → Ca(NO3)2 + CO2 ↑ + H2O; 2HNO3 + MgCO3 → Mg(NO3)2 + CO2 ↑ + H2O. It offers advantages such as rapid scale dissolution, high solubility of the resulting nitrates in water, and ease of use. Since nitric acid solution itself has a strong corrosive effect on metals, a certain amount of corrosion inhibitors and other additives must be added during the pickling process to protect the metal surface. Cleaning is carried out to meet production requirements. This method has met the requirement that certain heat exchange equipment cannot be taken out of service for cleaning. It saves labor, materials, and time; ensures excellent cleaning quality; and increases work efficiency by over tenfold. For example, if 11 coolers covering 1,350 square meters are cleaned in 1990, a direct benefit of 145,800 yuan can be obtained. (3) Use of online cleaning technology: The online automatic cleaning technology (also known as spring-based automatic online cleaning) is a mechanical method. At its core, it is a simple mechanical system composed of a helical spring and solid components, which is installed inside the heat exchanger tubes. Under the action of the fluid, continuous radial and axial vibrations are generated, disturbing the laminar layer at the inner wall of the tube and increasing the degree of turbulence. This effectively prevents the accumulation of dirt, thereby reducing the thermal resistance within the tube and enhancing its heating efficiency. Furthermore, the vibration of the helical spring and the repeated friction against the tube wall also help to remove dirt. 3.2 Addressing the issues of corrosion on the outer wall of the tube bundle and rust deposits: Generally, light-grade oils are used as the fluid flowing through the shell of heat exchangers; however, due to the harmful impurities present in these oils, severe corrosion occurs on the outer wall of the tube bundle. As observed on the corroded surface, the spaces between the tube bundles were blocked by loose corrosion products and debris, and pits appeared on the metal surface. At the same time, corrosion products increase fluid resistance and thermal resistance, reducing the heat exchange efficiency of the equipment. 3.2.2 The tube bundles of the heat exchange equipment are coated with a chemical “Ni-P” coating. Since in some coolers the medium temperature is relatively high (t > 160°C, pressure P > 1 MPa), the use of 5454 tube bundles is not suitable; however, anti-corrosion measures are still necessary. Since 1994, the tube bundles of the cooling equipment in some units have been coated with a Ni-P electroless coating, yielding excellent results. Since the Ni-P coating is a metallic layer with an amorphous structure, it lacks crystal defects such as grain boundaries and dislocations; it has a uniform structure that makes point corrosion unlikely, thereby granting it high corrosion resistance. In some media, the Ni-P coating performs better than titanium alloys; it does not exhibit tendencies such as pitting, intergranular corrosion, stress corrosion, or local corrosion. Coating low-carbon steel with a chemically deposited Ni-P alloy coating can replace some stainless steels, thereby **reducing costs**. At the same time, the Ni-P coating features good uniformity, strong adhesion, high hardness, and excellent wear resistance. 3.3 Use of chemical cleaning techniques: As mentioned earlier, using nitric acid to clean the scale on the inner walls of the tubes in heat exchange equipment has yielded very good results. However, using nitric acid to remove the rust layer from the outer wall of the tube bundle does not yield satisfactory results. Because the outer surface of the tubes without anti-corrosion measures is covered with many corrosion products, this not only affects heat transfer efficiency but also accelerates the corrosion of the metal surface. Although high-pressure water jet technology is used for core extraction during maintenance, the results are not satisfactory. To address this issue, X-ray diffraction analysis was conducted on the corrosion products on the outer surface of the tubes; the main components were FeS2, Fe3O4, and Fe2O3. Comparative tests show that using a hydrochloric acid solution with an appropriate corrosion inhibitor yields the best results, while also being the most cost-effective. An aqueous hydrochloric acid solution has a dissolving effect on iron oxides; the mechanism is as follows: FeO + 2HCl → FeCl2 + 2H2O; Fe2O3 + 6HCl → FeCl3 + 3H2O; Fe3O4 + 8HCl → FeCl2 + 2FeCl3 + 4H2O; Fe + HCl → FeCl2 + H2 ↑ Hydrochloric acid is an inorganic acid commonly used for the chemical cleaning of metal equipment. It exhibits a high dissolution rate and capacity for various iron oxides; the resulting salts are readily soluble, the process is simple and safe, and the equipment remains in excellent condition after cleaning. Therefore, based on the corrosion and scaling conditions of the heat exchange equipment, it is necessary to adopt a method of „first diagnosing the problem, then determining the appropriate solution, and finally addressing the issue“. In other words, by adopting different protection methods based on the usage and corrosion conditions of the heat exchange equipment, the best economic benefits can be achieved. To determine the appropriate methods, we need to continuously conduct research and experimentation in practical work, and keep summarizing experience in order to minimize the corrosion losses of heat exchange equipment.

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